DSS/ 007 / 010 Code of Practice for the Protection and ... · Document reference DSS/ 007 / 010 Version:-1.1 Date of Issue:-Jan 2011 Page 3 of 30 CAUTION! - This document may be out

Document reference DSS/ 007 / 010

Version:- 1.1 Date of Issue:- Jan 2011 Page 1 of 30

CAUTION! - This document may be out of date if printed

DSS/ 007 / 010 – Code of Practice for the Protection and Control of HV Circuits

1 Purpose

This code of practice details the network protection philosophy covering the High Voltage (HV) networks. There is a legal obligation to provide protection to meet the requirements of the Electricity Safety, Quality and Continuity Regulations 2002 and the Electricity at Work Regulations 1989. This code of practice is driven by our Quality of Service (QoS) performance, specifically reduction in number of faults, reduction in number of customers per fault and improvements in fault restoration times. The document should be considered in conjunction with IMP/001/912 – Code of Practice for the Economic Development of the HV system.

This document supersedes the documents detailed in Section 3.13

2 Scope

This document applies to both rural and urban HV circuits (main line and spur protection) with a nominal operating voltage of between 1kV and 22kV. All other voltages are covered by DSS/007/001 – Code of Practice for the Protection of High Voltage Networks (TS1).

This code of practice shall be applied to all HV system development including new connections, system reinforcement and asset replacement. It is not intended to apply this code of practice retrospectively.

3 Policy

3.1 Assessment of Relevant Drivers

The key internal business priorities relating to the protection and control of HV circuits are:

Employee Commitment – achieved by developing a safe HV system to ensure that members of the public and employees are not exposed to risks to their health as far as reasonably practicable;

Financial Strength – contribute towards maximising IIS rewards and minimising operating costs;

Customer Service – achieved by reducing the impact of fault incidents on customers

Operational Excellence – achieved by reducing the potential number of customer interruptions and customer minutes lost

The external business drivers relating to the protection of HV systems are detailed in the following sections.

3.1.1 Requirements of the Electricity Safety, Quality and Continuity Regulations

The Electricity Safety, Quality and Continuity (ESQC) Regulations 2002 impose a number of obligations on Northern Powergrid, mainly relating to quality of supply and safety. All the requirements of the ESQC Regulations that are applicable to the protection of HV circuits shall be complied with and the Northern Powergrid distribution systems shall be designed to comply with these requirements.

Regulation 3 states that “generators, distributors and meter operators shall ensure that their equipment is … so constructed, installed, protected (both electrically and mechanically), used and maintained as to prevent danger, interference with or interruption of supply, so far as is reasonably practicable”. This code of practice details the processes to be followed in order to ensure that the network is designed to limit the number of people that are affected by a fault and hence comply with these requirements.

Regulation 6 states that “A … distributor shall be responsible for the application of such protective devices to his network as will, so far as is reasonably practicable, prevent any current, including any leakage to earth, from flowing in any part of his network for such a period that part of his network can




no longer carry that current without danger.” This code of practice details the process to be followed in order to ensure that the protection fitted to HV circuits meets these requirements.

3.1.2 Requirements of the Electricity at Work Regulations 1989

Regulation 5 of the Electricity at Work Regulations 1989 states: “No electrical equipment shall be put into use where its strength and capability may be exceeded in such a way as may give rise to danger” and places obligations on the business relating to the safety of plant and equipment used on the distribution system. It requires that plant and equipment is designed and operated within the limits of its capability.

3.1.3 Requirements of Distribution Licences

The Distribution Licenses held by Northern Powergrid contain a number of conditions to be complied with which are relevant to system design. In particular, Standard Licence Condition 24 requires the distribution system to be planned and developed to a standard not less than that set out in Engineering Recommendation P2/6 (2006) – Security of Supply. This CoP requires that the HV distribution system is designed to at least the standard required by ER P2/6.

In addition, Standard Licence Condition 21 refers to the Distribution Code and requires that “the licensee must take all steps within its power to ensure that the Distribution Code in force …remains a code approved by the authority that complies with … the requirement that the Distribution Code … must be designed so as to permit the development, maintenance and operation of an efficient, co-ordinated, and economical system for the distribution of electricity.” The code of practice requires that the HV distribution system is designed in line with the Distribution Code and therefore in an efficient, co-ordinated and economical manner as required by the Electricity Act.

3.2 Key Requirements

The general objective in developing the protection for the HV network is to ensure the safety of public and staff and to minimise QoS indicators through the management of the number of customer interruptions, the number of customers affected and the restoration time of faults within the constraint of the customers’ willingness to pay,

This code of practice helps ensure that all HV circuits are protected in a manner which:

Prevents, as far as reasonably practicable, danger to members of the public and staff;

Optimises network security and availability; and

Satisfies all other relevant obligations.

3.3 Background

3.3.1 Design Policy

The objective of this code of practice is to ensure safety and contribute to achieving our strategy to deliver an economic level of reliability in line with the willingness to pay of our customers. This is achieved through the management of the four variables:

Number of interruptions;

Customers per fault;

Restoration time, and

Cost.

Different design approaches can be used to improve these base measures and this code of practice aims to detail the approaches to be used.




3.3.2 Design Guidelines

This code of practice shall be read in conjunction with the relevant Engineering Recommendations and other CE Electric UK documents including the following:

Code of Practice for the Economic Development of the HV System (IMP/001/912)

Code of Practice for the Application of Lightning Protection (IMP/007/011)

Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001)

Code of Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007)

Design of HV circuits should be performed using the following design process:

A summary of the process to be followed can be found in Appendix 1.

3.3.3 Network Configuration

The main purpose of the HV system is to distribute electricity in localised urban and rural areas in an economic, efficient, safe and secure manner, meeting the needs of electricity supply to customers currently and likely to be connected to it. The HV system supplies HV/LV substations, larger demand and generation customers at HV, systems owned and operated by Independent Distribution Network Operators (IDNOs) and is designed in line with IMP/001/912 – Code of Practice for the Economic Development of the HV system.

