Executive summary - Energy Web viewThe Victorian Government is committed to leading ground-breaking research and accelerating the development of innovative technologies in the field

POWERLINE BUSHFIRE SAFETY PROGRAMREFCL Technologies Test Program – Final Report4 December 2015

Report and analysis by Dr Tony Marxsen, Marxsen Consulting Pty Ltd

This report was commissioned and produced with the authorisation of the Powerline Bushfire Safety Program, Department of Economic Development, Jobs, Transport and Resources.

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Centre of Excellence - Electricity Bushfire Safety

The Victorian Government is committed to leading ground-breaking research and accelerating the development of innovative technologies in the field of powerline bushfire safety.

The 10 year, $750 million Powerline Bushfire Safety Program (PBSP) was established in December 2011 for the purpose of implementing the recommendations from the Victorian Bushfires Royal Commission and subsequent Powerline Bushfire Safety Taskforce.

The PBSP’s primary purpose is to reduce the harm to people and property from bushfires started by electricity assets. This includes a five year, $10 million Research and Development Fund that invests in R&D initiatives aimed at reducing the risk of fires starting from powerlines.

The Victorian Government is committed to ensuring that the fight against the threat of bushfires uses the best proven technologies available and draws upon the know-how of those at the forefront of innovation. The critical areas of funding priority include:

bushfire mapping and modelling;

powerline faults and fire ignition; and

improved powerline conductor technology.

Working closely with industry, regulators and the research community the PBSP has undertaken projects which aim to improve the knowledge of bushfire behaviour, bushfire risk and advancing technological solutions that both contribute to enhanced safety on Victoria’s electricity distribution network and leave a legacy that will continue well after the life of the program.

The world first research in this report is one of these priority projects. The 2015 Kilmore trial of Rapid Earth Fault Current Limiter (REFCL) technologies built on the 2014 Frankston REFCL test program, which proposed a technology-neutral performance standard for ‘wire on ground’ earth faults. If this performance level were achieved, the risk of a fire starting even in worst case fire weather conditions would be greatly reduced. The report also draws on the findings of the 2015 Springvale Vegetation conduction ignition test program.

The 2015 Kilmore test program sought to:

Generate reliable information on the ability of four REFCL technologies of interest to meet the performance standard, as well as to generate understanding of the implementation issues associated with each technology; and

Confirm and refine the draft performance standard to improve its suitability for inclusion in an appropriate regulatory mechanism.

The Victorian Government is making available the findings from this research available in order to foster ongoing research and commercial development of new or enhanced products to further technology that prevents bushfires from powerlines.

© Marxsen Consulting Pty Ltd document.docx Friday, 4 December 2015

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Disclaimer

This report outlines the results of tests carried out for the Powerline Bushfire Safety Program at a purpose-built facility in Kilmore Victoria in the first half of 2015 in accordance with an Agreement between Marxsen Consulting Pty Ltd and the Victorian Department of Economic Development Jobs Transport and Resources. This report contains test results, observations, analysis, commentary and interpretation.

Subject to the Agreement, no warranty can be offered to third parties for:

The application of anything in this report for any purpose other than those required by the specific objectives of the test program outlined in the body of this report.

The direct application of anything contained in this report to any situation other than those that were recorded in the tests.

A complete set of test records is available in the public domain or (in the case of very large video files) upon request from the Powerline Bushfire Safety Program. Readers are advised to rely on their own analysis of these records if they wish to use this report for any purpose other than the specific objectives of the test program outlined in the body of this report.

Readers should in particular note the following qualifications:

The information in this report relates to ‘wire on ground’ and ‘branch touching wire’ 22kV powerline earth faults only. Readers who wish to use these results to derive conclusions for other types of network earth fault should rely on their own investigations.

Definitions of worst case fire risk conditions were derived from limited data. No warranty can be offered that even worse fire risk condition will not occur in practice.

Reasonable care has been taken to outline the rationale and evidence for findings. Readers should make their own judgements of the merits of any findings before relying on them.

Where a level of statistical uncertainty is stated, this is a statistical measure which cannot be applied to individual cases or small numbers of cases, but can only be validly applied to cohorts of cases large enough to meet the criteria set out in statistical theory.

Quantification of statistical certainty has not been possible for many findings due to test-to-test variation of factors that influence test outcomes. In such cases, readers should form their own judgement of the level of confidence they can place on the findings concerned.

Definition of typical and worst case ‘wire on ground’ and ‘branch touching wire’ earth faults has been done with the sole purpose of illustration of potential application of test results to real situations. Readers should rely on their own investigations to define the faults to which they wish to apply the tests results and to define the method they use to do so.

Many assumptions were used to generate insights, derive findings and interpret results. All reasonable care has been taken to explicitly document these assumptions and explain the rationale in each case, but no warranty can be offered that such documentation is complete or that any implicit or explicit assumptions used are valid.

Where mathematical theory has been used to derive insights from test results, care has been taken to outline the theory and how it was applied. However, no warranty is offered that the theory employed is valid or correctly applied.

Advice to the reader

Readers who are not familiar with REFCL technology are advised that prior reading of Appendices A and B of the 2014 REFCL Trial report may provide potentially useful perspective and background to assist a sound understanding of the findings and analysis set out in this report. Readers are also advised that reading of the report of the 2015 Vegetation Conduction Ignition test program carried out at Springvale is necessary to fully understand the background and implications of the vegetation earth fault test results set out in this document.


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Contents

Contents

1 EXECUTIVE SUMMARY.......................................................................................................................... 7

1.1 SUMMARY OF FINDINGS....................................................................................................................................71.2 SUMMARY OF RECOMMENDATIONS......................................................................................................................9

1.2.1 Recommendation 1: amend the draft REFCL performance standard................................................91.2.2 Recommendation 2: REFCL product development and testing.......................................................10

2 THE PROJECT...................................................................................................................................... 11

2.1 GENESIS AND OBJECTIVES................................................................................................................................112.2 GOVERNANCE................................................................................................................................................122.3 PROJECT TIMELINE..........................................................................................................................................122.4 PROJECT TEAM..............................................................................................................................................14

3 EXPERIMENT DESIGN.......................................................................................................................... 15

3.1 DESIGN ELEMENTS FROM THE 2014 REFCL TRIAL...............................................................................................153.2 DESIGN ELEMENTS FROM THE 2015 VEGETATION CONDUCTION IGNITION TESTS........................................................153.3 THE DIMENSIONS OF EARTH FAULT PERFORMANCE................................................................................................163.4 RELATING ‘WIRE DOWN’ TEST RESULTS TO REAL NETWORK EARTH FAULTS..................................................................163.5 KEY ASSUMPTION: WORST-CASE LENGTH OF CONDUCTOR ON THE GROUND...............................................................173.6 THE TEST NETWORK........................................................................................................................................17

4 THE DRAFT REFCL PERFORMANCE STANDARD.....................................................................................19

4.1 THE INITIAL DRAFT REFCL PERFORMANCE STANDARD............................................................................................194.2 THE PERFORMANCE STANDARD AS AMENDED BY THE RECOMMENDATIONS OF THIS REPORT...........................................19

5 REFCL TECHNOLOGIES......................................................................................................................... 21

5.1 ARC SUPPRESSION COIL (ASC).........................................................................................................................215.2 GROUND FAULT NEUTRALISER (GFN)................................................................................................................225.3 SOLID STATE FAULT CURRENT LIMITER (SSFCL)...................................................................................................235.4 FAULTED PHASE EARTHING (FPE).....................................................................................................................245.5 REFCL CONFIGURATIONS.................................................................................................................................25

5.5.1 GFN configuration..........................................................................................................................255.5.2 ASC+FPE configuration...................................................................................................................255.5.3 SSFCL+FPE configuration................................................................................................................25

6 FAULT DETECTION PERFORMANCE...................................................................................................... 26

6.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS................................................................................................266.2 IS IT NECESSARY TO INCLUDE FAULT DETECTION PERFORMANCE IN THE STANDARD?......................................................266.3 IS THE SPECIFIED LEVEL OF FAULT DETECTION PERFORMANCE RIGHT?........................................................................27

6.3.1 Calculation of fault detection sensitivity required to prevent fires.................................................276.3.2 Lineal soil current intensity threshold for ground ignition..............................................................276.3.3 Insights from test results into ‘wire down’ ground ignition processes............................................316.3.4 Length of fallen conductor in contact with the ground..................................................................326.3.5 ‘Time to detect’ performance........................................................................................................33

6.4 IS THE SPECIFIED LEVEL OF FAULT DETECTION PERFORMANCE ACHIEVABLE?................................................................336.4.1 Fault detection test results from Kilmore South.............................................................................336.4.2 Achievement of fault detection performance in larger networks...................................................346.4.3 Achievement of network capacitive balance..................................................................................36

6.5 ARE THE BEST WORDS USED TO SPECIFY FAULT SENSITIVITY?...................................................................................36

7 FAULT RESPONSE: SPEED AND COMPLETENESS OF VOLTAGE CANCELLATION........................................37

7.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS................................................................................................377.2 IS IT NECESSARY TO INCLUDE FAULT RESPONSE IN THE DRAFT STANDARD?..................................................................37


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7.3 IS THE SPECIFIED LEVEL OF FAULT RESPONSE RIGHT?..............................................................................................387.3.1 Fault response required to extinguish bounce arcs........................................................................387.3.2 Fault response speed required to prevent bounce ignition.............................................................407.3.3 Fast fault response required in larger networks.............................................................................417.3.4 Choice of fault resistance in tests for fast voltage cancellation.....................................................437.3.5 Fault response required to prevent ground ignition.......................................................................437.3.6 Fault response performance measurement challenges..................................................................437.3.7 Recommendation – fault response performance standard............................................................44

7.1 IS THE SPECIFIED LEVEL OF FAULT RESPONSE ACHIEVABLE?......................................................................................447.2 ARE THE BEST WORDS USED TO SPECIFY FAULT RESPONSE?.....................................................................................47

8 FAULT MANAGEMENT: LIMIT ON RESIDUAL CONDUCTOR VOLTAGE......................................................48

8.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS................................................................................................488.2 IS IT NECESSARY TO INCLUDE A LIMIT ON RESIDUAL CONDUCTOR VOLTAGE IN THE DRAFT STANDARD?..............................488.3 IS THE SPECIFIED LIMIT ON RESIDUAL CONDUCTOR VOLTAGE RIGHT?.........................................................................48

8.3.1 Worst-case conditions for a fire from residual voltage...................................................................498.3.2 The KMS test results – ignition from residual voltage....................................................................508.3.3 Derivation of the limit on residual voltage.....................................................................................528.3.4 Inclusion of a safety margin to cover remote faults.......................................................................53

8.4 IS THE SPECIFIED LIMIT ON RESIDUAL CONDUCTOR VOLTAGE ACHIEVABLE?..................................................................548.4.1 Reduction of residual conductor voltage by the GFN.....................................................................548.4.2 Reduction of residual conductor voltage by FPE.............................................................................55

8.5 ARE THE BEST WORDS USED TO SPECIFY THE RESIDUAL CONDUCTOR VOLTAGE LIMIT?...................................................56

9 FAULT MANAGEMENT DIAGNOSTIC TESTS: LIMIT ON CURRENT AND I2T...............................................57

9.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS................................................................................................579.2 IS IT NECESSARY TO INCLUDE LIMITS ON DIAGNOSTIC TEST CURRENT AND I2T IN THE DRAFT STANDARD?...........................579.3 IS THE SPECIFIED LIMIT ON DIAGNOSTIC TEST CURRENT RIGHT?................................................................................579.4 IS A COMPLIANCE TEST USING A RESISTIVE FAULT ADEQUATE?..................................................................................599.5 IS THE LIMIT ON I2T RIGHT?..............................................................................................................................659.6 ARE THE SPECIFIED LIMITS ON DIAGNOSTIC TEST CURRENT AND I2T ACHIEVABLE?........................................................669.7 ARE THE BEST WORDS USED TO SPECIFY THE LIMIT ON DIAGNOSTIC TEST CURRENT?....................................................67

10 GFN COMPLIANCE TESTS..................................................................................................................... 68

10.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS..............................................................................................6810.2 PERFORMANCE REQUIREMENT: DETECT 25,400Ω EARTH FAULT IN 1.5 SECONDS......................................................6810.3 PERFORMANCE REQUIREMENT: REDUCE VOLTAGE TO 250V IN 2 SECONDS..............................................................7010.4 PERFORMANCE REQUIREMENT: FAST RESPONSE TO LOW IMPEDANCE FAULTS............................................................7010.5 PERFORMANCE REQUIREMENT: MAXIMUM 0.5A CURRENT IN DIAGNOSTIC TESTS......................................................7110.6 PERFORMANCE REQUIREMENT: I2T<0.1A2S IN DIAGNOSTIC TESTS..........................................................................71

10.6.1 I2t in high-impedance fault tests................................................................................................7110.6.2 I2t in low-impedance fault tests.................................................................................................73

10.7 COMPLIANCE TEST PROCEDURE.......................................................................................................................75

11 REFCL MANAGEMENT OF HARMONICS................................................................................................79

11.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS..............................................................................................7911.2 FIRE RISK FROM HARMONICS IN NETWORK EARTH FAULTS.....................................................................................7911.3 INCREASED GROUND IGNITION FIRE RISK FROM HARMONICS.................................................................................8111.4 FIRE RISK FROM HARMONICS IN DIAGNOSTIC TESTS.............................................................................................8211.5 INCREASED BOUNCE IGNITION FIRE RISK FROM HARMONICS..................................................................................8311.6 MEASUREMENT OF HARMONICS BY THE GFN....................................................................................................8311.7 COMPENSATION OF HARMONICS BY THE GFN....................................................................................................8411.8 GENERATION OF HARMONICS BY THE GFN........................................................................................................88

11.8.1 How a non-linear ASC coil generates harmonic earth fault currents..........................................8811.8.2 Factors that determine the magnitude of harmonic fault current.............................................8811.8.3 Confirmation in simulations.......................................................................................................8911.8.4 Confirmation in test results........................................................................................................90


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11.8.5 Relevance to fire risk..................................................................................................................91

12 REFCL MANAGEMENT OF TWO-PHASES-TO-GROUND EARTH FAULTS...................................................92

12.1 SUMMARY OF FINDINGS AND RECOMMENDATIONS..............................................................................................9212.2 RELEVANCE TO FIRE RISK................................................................................................................................92

12.2.1 Back-fed faults (wire down, ‘downstream’ end remains connected)..........................................9212.2.2 Two-phases-to-ground earth faults (‘tree in powerline’ faults)..................................................93

12.3 GFN TREATMENT OF TWO-PHASES-TO-GROUND EARTH FAULTS.............................................................................9412.4 GFN TEST RESULTS: TWO-PHASES-TO-GROUND EARTH FAULTS...............................................................................9512.5 FIRE RISK REDUCTION WITH THE CURRENT GFN DESIGN.......................................................................................96

12.5.1 Back-fed faults...........................................................................................................................9612.5.2 Tree faults..................................................................................................................................97

12.6 APPROACHES FOR LOWER FIRE RISK IN TWO-PHASES-TO-GROUND EARTH FAULTS.......................................................98

13 REFCL MANAGEMENT OF VEGETATION EARTH FAULTS.........................................................................99

13.1 SUMMARY OF FINDINGS................................................................................................................................9913.2 THE KILMORE VEGETATION EARTH FAULT TESTS...................................................................................................9913.3 THE ‘FLARC’ – A QUALITATIVE DIFFERENCE WITH REFCL PROTECTION?.................................................................101

14 REFCL TECHNOLOGIES TEST PROGRAM DESIGN.................................................................................104

14.1 USE OF 2014 REFCL TRIAL DESIGN..............................................................................................................10414.2 USE OF 2015 VEGETATION CONDUCTION IGNITION TEST PROGRAM DESIGN..........................................................10514.3 THE TEST NETWORK – FEEDER KMS21..........................................................................................................105

14.3.1 Network capacitive balance.....................................................................................................10614.3.2 Network voltage harmonics.....................................................................................................10814.3.3 Network load...........................................................................................................................110

15 APPENDICES..................................................................................................................................... 111

15.1 APPENDIX A: TEST RECORDS........................................................................................................................11215.1.1 Valid ignition tests...................................................................................................................11215.1.2 Bolted resistive fault tests........................................................................................................12015.1.3 Setup and invalid tests.............................................................................................................123

15.2 APPENDIX B: HRL TECHNOLOGY REFCL TRIAL REPORT.....................................................................................129


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List of abbreviations

Acronym Explanation

22kV 22,000 volts – the ‘wire to wire’ voltage on most of Victoria’s electricity networks

50Hz 50 cycles per second – the frequency of electricity distributed in Victoria

ACR Automatic Circuit Recloser – a remote control pole-mounted high voltage switch

Amp, A Amperes – the unit used to measure flow of electric current

ASC Arc Suppression Coil – a REFCL component used in all resonant earthing schemes

DBRG The Distribution Business Reference Group – network owner senior executives

DEDJTR The Victorian Department of Economic Development Jobs Transport and Resources

ESV Energy Safe Victoria – Victoria’s energy safety regulator

FCT Fault confirmation test – a function of the GFN to test for a sustained earth fault

FMC Fuel moisture content – the % moisture of grass and vegetation samples

FPE Faulted Phase Earthing – a type of REFCL technology

GFN Ground Fault Neutraliser – a REFCL product manufactured by Swedish Neutral AB

HMI Human Machine Interface – used to monitor and manage protection systems

NER Neutral Earthing Resistor – a non-REFCL network earthing approach used in Victoria

KMS Kilmore South – the location of the test program described in this report

Ohm The unit of measurement of electrical resistance (ratio of voltage to current)

PBST, PBSP Powerline Bushfire Safety Taskforce (2011) and Program (current)

RCC Residual Current Compensator – a component of the GFN product

REFCL Rapid Earth Fault Current Limiter – a technology that quickly limits earth fault current

SSFCL Solid State Fault Current Limiter – a high-voltage transistor switch used in a REFCL

TWG The PBSP Technical Working Group – network owners technical experts

VESI The Victorian Electricity Supply Industry


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1 Executive summaryThis Final Report sets out findings and recommendations arising from the four tranches of tests in the Powerline Bushfire Safety Program’s 2015 REFCL Technologies test program1 carried out at a custom-built test facility adjacent to AusNet Services’ Kilmore South zone substation in May, June, September and October 2015.