Collect circuit

information

Identify main and spur lines, customer numbers and their distribution

Design protection for main line

- design source protection (see section 3.4):

- position pole mounted autoreclosers (section 3.4.1)

Design protection for spurs (see section 3.6):

- position auto sectionalising links (section 3.6.1)

- design required fuse protection (section 3.6.2)

Complete additional circuit protection:

- ferreoresonance assessment (section 3.7)

- position lightning protection (section 3.8.1)

- position fault passage indicators (section 3.8.2)




Urban HV systems normally comprise underground cables and looped ground mounted HV to LV substations. Rural HV systems normally comprise of a mixture of underground cables, ground mounted HV to LV substations and overhead lines with teed pole mounted HV to LV transformers.

The Northern Powergrid HV systems predominantly operate at 20kV and 11kV, with a small proportion of lower voltage (5.25kV to 6.6kV) assets associated with specific customer supplies or small pockets of old industrial areas of urban centres.

The majority of 20kV systems serve more sparsely populated rural areas, typically Northumberland whereas the 11kV systems serve more urban areas such as Tyne and Wear, Durham, Cleveland and most parts of Yorkshire.

3.4 Main Circuit Protection

Details of the process and requirements for source circuit breaker protection can be found in:

Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001)

Code of Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007)

Protection of the main circuit should be considered first. The protection of spurs should be excluded in this section and protection of the main line should be implemented in the following manner.

3.4.1 Auto-Reclosers

3.4.1.1 Functional Specification

The functional specification for auto-reclosers shall be in line with the latest version of NPS/001/009 – Technical Specification for 11kV, 20kV and 33kV Pole Mounted Auto-Reclose Circuit Breakers.

The protection functional requirements are as follows:

4 trips to lockout with any combination of tripping characteristics.

Over Current / Earth Fault (OC/EF) and Sensitive Earth Fault (SEF) sequences to be independent of each other.

Selectable Sequence Co-ordination.

Individually adjustable Dead Times

Adjustable reclaim times

Sequence co-ordination.

Cold Load Pickup.

Magnetising Inrush Restraint.

Voltage Measurement on the incoming terminals with provision for an external Voltage Transformer (VT) for measurement of voltage on the load terminals.

All auto-reclosers fitted onto main line circuits shall be equipped with remote control.

3.4.1.2 Design Rules

A summary of the design process can be found in Appendix 1.

Source Protection Zone

All HV circuits with greater than 1km of overhead line (including spurs) shall be equipped with auto-reclose facilities either at the source or at another circuit breaker (see below).

The functionality and operation of the primary breaker should then be considered. If the breaker has a multi shot capability, the use of this should be used.




Note that consideration should be given to disabling auto-reclose on the primary breaker and installing an auto-recloser at the beginning of the first main overhead line section where the following applies:

the first leg from the Primary substation consists entirely of underground cable which feeds more than 50 customers;

the primary substation breaker does not have multi shot capability.

Primary CBs and Pole Mounted Auto Reclosers (PMARs) being used to provide the source zone auto-reclose facilities should be set in accordance with section 3.4.1.3

Further Protection Zones - Location of PMARs

In determining possible locations for a PMAR the following should be considered:

the current capacity and breaking capacity of the auto-recloser; and

staff access requirements.

In general the provision of a PMAR on an overhead line distribution circuit will result in a reduction in the number of customers per fault and the fault restoration times on that circuit. The improvement in performance is a function of the location of the PMAR, and is greatest at a particular point in the line. However, it may not be desirable to locate the recloser at this point, for example accessibility may be difficult, and so it is necessary to estimate the benefit of a number of alternative locations and select the most advantageous.

The performance benefit is a function of many parameters with wide tolerances, including fault rates, outage times, customer distribution etc. To simplify the criteria for provision of PMARs, guideline rules have been developed to describe the number and location of reclosers that should be installed on a given circuit depending upon its make up and configuration. The objectives include:

No customer should experience more than four interruptions of three minutes or longer per annum.

No more than 500 customers between remote control points (note that all main-line PMARs are to be fitted with remote control).

No more than 2000 customers per circuit.

A maximum of three auto-reclose devices should be used in series at 11kV and four at 20kV.

No more than 200 customers between switching points.

Additional auto-reclose protection zones should be installed where the potential annual gain in customer interruptions (CI) and customer hours lost (CHL) as calculated by approved software meets the following requirements (calculated for DPCR5 IIS incentive rates):

(CI saved) + 2*(CHL saved) > 100

Any situation outside of these parameters should be referred to the Design Manager for guidance.

3.4.1.3 Application of Settings

Selecting protection settings on overhead line circuits is a balance between speed of operation to reduce the amount of energy released at the point of fault and the deliberate insertion of time delays to create grading points and so reduce the number of customers affected by a fault isolating only the part of the network necessary to clear the fault. There is also the need to ensure that spur-line fuses and sectionalisers operate as intended and a delay may be required for this.




For new overhead lines that are of short or medium length the deliberate delaying of protection to create grading points shall be employed but should be restricted to one grading point. Standard settings will be adopted on short and medium length lines.

Where an existing short or medium line is being redeveloped the minimum time delay policy set out in the preceding paragraph shall be implemented.

For new lines that fall into the long category the protection shall be graded using both time and current settings. Opportunity shall be taken whenever an existing long line is being refurbished or reconfigured to implement this protection policy.

In the case of overcurrent and earth faults, fuses and sectionalisers should, where possible, discriminate with up-stream protection. It is accepted that in some cases 100% discrimination can not be achieved and in most cases discrimination with the SEF settings we normally use will not be possible.

General

Where the protection in use is currently instantaneous, fault sequencing should not be used however, the facility to employ this in the future should be included to allow for its use should the current drivers on quality of supply change. Where sequencing is currently employed in conjunction with instantaneous settings with the intention of minimising the number of transient interruptions seen by customers, then this should be maintained. Where auto-reclosers are connected in series and delayed protection is enabled then fault sequencing should be used on the recloser with the time-delay settings.