In the first three tranches of tests in May and June 2015, a total of 538 tests were performed on three REFCL configurations:

1. Ground Fault Neutraliser (GFN) a product of Swedish Neutral AB.2. Solid State Fault Current Limiter (SSFCL) with Faulted Phase Earthing (FPE). The SSFCL is a

product of Applied Materials Inc. of Gloucester Mass. USA. The FPE is a product of Noja Power of Brisbane.

3. Arc Suppression Coil (ASC, a component of the GFN) with FPE.

The fourth and final tranche of tests in September and October 2015 comprised a further 297 tests focused on GFN compliance with the amended performance standard and its performance in management of ‘branch touching wire’ vegetation faults, network harmonics, back-fed earth faults and two-phases-to-ground earth faults.

Two types of tests were performed: ‘bolted’ resistive earth fault tests ranging from zero Ohms (a direct short circuit) to 40,000 Ohms; and, ignition tests using either a ‘wire on ground’ earth fault simulation or a ‘branch touching wire’ earth fault simulation, both in worst case fire risk conditions.

This report addresses three project objectives:

Review and finalise the draft REFCL performance standard proposed for application in areas of Victoria that pose extreme risk from powerline related bushfires;

Test the performance of the three REFCL technologies against the draft performance standard and any recommended amendments to it; and

Confirm the fire risk benefits of REFCL technology in vegetation earth faults.

The project also delivered major benefits in building deeper understanding of how REFCL technologies work and how they can be applied to Victoria’s rural electricity distribution networks to reduce fire risk.

The tables in the following two sections summarise the findings and recommendations developed from the test results.

1.1 Summary of findings

Table 1 summarises findings derived from the Kilmore South test results, taking into account results from the PBSP’s earlier 2014 REFCL Trial and 2015 Vegetation Conduction Ignition test program.

Table 1: summary of findings – REFCL Technologies test program

1. Fault detection1a

Fault detection performance is an essential part of the performance standard if fire risk reduction is to be achieved.

1b

The fault detection performance (detection of a 25,400 ohm earth fault within 1.5 seconds) specified in the draft standard is appropriate, realistic and necessary to achieve target fire risk reduction.

1c The fault detection performance specified in the draft performance standard has been demonstrated in tests at Kilmore South.

1 The test program also included tests of the fire performance of lo-sag covered conductor in ‘wire down’ faults. The results of these tests are the subject of a separate report: Ignition tests: lo-sag conductor, 4 December 2015.


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1d

Tests confirm that theoretical calculation methods for analysis of fault detection performance are reliable, but a key network parameter required for such analysis (total system damping) cannot be determined with any certainty before REFCL commissioning.

1e

The specified level of fault detection performance is likely to be achievable in most of Victoria’s rural electricity networks using currently available technology.

1f A high degree of network capacitive balance will generally be required for achievement of the fault detection performance specified in the draft standard and realistic engineering options to achieve this are available.

1g

To achieve the performance standard, some substations supplying larger networks may have to be fitted with multiple REFCLs and required to operate with split busbars during high fire risk periods.

1h

There may be a very small number of the largest rural networks that may struggle to meet the fault detection performance standard and a regulatory process should be available to manage such cases.

1i Network owners should take care to preserve fault detection performance when designing future network augmentations. This may require changes to design policies and possibly the use of over-rated cable in new housing estates or other major undergrounding projects.

2. Fault response2a

Fault response performance (speed and completeness of cancellation of the voltage on the faulted conductor) is an essential part of the performance standard if fire risk reduction is to be achieved.

2b

The fault response performance required to prevent ignition depends on fault current and ignition type. For high current faults, speed is likely to be more important than completeness; for lower current faults, completeness can be more important than speed.

2c To prevent bounce ignition, it is necessary that voltage on the faulted conductor be limited to less than 1,900 volts within 85 milliseconds of an earth fault comprising a 400 Ohm resistor between any powerline conductor and earth.

2d

To prevent ground ignition, it is necessary that voltage on the faulted conductor be limited to less than 750 volts within 500 milliseconds of an earth fault comprising a 400 Ohm resistor between any powerline conductor and earth.

2e

To prevent ground ignition, it is necessary that voltage on the faulted conductor be limited to less than 250 volts within two seconds of any detectable earth fault.

2f The recommended performance standard for fault response can be met by available technology. Specifically, the GFN is fully compliant and the SSFCL+FPE and ASC+FPE are at least partially compliant to the extent they could be tested.

3. Fault management (residual conductor voltage)3a

A limit on residual conductor voltage in the case of a sustained powerline fault is an essential part of the performance standard if fire risk reduction is to be achieved.

3b

The proposed limit of 250 volts at the substation is appropriate and realistic for Victoria’s rural networks.

3c All three of the tested REFCL technologies can achieve the 250 volt limit.4. Fault management (diagnostic tests)4a

Limits on current and energy release in diagnostic tests are an essential part of a performance standard if fire risk reduction is to be achieved.

4b

The existence of harmonic voltages on networks prevents effective compliance tests of fault management in low impedance faults but does not materially affect the validity of tests using a 25,400 Ohm resistor.

4c The 0.5 Amp limit on fault current during diagnostic tests is appropriate, realistic and necessary for the target reduction of fire risk.

4d

The 0.5 Amp limit can be achieved with available technology, specifically by a GFN.

4e

The 0.04 A2s limit on I2t for prevention of fires in diagnostic tests is over-conservative and can be relaxed. A more appropriate value would be 0.1 A2s.

4f The SSFCL+FPE and ASC+FPE configurations did not provide diagnostic test functionality so their performance against the associated performance criteria could not be tested.

5. GFN compliance tests


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5a

The GFN was found to comply with all elements of the draft performance specification as amended by the recommendations in this report.

6. REFCL management of harmonics6a

Harmonic currents generated by customer loads can flow in a network earth fault and may have the potential to increase fire risk from such faults.

6b

It is likely the contribution of network harmonics to fire risk is second order.

6c Harmonics do not significantly increase fire risk from GFN diagnostic tests.6d

The GFN has the capability to reduce low-order harmonic components in earth fault current.

6e

Further product development is required if the GFN harmonic compensation capability is to provide its full potential fire risk benefits.

6f The GFN appears to produce harmonic components in earth fault current when it acts to fully displace network voltages – this is likely to be a property inherent in all ASC-based REFCLs

7. REFCL management of back-fed and two-phase-to-earth faults7a

The current GFN design is likely to achieve significant (50-90%) mitigation of fire risk from complex tree faults that present as two-phases-to-ground earth faults.

7b

The test results confirmed that the GFN product is not yet engineered to minimise fire risk in two-phases-to-ground earth faults, but there is clear potential to achieve this outcome.

8. REFCL management of vegetation earth faults8a

REFCL network protection can cut fire risk from ‘branch touching wire’ earth faults, averaged over a wide range of species, by a factor of twelve, i.e. from 100% to 8%.

8b

A GFN can detect current in a ‘branch touching wire’ earth fault at a level of 0.5 amps and upon detection, reduce current through the vegetation to a level close to zero.

8c Vegetation earth fault fire probability is not materially affected by diagnostic tests that comply with the REFCL performance standard.

1.2 Summary of recommendations

The following recommendations have been derived from the findings and test results:

1.2.1 Recommendation 1: amend the draft REFCL performance standard

The draft performance benchmark for high fire risk areas should be amended to be:

1. For a high-impedance fault:a. The REFCL must detect the fault within 1.5 seconds of its occurrenceb. Within two seconds of fault occurrence, the REFCL must limit the voltage on the

faulted conductor to less than 250 volts except during diagnostic testsc. During diagnostic tests to confirm if the fault is sustained or not or to identify which

powerline it is on, the REFCL must:i. Limit the fault current to less than 0.5 amps

ii. Limit the I2t to less than 0.1 A2s

2. For a low impedance fault:a. Within 85 milliseconds of fault occurrence the REFCL must limit the voltage on the

faulted conductor to less than 1,900 volts b. Within 500ms of fault occurrence the REFCL must further limit the voltage on the

faulted conductor to less than 750 volts


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c. Within two seconds of fault occurrence the REFCL must further limit the voltage on the faulted conductor to less than 250 volts except during diagnostic tests.

The definitions of high-impedance fault and low-impedance fault should be:

High impedance fault: a resistance from any high voltage powerline conductor to earth of value equal in ohms to twice the nominal phase-to-earth voltage in volts (25,400 Ohms in a 22kV network); and

Low impedance fault: a resistance from any high voltage powerline conductor to earth of value equal in ohms to the nominal phase-to-earth voltage in volts divided by 31.75 (400 Ohms in a 22kV network).

Notes on the standard: The requirement set by the standard should be that of a compliance test. The voltages stated in the standard should be measured at the network’s source substation. Where there is no practicable engineering remedy, requests for adjustment of the fault

detection requirement should be considered for approval by the appropriate regulatory authority.

The application of the standard should recognise the possibility of an increase in the residual voltage measured at the substation above the 250 volt limit if required by the action of enhanced technology to reduce the voltage at the fault location below 250 volts.

1.2.2 Recommendation 2: REFCL product development and testing

The VESI should continue to work with REFCL manufacturers to encourage the following product developments in the short to medium term:

GFN:

Greater I2t safety margin to increase assurance of compliance in larger networks. Mitigation of risks posed by false-positive identification of back-fed earth faults when

operating with high fault detection sensitivity. Development of the harmonic compensation capability to further reduce fire risk in network

earth faults. Development of the treatment of two-phases-to-ground earth faults to further reduce fire

risk in these types of faults. The GFN should apply effective compensation to reduce earth fault current in all earth fault types without exception and should apply the same fault-confirmation test (proven to have low fire risk) to allow fast disconnection of sustained faults.

SSFCL, FPE, ASC:

Development of fault response and fault management capability to meet the REFCL performance standard.

Once a GFN is installed on a larger, more heavily loaded network, the VESI should test the GFN’s capability to reduce voltage at the location of a sustained remote fault.

The VESI should continue to use primary-side tests, i.e. application of resistive earth faults to the high voltage network, when commissioning REFCLs and for compliance tests.


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2 The projectThe REFCL Technologies test program was established by the Powerline Bushfire Safety Program (PBSP) as a limited duration research project. The field test phase of the project commenced on 11 th May 2015 and finished on the 16th October 2015.

2.1 Genesis and objectives

In September 2011, the Powerline Bushfire Safety Taskforce (PBST) recommended that REFCLs be installed on Victoria’s rural electricity distribution networks.

Although REFCL installation and requisite network hardening and balancing works are relatively expensive, the delivered fire risk reduction benefit per dollar spent is comparatively attractive because each REFCL provides protection against earth fault fires on all multi-phase powerlines in an entire substation network – on average 400-500km of powerline route length.

In December 2011, the Victorian Government established the PBSP for the express purpose of implementing Recommendations 27 and 32 of the 2009 Victorian Bushfires Royal Commission (VBRC) and the six recommendations of the PBST that gave specific direction to the VBRC recommendations.

In 2014, the PBSP carried out the REFCL Trial2 at Frankston to rigorously test REFCL technology on a real network. The project confirmed that a REFCL is effective in reducing fires started by electric arcs in ‘wire down’ single phase to earth faults on multi-wire 22kV powerlines.

Based on results from the 2014 REFCL Trial a technology-neutral performance standard was proposed for ‘wire on ground’ earth faults, such that if this performance level were achieved, the risk of a fire starting even in worst case fire weather conditions would be greatly reduced. The key elements of this draft performance benchmark were:

1. Detection within 1.5 seconds of an earth fault that draws 0.5 amps of current and faster detection of faults that draw currents higher than this.

2. Fast limitation of voltage on the faulted conductor (as measured at the source substation) to a residual voltage less than 250 volts except during diagnostic tests to locate and confirm a sustained fault.

3. During network diagnostic tests, limitation of fault current to less than 0.5 amps and limitation of I2t in the fault.

In early 2015, the PBSP carried out a project at Springvale to understand ignition of fires by conduction of high voltage electricity through vegetation3. This project confirmed that fire risk could be reduced if the current in such faults was interrupted once it had reached a level of half an amp. This project also identified Willow (Salix Sp.) and Desert Ash (Fraxinus Angustifolia) as two tree species with particularly high fire risk in powerline faults.

The REFCL Technologies test program reported here was established by the PBSP to:

Generate reliable information on the ability of four REFCL technologies of interest to meet the performance standard, as well as to generate understanding of the implementation issues associated with each technology.

2 The report of the 2014 REFCL Trial is available at: http://www.energyandresources.vic.gov.au/energy/safety-and-emergencies/powerline-bushfire-safety-program/r-and-d/rapid-earth-fault-current-limiter/refcl-trial-report. 3 The report of the 2015 Vegetation Conduction Ignition tests is available at: http://www.energyandresources.vic.gov.au/__data/assets/pdf_file/0008/1192607/R_D_Report_-__Marxsen_Consulting_-_Vegetation_conduction_ignition_tests_final_report_15_July_2015_DOC_15_183075_-_external_.PDF


consultingmarxsen

http://www.energyandresources.vic.gov.au/__data/assets/pdf_file/0008/1192607/R_D_Report_-__Marxsen_Consulting_-_Vegetation_conduction_ignition_tests_final_report_15_July_2015_DOC_15_183075_-_external_.PDF

http://www.energyandresources.vic.gov.au/__data/assets/pdf_file/0008/1192607/R_D_Report_-__Marxsen_Consulting_-_Vegetation_conduction_ignition_tests_final_report_15_July_2015_DOC_15_183075_-_external_.PDF

http://www.energyandresources.vic.gov.au/energy/safety-and-emergencies/powerline-bushfire-safety-program/r-and-d/rapid-earth-fault-current-limiter/refcl-trial-report

http://www.energyandresources.vic.gov.au/energy/safety-and-emergencies/powerline-bushfire-safety-program/r-and-d/rapid-earth-fault-current-limiter/refcl-trial-report

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Confirm and refine the draft performance standard to improve its suitability for inclusion in an appropriate regulatory mechanism.

2.2 Governance

The PBSP team within the Department of Economic Development Jobs Transport and Resources (DEDJTR) was given oversight of the project with the support of Energy Safe Victoria (ESV) and each of the electricity distribution businesses in the Victorian Electricity Supply Industry (VESI). A Technical Working Group (TWG) of technical expert representatives from these organisations supported the research. Project governance arrangements are shown in Figure 1.

Figure 1: REFCL Technologies test program - governance arrangements

Technical Working Group

Distribution Business Reference Group

PBSP Team ESV

Program Control Board

Lead Researcher REFCL Technical Support

AusNet Services

Research Team

Research Program Sponsor

Strategic Decisions

Operational Control

Operational Activities

Technical Advice

The PBSP Team, ESV and the DBRG exercised oversight of delivery against key milestones and authorised any required strategy changes. The PBSP team in turn reported to a Program Control Board, which sponsored the research program and was the ultimate decision-maker.

The host distribution business had operational decision-making power on all aspects of the work that had potential to affect its broader distribution network and connected customers.

The Lead Researcher had operational control over the research program, including responsibility for managing consortium partners and direct engagement with the host distribution business for each stage of the testing, although all contracts were made directly with the PBSP team in DEDJTR.

2.3 Project timeline

The REFCL Technologies test program commenced with the mid-February 2015 engagement of the Lead Researcher and the first three tranches of field test activity started on 11th May 2015 and were completed by 25th June 2015. Tranche 4 started on 24th September 2015 and was completed on 16th October 2015. The broad timeline of project activities is shown in Figure 2.


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Figure 2: Schedule of major activities

2015 REFCL technologies test programMonth: February March April May June July August September October November December

Project start-upEngage test lead services providerEngage research services providerExecute formal network access agreement

Theoretical modellingEstablish data procurement channelsConstruct modelsFault modelling of REFCL technologiesDocumentation of models and results

Establishment of test facilityDefine test concept and high level test programDesign test facility including REFCL configurationsConstruct and commission test facility

Tests and analysisTranche 1 tests: fault current tests (5 series)Tranche 2 tests: ignition tests (5 series)Tranche 3 tests: 'second chance' tests (5 series)

Program planning and deliveryDraft interim report

Vegetation ignition tests Define test concept and high level test programConstruct and commission test facilityTranche 4 tests

Program planning and deliveryDraft final reportFinal report

Activity on site


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2.4 Project team

The key project team members were:

Role Name Affiliation

DEDJTR Program Director Ashley Hunt PBSP Director, DEDJTR

DEDJTR Project Manager Neil Saul PBSP project manager, DEDJTR

Lead Researcher Tony Marxsen Marxsen Consulting

Test Manager Blake Stewart HRL Technology

Host Network Test Lead Jon Bernardo AusNet Services

Network Protection & Control Engineers Graeme McClure, Brodie Stephenson AusNet Services, CitiPower/Powercor

Network Tester Sam Creamer AusNet Services

Site Controllers Tim Wall, Jason Ward AusNet Services

Data Managers Marc Listmangof, Sam Creek HRL Technology

Test Rig Operator Adrian Graves HRL Technology

High-speed Camera Operators David Adermann, Alexis Di Giovine MACS Images


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3 Experiment designThe 2015 REFCL Technologies test program drew extensively on concepts and designs developed in previous PBSP projects, starting with the three guiding principles:

1. Realism – tests must to the extent possible replicate the conditions in real powerline faults known to start fires

2. Worst case – tests must be carried out in worst case fire risk conditions (air temperature, wind speed, fuel moisture content, etc.)