Due to the different protection characteristics available on each of the reclosers currently in use and on those proposed for purchase, only units of the same fault interruption type and fitted with configurable relays having suitable characteristics shall be installed in series.

Overcurrent Settings

These should be set in accordance with DSS/007/007.

Earth Fault Settings

These should be set in accordance with DSS/007/007.

Note that earth fault settings should be a minimum of 80 amps and discriminate with any fuses and sectionalisers (20% margin on actuating current) installed in the zone.

Sensitive Earth Fault Settings

All HV circuits greater than one span of overhead line shall be equipped with SEF protection.

The operational policy for try-in post a confirmed SEF protection operation is currently under review at a national level in conjunction with the HSE and therefore the auto-close arrangements shall continue to be consistent with the operational procedure applied by the control centre for each circuit.

Where the control room operational procedure is to wait at least 30 minutes prior to try-in of the circuit then the SEF protection shall be set to inhibit a re-closer sequence so that SEF operation causes a trip and lockout of the circuit breaker.

Where the control room operational procedure is to initiate a try-in immediately after an earth fault operation then the SEF protection shall be set to initiate an autoreclose sequence. Where a separate SEF sequence is available it should be selected to 2 trips to lockout.

Short and medium line lengths

For protection purposes a line will be classified as short if the phase to phase or three phase fault current at the most remote point on the feeder is 800A or greater at 11kV or 1600A or greater at 20kV and of medium length if the fault current at the most remote point on the feeder is 400A or greater at 11kV or 800A or greater at 20kV. Other than the setting for the source Inverse Definite Minimum Time (IDMT) relay which will be calculated, settings at all other relaying points will selected from a standard




list approved by the Technical Services Manager and issued in DSS/007/007 – The setting of protection and associated equipment.

The operating sequence for all source zone auto-reclose protection equipment on the HV system will be 2 instantaneous trips followed by one IDMT trip to lockout with the IDMT set to a 0.7 second clearance time in accordance with DSS/007/007. Where there is one pole mounted circuit breaker in series with the source circuit breaker the pole mounted circuit breaker will be set to 4 instantaneous shots to lock out. Where there is a second PMAR in series with the first, the first auto-reclose circuit shall be set to 4 instantaneous shots to lockout with a 120millisecond delay and sequencing selected for each trip. The second most remote PMAR shall be set at 4 instantaneous trips to lock out. Any further down stream grading points will be achieved through the use of sectionalisers with different pulse settings to trip i.e. grading shall be achieved by trips to lockout. Diagram 1 below illustrates the basic configuration for the maximum number of circuit breakers in series.

3 Stage Auto-Reclose Scheme - Diagram 1

Pole Mounted Auto-Recloser

Pole Mounted Auto Recloser

Protection 2 Instantaneous 1 IDMT SEF 20 Amp 10 Second Delay

Ground Mount Auto Reclosing Circuit

Protection 4 Definite Time Each 120millisec Delay and sequencing SEF 16 Amp 7.5 Second Delay

Protection 4 Instantaneous No Delay SEF 16 Amp 5 Second Delay

Sectionaliser

Protection 3 Instantaneous Opens in third Dead Time




Long Lines

Long lines where the phase to phase or three phase fault current at the most remote point from the source is below 400A at 11kV and below 800A at 20kV will have each protection point individually calculated to ensure that the customers at the end of the lines are adequately protected. Additional protection points may be added to provide further sectionalisation where this is considered economically beneficial. Each additional protection point will require increasing the time delay of the instantaneous settings of the relevant protection by 120milliseconds to ensure grading. The maximum permitted time delay of the instantaneous protection at the source circuit breaker is 360millisec. Where time delays are used in conjunction with instantaneous settings protection sequencing will also be used.

Auto-Reclose Sequences

The dead time and reclaim time for auto-reclose relays at source protection points with solenoid or motor charged circuit breakers will be set as detailed below:

For multishot relays

Dead Time 10.0 seconds

Reclaim Time 7.5 seconds

The dead time and reclaim time for pole mounted auto-reclose relays not at source protection locations will be set as detailed below:

For multishot relays

Dead Time 5.0 seconds

Reclaim Time 10.0 seconds

The Use of Magnetising Inrush Settings

Magnetising inrush restraint settings will not normally be applied at protection points where instantaneous protection is activated however the use of instantaneous protection settings may cause problems when attempting to restore supplies after a transient fault due to high transformer magnetising inrush currents in some networks. Where problems are identified due to the transformer capacity on a section of network the magnetising inrush restraint feature shall be activated.

Network conditions where consideration should be given to applying a magnetising inrush restraint setting are:-

PMARs with a 400A setting will have a magnetising inrush restraint setting applied if the installed transformer capacity is above 5MVA.

PMARs at positions in the network with lower fault levels and a 200A setting will have a magnetising inrush restraint setting applied if the installed transformer capacity is above 2.5MVA.

Notes:

Gas-filled Vacuum Recloser (GVR) PMARs do not have facility for selecting a magnetising inrush restraint setting.

All MVA values relate to summated transformer capacity not load.




3.5 Arc Suppression Coils

Arc Suppression Coil (ASC) earthing works in conjunction with the existing overhead line protection. The coil will minimise the fault current for all single phase to earth faults whilst it is not bypassed. If the fault current disappears within bypass time no other action is initiated and the supply will not be disconnected, resulting in customers experiencing fewer interruptions and an improved quality of service. After a set period of time, the coil will be bypassed which will allow the protection to deal with interruptions caused by permanent single phase to earth faults and interruptions due to all other types of faults.

During the time a single-phase to earth fault is on the system and before the ASC is bypassed most faults will fully displace the system neutral. This will cause a large unbalance in the capacitive leakage to earth between the three phases. Standard earth fault relays (i.e. non-directional) will detect this unbalance as earth fault current. Normal earth fault settings are usually too high to be affected by this unbalanced current however, SEF settings can be in range and care must be taken to set the bypass time within SEF operating time where this occurs.