3. Direct comparison – tests on different network protection configurations must to the extent possible be designed to support direct comparison of their fire risk performance.

The experiment design was based on the 2014 REFCL Trial (for the ‘wire down’ earth fault simulations) and the 2015 Vegetation Conduction Ignition test program (for the ‘branch touching wire’ earth fault simulations).

3.1 Design elements from the 2014 REFCL Trial

The following elements of experimental design were carried over from the 2014 REFCL Trial carried out at Frankston:

1. Classes of ‘wire down’ ignition: bounce ignition and ground ignition2. Test rig – with modifications to fit lo-sag covered conductor for Tranche 4 tests3. Data acquisition system and data processing procedures4. Soil bed and fuel sample preparation and management procedures5. Site safety architecture and safe operating procedures.

In nearly all respects the ‘wire down’ earth fault experiment design was identical to that used in the 2014 REFCL Trial. The only material difference was that the parallel current path via a ‘sandpit’ was not used in the 2015 tests. This had the effect that some tests at low current levels did not result in detection of the fault on the first bounce. These were marked as unrealistic and potentially invalid and results were excluded from some specific statistical analyses. In nearly all ‘wire down’ ignition tests, soil-bed management procedures maintained bounce current at four amps or greater so faults were detected quickly and this issue did not have a material effect on the rigour of the test program.

3.2 Design elements from the 2015 Vegetation Conduction Ignition tests

The following elements of experimental design were carried over from the 2015 Vegetation Conduction Ignition test program carried out at Springvale:

1. Test rig2. Vegetation sample preparation, management and analysis procedures3. Safety architecture and safe operating procedures.

In nearly all respects the ‘branch touching wire’ earth fault experiment design was identical to that used in the 2015 Vegetation Conduction Ignition test program. The only material differences were that the tested species were limited to those identified as ‘worst case’ in the earlier program (Salix Sp. and Fraxinus Angustifolia), the series resistance in the test rig high-voltage supply was 400 Ohms rather than 200 Ohms, and the test network was protected by the GFN set to a fault detection sensitivity close to 0.5 amps.

High-frequency fault signatures were not collected in the Kilmore tests. This allowed the same instrumentation and data acquisition configuration to be used in both the ‘wire down’ earth fault simulation and the ‘branch touching wire’ earth fault simulation.


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3.3 The dimensions of earth fault performance

For a fire to be avoided when an earth fault occurs in worst case fire weather conditions, the REFCL must exhibit a multi-stages response:

1. Detect the fault2. Ascertain which phase of the network is faulted3. Compensate to reduce the voltage of the faulted phase to a level close to zero 4. After a short delay, test to see if the fault is still present 5. If the fault is still present, identify the powerline on which the fault is located6. Disconnect the faulted powerline to remove the fault from the network7. Switch off the compensation to restore all network voltages to normal levels.

For the purposes of discussion and analysis of test results, these steps were divided into three broad groups as follows:

Fault detection: Detection of the fault and identification of the faulted phase Fault response: Cancellation of the voltage on the faulted phase Fault management: Maintenance of low voltage on the faulted phase, tests to confirm and

locate the fault (i.e. identify the faulted powerline), plus action to restore network voltages to normal once the fault is removed from the network.

3.4 Relating ‘wire down’ test results to real network earth faults

Two different types of ‘wire down’ earth fault simulations were used:

1. Bolted resistive fault tests: the test rig was shorted out and a series resistor in the HV supply to the test rig was used to simulate the earth fault.

2. ‘Wire down’ ignition test: the test rig operated in the normal way to cause a length of live powerline conductor to impact a soil-bed covered in standing dry grass. A low-value series resistor was used in the test rig high-voltage supply solely to limit current should there be a flashover within the rig.

These two arrangements approached the realism principle in different ways. Each had both realistic features and departures from realism as listed in Table 2. Taken together, with careful consideration of the non-realistic features of each, they allowed test results to be reliably related to real network earth fault scenarios.

Table 2: bolted resistive fault tests and ignition tests

Test type Realistic features Non-realistic features

Bolted resistive fault test Test current equals fault current seen at the source substation

Fault current is continuous.Fault resistance is linear.

Ignition test Fault current is intermittentFault resistance is non-linear

Test current does not equal current flowing in an equivalent real fault

To relate a particular test result to a real earth fault, two different currents must be considered:

a. Test current: the current into the test rig’s 400 millimetre (0.4 metres) long soil bed in an ignition test – this was the current measured on-site in the tests.

b. Fault current: the current that would flow in a real worst-case ‘wire on ground’ network earth fault – this is the current used to categorise faults into low impedance, high impedance, very high impedance, etc.


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The key enabling hypothesis is that fault current into the earth is evenly distributed over the length of conductor lying on the ground.

3.5 Key assumption: worst-case length of conductor on the ground

The test current and fault current are related by the length of conductor lying on the ground in the fault as follows:

Real network earth fault:

( lineal soil current intensity∈amps per metre )= ( fault current∈amps )( length of conductor on ground∈metres )

Fire risk variation with soil current intensity can be derived from tests on 0.4 metres of soil-bed conductor length using:

Ignition test:

( lineal soil current intensity∈amps per metre )= (recorded test current∈amps )(0.4metres )

Worst case fire risk tends to occur with a short length of conductor on the ground as this concentrates the fault current and increases the lineal soil current intensity. Worst case fire risk is realised when fault current is too low to be easily detected by powerline protection systems but lineal soil current intensity is high enough to start a fire. Since lineal soil current intensity varies inversely with the length of conductor on the ground, for any given value of fault current, worst case is the shortest realistic length of conductor on the ground for that type of fault.

A key question is therefore: what is the shortest realistic length of conductor to be lying on the ground in a ‘wire-down’ fault? There is no certain answer, but the following logic was used to derive a key assumption that the worst case is three metres of conductor on the ground:

The shortest realistic powerline span in rural areas is 25 metres long Sag in worst case fire weather will add one metre to make total conductor length 26 metres Typical powerline pole height is 10 metres A mid-span break4 will result (at each end of the span) in:

o 13 metres of conductor attached to the pole o 10 metres of conductor off the ground, i.e. between the ground and the pole-topo Three metres of conductor lying on the ground near the base of the pole.

In most ‘wire-down’ faults the length of conductor on the ground exceeds three metres – sometimes it is hundreds of metres - but this short length was assumed as a potentially realistic worst case scenario for fire risk.

The test results and analysis presented in this report can easily be reprocessed by network owners to provide guidance for any alternative assumed worst case conductor-on-ground length.

3.6 The test network

The tests were performed on the Kilmore South (KMS) 22kV distribution network. The test facility, though physically located immediately adjacent to the KMS zone substation, was connected to its 22kV distribution network at the end of 9.7 kilometres of feeder KMS21.

4 Anecdotal reports indicate that conductor breaks tend to occur either at one end of the span or at mid-span (due to corrosion from moisture collection there). A mid-span break can create both a direct-fed earth fault and a back-fed earth fault if both portions of conductor are on the ground. The back-fed fault can potentially make the direct-fed fault harder to detect. However, the materiality of this effect depends on many factors and it is assumed only the direct-fed fault exists in this scenario. See also Section 6.3.1.


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The total KMS21 feeder parameters during the tests were:

21 kilometres of overhead three-phase powerline 24 kilometres of single-phase (two-wire) spurs 2.2 kilometres of underground cable (substation exit, road/rail crossings, housing estates) 112 distribution substations 340 connected customers.

The KMS test network had features that were not reflective of the majority of rural networks in Victoria as indicated in Table 3 and these precluded some types of tests. The tests were carefully planned to minimise any risk of loss of validity that might arise from the potentially unrepresentative features of the KMS21 test network.

Table 3: KMS21 test network

Parameter KMS Average rural network in Victoria

Route length 45 km 450 km

Peak feeder load 10-15 amps 180 amps

Number of feeders 1 5

Total capacitance 2.5 µF 30-50 µF

Voltage harmonics 0.5% THD5 2% THD

5 This level of harmonics was measured in Tranche 2 tests. Harmonic levels in Tranche 4 tests were higher. See Section Error: Reference source not found for details.


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4 The draft REFCL performance standardFollowing the 2014 REFCL Trial, a technology-neutral REFCL performance standard was developed from the 2014 ignition test results such that if this standard was met, powerline fire risk even in worst-case fire conditions would be reduced.

A primary objective of the Kilmore test program reported here was to assess the degree to which each REFCL configuration met the criteria set out in this initial draft performance standard. At the same time, it also tested the draft performance standard itself to ensure it remained appropriate and to identify any opportunities for improvement.

The separate elements of the draft performance standard were assessed against the goal of fire risk reduction in the short/medium term, i.e. using currently available technologies. Specific questions were considered in respect of each element of the draft standard:

Is this element of the draft performance standard directly necessary to the achievement of the fire risk reduction objective?

Is the level of performance specified in this element of the draft performance standard appropriate, over-conservative or not demanding enough to achieve the fire risk reduction objective?

Is this element of the draft performance standard realistically achievable using currently available technology in a way that can lead to fire risk reduction?

Are the words used to describe this element of the draft performance standard effective?

Sections 6 to 9 below set out the answers to these questions together with associated rationale and supporting evidence.

In this assessment of the draft performance standard and in the development of recommendations for its refinement, it was recognised that the performance of a REFCL against the standard depends very much on parameters of the network in which it is installed. Differences between the Kilmore South network on which the tests were done and other rural networks in Victoria were kept in mind in addressing the four questions stated above. Some of these differences are set out in Section Error: Reference source not found below.

4.1 The initial draft REFCL performance standard

The draft REFCL performance standard comprised the following elements:

1. Fault detection: Detect within 1.5 seconds, resistive earth faults that have fault current of 0.5 amps; and

2. Fault response: Within 70 milliseconds of fault detection, limit voltage on the faulted conductor to 250 volts or less (measured at the zone substation); and

3. Fault management: Maintain 250 volt voltage limit while the fault remains present except during any diagnostic test; and during any diagnostic test:

a. Limit fault current to 0.5 amps or less; and b. Limit I2t to 0.04 amps-squared seconds or less.

For assessment of compliance with this performance standard, a resistive fault was defined as a fault comprising a linear resistor placed between a high voltage powerline conductor and earth.

4.2 The performance standard as amended by the recommendations of this report

Taking into account the recommendations set out in this report, the draft performance standard for high fire risk areas has been refined to be:


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1. For a high-impedance fault:a. The REFCL must detect the fault in 1.5 secondsb. Within two seconds and for as long as the fault is present, it must limit the voltage

on the faulted conductor to less than 250 volts except during diagnostic testsc. During diagnostic tests to confirm if the fault is sustained or not or to identify which

powerline it is on:i. It must limit the fault current to less than 0.5 amps

ii. It must limit the I2t to less than 0.1 A2s

2. For a low impedance fault:a. Within 85 milliseconds of fault occurrence the REFCL must limit the voltage on the

faulted conductor to less than 1,900 volts b. Within 500ms of fault occurrence it must further limit the voltage on the faulted

conductor to less than 750 volts c. Within two seconds of fault occurrence and for as long as the fault is present, it

must further limit the voltage on the faulted conductor to less than 250 volts except during diagnostic tests.

The definitions of high-impedance fault and low-impedance fault are:

High impedance fault: a resistance from any high voltage powerline conductor to earth of value equal in ohms to twice the nominal phase-to-earth voltage in volts (25,400 Ohms in a 22kV network); and

Low impedance fault: a resistance from any high voltage powerline conductor to earth of value in ohms equal to the nominal phase-to-earth voltage in volts divided by 31.75 (400 Ohms in a 22kV network).

The conductor voltages specified in the standard should be measured at the powerline network’s source substation.

In Tranche 4 of the test program, the compliance of one REFCL technology (the GFN) was rigorously tested against this amended standard with the results set out in Section 10 below.


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5 REFCL technologiesFour different REFCL technology components were selected for tests in the project:

5.1 Arc Suppression Coil (ASC)

An ASC is an inductor that connects the neutral of the source substation transformer(s) to earth. The inductance value is tuned to achieve near-resonance with the total network capacitance to earth. In this state the connection between the neutral and earth has very high impedance at 50Hz, so earth fault currents are greatly reduced – typically from more than 1,000 amps to less than ten amps. This technology is relatively simple and fully mature. It is widely used in many areas of the world and ASC products are offered by multiple competing suppliers.

In these tests, the ASC was that in the GFN system. It comprised a 505mH inductor (to provide 80 amps of inductive current at full neutral voltage displacement) in parallel with a variable capacitor which ranged from zero to 20.05 microfarads as required for network tuning. The test network tuned at a setting around ten amps, i.e. the parallel capacitor provided about 70 amps of capacitive current at full neutral displacement, which meant it had a value around 17.5 microfarads.

The GFN/ASC is shown in Figure 3 and the switched capacitor bank (located immediately above the ASC coil) used to tune the coil to resonate with the network capacitance is shown in Figure 4.

Figure 3: the ASC component of the Kilmore South GFN

It was recognised this type of ASC is not the same as many ASCs installed around the world which have a variable inductor with no parallel switched capacitors. Such devices may well produce test results that differ from those reported here.


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Figure 4: the switched tuning capacitors mounted above the ASC coil

5.2 Ground Fault Neutraliser (GFN)

The GFN is a proprietary product of Swedish Neutral AB (SN). It combines an ASC with a controlled solid-state inverter called a Residual Current Compensator (RCC). The ASC cancels the network capacitive current in the normal way, leaving just the network’s resistive damping current flowing in the fault plus any tuning mismatch current. The RCC acts to cancel this residual current. In this way, the fault current is further reduced to levels usually well below one amp. There are perhaps 100 GFNs in service around the world, including about 25 in service in New Zealand. The REFCL tested in the PBSP’s 2014 REFCL Trial at Frankston was a GFN. The performance of the GFN is determined by firmware embedded in dual (master-slave) digital processors which together occupy the GFN control cubicle, an example of which is shown in Figure 5.

Figure 5: KMS GFN control cubicle


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The Kilmore South RCC comprised a solid state three-phase inverter with its own 415 volt power supply from the station service transformer6. The GFN’s inverter technology is AC-DC-AC with the injected voltage synthesised using pulse-width modulation at a switching frequency of 8 kilohertz. The voltage was injected into a low-voltage (345 volts) winding on the ASC, so there was an approximately 37 times turns ratio between inverter current and current injected into the 22kV high voltage network neutral. At Kilmore South, the inverter had an output current limit of 437 amps, meaning it could supply up to 12 amps into the high-voltage network. The reference angles for the RCC’s synthesised 50Hz output voltage were taken from the power supply to the inverter. Apart from compensation, the inverter was used in a number of other GFN functions such as network voltage soaking, admittance measurement for fault confirmation and faulted powerline identification, and RCC tuning.

Figure 6: Kilmore South GFN RCC cubicle (with and without cover removed)

5.3 Solid State Fault Current Limiter (SSFCL)

The SSFCL is an emerging product developed by Applied Materials Inc. of Gloucester, Massachusetts, USA (AM). The SSFCL at Kilmore South comprised a high-voltage transistor switch in the neutral to earth connection of the substation. This switch could be set to open when neutral current exceeded a set threshold. By opening this connection, the earth fault current path was interrupted and the earth fault current was limited to the capacitive current of the network – for rural networks in Victoria this is typically between ten and 200 amps. The SSFCL operated at solid state speeds, i.e. in microseconds. The SSFCL tested in this project was AM’s development prototype. However, the components in it have been proven in AM’s other power network products.

The SSFCL switch comprised a ladder of six insulated gate bipolar transistor switches. A series RC over-voltage damping circuit was connected across the switch comprising a 150 ohm resistor and a 0.125 microfarad capacitor. A 24 kilovolt metal-oxide varistor was also connected across the switch for power frequency over-voltage protection. A current transformer provided an analogue current signal to the control electronics for the six transistor switches. Two control settings were available: detection sensitivity (the instantaneous current level at which the switch would open) and re-try

6 The Frankston South RCC was supplied by a dedicated separate station service transformer.


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period (the interval before each re-attempt to close). A re-close attempt would immediately (in 50 microseconds) re-open if the switch detected neutral current in excess of the detection threshold.

Figure 7: The Kilmore South SSFCL enclosure

5.4 Faulted Phase Earthing (FPE)

An FPE is a switch that closes to connect a faulted phase to earth within the source substation, thereby cancelling the voltage on the faulted phase and reducing the earth fault current flow. For FPE to function, the neutral connection to earth cannot be low impedance – otherwise very high currents will flow in the FPE switch – so it is usually combined with resonant earthing or high resistance earthing. It is a well-understood, mature technology used in a number of areas of the world, e.g. in Ireland where it is the local standard for all 10kV and 20kV networks.

The Kilmore South FPE used in the test program was a development prototype from Noja Power of Brisbane Queensland. It comprised three ACRs with a common controller that performed continuous faulted-phase identification logic. The operating close time of the ACRs was around 55 milliseconds. A signal to close the FPE was sent via a separate ABB REF620 relay with a typical transit time of ten milliseconds. The faulted phase was identified using continuous monitoring and comparison of the voltages on the three phases. The FPE equipment is shown in Figure 8.

Figure 8: The Kilmore South FPE switches and SSFCL+FPE control relay


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5.5 REFCL configurations

The following three REFCL configurations were defined for the KMS tests:

5.5.1 GFN configuration

The GFN product is a proprietary packaged all-in-one solution.

Most aspects of the GFN could be adequately tested on the KMS network. The exceptions were:

1. Harmonic compensation: The test network voltages had relatively low levels of harmonic distortion (see Section Error: Reference source not found). The GFN’s capability to measure and cancel harmonic components of the residual voltage driving fault current was tested in Tranche 4 tests (see Section Error: Reference source not found below), but its automated performance of this function could not be tested.

2. Reduced voltage at the fault location: The GFN has logic that can adjust the RCC compensation to progressively reduce the voltage at the location of a remote fault. The process takes about 20-30 seconds to complete. Due to the short feeder length and light load on the test network the voltage at the fault location was always close to the voltage at the substation and this function could not be adequately tested. It has potentially significant benefits in fire risk reduction and clear benefits for public safety in ‘wire down’ network faults.

The GFN was tested in all four tranches of the test program.

5.5.2 ASC+FPE configuration

The ASC reduces earth fault current but does not use active cancellation of the voltage on the faulted conductor to compensate for residual current. FPE adds this function.

The ASC used in tests was the ASC incorporated in the GFN, i.e. the RCC was disabled to leave the ASC as the only active component. This is not identical to a variable-inductance ASC as the fixed-inductance GFN coil has a variable tuning capacitor connected across it, whereas other types of ASC do not have such capacitors.

The FPE was able to be tested successfully on the test network. However, the short feeder length and the light load on the network meant the voltage at the fault location with the FPE active was essentially zero (less than 50 volts), so this aspect of the tests was potentially unrealistic.

It was found that the ASC+FPE configuration posed an unacceptable risk of interrupting customer supply. This was due to the presence of a Restricted Earth Fault system in the substation which interpreted the FPE current in the neutral connection of the transformer as evidence of an earth fault inside the substation transformer.

As a result, testing of the ASC+FPE configuration was very limited and performed in Tranches 1 and 2 only.

5.5.3 SSFCL+FPE configuration

The SSFCL opens the connection between the transformer neutral and earth to limit fault current to that driven by the network capacitance and resistive leakage. It then uses FPE to cancel the voltage on the faulted conductor and reduce the fault current further. The SSFCL continues to attempt to reconnect at regular intervals and immediately opens again if neutral current is still present. Each of these connection attempts lasts a few tens of microseconds.

Testing of the SSFCL+FPE configuration was done in Tranches 1, 2 and 3.


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6 Fault detection performanceThe draft performance standard specifies the capability to detect 0.5 amp resistive earth faults, i.e. those caused by a linear resistor between any powerline conductor and earth, within 1.5 seconds.

6.1 Summary of findings and recommendations

This report section sets out the rationale and evidence to support the following findings:

1. Fault detection performance is an essential part of the performance standard if fire risk reduction is to be achieved.

2. The fault detection performance specified in the initial draft standard is appropriate, realistic and necessary to achieve target fire risk reduction.

3. The fault detection performance specified in the initial draft performance standard has been demonstrated in tests at Kilmore South.

4. The wording of fault detection performance part of the draft performance standard should unambiguously specify the fault resistance to be used in compliance tests.

5. Tests have shown that theoretical calculation methods for analysis of fault detection performance are reliable, but a key network parameter required for such analysis (total damping) cannot be determined with any certainty before REFCL commissioning.

6. The specified level of fault detection performance is likely to be achievable in most of Victoria’s rural electricity networks using currently available technology.

7. A high degree of network capacitive balance will generally be required for achievement of the fault detection performance specified in the draft standard and engineering options to achieve this are available.

8. To achieve the performance standard, some substations supplying larger networks may have to be fitted with multiple REFCLs and required to operate with split busbars during high fire risk periods.

9. There may be a very small number of the largest rural networks that may struggle to meet the performance standard and a regulatory process should be available to manage such cases.

10. Network owners should take care to preserve fault detection performance when designing future network augmentations. This may require changes to design policies and possibly the use of over-rated HV cable in new housing estates or other major undergrounding projects.

Recommendations include:

1. The fault detection requirement in the performance standard should be expressed as the capability to detect within 1.5 seconds, an earth fault comprising a 25,400 Ohm resistor connected between any 22,000 volt powerline conductor and earth. Proportionately lower resistance values should apply for high voltage networks that operate at lower voltages.

2. In exceptional circumstances where there is no practicable engineering remedy, requests for adjustment of the applicable fault detection performance requirement should be considered for approval by the appropriate regulatory authority.

The following sections set out the rationale and evidence for each of the above statements.

6.2 Is it necessary to include fault detection performance in the standard?

This element of the draft standard is necessary for the goal of fire risk reduction to be achieved. If faults capable of causing fires cannot be detected by powerline protection systems, then the associated fire risk cannot be reduced by the response of such systems. The 2014 REFCL Trial proved that faults at current levels lower than are normally detected by powerline protection systems can cause fires. The inclusion of a specified level of fault detection sensitivity addresses this risk. Second,


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it is known that slow detection of a fault can result in a fire before any fault response can occur to prevent it, so a measure of fault detection speed is a necessary inclusion to ensure sensitive detection of faults occurs quickly enough to prevent fires.

6.3 Is the specified level of fault detection performance right?

Is the ‘0.5 amps in 1.5 seconds’ criterion appropriate or is it either over-conservative or too undemanding? A thorough review of this question confirmed it is appropriate. Details of this review are:

6.3.1 Calculation of fault detection sensitivity required to prevent fires

The specified fault detection level of 0.5 amps was originally derived by combining two elements:

The finding in the 2014 REFCL Trial that lineal soil current intensity below 0.15 amps per metre of fallen conductor did not lead to ignition of dry grass in worst case conditions; and

The assumption that ‘wire on ground’ faults will rarely have less than three metres of conductor lying on the ground.

The ignition threshold of 0.15 amps per metre was multiplied by the worst-case three metre length of conductor on the ground to give 0.45 amps, which was rounded up to produce the 0.5 amps fault detection limit and avoid any misleading impression of greater precision than was available given the degree of uncertainty in the two key constituent numbers.

6.3.2 Lineal soil current intensity threshold for ground ignition

The 2014 REFCL Trial at Frankston South was the authoritative study that first defined the value of lineal conductor-soil current density that forms the boundary between fire and non-fire test results in worst-case fire risk conditions. The specific 2014 test results that define 0.15 amps per metre as that boundary value are shown in Table 5 on the next page.

The 2015 REFCL Technologies tests at Kilmore South performed prior to Tranche 4 differed from those in the 2014 REFCL Trial at Frankston South. The differences are set out in Table 4.

Table 4: Comparison of Frankston and Kilmore (prior to Tranche 4) ground ignition tests

2014 Frankston ground ignition tests 2015 Kilmore T2 and T3 tests

More 40µm ‘fines’ in soil Water was rarely used to increase current Beds conditioned in 45°C air, felt ‘dryer’ with

a dry ‘crust’ Very little smoke production in most tests

and generated smoke was darker Current tended to rise during many tests NER in service and series and shunt resistors

used to manage residual voltage and current

Less 40µm ‘fines’ in soil Water was routinely used to manage current Beds conditioned in 45°C air, felt ‘wetter’

with no dry ‘crust’ Copious white smoke in some tests,

white/grey smoke produced in most tests Current tended to fall during many tests GFN in service and RCC calibration used to

manage residual voltage and current

These differences are consistent with the Kilmore South Tranche 2 and 3 tests having higher grass moisture content at the base of the grass adjacent to the soil than was the case at Frankston South even though Kilmore South grass sample moisture analysis (which took grass from between 15-20mm above the soil to the top of the grass) showed 5-6% moisture content, i.e. similar to levels measured in the Frankston South tests.


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Many ground fires start at low to middle grass height, so increased grass moisture close to the soil is reflected in a higher current and longer times required to start a fire in the Kilmore South Tranches 2 and 3 tests as shown in Table 5.

Table 5: 2014 Frankston and 2015 Kilmore T1, T2 and T3 ground ignition test results7

Test Current Fire? Time Test Current Fire? Time 436 0.003 N - 249 0.0015 N -517 0.02 N - 253 0.005 N -339 0.05 N - 219 0.032 N -438 0.08 N - 224 0.04 N -440 0.08 N - 255 0.05 N -433 0.1 N - 233 0.06 N -441 0.1 N - 229 0.06 Y 31434 0.12 N - 225 0.07 Y 32334 0.13 N - 248 0.07 Y 8355 0.13 N - 232 0.09 Y 11435 0.13 N - 247 0.1 Y 2.5437 0.13 N - 256 0.12 N -349 0.15 N - 227 0.12 Y 3.8361 0.15 N - 258 0.12 Y 6431 0.15 N - 259 0.12 Y 2442 0.15 N - 218 0.15 N -354 0.17 N - 244 0.15 Y 2343 0.18 N - 226 0.17 Y 3.2359 0.18 N - 221 0.18 Y 1341 0.18 Y 9 241 0.18 Y 32340 0.2 N - 257 0.22 Y 31357 0.2 Y 44 231 0.23 Y 3.6345 0.24 N - 242 0.24 Y 5350 0.25 N - 228 0.25 Y 54352 0.25 N - 230 0.28 Y 2.3353 0.27 N - 222 0.31 Y 9.3342 0.28 Y 50 220 0.37 Y 5348 0.28 Y 10 223 0.4 Y 1.7358 0.28 Y 6.3335 0.29 Y 9.5360 0.31 N -351 0.32 Y 21356 0.33 Y 37347 0.4 Y 48336 0.45 Y 7338 0.55 Y 6344 0.83 Y 15346 0.98 Y 18337 1.2 Y 12444 N -430 Y 14

2014 Frankston2015 Kilmore

In preparation for Tranche 4 of the Kilmore tests, enhancements were made to more closely approach the worst-case conditions achieved in the 2014 Frankston tests:

Soil beds were dried naturally and no water was applied to manage current; and Grass/hay ‘fines’ were spread around the vertical straw stalks to more closely resemble the

Frankston fuel mix and ensure fuel moisture content close to the soil surface was low.

7 Current shown is amps into 0.4m of soil averaged during the five second period immediately before ignition, or if there is no ignition, the maximum five-second average during the test. Time to ignite is in seconds from fault occurrence.


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The result of these changes was that the Kilmore Tranche 4 tests very closely resembled the 2014 Frankston tests. FMC was maintained below 5% and the soil beds felt dry and had a hard dry crust. Soil moisture content was below 2%. Less smoke was produced in tests and it was dark rather than white. On average, soil current tended to rise rather than fall over the course of any particular test. The results of the 26 valid Kilmore T4 ‘wire down’ ground ignition tests are shown in Table 6, again compared to the 2014 Frankston tests.

Table 6: Kilmore Tranche 4 test results compared to 2014 Frankston test results

Test Current Fire? Time 798 0.04 N -821 0.04 Y 85784 0.05 N -783 0.05 N -823 0.06 Y 100805 0.07 Y 90818 0.08 Y 99820 0.09 Y 65822 0.09 N -803 0.09 N -800 0.10 N -781 0.10 N -799 0.11 N -806 0.11 N -804 0.12 Y 17782 0.13 N -793 0.13 N -796 0.13 N -801 0.14 N -808 0.15 N -819 0.17 N -794 0.19 N -795 0.19 Y 23807 0.21 Y 55779 0.31 N -802 0.33 Y 34

KilmoreTest Current Fire? Time 249 0.0015 N -253 0.005 N -219 0.032 N -224 0.04 N -255 0.05 N -233 0.06 N -229 0.06 Y 31225 0.07 Y 32248 0.07 Y 8232 0.09 Y 11247 0.1 Y 2.5256 0.12 N -227 0.12 Y 3.8258 0.12 Y 6259 0.12 Y 2218 0.15 N -244 0.15 Y 2226 0.17 Y 3.2221 0.18 Y 1241 0.18 Y 32257 0.22 Y 31231 0.23 Y 3.6242 0.24 Y 5228 0.25 Y 54230 0.28 Y 2.3222 0.31 Y 9.3220 0.37 Y 5223 0.4 Y 1.7

Frankston

The probability of fire at low soil currents was calculated from all 95 tests performed at both Kilmore and Frankston and is shown in Figure 9.


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Figure 9: fire probability from ground ignition - combined Kilmore and Frankston results

0

10

20

30

40

50

60

70

80

90

100

0-59 60-99 100-149 150-199 200-249 250-299 300-349

Fire

pro

babi

lity

(%)

Current into 400mm soil bed (mA)

The analysis reflected in Figure 9 supports the 60mA ignition threshold derived from the 2014 Frankston tests for the purpose of defining the initial draft performance standard. Only a single fire resulted from 12 tests performed with pre-fire soil currents less than 60mA, confirming that fires are unlikely to occur if lineal soil current intensity is less than 0.15 amps per metre of fallen conductor on the ground, equivalent to about 0.5 amps fault current in a worst-case ‘wire down’ earth fault.

A second important question is ‘how many tests would have produced a fire if the fault detection sensitivity was set to detect soil current of 60mA?’ In the tests, despite constant arcing between the fallen conductor and the soil, grass fires often took many tens of seconds to ignite. During the extended pre-fire period, the current sometimes exceeded 60mA. This is illustrated in Figure 10 which shows the soil current variation during Kilmore Test 821 - the single test in which a fire started during a period of less than 60mA of pre-fire soil current. If the fault detection sensitivity had been 60mA, no fire would have occurred in this test because the power supply would have been interrupted within a few seconds, i.e. well before the fire started 80 seconds later.


60mA soil current, equivalent to 0.5 amps in a

worst case ‘wire down’ earth fault

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Figure 10: Kilmore test 821 – rms soil current (fire started at 85 seconds)

Analysis of all tests that resulted in fires showed that if fault detection sensitivity had been set to 60mA no fires would have resulted in any of the 95 valid Kilmore and Frankston tests. Measured fire probability would be zero (less than 3% with 95% confidence).

Table 7 shows that a fault detection threshold of less than about 85mA would have been sufficient to have prevented all the fires observed in tests. This could be taken to imply that use of the 60mA ignition threshold to define fault detection performance may provide a 40% safety margin. However, such a margin is required to account for two possibilities:

1. In real network faults, soil current variation in the period before ignition may not follow the particular patterns of rising and falling current levels observed in the tests, i.e. a fire may result at 60mA soil current with no pre-fire current level exceeding this value.

2. In a real fault produced by a mid-span conductor breakage, there may be a concurrent back-fed fault which may reduce the fault current seen by the REFCL.

It was concluded that for maximum fire risk benefits, fault detection sensitivity should continue to be based on an ignition threshold of 60mA, i.e. 0.15 amps per metre of fallen conductor.


5 seconds pre-fire: 40mA

Defined ignition

limit: 60mA

Fault detection with 60mA sensitivity

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Table 7: maximum pre-fire current in 44 tests that produced fires (both Kilmore and Frankston tests are shown)8

Project Test Current Fire? Time Pre-fire ampsK 818 0.077 Y 99 0.086F 248 0.070 Y 8 0.087K 805 0.065 Y 90 0.108F 232 0.090 Y 11 0.111F 225 0.070 Y 32 0.120F 247 0.100 Y 2.5 0.125F 227 0.120 Y 3.8 0.125F 229 0.060 Y 31 0.139K 820 0.085 Y 65 0.139K 804 0.115 Y 17 0.145K 823 0.060 Y 100 0.151K 821 0.040 Y 85 0.155F 258 0.120 Y 6 0.160F 244 0.150 Y 2 0.160F 226 0.170 Y 3.2 0.190F 221 0.180 Y 1 0.190K 795 0.190 Y 23 0.200K 341 0.180 Y 9 0.215K 807 0.210 Y 55 0.220F 259 0.120 Y 2 0.225F 241 0.180 Y 32 0.267K 335 0.290 Y 9.5 0.320K 348 0.280 Y 10 0.330F 242 0.240 Y 5 0.350F 230 0.280 Y 2.3 0.350K 342 0.280 Y 50 0.350F 228 0.250 Y 54 0.351F 231 0.230 Y 3.6 0.360K 802 0.325 Y 34 0.360F 257 0.220 Y 31 0.370K 430 0.250 Y 14 0.430K 357 0.200 Y 44 0.440F 222 0.310 Y 9.3 0.470K 351 0.320 Y 21 0.470F 220 0.370 Y 5 0.490K 356 0.330 Y 37 0.530K 358 0.280 Y 6.3 0.550K 336 0.450 Y 7 0.580F 223 0.400 Y 1.7 0.600K 338 0.550 Y 6 0.690K 347 0.400 Y 48 0.720K 344 0.830 Y 15 1.000K 346 0.980 Y 18 1.150K 337 1.200 Y 12 1.200

The comprehensive tests at Kilmore reinforced the 0.15 amps per metre lineal fault current intensity as the limit for prevention of fires in ‘wire down’ earth faults.

6.3.3 Insights from test results into ‘wire down’ ground ignition processes

Observations of test video records and of soil-bed samples after tests provided additional insights into fallen powerline ignition processes at low levels of lineal soil current intensity:

Long periods of low intensity arcs appear to ‘prune back’ the grass next to the fallen conductor – tests with this characteristic often failed to produce ignition.

The thermal mass of the conductor inhibits ignition under and next to it – in a fire, the grass under and next to the conductor was often unburned though all the rest of the grass was reduced to ash.