The functional specification for the installation of new arc suppression coils shall be as follows:

An automatically tuned ASC with associated controller. The HV network connected to the primary substation determines the coil size required. The actual coil rating chosen will be 50% greater than the initial requirement, in order to future-proof the site and allow for operational switching.

Shunt circuit breaker with auto-opening and auto-closing protection scheme.

Standby earth fault switching scheme as required to compensate for the lower earth fault levels with a single Neutral Earthing Resistor (NER).

Fitting of disturbance recorders as necessary.

Replacement / addition of neutral earthing resistors as necessary. Liquid filled resistors in a poor condition will be replaced with dry type resistors.

Protection panel for housing of controller and associated protection scheme

Four isolators to allow for maintenance of the ASC equipment and transformer outages

Balancing of single phase spur lines as necessary to reduce the standing coil voltage to an acceptable level.

SCADA alarm of coil operation

SCADA control of by-pass CB.



ASCs are currently installed to cover a substantial part of the overhead line network. When any changes are made to feeders out of a site which contains an ASC, then the impact of the changes on the ASC should be considered.

ASCs will be installed at selected worst performing primary substations. These investments are driven purely by quality of service performance. Installation is only beneficial at sites where the feeders are made up from a high percentage of overhead line (i.e. cable forms typically no greater than one third of the network). The performance of the feeders out of the sites will be assessed and the decision made to apply ASC earthing to those sites which have the highest number of overhead line faults and the greatest length of normally connected overhead lines.

The expected benefits should then also be considered. As the more effective sites are equipped with these installations, the per site benefits can reasonably be expected to decrease. The utilisation of ASC earthing is expected to provide a 10% reduction in permanent faults on overhead lines and to reduce short interruptions by 50% at the selected substations.




3.6 Spur Protection

Where economical, all spurs, apart from those in the final zone, need to be individually protected at their point of connection to the main line.

Major spur – 5km or more of overhead line. Major spurs should be protected by auto-reclosers wherever possible provided this does not compromise the main line grading points.

Medium spur – Spurs between 0.5km and 5km in length. Medium spurs should be protected by an sectionaliser unless prospective fault currents coupled with clearance times are too high for the spur conductors.

Minor spur – Spur with less than 0.5km of overhead line. The fault risk associated with these is relatively small and it is difficult to justify the costs of retrofitting protection to existing sites. Therefore where there are no existing links, minor spurs should be left solid. Where solid links are currently fitted and in new locations, minor spurs should be protected by fuses unless they are in the final auto-reclose section where an sectionaliser should be used. During an overhead line rebuild the opportunity should be taken to protect all minor spurs with fuses or sectionalisers as appropriate.

The following table shows the appropriate protection to be used at the main line connection point.

Spur Type Source A/R Zone Middle A/R Zone Final A/R Zone

Major Spur A/R A/R 3 shot A/S

Medium Spur 2 shot A/S 2 or 3 shot A/S 2 or 3 shot A/S

Minor Spur 2 shot A/S or fuse 2 shot A/S or fuse 2 shot A/S

*Check fault currents and clearance times are acceptable for the conductor sizes in use. If not suitable for the use of sectionalisers then fuse, or exceptionally, a recloser should be used.

Diagrams presenting an overview of the protection to be applied to spurs can be found in Appendix 4.

Consideration should then be given to adding further discrimination on major and medium spur lengths.

3.6.1 Auto-Sectionalising Links


The functional specification for auto-sectionalising links shall be in line with the latest version of NPS/001/032 – Technical Specification for 11kV, 20kV, Pole Mounted Auto-Sectionalising Links (ASLs).

With regard to the protection functional requirements, these are as follows:

ASL’s are designed to recognise a pre-determined sequence or fault currents and then, during a period when the upstream protective device is open, to disengage and drop-out to the isolating position.

All actual sectionalisers deviate to varying degrees from the ideal characteristics required of a sectionaliser. The deviations can take the form of requiring a pre-charge from load current to operate correctly or requiring the fault current to be on for a minimum time. Any such deviations will be determined at the time of tendering to ensure that the devices remain suitable for the locations where they are to be fitted.

The ASL’s shall have load current ratings available of between 15 and 112 Amps.

ASL’s shall have a reclaim time of 25 to 30 seconds

ASL devices are required to be capable of either two or three shot operation.





A summary of the design process can be found in Appendix 1 and a guide to calculating the required settings found in Appendix 5.

The location of the sectionalisers on an overhead circuit selected for auto-reclosing shall comply with the following guidelines but, when applying them, consideration must be given to local conditions affecting access. Note that additional rules may be introduced from time to time to cover any restrictions as to where the types of sectionalisers being purchased can be used.

Normally spurs with more than 5km of overhead line shall be protected by an auto-recloser unless they are in the final main-line protection zone in which case a 3-shot sectionaliser should generally be used

Normally spurs between (0.5 km) and 5km shall be protected by an sectionaliser.

In new locations, where solid links are currently fitted and during a major rebuild of the main line normally spurs less than 0.5 km long shall be fused however consideration shall be given to the installation of a sectionaliser if the number of connected customers exceeds 50. Note that minor spurs in the final protection zone should be protected by a 2-shot sectionaliser as fuses will not operate with all instantaneous shots on the auto-recloser.

Where spurs less than 0.5 km are solidly connected to main lines then these should generally be left solid. However, the opportunity should be taken during an overhead line rebuild of the main line to apply protections to all short spurs.

Fuses should not normally be used to protect spurs containing TSG’s.

Where a spur is located in the first protection zone from the primary, sectionalisers should be limited to a maximum of two counts.

Care must be taken to ensure that the maximum fault current and prospective clearance times are compatible with the conductor sizes in use. Particularly in the source protection zone this may limit the use of sectionalisers and fuses will have to be used instead.

Care must be taken to ensure that devices are only used within their fault rating e.g. new sectionalisers are limited to 10kA maximum through fault current for one second.

Care must be taken to ensure that the manufacturer’s recommendations on the discrimination between actuating currents and minimum auto-reclose relay settings are maintained., e.g. the AB Chance 25 amp (load current) rated sectionaliser has an operating current of 1.6 times this, i.e. 40 amps and the recommended minimum recloser setting is 50 amps

Details of the fault rating of small overhead conductors are shown in the section on design rules in Appendix 1.