8 The column labelled ‘Pre-fire amps’ is the maximum value of current in the whole fault period before ignition. The column labelled ‘Current’ is the average value of current in the five seconds immediately prior to ignition.


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Ignition seemed critically dependent on the build-up of pyrolysis gases to reach the flammability threshold of around 4% in air. In some tests, sudden very brief gas flares could be seen to ignite only to immediately extinguish again as soon as the local gas concentration had been burned. This was also occasionally seen in Frankston tests with low soil current.

Illustrative examples of these observations are shown in Figure 11 and Figure 12.

Figure 11: Kilmore Test 803 – protection of grass by thermal mass of fallen conductor

Figure 12: Kilmore Test 819 – brief gas flare at 84 seconds (current at 140mA, FMC=4.2%) – no ignition

6.3.4 Length of fallen conductor in contact with the ground

The assumption of three metres as the worst case length of conductor on the ground was based on the logic set out in Section 3.5 above. Whilst there are anecdotal field reports of earth faults that involve a much longer length of conductor on the ground than three metres, no compelling body of evidence was identified that would warrant adoption of a longer length as worst-case. It was concluded the assumption of three metres remains appropriate.

After consideration of the ignition test results and the logic behind the assumption of a three metre worst-case conductor-on-ground length, it was concluded that the 0.5 amp fault detection criterion was not over-conservative and it remains appropriate for fire risk reduction purposes.

6.3.5 ‘Time to detect’ performance

The initial draft performance standard also included a requirement that a fault at the 0.5 Amp current detection threshold must be detected within 1.5 seconds. This figure was also reviewed. It was based on 2014 REFCL Trial ‘time to ignite’ test results and the 2014 Theoretical Modelling project


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which indicated that on the Frankston South network a half-amp fault would be detected in about one second.

From Table 7 above, it can be seen that low fault current levels can take some seconds or even many tens of seconds to start a fire, though there are one or two tests in which ignition took place in about two seconds. On this basis considered in isolation, the 1.5 second ‘time to detect’ criteria might appear marginally over-conservative. However, fault detection is only the first step in prevention of a fire and time must be allowed for other processes necessary to safely manage a low-current fault.

Taking all factors into account, it was concluded that there may be room for some slight relaxation of the 1.5 second ‘time to detect’ criterion. However, slower fault detection is likely to increase fire risk and the materiality of this increase cannot be estimated with any certainty.

On balance, it was concluded that the 1.5 seconds criterion should be retained with an extension possible in special circumstances, subject to an expert review process to confirm there is no practicable engineering alternative available.

6.4 Is the specified level of fault detection performance achievable?

The 2015 REFCL Technologies test program at Kilmore South demonstrated that available REFCL technology can detect 0.5 amp earth faults in a way that can cut fire risk.

6.4.1 Fault detection test results from Kilmore South

The Kilmore South GFN was tested at less than 0.5 amps fault current level and the results are shown in Table 8.

Table 8: GFN bolted 25,400 Ohm resistive fault tests for 0.5 amp detection

Test No Faulted Phase Initial fault current Time to detect

300 White 0.48A 0.24s

301 White 0.47A 0.25s

302 White 0.47A 0.23s

303 White 0.48A 0.29s

Because of network capacitive imbalance, the Kilmore South GFN had a standing neutral voltage displacement of 800-900 volts prior to Tranche 4 and about 500 volts in Tranche 4 tests (with the GFN U0 Injectors in service). This meant fault detection sensitivity and speed depended on which phase was faulted, with Blue phase being the least sensitive and Red and White phases being roughly matched in sensitivity. Variation of detection speed due to imbalance was verified as shown in Table 9. Even though the fault current was higher in a Blue phase fault, the detection time was longer.

Like the 2014 Frankston tests, test results from Kilmore South generally confirmed the validity of theoretical models developed by REFCL suppliers and in the 2014 Theoretical Modelling project.

Table 9: GFN bolted 15,600 Ohm resistive fault tests on different phases


281 Blue 0.89A 0.33s

282 White 0.78A 0.17s


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The tests confirmed that variations of fault detection sensitivity and speed caused by network capacitive imbalance can be reliably predicted by theoretical analysis once network parameters are known following GFN commissioning. With lower network capacitive imbalance, the GFN fault detection threshold could have been reduced further to 10% (it was 21% during these tests) providing fault detection sensitivity on all phases of the Kilmore South test network well below 0.5 amps.

The SSFCL was tested at 0.8 amps of fault current9 and fault detection was found to be almost instantaneous as indicated in Table 10. For these tests a 10:1 ratio current transformer replaced the 100:1 unit originally fitted to the SSFCL. The detection threshold was set to 0.4 amps instantaneous, equivalent to about 0.3 amps rms. Test results were consistent with this and two other detection threshold settings (4 and 10 amps) used on the SSFCL during the test program.

Table 10: SSFCL bolted 15,600 Ohm resistive fault tests for 0.8 amp detection


59 Blue 0.80A <5ms

60 Blue 0.80A <5ms

The ASC was not separately tested for fault detection sensitivity as this would have simply repeated the GFN test since the GFN with RCC disabled was used to simulate an ASC.

It was concluded that on the Kilmore South test network, all three REFCL configurations could reliably detect a 0.5 amp fault in much less than 1.5 seconds.

6.4.2 Achievement of fault detection performance in larger networks

Can this level of fault detection performance be replicated on other (larger) networks in Victoria? This question involves a number of complex issues and the answer varies by REFCL type.

The detection of an earth fault by the SSFCL is simply detection of an increase in current flowing in the connection between the transformer neutral and earth, i.e. current through the SSFCL. The sensitivity and speed of fault detection using this approach could be expected to be largely independent of network parameters, so the KMS test results are likely to reflect the SSFCL performance that would be achieved on larger networks. However, it must be recognised this has not yet been demonstrated by test.

For REFCL performance to reduce fire risk, it must not only detect the fault, it must also identify which phase it is on. Otherwise, the only available response is to interrupt power supply to the entire network until the period of high fire risk has passed. This is not an acceptable outcome for rural communities during high fire risk periods. REFCLs based on resonant earthing do this identification as a natural consequence of their operating principle. The SSFCL when acting by itself does not. Additional measurement logic will be essential to provide this function in an SSFCL-based REFCL.

In contrast to the SSFCL, the detection of faults by a GFN or ASC depends strongly on network properties – the REFCL simply allows the 50Hz component of neutral voltage to move away from zero volts with respect to earth. It then monitors the network’s response (i.e. the neutral voltage rise) to any fault that occurs. The network’s response is determined primarily by network parameters, though some ASC parameters influence it.

9 Testing at 0.8 amps fault current was easily achieved with the test facility equipment. The 0.5 amps tests of the GFN were performed with additional borrowed equipment which had very stringent short-time thermal ratings. It was decided not to risk this equipment in tests on other REFCL configurations.


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The effects of network properties on GFN/ASC fault detection sensitivity are summarised in Table 11.

Table 11: Effect of network parameters on fault detection sensitivity

Network property

Effect on fault detection sensitivity Status of KMS test network (Tranches 1-3)

Capacitive imbalance

Reduces sensitivity by creating standing neutral voltage displacement. Detection sensitivity also varies by faulted phase.

Moderate imbalance (2,300 volts), reduced (to <900 volts) by GFN U0 Injector to allow more sensitive fault detection. Detection threshold was 21% (2,655 volts rms UEN).

Total Damping

Limits sensitivity by reducing the neutral voltage rise caused by a given level of fault current.

Low network damping (0.01-0.02mS) increased by core losses in the over-size GFN (to 0.08mS) reduced fault detection sensitivity.

Total Capacitance

Slows detection by increasing the network transient time constant

Small network capacitance (2.6µF), increased by over-size GFN (to 20.1µF), i.e. a 0.3s time constant for a 25.4kΩ fault.

Customer load

Has a second order effect compared to other factors.

Light load – did not affect fault detection sensitivity.

Network size can reduce fault detection sensitivity but there are ways to mitigate this effect. Both total network capacitance and network damping increase with network size, i.e. total route length of powerlines and cables supplied by the zone substation. Industry experience over many decades has shown that it can be more difficult to achieve sensitive REFCL fault detection on very large networks, whereas small networks can show a very sensitive response to high-impedance earth faults.

The established method to achieve sensitive fault detection on large networks is to split them into smaller networks by opening bus-tie circuit breakers in the source substation and to provide multiple ASC coils – one for each transformer supplying one of the smaller networks.

Most of Victoria’s rural networks will support sensitive fault detection. The test results confirm expectations and theoretical calculations that, provided networks are adequately balanced, GFN and ASC solutions will (like the SSFCL device) achieve 0.5 amp earth fault detection on networks of the size of those in rural Victoria. In some larger networks this may require splitting the substation busbar to create smaller networks each with its own REFCL.

Exceptions must also be managed. There may be a very small number of rural networks (no more than two or three) where bus-splitting and multiple REFCLs may still not quite suffice to achieve the mandated level of fault detection performance. This can only be known after commissioning as fault detection sensitivity depends critically on total network damping which cannot be known before a REFCL is installed.

Future customer demand growth may also present challenges. Network planning for REFCL installation at a particular substation is likely to take into account expected development of the network over the next decade or more. Where such development involves a significant amount of underground cable, the network’s total capacitance may dramatically increase over this period. This may happen in peri-urban areas where new housing estates have fully underground networks, or in bushfire risk areas where regulations require undergrounding of new lines.

Anticipated growth of network capacitance may require initial GFN/ASC coil over-sizing to provide for future development. As was seen in the Kilmore tests, an over-sized coil produces effects that can


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limit fault detection sensitivity by increasing total damping and can slow down fault detection by increasing total capacitance. To minimise this challenge, the option of providing for future network development by using underground cable with a higher than normal voltage rating may be warranted to ensure network damping does not increase in future to a level where earth fault detection performance is degraded.

6.4.3 Achievement of network capacitive balance

The achievement of the specified level of fault detection sensitivity will require that networks be balanced so the capacitance on each phase to earth is equalised to close tolerances. Engineering options are available to achieve this:

1. Re-phasing of two-wire spur take-off connections. This was done on the KMS network to reduce imbalance by a factor of four over the course of the project.

2. Addition of capacitance to the third phase at two-wire spur take-off points or more generally at any convenient location, to balance a switched network section.

3. Adding a third phase to two-wire spur lines, though this may be more costly than Option 2.

The U0 injector in the GFN can be used to balance the total network to increase fault detection sensitivity. However, identification of the faulted powerline can require that each substation feeder be separately balanced, so this option may be of limited value. It is likely that development of a standard pole-mounted transformer-isolated capacitance module for volume production will prove a cost-effective solution in any large scale roll-out of GFN/ASC REFCLs.

6.5 Are the best words used to specify fault sensitivity?

The wording of the initial draft standard referred to a 0.5 amp resistive earth fault and defined a resistive fault as a resistance connected between any high voltage conductor and earth.

The risk of any ambiguity in this specification can be further reduced. Use of a level of current to characterise a fault has long been standard network engineering practice in Victoria. However with a REFCL in service, fault current is not constant but varies with time – it may start at 0.5 amps but it will decrease with time as network voltages respond to its presence. The half-amp value may be ambiguous in this context.

It would be better to express the performance requirement proposed for Victoria’s high fire risk areas as the capability to detect an earth fault that comprises a 25,400 Ohm10 resistor between any high voltage powerline conductor and earth.

10 This value would apply to 22kV networks. Networks operating at other voltage levels would require a different value. It is notable that at least one other jurisdiction that regulates the performance of earth fault protection has chosen to use a specific fault resistance rather than a value of fault current to provide an unambiguous criterion for fault detection sensitivity.


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7 Fault response: speed and completeness of voltage cancellationThe initial draft performance standard specified a response to a resistive fault between any powerline conductor and earth such that the voltage on the faulted conductor (measured in the source substation) is limited to below 250 volts within 70 milliseconds of fault detection.


This section sets out evidence to support the following findings:

1. Fault response performance (speed and completeness of cancellation of the voltage on the faulted conductor) is an essential part of the performance standard if fire risk reduction is to be achieved.

2. The fault response performance required to prevent ignition depends on fault current and ignition type. For high current faults, speed can be more important than completeness; for lower current faults, completeness can be more important than speed.

3. To prevent bounce ignition, it is necessary that voltage on the faulted conductor be limited to less than 1,900 volts within 85 milliseconds of an earth fault comprising a 400 Ohm resistor between any powerline conductor and earth.

4. To prevent ground ignition, it is necessary that voltage on the faulted conductor be limited to less than 750 volts within 500 milliseconds of an earth fault comprising a 400 Ohm resistor between any powerline conductor and earth.

5. To prevent ground ignition, it is necessary that voltage on the faulted conductor be limited to less than 250 volts within two seconds of any detectable earth fault.

6. The recommended performance standard for fault response can be met by available technology. Specifically, the GFN is fully compliant and the SSFCL+FPE and ASC+FPE are at least partially compliant to the extent they could be tested.

Recommendation: The performance standard for fault response in extreme fire risk areas should be modified to ensure that:

1. When tested with a high impedance (25,400 Ohm resistive) earth fault, a REFCL limits the voltage on the faulted conductor at the substation to not more than 250 volts within two seconds of occurrence of the fault11 except during diagnostic tests.

2. When tested with a low impedance (400 Ohm resistive) earth fault, a REFCL limits the voltage on the faulted conductor at the substation to not more than:

a. 1,900 volts within 85 milliseconds of fault occurrence; b. 750 volts within 500 milliseconds of fault occurrence; and,c. 250 volts within two seconds of fault occurrence except during diagnostic tests.

The wording of the draft performance standard should be revised to reflect the above requirements.

The following sections set out the rationale and evidence to support the findings and recommendations above.

7.2 Is it necessary to include fault response in the draft standard?

This element of the draft standard is necessary for the goal of fire risk reduction to be achieved. The PBST’s 2011 arc-ignition research proved that powerline faults can cause ignition of dry grass almost (but not quite) instantly. If the response to a detected fault is slow, fires can be produced in realistic circumstances where a faster response may have prevented them. This was a finding of the 2014

11 Given the fault detection performance standard is that such faults must be detected in 1.5 seconds, this allows 0.5 seconds for voltage cancellation.


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REFCL Trial. The inclusion of a minimum fault response speed is necessary to avoid bounce ignitions in higher current faults, as well as to prevent ground ignitions at lower levels of fault current.

The 2014 REFCL Trial showed that an incomplete fault response results in substantial residual voltage on the faulted conductor creating a risk of subsequent ground ignition after the conductor has come to rest on the ground. Inclusion of a measure of fault response completeness is essential to ensure fault response is effective in preventing this type of fire.

In summary, both speed and completeness of fault response are required to prevent fires from ‘wire down’ powerline earth faults.

7.3 Is the specified level of fault response right?

The initial draft performance standard called for a fault response that reduces the voltage on a faulted conductor to less than 250 volts within 70 milliseconds of fault detection.

Fault response performance must prevent two different classes of ignition: bounce and ground ignition. The importance of speed and completeness of voltage cancellation to the management of fire risk varies by ignition type and fault type as outlined in Table 12.

Table 12: fault response - how speed and completeness of voltage cancellation prevent two classes of ignition

Fire risk Fault response performance measure

Ignition type High risk faults Speed Completeness

Bounce Higher fault current More important Less important

Ground Lower fault current Less important More important

The 2015 REFCL Technologies tests at Kilmore South prompted a wide ranging re-think of the best way to specify fault response performance to achieve fire prevention. This covered not only speed and completeness, but the type of test in which each could best be measured.

The following sections define fault response performance requirements to prevent bounce and ground ignitions and the requirements of a technology-neutral performance standard.

7.3.1 Fault response required to extinguish bounce arcs

Twelve tests in the 2014 REFCL Trial produced bounce ignitions and they are listed in Table 13. The record of each test has been analysed to derive the value of voltage12 in the half cycle coincident with arc extinction. The hypothesis was that if the voltage were lower than this value, the observed arc extinction would still have occurred, whereas if it were higher, the arc may have continued to exist.

In these tests, the lowest conductor voltage immediately prior to arc extinction was 3.8 kV in Test 155. In this test, 70% cancellation of the voltage on the faulted conductor within 88 milliseconds of fault occurrence extinguished the arc when peak bounce current was 14.8 amps. The bounce arc voltage and current waveforms in this test are shown in Figure 13.

Given the limited number of bounce fire results available for analysis, it cannot be safely assumed that voltages somewhat below 3.8kV would always guarantee arc extinction and a reasonable margin of safety must be applied to define a prudent standard for fast voltage cancellation. It is proposed the standard be 85% cancellation rather than the 70% in Test 155, i.e. the voltage on the conductor should be half the extinction voltage observed in Test 155, about 1,900 volts.

12 The rms value was estimated by dividing the peak-to-peak value by 2.83 (twice the square root of two).


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Table 13: bounce ignitions in the 2014 REFCL Trial

Protection TestBounce current (Arms)

Arc duration

(ms)

Conductor voltage (kVrms)

arc extinction 85ms post-fault

ASC

121 14.8 114 5.1 >5.1123 13.5 110 5.1 >5.1127 12.5 75 8.2 8.2128 12.5 76 7.8 7.8129 15.0 82 7.7 7.7

GFN

154 13.0 76 6.2 5.2155 14.8 88 3.8 3.8157 11.0 74 6.5 6.0159 13.5 120 4.2 >4.2161 17.0 92 6.9 >6.9176 14.0 79 7.7 7.7177 13.0 48 8.5 7.2

Figure 13: Frankston Test 155 voltage and current waveforms during bounce arc

It was concluded that to extinguish bounce arcs, the REFCL must reduce the conductor voltage below 1,900 volts.