3.6.2 Fuses


The functional specification for pole mounted expulsion fuses shall be in line with the latest version of NPS/001/004 – Technical Specification for 11kV, 20kV and 33kV Pole Mounted Expulsion Fuses and Solid Links.


A summary of the design process can be found in Appendix 1 and a guide to calculating the required settings found in Appendix 5.

Fusing shall be used in the first two protection zones only.

Normally spurs of less than 0.5 km length that are solidly connected to a main line should be left solid.

Spurs of less than 0.5 km in length that are fitted with solid links shall be fused however consideration shall be given to the installation of a sectionaliser if the number of connected customers exceeds 50.

Spurs containing TSG’s should not be fused unless this is unavoidable because of fault level and conductor size factors..

Any existing fuses protecting ground mounted transformers shall be replaced with pole or ground mounted circuit breakers.

Care must be taken to ensure that devices are only used within their fault rating.

3.6.2.3 Application of Settings

The maximum fuse size should be restricted to 50A for minor spurs to allow for grading to successfully occur. The fuse size should be matched to the size of transformers being protected.

3.7 Ferroresonance

Ferroresonance is a term used to describe series resonance in an alternating current circuit consisting of non-linear inductance (the transformer) and capacitance (the HV underground cable). In a distribution system, this condition is most likely to occur when lengths of underground cable are connected in an overhead system. The ferroresonant state can occur when energised and de-energised phases occur in the same section of line leading to the generation of overvoltages. When this occurs, damage to both customers’ equipment and the distribution system can result.

Ferroresonance is also possible during system fault conditions where blown fuses leave the system supplied single phase.

Tests indicate that the condition is most likely to occur during de-energising. However, only approximately 5% load is required to dampen ferroresonance.

The failure of surge arresters under these conditions is well known. It should be noted that with gapped surge arresters this effect could go unnoticed as the resonant overvoltage would be below the sparkover level of an 11kv surge arrester. However, in the case of metal oxide surge arresters some failures will occur. When a line is refurbished then gapped or porcelain surge arresters should be replaced. The failure mode with a polymeric housed surge arrester of proven short circuit capability is not a violent explosion but nevertheless injury could be caused if personnel are near to the failing arrester.




Some conditions when ferroresonance can occur are:

During single phase switching operations.

Operation of a single-phase over-current protective device.

When one or two phases of a three phase circuit become open-circuited (blowing of a fuse, breaking of a line conductor, etc).

The increase in the number of incidents of ferroresonance reported is due to modern trends in circuit parameters:

Increased use of underground cables in rural systems.

More three phase transformers on rural systems.

Highly seasonal three phase loads resulting in some transformers operating at very light load.

Improved material design of transformers (lower excitation currents).

Although measures to protect against ferroresonance should be considered, these measures will normally only be applied where the line is new build or significant alterations have occurred.

3.7.1 Design Rules


Where possible, avoid using HV underground cable in rural situations. Ferroresonance will not occur where the system being energised or de-energised comprises entirely of HV overhead lines.

Where HV underground cable exists or is installed, the following rules shall be adopted:

Where there are more than 10 transformers in the circuit to be energised (or de-energised), ferroresonance is unlikely to be a problem and no action is required.

Where there are less than 4 transformers in the circuit, each transformer should be equipped separately with solid links at the transformer position. These links will be removed before the tee is disconnected and replaced after the tee is re-connected.

Where there are between 4 and 10 transformers in the circuit, a 3 phase switching device, for example an isolator, should be installed at the tee off position in addition to any spur protection (sectionalisers). In exceptional circumstances (eg fault level above the rating of sectionalisers) an auto-recloser can be used in this position. In this case this should be set to instantaneous protection with the number of shots to lockout set to mimic a three or two shot sectionaliser as appropriate.




3.7.2 Critical Cable Lengths to avoid Ferroresonance

Critical cable lengths have been calculated for both the 11kV and 20kV systems. The following tables indicate the maximum aggregate cable length in section without the risk of Ferroresonance:

Critical 11 kV Cable Lengths to Avoid Ferroresonance

Cable Lengths in Metres

Aggregate transformer

capacity

0.04 0.06 0.1 95 mm 185 mm 300mm

5 1.2 1.1 0.83 0.9 0.7 0.5

10 2.4 2.1 1.6 1.7 1.4 1.1

15 3.6 3.3 2.5 2.5 2.0 1.7

25 6.0 5.5 4.2 4.0 3.4 2.7

50 12.0 11 8.3 8.0 6.6 5.4

100 23.8 22.2 16.7 16.0 12.0 11.0

200 47.7 44.4 33.4 32.0 23.0 22.0

315 75.0 69.9 52.6 50.0 35.0 34.0

500 119.0 111 83.5 75.0 55.0 52.0

750 178.7 166.6 125.3 115.0 90.0 82.0

1000 238.0 222.1 167.0 155.0 125.0 110.0

1250 298.0 276.6 208.0 194.0 156.0 137.0

1600 381.2 355 267.0 248.0 200.0 175.0

Critical 20 kV Cable Lengths to Avoid Ferroresonance

Cable Lengths in Metres


capacity

0.04 0.06 0.1 95 mm 185 mm 300mm

5 0.36 0.33 0.25 0.27 0.21 0.15

10 0.72 0.63 0.48 0.51 0.42 0.33

15 1.08 0.97 0.75 0.75 0.6 0.51

25 1.8 1.64 1.26 1.2 1.04 0.81

50 3.6 3.23 2.49 2.4 1.98 1.62

100 7.14 6.5 5.01 4.8 3.6 3.3

200 14.31 13.0 10.04 9.6 6.9 6.6

315 22.5 20.5 15.78 15.0 10.5 10.2

500 35.7 32.5 25.05 22.5 16.5 15.6

750 53.61 48.7 37.5 34.5 27.0 24.6

1000 71.4 65 50.1 46.5 37.5 33.0

1250 89.4 81.1 62.4 58.2 46.8 41.1

1600 114.36 104.1 80.1 74.4 60.0 52.5




Table for XLPE triplex cable

11kV cable lengths in Metres 20kV cable lengths in Metres


capacity

95 mm 185 mm 300 mm 95 mm 185 mm

5 2.8 2.2 1.8 1.2 1

10 5.7 4.4 3.7 2.5 2

15 8.5 6.7 5.5 3.7 3

25 14.2 11.1 9.1 6.2 5

50 28.3 22.2 18.3 12.4 9.9

100 56.7 44.4 36.5 24.9 19.9

200 113 89 73 50 40

315 179 140 115 78 63

500 283 222 183 124 100

750 425 333 274 186 149

1000 567 444 365 249 199

1250 709 555 457 311 249

1600 907 710 585 398 318

3.8 Ancillary Circuit Protection

3.8.1 Lightning Protection

Lightning protection should be applied in line with IMP/007/011 – Code of Practice for the Application of Lightning Protection.

3.8.2 Fault Passage Indicators


The functional specification for fault passage indicators shall be in line with the latest version of NPS/001/014 – Technical Specification for overhead line Fault Passage Indicators (FPIs).

With regard to the protection functional requirements, these are as follows:

Initially the FPI’s shall be supplied preset as follows:-

o Default current settings 21A

o Delay 150 mS

As an example the Bowden Pathfinders should be preset to:

o SEF and Instant 21 A

o Inrush 98 mS, Instant 148 mS, IDMT 150 mS

o 3 hour flash time and 60 seconds line trip

o Type D (transient fault reporting enabled)

o The above corresponds to setting codes of 99-16-207-2

The FPI’s shall be designed such that earth fault sensitivity levels can be amended from its default value to a user entered value. If fitted with GSM/GPRS communications, this amendment shall be achievable via the remote signalling system.

FPI’s shall be designed to discriminate between magnetic inrush current and fault current.






Locations for fitting pole mounted FPIs:

As near as possible to manual switching points

As near as possible to both ends of cable sections

Major spurs that are not protected with an auto-recloser – as near as possible to the point of connection with the main line.

To ensure that no more than 500 customers are between monitored points

To ensure that there is no more than 3km of main line without an FPI

Pole mounted FPIs should not be installed on poles:

With underground cables

With transformers

With double circuit lines

At tee-off positions

Closer than 300m to 275 – 400kV lines

Closer than 150m to 132kV lines



Closer than 100m to 25kV overhead conductors

All pole-mounted FPIs used on main lines shall be capable of indicating back to Network Management System (NMS) that they have operated.

3.9 Urban Circuits

Urban circuits are generally all underground (up to 1 km of overhead line is permitted) and will be equipped with standard overcurrent and earth fault protection in accordance with the Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001). Settings should be as the Code of Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007). Circuits with in excess of one span of overhead line will be equipped with SEF protection. Auto-reclose is not normally enabled. Where practical, all overhead spurs on urban circuits shall be fused.

On circuits normally supplying in excess of 2,000 customers consideration should be given to installing intermediate protection when this can be easily achieved, e.g. when replacing switchgear at a distribution substation and there is space available for a feeder CB. The economics of doing this should be checked against the interruption performance benefit that can be obtained using approved software tools.




3.10 Assumptions

It is anticipated that all calculations regarding design of protection for HV circuits will be carried out using GROND. A set of standard assumptions for use in these calculations can be found in Appendix 2 of this document.

Several aspects of the policy may be subject to subsequent review. This includes section 3.7 on Ferroresonance and the parts of section 3.4.1.2 relating to SEF protection.

Improved interruption performance has been evaluated using the IIS incentive rates for the DPCR5 period (2010/11 to 2014/15). This policy will be reviewed should there be a significant change in these values after DPCR5.

3.11 Implementation and Monitoring

The main accountabilities for implementation and monitoring of this policy lie with:

Designation Responsibility

Design Manager The manager appropriate to the part of the network where the policy is being applied, who is accountable for the implementation of this policy. Will ensure responsible persons are appointed to implement this policy

Protection and Technical Services Manager

The manager appropriate to the part of the business where the policy is being applied, who is accountable for the implementation of this policy. Will ensure responsible persons are appointed to implement this policy

Policy Manager The manager who is accountable for the derivation of this policy

Policy Production Manager Responsible for monitoring compliance of all business divisions with this policy

3.12 Planned Policy Review

This policy shall be proposed for review on a biennial basis or at any time when external or internal influences drive a change in policy e.g. a change in legislation, or learning points from the initial implementation stage.

The following responsibilities shall apply to policy control and review:

Designation Responsibility

Publication Manager Responsible for issuing a quarterly report to the Policy Production Manager (or representative) detailing policies scheduled for biennial review within the next six months

Policy Production Manager Responsible for assessing the continued applicability of this company policy and for amending this document and communicating any changes in policy.




3.13 Superseded Documentation

This document supersedes the following documents, all copies of which should be destroyed.

Ref Version Title

DD.554 Dec 2000 Code of Practice for the protection of HV rural lines DSS/007/010 Draft (Sept 1999) The protection of 11kV overhead lines




4 References

4.1 External Documentation

Reference Title Version and date

EAW Regulations The Electricity at Work Regulations 1989 Statutory Instrument 1989 No 635

ESQC Regulations

The Electricity Safety, Quality and Continuity Regulations 2002

As amended at the date of issue of this policy. Statutory Instrument 2002 No. 2665

4.2 Internal documentation

Reference Title Version and date

IMP/001/912 Code of Practice for the Economic Development of the HV System

V1.0; Jan 2009

DSS/007/001 The Protection of High Voltage Networks (TS1) V2.0; Mar 2000 DSS/007/007 The Setting of Protection and Associated Equipment

(TS16/17) V2.0; Jun 2001

DSS/007/009 Standard for the Application of Tele Auto-Reclose to the 66kV and 33kV Systems

V1.0; Mar 2001

IMP/007/011 Code of Practice for the Application of Lightning Protection

V1.0, Aug 2006

NPS/001/009 Technical Specification for 11kV, 20kV and 33kV Pole Mounted Auto-reclose Circuit Breakers

V2.0, Feb 2008

NPS/001/004 Technical Specification for 11kV, 20kV and 33kV Pole Mounted Expulsion Fuses and Solid Links

V2.0, Dec 2007

NPS/001/014 Technical Specification for Overhead Line Fault Passage Indicators

V2.0, Jan 2008

NPS/001/032 Technical Specification for 11kV, 20kV Pole Mounted Auto Sectionalising Links

V1.0 Dec 2010

05/61 An Investment Appraisal for Arc Suppression Coil Earthing

V3.0, Mar 2005

13808 An Investment Appraisal for Under Performing HV Feeders

V5.0, Mar 2008

12109 An Investment Appraisal for the Protection of 11kV Overhead Lines

V5.0, Dec 2008




5 Definitions

Term Definition

ASC Arc Suppression Coil ASL Automatic Sectionalising Link CoP Code of Practice EF Earth Fault FPI Fault Passage Indicator GVR Gas-filled Vacuum Recloser HSE Health and Safety Executive HV High Voltage (between 1,000 and 22,000 Volts) IDMT Inverse Definite Minimum Time Interruption A supply interruption with a duration in excess of three minutes NER Neutral Earthing Resistor NMS Network Management System OC Over Current PMAR Pole-mounted auto-recloser QoS Quality of Service; referring to interruptions performance in terms of CI and

CML SEF Sensitive Earth Fault Short Interruption A supply interruption with a duration of less than three minutes TSG Triggered Spark Gap VT Voltage Transformer




6 Authority for issue

6.1 Author

I sign to confirm that I have completed and checked this document and I am satisfied with its content and submit it for approval and authorisation.

Sign Date

Claire Thomas Asset Management Engineer Claire Thomas 16/12/2010

6.2 Policy Sponsor

I sign to confirm that I am satisfied with all aspects of the content and preparation of this document and submit it for approval and authorisation.

Sign Date

Andrew Ellam Policy & Risk Manager Andrew Ellam 16/12/2010

6.3 Technical Assurance

I sign to confirm that I am satisfied with all aspects of the content and preparation of this document and submit it for approval and authorisation.

Sign Date

Glen Hodges Programmes & Strategic Design Leader

Glen Hodges 17/12/2010

Jim Paine Protection & Technical Services Manager

Jim Paine 16/12/2010

6.4 CDS Assurance

I sign to confirm that this document has been assured for issue on to the DBD system.

Sign Date

Sean Johnson CDS Administrator Sean Johnson 16/12/2010

6.5 Approval

Approval is given for the content of this document.

Sign Date

Mark Nicholson Head of System Strategy Mark Nicholson 16/12/2010

6.6 Authorisation

Authorisation is granted for publication of this document.

Sign Date

Mark Drye Director of Asset Management Mark Drye 22/12/2010




Appendix 1 – Summary of Design Process

1. Collect circuit information and identify main and spur lines, customer numbers and distribution.

2. Design main line protection:

2.1. Source Circuit Breaker

2.1.1. If protected circuit has less than 1 km of overhead, no auto-reclose and settings as per TS1; else

2.1.2. Source CB does not have auto-reclose facilities. Fit PMAR as close as possible to the start of the first section of overhead line; else

2.1.3. Source CB should be set for auto-reclose unless there is a section of underground cable greater than 500m in length or supplying grater than 50 customers prior to the first section of overhead line. If so then consider placing the auto-recloser at the first section of overhead line rather than at the primary.

2.2. Position additional auto-reclosers on the circuit in line with the following guidelines using the approved software modelling tool to assist in achieving the optimal solution:

No customer should experience more than four faults per annum

Aim for no more than 500 customers between remote control points

Aim for no more than 2000 customers per circuit

Aim for a maximum of 200 customers between switching points

In general, a maximum of three auto-reclose devices should be used in series at 11kV and four at 20kV

2.3. Consider the impact of the changes on any arc suppression coil fitted at the primary substation.

3. Design spur protection

3.1. Define the type of spur (major, medium or minor) and refer to table in section 3.6 for detailed protection requirements

3.2. If spur is longer than 0.5 km then protect using a sectionaliser as long as the fault rating at the point of installation is below the rating of the conductor. Small conductor within the first protection zone may need to be protected by a fuse where fault ratings are exceeded. See appendix 5 for the detailed rules on sectionaliser locations and calculation of sizes.

3.3. If spur supplies more than 50 customers then consider protecting using an sectionaliser

3.4. If spur contains any triggered spark gaps then avoid using fuses unless fault level and conductor size considerations force the use of fuses.

3.5. If spur is located in the final protection zone then check the benefits of fitting a sectionaliser. Where there are a low number of customers in the final protection zone then it will be unlikely that spur line protection can be justified.

3.6. Otherwise protect with a fuse.

4. Consider protection of transformers.

5. Ensure circuit is protected against ferroresonance (where new build or a significant alteration only)

6. Design lightning protection in line with IMP/007/011 and re-check position against 3.4

7. Position fault passage indicators as required.




Appendix 2 – Summary of Modelling Assumptions for use in calculations

Fault rates to be applied when using GROND application for CI & CML performance figures.

All studies should be carried out using the Light, Medium & Heavy fault performance figures for both Underground cable and Overhead Lines as follows:

Light Medium Heavy

Underground 0.05 0.05 0.05

Overhead 0.1 0.08 0.07

All transient fault rates per km are set at zero.

Value per CI and CHL to be set as follows:

Type Value (£)

CI 5

CHL 10

Restoration times to be set as follows:

Switching Time (Mins)

Manual 80

Manual/EFI 80

Tele 10

Auto 2

Recloser 1

Repair times to be set as follows:

Type Time (Mins)

Underground 480

Overhead 480







Appendix 3 – Approved software modelling tools

Functionality Approved Software Tool

Network configuration and plant position GIS

Load flow and fault currents DINIS

Predicted Fault frequency and protection position planning

GROND




Appendix 4 – Overview of protection arrangements to be used on spurs.