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7.3.2 Fault response speed required to prevent bounce ignition

Fault response speed is possibly the most important factor in prevention of bounce ignition when fault current during the bounce is high. The 2014 REFCL Trial found that bounce arcs of duration less than 40 milliseconds did not start fires. The duration of a bounce arc is set by many factors as outlined on page 59 of the 2014 REFCL Trial report, one of which is the speed of the REFCL fault response.

The largest body of evidence on the fault response speed required to prevent bounce ignition comes from tests of the SSFCL+FPE configuration at Kilmore South. The SSFCL+FPE response to a fault as shown in Figure 14 below is qualitatively different to the GFN response shown in Figure 13 above.

When the SSFCL detected a fault and opened the connection between the transformer neutral and earth, the fault current was determined by the total network capacitance until the FPE acted to cancel the voltage on the faulted phase.

An example from the Kilmore test program is shown in Figure 14. In KMS Test 414, the fault current stayed at around seven amps until the FPE closed 73 milliseconds after the fault. Before the FPE closed, the conductor voltage varied somewhat randomly due to network voltage displacement allowed by isolation of the neutral from earth. KMS Test 414 did not produce a fire.

Tests of the SSFCL+FPE configuration were performed with different delays added to the FPE response time to assess the effect of a slower FPE response on fire probability. The results are shown in Figure 15. At the level of capacitive current on the test network (typically seven amps), FPE response times beyond about 80-90ms produced fire risk. Response times faster than this did not.

Figure 14: KMS Test 414 - typical SSFCL+FPE fault response

Some uncertainties remain about the most appropriate performance standard for REFCL designs that rely on FPE without resonant earthing. However, the available evidence supports an 85 millisecond timing requirement for fast voltage cancellation if bounce arcs are to be extinguished before ignition occurs.

It was concluded that bounce ignitions would be prevented if a REFCL reduced the conductor voltage to less than 1,900 volts within 85 milliseconds of fault occurrence.


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Figure 15: dependence of fire risk on FPE response time

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7.3.3 Fast fault response required in larger networks

The 35 valid GFN ignition tests performed at Kilmore South without artificial elevation of residual conductor voltage had a peak bounce current distribution shown in Figure 16. The median is 7.25 amps which equates to a lineal current density of 18 amps per metre or 54 amps fault current in a worst case ‘three metres on the ground’ earth fault.

Larger networks have higher capacitive current levels. However, the bounce current remains limited by the fault resistance offered by the conductor-soil path. Bounce ignition tests were performed on the Frankston network in the 2014 REFCL Trial with the NER in service as indicated in Table 13 above. Review of the Frankston test results indicated that the bounce arc situation in the larger Frankston network (capacitive current 144 amps) was not greatly different to that observed in the tests on the much smaller Kilmore network (capacitive current 9-10 amps).

Two of the 2014 Frankston tests provide an indication that a response time around 80-90 milliseconds is also appropriate for a larger network. The tests (Test 236 and 240) are outlined in Table 14. Bounce arc duration was in the 80-100 milliseconds range and ignition results were marginal despite conductor voltage through the bounce arc period staying at about 90% of nominal (these tests were performed with the NER in service and a series resistance of 100 Ohms in the rig supply).

The fault current in Test 236 is shown in Figure 17. The large negative spike at the end of the test is due to a fulgurite formed in the last ten milliseconds of the test. The record is clipped, but the peak current in this ten millisecond period would have been around 150 amps. This test did not produce sustained ignition.


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Figure 16: distribution of peak bounce currents in valid GFN ignition tests with low residual voltages

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Test Current (Arms) Duration (ms) Result236 11.6 (+ fulgurite) 99 Single tiny flame self-extinguished240 13.9 83 Three arcs, two points of ignition

Figure 17: Frankston Test 236 fault current


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After reviewing all the evidence from the Frankston and Kilmore tests, it was concluded that voltage cancellation to 1,900 volts within 85 milliseconds of the fault is an appropriate standard to prevent bounce ignitions across the diversity of Victoria’s rural networks.

7.3.4 Choice of fault resistance in tests for fast voltage cancellation

This ‘1,900 volts within 85 milliseconds’ standard must be achieved at fault current levels high enough to produce bounce ignitions when three metres of conductor hit the ground:

Based on all Kilmore South bounce ignition tests, this is would be a lineal current intensity of more than 18 amps per metre or 54 amps fault current; and

Based on all 2014 Frankston bounce ignition tests, this would be a lineal current intensity of more than 25 amps per metre or 75 amps fault current.

The higher the peak bounce current, the greater the natural voltage cancellation due to series impedance in the supply to the fault, including natural resonant ASC action. Compliance tests using a resistor that produces lower current levels than the above are therefore conservative. They provide a safety margin of additional assurance of REFCL fire risk reduction.

It is recommended that the higher current test of fast fault response be performed on 22kV networks with a resistor of 400 Ohms, equivalent to a fault current of 32 amps. This provides some safety margin and has the added benefit that REFCL fault detection at this current level is usually very fast which mitigates any issues with use of fault occurrence as the starting point for time measurements in compliance tests.

7.3.5 Fault response required to prevent ground ignition

Section 8 below sets out the rationale and evidence that supports a 250 volts limit on sustained conductor voltage to avoid ground ignitions. It also sets out the evidence that indicates higher voltages can be tolerated for short periods of time with low ignition risk, specifically up to about 750 volts for up to 500 milliseconds. A similar set of data was not produced in the 2014 REFCL Trial so the 2015 Kilmore South test results are the most authoritative available information.

It was concluded that the fault response requirement to prevent ground ignitions should be voltage cancellation to reduce the conductor voltage to less than 750 volts within 500 milliseconds of the fault for fault currents around 50 amps.

This should be accompanied by an overarching requirement to reduce conductor voltage to its longer term limit of 250 volts within two seconds - the minimum time to ignite for very low current faults as identified in Section 6.3.5 above.

7.3.6 Fault response performance measurement challenges

The Kilmore tests revealed a number of potential weaknesses in the initial draft performance standard for fault response speed:

1. Ambiguity of timing measurement: The initial draft performance standard called for completion of fault response within 70ms of fault detection. However, fault detection is an internal determination that takes place within the REFCL control systems and its precise timing cannot usually be monitored externally. There is no sign on test recordings to show the precise instant the fault is detected by the REFCL. The only visible signs on test recordings are occurrence of the fault, commencement of compensation (active voltage cancellation) and progress of that compensation to completion.


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2. Coverage of higher current faults: The initial draft performance standard implied (but did not specifically describe) a compliance test using a high resistance to produce an earth fault of nominal current 0.5 amps. The speed of compensation is not as critical to fire prevention in low current faults as it is for earth faults involving higher currents. The initial draft performance standard was silent on higher current faults.

3. The challenge of technology neutrality: Fault response is qualitatively different with different REFCL technologies. This presents a challenge when it comes to ensuring a fault response performance standard is technology-neutral. FPE takes time for the electro-mechanical earthing switch to operate but it then applies 100% complete compensation instantly as the faulted conductor is shorted to earth in the substation. The GFN uses a solid-state inverter to inject voltage to cancel the voltage on the faulted conductor and this cancellation takes time to complete because the current the inverter can supply is limited.

It was concluded that the performance standard can best achieve the goal of fire risk reduction by specifying REFCL performance for both low and higher current earth faults, i.e. a compliance test that uses two different values of fault resistance.

7.3.7 Recommendation – fault response performance standard

The recommended performance standard for fault response is:

1. When tested with a high impedance (25,400 Ohm resistive) earth fault, the voltage on the faulted conductor at the substation will be limited to not more than 250 volts within two seconds of fault occurrence, except during diagnostic tests.

2. When tested with a low impedance (400 Ohm resistive) earth fault, the voltage on the faulted conductor at the substation will be limited to not more than:

a. 1,900 volts within 85 milliseconds of fault occurrence; b. 750 volts within 500 milliseconds of fault occurrence; and,c. 250 volts within two seconds of fault occurrence, except during diagnostic tests.

The 400 Ohm compliance test would demonstrate the REFCL’s capability to prevent bounce ignitions at higher levels of fault current.

7.1 Is the specified level of fault response achievable?

The 2015 REFCL Technology test program demonstrated that today’s technology is capable of achieving the level of fault response performance specified in Section 7.3.7 above.

A compliance test of the Kilmore GFN using a 25,400 Ohm resistive fault recorded the response shown in Figure 18. The time to reduce the white phase voltage to less than 250 volts, measured from the occurrence of the fault, was 350 milliseconds. This performance was well within the two second standard.

Detection of the fault took the majority of the time: it took 250 milliseconds for the voltage to fall the required 17% for the neutral voltage displacement to exceed the fault detection threshold. Faster fault detection can be expected when network capacitance is better balanced and the detection threshold correspondingly reduced. The conductor voltage was 90% cancelled within 50 milliseconds of the commencement of RCC action. The effect of the RCC inverter current limit can be seen in the last 50-60milliseconds of the voltage cancellation process.


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Figure 18: Kilmore Test 301 GFN response to 25,400 Ohm earth fault

A compliance test of the Kilmore South GFN using a 400 Ohm resistive earth fault recorded the response shown in Figure 19.

Figure 19: Kilmore Test 93 – GFN with 400 Ohm earth fault

The conductor voltage was cancelled to less than 150 volts within 85 milliseconds of the fault occurrence. A similar test of the Frankston South GFN on 14th June 2014, showed the response in Figure 20. The conductor voltage was cancelled to less than 940 volts within 85 milliseconds. Though they exhibit a three-to-one ratio of total capacitance, both REFCL installations were well within the proposed performance standard (cancellation to less than 1,900 volts within 85 milliseconds) for higher current fault response.


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Figure 20: Frankston Test 189 - GFN with 400 Ohm earth fault

The Kilmore South ASC+FPE configuration was not tested with a 25,400 Ohm resistive fault. A compliance test using a 400 Ohm resistive earth fault recorded the response shown in Figure 21.

Figure 21: Kilmore Test 98 – ASC+FPE response to a 400 Ohm resistive fault

The test showed voltage cancellation to 876 volts within 85 milliseconds of fault occurrence and cancellation to less than 20 volts when the FPE operated 161 milliseconds after the fault, i.e. it was compliant with all three elements of the 400 Ohm performance standard.


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The SSFCL+FPE configuration was not tested with either a 25,400 Ohm resistive fault or with a 400 Ohm fault. A test using a 100 Ohm resistive earth fault recorded the response shown in Figure 22.

Figure 22: Kilmore Test 376 - SSFCL+FPE with 100 Ohm resistive fault

Taking into account the theoretical relationships of the voltages and currents concerned, it was concluded the SSFCL+FPE configuration would most likely comply with the fault response standard on the small KMS network. However, this was not actually demonstrated in the Kilmore South tests and theory would indicate results on a larger network may not be compliant without further SSFCL+FPE product development.

7.2 Are the best words used to specify fault response?

The proposal to add a second compliance test at a higher current using a 400 Ohm resistive fault requires the wording of the fault response element of the initial draft performance standard to be revised.

In addition, where response times are mentioned, the starting time for the measurement must be explicitly stated and it must be an event that is visible on the compliance test record, preferably fault occurrence. The resistance values specified in compliance tests should be proportionately reduced for tests on networks that operate at lower voltage levels.


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8 Fault management: limit on residual conductor voltage The initial draft performance standard specified that in the presence of a sustained earth fault the residual voltage on the faulted conductor (measured at the source substation) must be limited to not more than 250 volts except during diagnostic tests13.


This section sets out the rationale and evidence to support the following findings:

1. A limit on residual conductor voltage in the case of a sustained powerline fault is an essential part of the performance standard if fire risk reduction is to be achieved.

2. The proposed limit of 250 volts at the source substation is appropriate and realistic for Victoria’s rural networks.

3. All three of the tested REFCL technologies can achieve the 250 volt limit.

It is recommended that:

1. The standard should make clear the requirement is that of a compliance test. 2. The application of the standard should recognise the possibility of an increase in the residual

voltage measured at the substation above 250 volts when required by the action of enhanced technology that further reduces the voltage at the fault location below 250 volts.

The rationale and evidence for each of these findings and recommendations are set out below.

8.2 Is it necessary to include a limit on residual conductor voltage in the draft standard?

This element of the draft standard is necessary for the goal of fire risk reduction to be achieved. The 2014 REFCL Trial showed that heightened residual voltage on a fallen conductor could produce fault current and fires. Test results in the 2015 REFCL Technologies test program at Kilmore South confirmed this finding. The inclusion of a specified maximum residual voltage is therefore necessary to avoid fire outcomes.

It is also appropriate that the residual voltage be measured at the substation in compliance tests. The residual conductor voltage at the fault location depends on many uncertain variables including network parameters, the distance from the substation and the load on the network. To ensure compliance assessment is certain and unambiguous, it is necessary to specify the location at which the residual voltage is to be measured in compliance tests. The only location which can be specified unambiguously and consistently across the diversity of Victoria’s rural networks is the substation supplying the network.

8.3 Is the specified limit on residual conductor voltage right?

For fire risk reduction, the limit on residual voltage must be set to be lower than the levels of conductor voltage that produced ground ignition in long-duration ignition tests. The initial draft standard specified a limit of 250 volts.

The 2014 REFCL Trial did not provide a comprehensive data base for assessment of fire risk versus residual conductor voltage. Its limitations in this respect included:

Conductor voltage was not directly recorded. The voltage records collected in the tests were taken ‘upstream’ of the multi-kilo-ohm high-voltage resistor used to reduce the current.

13 Such tests include the confirmation that the fault is sustained and the identification of the location of a sustained fault, at least to the extent of identifying which powerline it is on so that powerline can be disconnected if necessary to remove the fire risk.


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In many of the tests, a shunt resistor was used to further reduce the fault current. The combination of series and shunt resistors meant that the source impedance supplying the test rig was thousands of ohms, whereas in real faults residual voltage is supplied by a low impedance source. The effect of high source impedance on fire risk is not known, but the conductor voltage would have varied substantially as current varied during the test.

There were not enough ground ignition tests completed in the 2014 test series to provide a reliable statistical correlation of fire results with a calculated conductor voltage.

Despite these limitations, a few tests at Frankston South indicated that residual voltages of less than 250 volts were achievable with a GFN and that figure was adopted for the initial draft performance standard.

8.3.1 Worst-case conditions for a fire from residual voltage

Comparison of the KMS ignition test results in Tranches 1 to 3 with those carried out in the 2014 REFCL Trial at Frankston and in Tranche 4 at Kilmore showed that the worst case conditions for fires from residual voltage were different to those used to derive the fault detection sensitivity criterion of 0.5 amps.

The essential prerequisites for a fire from low residual voltage on a fallen conductor were identified as:

1. Sufficient soil conductivity (moisture level) to reduce resistance in the earth fault current path to levels where conductor-soil arc energy was high enough to start fires; and

2. Grass moisture content sufficient to ensure that the grass did not act as an insulator between the conductor and the soil, but not so high as to prevent ignition.

The conditions in KMS Tranches 1-3 ignition tests met these requirements. Conditions in the ground ignition tests in the 2014 REFCL Trial at Frankston and in Tranche 4 of the KMS tests did not – both soil and grass were too dry and residual voltages of some kilovolts were sometimes required for ignition.

A typical Kilmore test that produced a fire at low residual conductor voltage was KMS Test 358 shown in Figure 23. Fault detection was not an issue in this test (the fault current on the first bounce would have been about 35 amps) – the challenge was limitation of the voltage on the fallen conductor to prevent fires after fault detection. In this test, a fire started after six seconds.

Figure 23: KMS Test 358 - fire in six seconds from residual voltage of 1600 volts


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Figure 23 shows that at the time of ignition, the fault current was close to 0.5 amps driven by a residual voltage on the conductor of around 1,600 volts14.

8.3.2 The KMS test results – ignition from residual voltage

In the 2015 REFCL Technologies test program at Kilmore South, 150 valid ignition tests were performed with sustained residual conductor voltage controlled by the GFN’s RCC with only a 100 Ohm series resistor in the test rig supply. These tests lasted for 60 seconds and each test included at least one instance of the GFN fault-confirmation test. Voltage was measured ‘upstream’ of the 100 Ohm resistor.

The test results can be characterised by the fire result and two voltages: the residual voltage (applied continuously) and the peak voltage during the fault-confirmation test (applied in 0.5 second pulses). The correlation of the fire result with each of these two voltages is shown in Figure 24 and Figure 25 below.

Figure 24 shows the fire probability was only poorly correlated with the continuous residual voltage on the conductor. As Figure 25 shows, the correlation with the peak voltage in the fault-confirmation test was much clearer.

It was concluded that at low levels of residual voltage, the fault-confirmation test ‘masked’ the variation of fire risk due to sustained residual voltage. Below about 1,000 to 1,500 volts residual voltage, the fire was always caused by the fault-confirmation test. At higher levels of residual voltage, the fire risk due to sustained residual voltage could be more readily seen, e.g. ignition timing did not correspond to the timing of the fault-confirmation test, indicating the fire was due entirely to the presence of an elevated residual voltage. Examples of this are listed in Table 15.

14 The worst-case conditions that define the limit on fault detection sensitivity confirmed in Section 6 above were quite different to these – they were centred on fault current into very dry soil in the presence of very dry grass. The tests in KMS Tranche 4 revealed that in these conditions, the 60mA ignition threshold could be exceeded with the normal 12,700 volts on the conductor reduced to 8,000 volts – a smaller voltage change that makes fault detection a challenge.