Fuses (≤50A)

More than One PMAR, AR at Source

CB

2 shot Inst +

IDMT

ASLs 2/3 shot

PMAR

4 shot

PMAR

4 shot

PMAR

4 shot

ASLs 2 shot

One PMAR, AR at Source Fuses (≤50A)

ASLs 2 shot

CB

AR 2 shot + IDMT

PMAR

4 shot

Fuses (≤50A) No PMAR, AR at Source

CB

AR 2 shot + IDMT

ASLs 2 shot

ASLs 2/3 shot




* Check compatibility of sectionalisers with spur-line fault current and clearance time capabilities.

Fuses (≤50A)

Additional PMAR to allow Source CB AR to be Disabled.

CB IDMT

ASLs 2/3 shot

PMAR

2 shot +

IDMT

PMAR

4 shot

ASLs 2 shot

Fuses (≤50A)

More than One PMAR, AR at Source. Sectionalizer in Main Line

CB

AR 2 shot +

IDMT

ASLs 2/3 shot

PMAR

4 shot

PMAR

4 shot

Sectionalizer

3 shot (PMAR with

AR disabled)

ASLs 2 shot *ASLs 2 shot




Appendix 5 – Calculation of Sectionaliser and Fuse Ratings.

Sectionaliser Location

Care needs to be taken where sectionalisers are proposed to be installed particularly within the first protection zone to ensure that any small section conductor is adequately protected.

Where the primary protection is instantaneous and two shot sectionalisers are to be installed to protect small section conductor of 0.0225Cu or less then sectionalisers must not be installed where the fault level exceeds 5kA. All locations above this level must be protected by either a pole mounted recloser or fuse.

Where the primary protection is time delayed then sectionalisers must not be installed where the fault level is above the following levels for the type of conductor being protected:

0.017Cu – Fault level must be below 2kA in all zones


50AAAC – Fault level must be below 4.5kA in all zones


Where sectionalisers are proposed in areas where the fault level is in excess of the conductor rating then consideration should be given to altering the primary protection on the circuit from IDMT to instantaneous.

Calculation of Sectionaliser Rating

The pick-up current of the sectionaliser (Actuating Current) must be greater than twice the full load current of the circuit. The full load current of the circuit will be diversified where 2 or more transformers are connected.

Full load current must take account of three phase and single phase connections along the tee under consideration. The maximum full load current can be calculated either:

Summation of the ‘Transformer Rating Amp’ values in tables below; or

Summation of the separate three phase and single phase transformer ratings and using

11kV – 5.3A per 100kVA (3-ph) and 9.1A per 100kVA (1-ph)

20kV – 2.9A per 100kVA (3-ph) and 5.0A per 100kVA (1-ph)




Example

Full load current of tee = 40Amps Diversified current = FLC/1.5 40/1.5 = 26.6 Amps Minimum Actuating Current 26.6 x 2 = 53.3 Amps

Next highest sectionaliser actuating current is 57.6A (from table in DD554)

Sectionalisers must be specified on with continuous rating and number of shots (2 or 3) therefore a 36A rated unit is required (from table below)

Actuating Current (A) Continuous Rating (A)

179.2 112

89.6 56

64 40

57.6 36

48 30

40 25

32 20

Transformer single phase currents for use in calculating sectionaliser and fuse sizes are shown in the following tables:

Transformer Rating Linegear 2000 Rating LV Fuse Rating

Voltage Phases kVA Amp Pole

Mounted Substation

Ground Mounted

Substation

Pole Mounted

Substation

Ground Mounted

Substation

11 kV

Three Phase

25 1.3 25 A 100 A

50 2.6 25 A 100 A

75 4.0 25 A 160 A

100 5.3 25 A 10 A 200 A 160 A

150 7.9 25 A 12 A 315 A 200 A

200 10.5 25 A 15 A 400 A 250 A

300 15.8 25 A 400 A

315 16.5 25 A 400 A

500 26.5 40 A 500 A

800 42.4 65 A 630 A

1000 53.0 65 A 630 A

Single Phase Three Wire

25 2.3 25 A 100 A

50 4.6 25 A 160 A

75 6.8 25 A 315 A

100 9.1 25 A 15 A 315 A 315 A

150 13.6 40 A 15 A 400 A 400 A

167 15.2 40 A 25 A 400 A 400 A

200 18.2 40 A 25 A 400 A 400 A

225 20.5 30 A 400 A

375 34.1 40 A 500 A

580 52.8 65 A 630 A

Single Phase

Two Wire

5 0.5 25 A 100 A

15 1.4 25 A 160 A

25 2.3 25 A 200 A




Transformer Rating Linegear 2000 Rating LV Fuse Rating

Voltage Phases kVA Amp Pole

Mounted Substation

Ground Mounted

Substation

Pole Mounted

Substation

Ground Mounted

Substation

20 kV

Three Phase

25 0.7 25 A 100 A

50 1.5 25 A 100 A

75 2.2 25 A 160 A

100 2.9 25 A 6 A 200 A 160 A

150 4.3 25 A 10 A 315 A 315 A

200 5.8 25 A 10 A 400 A 315 A

300 8.7 15 A 400 A

315 9.1 15 A 400 A

500 14.5 25 A 500 A

800 23.2 40 A 500 A

1000 29.0 40 A 630 A

Single Phase Three Wire

25 1.3 25 A 100 A

50 2.5 25 A 160 A

75 3.8 25 A 315 A

100 2.0 25 A 10 A 315 A 315 A

150 7.5 25 A 10 A 400 A 400 A

167 8.4 25 A 10 A 400 A 400 A

200 10.0 25 A 15 A 400 A 400 A

225 11.3 15 A 400 A

333 16.7 15 A 500 A

375 18.8 25 A 500 A

580 29.1 40 A 630 A

Single Phase

Two Wire

7.5 0.4 25 A 100 A

15 0.8 25 A 160 A

25 1.3 25 A 200 A

Documents

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