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Figure 24: probability of fire versus elevated residual voltage on conductor

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Table 15: typical tests where fire resulted from residual voltage rather than fault-confirmation test (FCT)

Test No Residual (V) Peak FCT (V) Ignition timing (s) FCT timing (s)

335 2220 2584 9.5 13336 2200 2374 7 11337 2000 2175 12 Not clear338 2200 2335 6 11341 2300 2900 9 16342 2250 2942 50 13344 2050 2167 15 8346 1860 2064 18 8347 2060 2408 48 11351 1620 1722 21 11356 1620 1637 37 11357 1610 1743 44 11358 1600 1705 6 11

8.3.3 Derivation of the limit on residual voltage

The approach used to estimate the variation of fire risk with lower values of sustained residual voltage was to derive a ratio of ‘residual/FCT’ fire probability from the data at high values of residual voltage and apply this ratio at lower values of residual voltage. In effect this is a ratio of ignition probability between sustained (60 seconds) exposure to voltage and very brief (0.5 seconds) exposure to the same voltage. The approach adopted in this analysis assumes this ratio is constant for all levels of residual conductor voltage.

This estimation process was as follows:

At 2,000 volts residual voltage the probability of fire was about 65% and nearly all fires were due to sustained high residual voltage; and

At 2,000 volts peak FCT voltage, the probability of fire was about 22%. Based on these two data points, it was hypothesised that the probability of fire due to

sustained residual voltage was about three times the probability of fire for the same voltage applied for only 0.5 seconds at the peak of the FCT.

This approach produced the probability curve shown in Figure 26, indicating low fire risk for sustained residual voltages below about 1,000 volts.


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Figure 26: estimated probability of fire from elevated residual voltage in the absence of the FCT

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This was consistent with test records for low residual voltage which show soil-bed current in between the pulses of the fault-confirmation test. Examples are shown in Figure 27.

Figure 27: typical non-FCT soil-bed current at low levels of residual voltage

Test Residual voltage

Soil-bed current (non-FCT)

310 110V Zero468 220V Zero447 290V Zero450 360V 25mA456 550V 14mA316 1,000V 200mA

Taking into account the ignition threshold current over the length of the soil-bed is 60 mA, this data could be taken as an indication that the 250 volt limit may be over-conservative, though it can be seen that sustained current starts to appear at voltages only slightly higher than this limit. However, the voltage drop along the length of powerline between the substation and the fault location must also be taken into account.

8.3.4 Inclusion of a safety margin to cover remote faults

The initial draft performance standard specified conductor voltage must be measured at the source substation. However if an earth fault occurs some distance (perhaps some tens of kilometres) from the substation on a heavily loaded powerline, the voltage at the fault location will be higher than that at the source substation and this will increase the fire risk for that particular fault. This effect of


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distance and customer load in increasing the residual voltage at the fault location erodes the basis of any argument that the limit should be relaxed to be higher than 250 volts.

It was concluded that the 250 volt limit was appropriate and necessary for fire risk reduction.

One REFCL product (the GFN) offers enhanced functionality to progressively reduce the voltage at the fault location over a period of 20-30 seconds. This functionality was not seen in the Kilmore South tests. If it works as described15, it would reduce fire risk but increase the conductor voltage at the source substation, perhaps beyond the 250 volt limit.

It was concluded the compliance test should remain a simple measurement at the substation but the possible use of such enhanced technology should be recognised in the application of the standard.

It is recommended the performance standard be refined to:

1. Make clear the voltage measurement location (the substation) is for the purposes of compliance tests only; and

2. Recognise the possibility that a network owner may install enhanced technology to reduce residual voltage at the fault location even though this might increase the voltage in the substation beyond the 250 volts limit for real powerline faults.

8.4 Is the specified limit on residual conductor voltage achievable?

The 2015 RECL Technologies test program demonstrated compliance of all three REFCL technologies with the draft standard of not more than 250 volt residual conductor voltage at the substation.

8.4.1 Reduction of residual conductor voltage by the GFN

In the 2015 REFCL Technologies test program the voltage measurements were taken at the fault site, i.e. at the test facility. This was located 10 kilometres along 22kV feeder KMS21.

Test 301 was a GFN compliance test using a 25,400 Ohm fault. It produced the residual conductor voltage record shown in Figure 28. The voltage spikes reflect the GFN’s fault-confirmation test16 and the spaces between are the periods of residual conductor voltage.

Figure 28: Test 301 (GFN with 25,400 Ohm resistive fault) faulted phase conductor voltage

15 Checks with users of this technology (the GFN product) in other countries indicate it works as described by the supplier.16 In this test, a two-second delay was set between each step of the FCT, during which full compensation was reapplied.


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The residual voltage measured in the substation would be different to that measured at the test facility due to the series impedance of the powerline and the powerline currents due to customer load. Load on feeder KMS21 was generally below 12 amps (see Section Error: Reference source not found). Tests with the FPE switch on the faulted phase closed showed typical residual voltage at the test facility of less than 30 volts (see Section 8.4.2 following). This means that the maximum uncertainty in the KMS compliance tests due to measurement at the fault location rather than at the substation was about 30 volts.

In Test 301, the residual voltage on the faulted conductor at the test location varied between 122 volts and 155 volts. Taking into account the 30 volts uncertainty due to measurement at the test site at the end of feeder KMS21, this means the maximum residual voltage measured at the substation could not have been more than 185 volts, i.e. the GFN complied with the draft performance specification of 250 volts or less.

It was noticed that the GFN performance in this regard varied somewhat with time. Good cancellation was achieved shortly after recalibration of the RCC and re-tuning of the GFN. However, the value of residual voltage was sometimes observed to drift away from this initial good result. It was not clear to what extent this was due solely to measurement away from the substation (i.e. the effects of variations in customer load current) and to what extent, if any, it was due to the characteristics of the GFN compensation calibration scheme.

8.4.2 Reduction of residual conductor voltage by FPE

When FPE operates it directly connects the faulted conductor to the substation earth grid, i.e. the residual voltage on the faulted phase measured at the substation will by definition be zero, so the SSFCL+FPE and ASC+FPE configurations will always comply with the draft specification. Tests with FPE compensation typically had residual voltage profiles measured at the test facility similar to those shown in Figure 29.

Figure 29: typical residual voltage records for FPE compensation of faults17

17 The transient decay evident in some of the records shown in Figure 29 was an artefact of the high-pass frequency response of the capacitive voltage dividers used to measure network voltages at the test site. The final value just before the FPE compensation is removed is a reliable measurement.


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The measured residual voltage varies from 25 to 28 volts. Test results such as those shown in Figure 29 confirmed the expected compliance of FPE with the draft performance standard.

FPE response to a remote fault on a long, heavily loaded powerline would produce a higher residual voltage at the fault location due to the same powerline voltage drop described in Section 8.4.1 above. Calculation of residual voltage for any particular combination of powerline load and distance to the fault is relatively straightforward. This value can then be used with Figure 26 above to assess fire risk and public safety risk for such faults.

8.5 Are the best words used to specify the residual conductor voltage limit?

In line with the recommendation set out in Section 8.3 above, the wording in the performance standard should be reviewed to:

Make it clear the measurement of less than 250 volts residual voltage at the substation is for compliance test purposes; and

Ensure application of the standard allows for the use of enhanced technology to reduce the residual voltage at the fault location, even when this increases the voltage at the substation beyond the 250 volt limit during real sustained faults.


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9 Fault management diagnostic tests: limit on current and I2tThe initial draft performance standard specified that, when tested with a resistive earth fault, during any diagnostic tests, e.g. to confirm a sustained fault or identify the faulted powerline:

Fault current must be limited to not more than 0.5 Amp. I2t (where I is the fault current and t is the time for which it flows) must be limited to not

more than 0.04 A2s.


This section sets out the rationale and evidence to support the following findings:

1. Limits on current and energy release in diagnostic tests are an essential part of a performance standard if fire risk reduction is to be achieved.

2. The 0.5 Amp limit on fault current during diagnostic tests is appropriate, realistic and necessary for reduction of fire risk.

3. The 0.5 Amp limit can be achieved with available technology, specifically by a GFN.4. Provided it is clear that the 0.5 Amp limit applies in a compliance test using a 25,400 Ohm

resistor to create an earth fault, the current wording of the limit is appropriate.5. The 0.04 A2s limit on I2t in diagnostic tests is over-conservative and can be relaxed. A more

appropriate limit for prevention of fires would be 0.1 A2s.6. The SSFCL+FPE and ASC+FPE configurations did not provide diagnostic tests so their

performance against these criteria could not be tested.

Recommendation:

1. The limit in the performance standard on I2t in diagnostic tests should be relaxed from 0.04 A2s to 0.1 A2s.

The following sections set out the rationale and evidence for each of the above statements.

9.2 Is it necessary to include limits on diagnostic test current and I 2t in the draft standard?

This element of the draft standard is necessary for the goal of fire risk reduction to be achieved. The 2014 REFCL Trial showed that diagnostic tests to check for a sustained fault and identify the faulted powerline will lead to heightened residual voltage on a fallen conductor which in turn can produce enough fault current to cause a fire. The inclusion of a specified maximum current during diagnostic tests is necessary to avoid this outcome.

Further, the 2014 REFCL Trial showed that fires caused by diagnostic fault current did not start immediately and could be prevented if the current lasted only for a very brief time. The inclusion of a limit on energy released into the fault location environment by the diagnostic test is also required. I2t is an industry-recognised indicator of energy release from current flow. A limit on I2t in diagnostic tests is appropriate and necessary to ensure the energy released is insufficient to start a fire.

9.3 Is the specified limit on diagnostic test current right?

The initial draft performance standard set the limit on fault current during any diagnostic tests at 0.5 amps. All of the rationale and evidence set out in Section 6 above regarding fault detection sensitivity can also be directly applied to the definition of the limit for diagnostic tests.

Tranches 2 and 3 of the 2015 REFCL Technologies test program at Kilmore South included 150 ignition tests of the GFN’s combined fault-confirmation and powerline-identification test at different levels of artificially elevated residual voltage on the conductor. The results provide additional evidence to clarify fire risk from the short (0.5 seconds) pulses of voltage used in diagnostic tests.


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Analysis of these test results relied on assumptions because the length of conductor in the tests (0.4 metres) was much shorter than the assumed worst-case length of fallen conductor in real powerline faults (three metres).

Specifically:

The short length of conductor in the test rig may not draw enough current for the diagnostic test to ‘find’ the fault; and

The same current in the test rig as in a real fault would mean the test rig lineal current intensity (amps per metre flowing into soil) would be about seven times that in the real fault.

Using the same hypothesis of uniform distribution of current from the conductor into the soil as used in Section 6 above, Figure 30 shows that no fires in the Kilmore Tranches 2 and 3 tests were started at currents below the equivalent of 2.0 amps into a three metre length of fallen conductor.

Figure 30: fire risk as a function of peak current in GFN diagnostic tests (Kilmore Tranches 1, 2 and 3)

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Using the evidence presented in Section 6 above (specifically Table 5), there is a ratio of about three to one in the fire/non-fire boundary current values in the 2014 Frankston South and 2015 Kilmore South Tranches 2 and 3 tests. This implies that if the 150 tests had been done under Frankston or Kilmore Tranche 4 conditions, the current limit in diagnostic tests might have been set at 0.65 amps. However, to use the Kilmore South test results as a basis for relaxation of the proposed 0.5 amp limit would require reliance on too many assumptions.

The most reliable conclusion is that the 2015 Kilmore South Tranche 2 and 3 test results are consistent with the 0.5 amp limit on current during diagnostic tests, i.e. fires are unlikely if the 0.5 amp limit is observed. This is also consistent with the conclusion drawn in Section 6 above based on the analysis outlined in Figure 9 on page 30.

It was concluded that the 0.5 amp limit in diagnostic tests was appropriate to achieve the target reduction of fire risk.


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9.4 Is a compliance test using a resistive fault adequate?

There are compelling factors of cost, simplicity, repeatability and practicality that make compliance tests using resistive faults an attractive way to specify the performance required to prevent fires from powerline earth faults. The validity of such a test as a measure of fire risk was reviewed using the KMS ignition test results.

Some Kilmore South tests could be interpreted as indications that a compliance test using a resistive fault may not provide strong enough assurance of low fire risk. The GFN proved compliant in resistive fault tests and 112 valid ignition tests with the GFN on normal settings (i.e. not deliberately mis-calibrated to produce elevated residual conductor voltage) did not produce any fires. However, six ignition tests with the GFN on normal settings did produce fires.

The six tests that produced fires are listed in Table 16. Each of them was reviewed in detail to assess whether they provided strong evidence that a compliance test using a resistive earth fault was unsuited to assurance of fire risk reduction. However, each of them was found to be an unrealistic representation of real worst-case ‘wire down’ earth faults due to the trade-offs necessary in experiment design mentioned in Section 3.4 above.

Table 16: KMS GFN ignition tests that produced fires on normal GFN settings

Test No

Peak Bounce Amps

Bounce Arc

Duration (ms)

Peak FCT

Amps

Peak FCT Volts

Residual Volts

I2t (Amps2-s) Assessment

305 >2.5 54 0.87 3485 95 0.28 Unrealistic A306 6.78 49 1.86 5541 95 2.06 Unrealistic B307 0.11 18 0.5 11000 100 0.33 Unrealistic C313 6.41 65 0.62 3609 90 0.13 Unrealistic A467 12.89 44 1.34 2298 190 0.75 Unrealistic B532 5.59 37 0.55 4601 100 0.12 Rig problem

The assessment codes are:

Unrealistic A: extreme non-linear fault resistance, no current in the early stages of the FCT Unrealistic B: GFN didn’t find the fault on the first substantial FCT current pulse Unrealistic C: GFN did not find the fault and turned off the RCC, whereupon the fault

reappeared. This is a more extreme form of A above.

Reviews of selected tests that illustrate each of these assessment codes are set out here:

Test 313 (Unrealistic A – extreme non-linear fault resistance)

Test 313 resulted in a fire even though the GFN was operating on normal settings (the residual conductor voltage in the test was 92 volts). The recorded voltage and current during the fault-confirmation test are shown in Figure 31 and Figure 32 below.

The fault-confirmation test proceeded normally with the voltage progressively increasing until the conductor voltage was greater than 3.6kV at Step 10. Up to Step 10 no current larger than the measurement system noise level (10mA) had flowed. At Step 10, current suddenly started to flow and because the conductor voltage was so high, it averaged 0.5-0.6 amps over the half-second admittance measurement period with an I2t value of 0.134 A2s. This pulse of current produced a fire.


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Figure 31: Kilmore Test 313 voltage in the fault-confirmation test

Figure 32: Kilmore Test 313 current in the fault-confirmation test

This type of extreme non-linear fault resistance is typical of an insulating layer around the conductor that is suddenly broken down by the increasing high voltage between the conductor and the soil-bed. In ignition tests, the insulating layer was the dry grass in which the conductor was lying. After the bounce, the conductor tended to come to rest in a ‘tunnel’ of dry grass as shown in Figure 33.


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Figure 33: KMS Test 159 - grass after test, showing ‘tunnel’ formed by conductor drop

It was concluded this test was not a realistic simulation of a real earth fault. For this test to be a realistic reflection of a worst-case earth fault situation, it would have to be assumed that the whole three metre length of conductor lying on the ground was similarly insulated from earth by dry grass. Since the length of fallen conductor is more than seven times the length of the soil-bed in the test rig and it would be lying on ground that, unlike the soil-bed, has not been smoothed and uniformly populated with tufts of dry grass, this was considered an unsafe assumption. At some point along the three metre length of fallen conductor it is likely to have been in contact with soil and some current would have flowed in the earlier phases of the fault-confirmation test, causing the test to terminate before the conductor voltage reached 3.6kV and the current exceeded 0.5 amps.

It was concluded that this test did not reflect the typical reality of rural ‘wire down’ earth faults in extreme fire risk conditions. Whilst this test outcome remains theoretically possible in a real fault, it is considered to be somewhat beyond the boundary of ‘realistic worst-case’. As a result, Test 313 was not seen as providing strong evidence against the use of a resistive earth fault for performance compliance tests.

Test 467 (Unrealistic B – did not find fault on first substantial FCT current pulse)

Test 467 resulted in a fire though the GFN was operating on normal settings (the residual conductor voltage in the test was 198 volts). The recorded voltage and current during the fault-confirmation test in Test 467 are shown in Figure 34 and Figure 35. The final two FCT current pulses are shown in more detail in Figure 36.

The fault-confirmation test in Test 467 proceeded normally with the conductor voltage progressively increasing. At the third step the current increased over the 0.5 second measurement cycle from zero to around 0.07 amps. In Step 4, current started to flow at a rate of up to 0.65 amps with an average value over the measurement period of around 0.45 amps and an I2t of 0.11 A2s. However, this was insufficient for the GFN to ‘find’ the fault and the fault confirmation test proceeded to the fifth step in which the conductor voltage was 2.3kV and the current reached 1.34 amps, adding a further 0.64 A2s to the I2t.

At this increased current level, the GFN found the fault and stopped the fault-confirmation test, but by then ignition had occurred.


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Figure 34: Kilmore Test 467 voltage in the fault-confirmation test

Figure 35: Kilmore Test 467 current in the fault-confirmation test

Figure 36: Kilmore Test 467 - current and I2t detail in fault-confirmation test


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.

In a real fault with three metres of conductor on the ground, the current in Step 4 would have been around seven times the 0.65 amps test current into the soil-bed, i.e. about 4-5 amps. The GFN would certainly have found the fault at that point, given that it found it at a current of only 1.3 amps 2.5 seconds later in Step 5 of the test. This means that in a real fault, the test would never have gone beyond Step 4 and a fire would most likely not have occurred given the two-to-one ratio between the currents in Step 4 and Step 5.

Again, it was concluded this test was not reflective enough of real worst-case ‘wire down’ earth faults to pose strong evidence against the use of a resistive fault in the performance compliance test.

Test 307 (Unrealistic C – FCT did not find the fault, RCC turned off)

In Test 307, the initial soil current was only 47mA. The GFN detected the fault after 13 seconds when the fault current had risen to 140mA. The GFN used RCC compensation to collapse the white phase voltage to 100 volts, whereupon fault current ceased to flow. The fault current and conductor voltage are shown in Figure 37.

Figure 37: Kilmore Test 307 fault current and conductor voltage


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The fault-confirmation test proceeded through all 13 steps, increasing the residual voltage on the conductor to 6.4 KV without any current flow. The GFN logic concluded the fault was not sustained and removed RCC compensation, restoring network voltages to normal. Shortly after it did so, the fault current reappeared rising to 0.5 amps before falling to 0.1 amps at the end of the 60 second test period. This final burst of fault current produced a fire.

This test illustrates an extreme form of non-linear fault resistance and for the reasons given in the discussion of Test 313 above, it is unrealistic.

Discussions with the GFN manufacturer provided valuable insights into possible initiatives to further reduce fire risk in such a situation. These included:

1. Guidance on the choice of settings to determine the sequence of steps in the FCT. The FCT can progressively increase the conductor voltage to a much higher level than the standard 50% before a decision is made to switch off the RCC.

2. GFN firmware modification to switch off the RCC without first returning to full neutral voltage displacement, i.e. the RCC removal simply becomes the final step of the FCT, increasing the conductor voltage to 100% of nominal.

3. Clarification of the necessary ‘dead time’ during which fault detection must be disabled after the RCC is switched off, i.e. the time taken for the neutral voltage to collapse to a level below the fault detection threshold. There is a further product development opportunity to reduce this time by using the RCC to more rapidly collapse the neutral displacement.

While these insights are likely to be useful to optimise the approach to FCT settings, this type of fault behaviour is still judged unrealistic for the reasons given in the discussion of Test 313 above.

Test 532 review (Assessment: Rig problem)

Test 532 resulted in a fire though the GFN was operating on normal settings (the residual conductor voltage in the test was 97 volts). Review of the video record showed the conductor had not settled flat on the soil-bed after the bounce so the soil current was concentrated into a small length (probably not more than 50-70 millimetres) at one edge of the soil-bed increasing lineal current intensity over that short length of contact about tenfold. This led to ignition. This test and others with similar mechanical problems were marked as invalid.


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Figure 38: Kilmore Test 532 - conductor misalignment, leading to concentration of current

In summary, review of these apparent exceptions supported the proposed compliance test and performance standard as sufficient to prevent fires in realistic ‘wire on ground’ earth faults. No other tests with the GFN operating on normal settings produced a fire.

9.5 Is the limit on I2t right?

The 2014 REFCL Trial tests at Frankston South did not include enough ignition tests of fault management diagnostic tests to reliably quantify the energy release required in such tests to start a fire. Tranches 2 and 3 of the 2015 REFCL Technologies test program at Kilmore South incorporated a much more extensive range of tests and these results are the most authoritative available information on fire risk from this mechanism.

The correlation between I2t in tests and probability of fire is shown in Figure 39.

Figure 39: probability of fire versus I2t in diagnostic tests

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In 40 valid ignition tests with an FCT I2t of 0.1 A2s or below, there were no fires produced. This would indicate that the draft performance standard criterion of 0.04 A2s is over-conservative. Two further considerations would support this assessment:

The differences between the simulation of earth faults in the test rig and real earth faults involving three metres of conductor on the ground, as outlined in Section 9.4 above, would imply that ignition tests produce a higher probability of fire for the same I2t than real earth faults because they involve a higher lineal current intensity by a factor of about seven and because the FCT would terminate earlier in a real earth fault at the same lineal current intensity.

If FCT peak current is limited to 0.5 amps and the typical admittance measurement cycle time in an FCT is not more than 0.5 seconds, then I2t will normally be limited to less than 0.125 A2s anyway.

It was concluded that the I2t limit should be relaxed to a value of 0.1 A2s, even though the GFN was found to comply with the 0.04 A2s limit (see Figure 40 below).

9.6 Are the specified limits on diagnostic test current and I 2t achievable?

The Kilmore South tests demonstrated that available technology can comply with the draft performance standard in these two areas. The draft standard calls for the current limit during diagnostic tests to be demonstrated using a resistive fault.

KMS Test 301 with the GFN in service and a 25,400 Ohm earth fault produced the current and I2t record shown in Figure 40. The peak fault current prior to fault detection was 0.47 amps. The peak current in the diagnostic test was 0.13 amps, well below the 0.5 amp limit. The I2t due to the FCT is the difference between the initial (post-fault) value and the final value shown in Figure 40. The fault-confirmation test I2t was 0.03 A2s, within the 0.04 A2s draft limit and well within the recommended relaxed limit of 0.1 A2s.

Section 10 below describes further GFN compliance tests that confirm that the criteria for diagnostic tests are achievable with current technology.

Figure 40: KMS Test 301 - current and I2t during GFN diagnostic tests with 25,400 Ohm resistive earth fault


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Similar tests were not performed for the other two REFCL configurations as neither of them provided diagnostic test functionality at this stage of their product development.

9.7 Are the best words used to specify the limit on diagnostic test current?

Provided it is clear the diagnostic test limits apply in a compliance test using a 25,400 Ohm resistor to create an earth fault, no changes to the wording were seen as necessary.


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10GFN compliance tests After Tranches 1, 2 and 3 (Tests 1 to 538) a two-month break in the field test program allowed some enhancements to be made prior to resumption of tests in Tranche 4. These included:

1. Further balancing of the test network (re-phasing of a two-wire spur line) which reduced the standing value of neutral current with the NER in service from 187mA to 117mA.

2. Installation of a second U0 injector in the GFN to provide a greater range of balancing options to reduce the standing level of neutral voltage displacement.

3. Enhanced GFN firmware that addressed some issues identified in Tranches 2 and 3.4. Modification of the ‘wire down’ test rig to enable tests on lo-sag covered conductor18.5. Commissioning of the ‘branch touching wire’ vegetation earth fault test rig.6. Agreement in principle on the recommended changes to the draft performance standard

contained in the Interim Report on Tranches 1 to 319.

As the GFN was very close to compliant in Tranche 3 tests, it was re-subjected to a full set of compliance tests in Tranche 4. The results of these tests are set out in the following sections. The other two REFCL configurations were not tested for compliance at this time, pending further product development by their manufacturers.


The sections below set out the evidence and rationale to support the following findings:

1. The GFN was found to comply with all elements of the amended draft performance specification set out in Section 4.2 on page 19.

2. The GFN compliance with the I2t criterion in high-impedance faults was confirmed by root-cause analysis of test results to allow for the effects of higher than target levels of network capacitive imbalance.

3. Testing of the I2t criterion in low-impedance faults proved impractical due to the effects of network harmonics.

It is recommended that:

1. Compliance testing for I2t performance in diagnostic tests be restricted to high-impedance fault tests;

2. Industry continue to work with the GFN manufacturer to improve I2t performance to provide a larger safety margin and provide assurance of compliance in installations on larger networks than the KMS test network; and

3. The GFN manufacturer be requested to review and advise on the risks posed by false detection of back-fed faults when the GFN operates at high fault detection sensitivity.

These findings and recommendations are set out in greater detail in the following sections.

10.2 Performance requirement: detect 25,400Ω earth fault in 1.5 seconds

Conclusion: The GFN was found to fully comply with this performance requirement.

The GFN was tested with a 25,000Ω resistor20 to earth on each phase. The compliance test results are set out in Table 17.

18 The results of tests on lo-sag covered conductor are the subject of a separate report.19 These included the changes described in recommendation 1.2.1 in the Executive Summary on page 9 above.20 The 1.6% deviation from the specified 25,400Ω test resistance was considered immaterial except in the I2t tests described in Section 10.6.1 below where some results were adjusted to take it into account.


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Table 17: GFN compliance tests - fault detection performance, high-impedance fault

Test Phase Current (A) Time to detect (ms)Required: <1,500ms

621 White 0.48 205622 White 0.47 153623 White 0.48 174644 White 0.47 171624 Blue 0.51 270625 Blue 0.51 286626 Blue 0.51 280643 Blue 0.51 266645 Blue 0.51 273646 Blue 0.51 279647 Blue 0.50 283627 Red 0.47 181628 Red 0.47 185629 Red 0.47 184642 Red 0.48 169

The KMS test network was much smaller (and hence, more inherently fault-sensitive) than the average rural network in Victoria. To check how much margin was available for sensitive fault detection on larger networks, the KMS GFN was also tested with a 40,000Ω resistor connected from each phase to earth. The test results are shown in Table 18.

NB: this was an extreme test that far exceeded any requirements of the proposed regulatory compliance test.

Table 18: GFN bolted resistive fault tests – extreme fault detection performance (40,000Ω earth fault)

Test Phase Current (A) Time to detect (ms)634 White 0.31 293635 White 0.30 286636 White 0.30 253637 Blue 0.32 473638 Blue 0.32 489639 Red 0.30 268640 Red 0.31 314641 Red 0.31 307

The GFN identified two of these faults as back-fed earth faults21 (Tests 640 and 641). This is undesirable as the GFN is designed to immediately disconnect the 22kV powerline in such a case. Diagnostic tests confirmed that at very low levels of resistive fault current, capacitive imbalance and tuning mismatch current may swing the phase of the neutral displacement into the GFN’s ‘back-fed fault’ detection zone.

21 Earth faults where a broken conductor remains connected to the powerline on the downstream side of the break and is energised by downstream customer load current – refer to Sections Error: Reference source not found and Error: Reference source not found.


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It is recommended the GFN manufacturer be asked to study this risk and report on its materiality given the fault detection settings expected to be required in Victoria’s high fire risk regions.

10.3 Performance requirement: reduce voltage to 250V in 2 seconds


The GFN was tested with a 25,000Ω resistor to earth on each phase. Results are set out in Table 19.

Table 19: GFN compliance tests – fault response, high-impedance fault

Test Phase Voltage at 2,000msRequired: <250V

Test Phase Voltage at 2,000msRequired: <250V

621 White 95 645 Blue 175622 White 103 646 Blue 142623 White 79 647 Blue 145644 White 135 627 Red 87624 Blue 64 628 Red 91625 Blue 138 629 Red 102626 Blue 123 642 Red 118643 Blue 163

The GFN includes capability to progressively reduce the voltage at the fault location to a very low level by manipulating the voltage at the zone substation. Because the KMS21 powerline on which the tests were carried out was very short and lightly loaded, this capability could not be tested. It is recommended this capability be tested at some future time if the GFN product is installed in rural substations with longer, more heavily loaded powerlines than KMS21.

10.4 Performance requirement: fast response to low impedance faults


The GFN was tested with a 400Ω resistor to earth on each phase. The results are set out in Table 20.

Table 20: GFN compliance tests – fault response, low-impedance fault

Test Phase Voltage at 85msRequired: <1,900V

Voltage at 500msRequired: <750V

Voltage at 2,000msRequired: <250V

630 White 107 117 114631 White 114 126 139632 White 130 130 136651 White 138 96 101652 White 168 114 121633 Blue 74 82 81649 Blue 112 114 112650 Blue 137 170 161653 Red 117 80 77654 Red 91 82 81

On a larger network than the KMS21 test network, the natural decay of conductor voltage is likely to be slower and the sizing of the RCC inverter may have to be specifically selected to meet the ‘<1,900V in 85ms’ criterion and to a lesser extent, the ‘<750V in 500ms’ criterion.


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10.5 Performance requirement: maximum 0.5A current in diagnostic tests


The GFN was tested with a 25,000Ω resistor to earth on each phase and the maximum current in the fault-confirmation test was recorded. The results are set out in Table 21.

Table 21: GFN compliance tests – maximum current in diagnostic tests, high-impedance fault

Test Phase Peak FCT currentRequired: <0.5A

Test Phase Peak FCT currentRequired: <0.5A

621 White 0.16 645 Blue 0.21622 White 0.16 646 Blue 0.24623 White 0.16 647 Blue 0.21644 White 0.17 627 Red 0.20624 Blue 0.23 628 Red 0.20625 Blue 0.23 629 Red 0.19626 Blue 0.23 642 Red 0.21643 Blue 0.25

10.6 Performance requirement: I 2t<0.1A2s in diagnostic tests

Conclusions:

GFN was found to comply with this performance requirement for high-impedance faults on two out of the three network phases.

The GFN exceeded the performance criterion by 2% and 8% in two out of seven tests on the remaining phase and complied in the other five tests. After root-cause analysis, the GFN was assessed as compliant overall.

In tests with low-impedance earth faults, network harmonics rendered compliance testing impracticable.

10.6.1 I2t in high-impedance fault tests

The GFN was tested with a 25,000Ω resistor to earth on each phase and the I2t increase during the fault-confirmation test was calculated. The results are set out in Table 22.

Table 22: GFN compliance tests – maximum I2t in diagnostic tests, high-impedance fault

Test Phase FCT I2tRequired: <0.1A2s

Test PhaseFCT I2t

Required: <0.1A2s

621 White 0.048 645 Blue 0.072622 White 0.046 646 Blue 0.102623 White 0.048 647 Blue 0.072644 White 0.045 627 Red 0.067624 Blue 0.099 628 Red 0.067625 Blue 0.094 629 Red 0.067626 Blue 0.094 642 Red 0.071643 Blue 0.108


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Fault tests on white and red phases showed the GFN to be compliant with the performance standard. Results of tests on the blue phases varied, with five results compliant and two not compliant. Investigations showed that compliance occurred when the GFN found the fault at the fifth step of the FCT’s Stage 2, while non-compliant results occurred when the fault was not found until the sixth step. This difference is shown in Figure 41.

Figure 41: KMS Tests 643 and 645 – Blue phase high-impedance fault, GFN compliance with I2t criterion

It appeared that the 25,000Ω fault resistance put fault detection right on the boundary between these two steps of the FCT. It would have been a simple matter to reduce the admittance delta threshold setting in the FCT logic to achieve compliant performance but this was not done as it was not certain that it would validly indicate likely GFN performance in larger networks.

Adjusting for the use of a 25,000Ω resistor22 and averaging the seven Blue phase results, the result was 0.090A2s which is 10% below the 0.1A2s limit and can reasonably be argued to constitute compliance, particularly as a better balanced network would have reduced the Blue phase I2t results significantly. It is recommended the GFN manufacturer be encouraged to continue to develop the FCT algorithm to provide greater safety margin in this particular test.

22 Blue phase I2t results shown in Table 22 for Tests 624, 625, 626 and 643 were reduced by 3.2%. Tests 645, 646 and 647 were performed with the specified 25,400Ω fault resistance and did not require adjustment.


Fault detected on sixth step of FCT Stage 2: potentially

non-compliant.

Fault detected on fifth step of FCT Stage 2: compliant.

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10.6.2 I2t in low-impedance fault tests

Measurement of I2t in low-impedance fault tests demonstrated that the presence of network harmonic voltages makes this test unsuitable for use as a regulatory compliance test.

The problem is illustrated by the fault current and I2t charts for typical high-impedance and low-impedance tests shown in Figure 42 and Figure 43.

Figure 42: KMS Test 644 white phase high impedance fault test

In the high-impedance earth fault Test 644 (Figure 42) harmonic currents were limited to low levels by the high fault impedance. As a consequence, the I2t record was a clean representation of the fire risk produced by the FCT. The situation proved different in the low-impedance fault tests as shown in Figure 43:


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Figure 43: KMS Test 651 white phase low-impedance fault test

In Test 651, the low fault impedance allowed harmonic current of approximately one quarter of an amp to flow. Whilst the FCT is visible in the test records, the I2t result is dominated by this harmonic current and the FCT makes only a minor contribution to the total.

To avoid this uncertain outcome, it is recommended that I2t compliance assessment of REFCLs be limited to high-impedance earth fault tests as described in Section 10.6.1 above.

More information on network harmonics and the GFN capability to manage them is contained in Section Error: Reference source not found on page Error: Reference source not found below.


FCT

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10.7 Compliance test procedure

In compliance tests, the following sequence of actions was applied to the GFN-protected test network:

1. A bolted resistive fault was applied, either 25,000Ω or 400Ω, to one phase of the network.2. The GFN response was recorded while the fault remained on the network for 20 seconds –

including initial response to the fault and the subsequent automatic fault-confirmation test.3. The fault was removed from the network after 20 seconds.4. A GFN fault-confirmation test was manually initiated.5. The GFN response was recorded while the fault-confirmation test failed to find the fault and

the GFN switched off RCC compensation to restore network voltages to normal levels.

Figure 44 and Figure 45 illustrate typical high-impedance and low-impedance fault tests:

Figure 44: KMS Test 621 – typical high-impedance fault compliance test: 25,000Ω fault on White phase


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Figure 45: KMS Test 630 – typical low-impedance fault compliance test: 400Ω fault on White phase


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The 25,000Ω resistor used in the tests was made up of three 16,700Ω/0.25A woven-Constantin resistors connected in series/parallel across an in-line insulator in the test cell high-voltage supply as shown in Figure 46. The test rig was short-circuited for these tests as it was for all ‘bolted resistive fault’ tests.

Figure 46: high voltage 25,000Ω resistor


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Executive summary - Energy Web viewThe Victorian Government is committed to leading ground-breaking research and accelerating the development of innovative technologies in the field