the rcc ground fault neutralizer – a novel smart grid protection

International Conference on Electricity Distribution September 22, 23 and 24, 2014, Hotel Panamericano, CIDEL 2014 Buenos Aires, Argentine

Trends in electricity distribution for a sustainable future _________________________________________________________________________________________________

Full Paper Presentation

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Klaus WINTER

Swedish Neutral AB, Stockholm Sweden

[email protected]

INTRODUCTION

In utilizing the inherent - but so far unused – zero

sequence properties of the 3-phase system, the RCC

Ground Fault Neutralizer (GFN) elevates grid

protection to new levels of performance and takes all

the benchmarks of good protection – namely speed,

selectivity and last but not least detection sensitivity.

Moreover, the GFN action is smart in the way that

voltage injection and fault current can be cancelled out

completely without interrupting the power supply – a

truly smart grid solution. Instead of feeder tripping

“fault surgery” with lots of outages, the GFN now

offers “smart medication”. The “antidote” – a current

equal but opposite to the fault current - is simply

injected into the neutral. No immediate interruption of

any power supply is required.

The GFN protects all parts of an interconnected grid –

from the power transformer over the bus bar and the

outgoing feeders down to the last corner of the grid – a

truly overall protection scheme. This overall action is

accomplished in a very cost efficient way from one

point – the neutral.

Figure 1 GFN basic approach to earth fault protection

A PARADIGM SHIFT IN PROTECTION BY

SMART NEUTRAL TREATMENT

Protection is about safety and minimizing risks to

people and property. The risk with an electrical fault is

generally described as proportional to the energy

injected into the fault site..

Figure 2 GFN benchmarks for earth fault protection

by neutral treatment

Therefor the IEC standard defines the risk to be

proportional to fault current and interception time – the

well-known I2t criteria. Everybody understands -

protection must be fast. But fault current levels

included by their square are even more decisive to

minimize consequential damages. The good news is -

both fault current and interception time can be

influenced from the neutral.

A benchmarking for different types of system

grounding and neutral treatment with respect to fault

current levels and interception time clearly proofs the

superiority of the Ground Fault Neutralizer. Even

better - almost all existing grids can be converted to

GFN grounding. This conversion offers one of most

cost efficient investments in grid performance,

improved SAIDI figures (9) and, last but not least

safety. .




THE GFN GROUND FAULT NEUTRALIZER

The Ground Fault Neutralizer is normally connected to the

neutral of the supplying power transformer (Y-winding) or a

separate grounding transformer (Z-winding). A complete

GFN-system is composed of a modern solid core arc

suppression coil (ASC), see Figure 4, a cabinet with power

electronics for voltage/current injection (RCC - Residual

Current Compensator) and the GFN control cabinet.

Beside the controls for the RCC voltage/current injection

the GFN also provides automatic retuning for the arc

suppression coil and a new twin-scheme fault locator with

superior detection capabilities. Distance-to-fault information

can be obtained by feeder looping.

The arc suppression coil forms a parallel resonant circuit

with the phase-to-ground capacitive leakage (Co) of the

network. By this resonant circuit the source impedance for

single phase-to-ground faults increases in the order of ten to

twenty times, sufficient to quench single-phase flashover

faults on overhead lines. But it was still necessary to trip the

faulty feeder, in order to minimize the risk for fire and

personal hazards due to the remaining active current.

The GFN Ground Fault Neutralizer now provides fast and

complete compensation of all remaining earth fault currents

– both fundamental and harmonics – by injecting a 180

degree opposite current into the neutral. This is beneficial

especially in industry- and urban cable grids where almost

all faults start single phase-to-ground (cable screen). If not

properly compensated, a re-striking cable fault quickly

develops into a multi-phase or cross-country fault with

subsequent long term outages.

Figure 3 GFN adaptive zero sequence admittance scheme

Figure 4 Modern dry-type fast tuning arc suppression coils,

forming the high voltage part of the GFN system.

Also with respect to personal safety and fire prevention the

GFN offers premium protection [3]. A fault interception in

less than three cycles in praxis can never be reached by

traditional protection schemes working on breakers.

FAULT FINDING - A MATTER OF MINUTES

Even though the GFN immediately eliminates voltage

injection and fault current - independent of the location of

the fault - the actual fault site must be located for further

inspection or repair.

The integrated GFN fault locating is based on two superior

detection schemes – a very fast transient detection mainly

for re-striking cable faults and a highly sensitive adaptive

zero sequence admittance scheme to detect both low and

high impedance faults on overhead lines and cables.




Figure 5 GFN fault interception in less than 3 cycles

In urban grids distribution feeders are normally arranged in

open rings with ring main units to provide safe power

supply. For these grids the GFN works with its well proven

distance-to-fault scheme. After closing the NOP - normally-

open-point – the fault site can be pin pointed with high

accuracy.

Figure 6 GFN Cabinet – Everything at your fingertips

Rural distributions generally have radial structures with

many lateral spurs. Earth fault locating in these grids is a

kind of difficult and often based on either time consuming

trial & error methods or overcurrent fault passing indicators

with insufficient sensitivity.

The new GFN FPI working with access to modern internet

communication technology provides a substantial

improvement. Due to its adaptive scheme, the GFN FPI can

detect much more sensitive than traditional FPI’s. This is an

important step ahead, as many of the line faults in overhead

grids are high impedance mid-span faults with poor ground

contact.

The GFN FPI can be combined with any type of

sectionalizing equipment. All fault confirmation and

sectionalizing controls are available at the central GFN

terminal, but can also be transmitted to a dispatch center or

a handheld field monitor.

GFN PD-MONITORING & CONTROL – THE

NEXT STEP IN GRID PROTECTION

The GFN Ground Fault Neutralizer, originally developed to

solve the problem with re-striking cable faults in resonant

grounded networks, offers combined with state of the art on-

line PD measurement a new powerful tool for grid

insulation monitoring and pre-fault protection. By means of

voltage/current injection into the neutral, the GFN controls

all phase-to-ground voltages and - if necessary - quickly

quenches discharge activities by lowering the voltage in the

suspected phase, thus preventing further development into a

full dielectric breakdown.

Furthermore the full control of phase-to-ground voltages

during plant operation also admits for new online PD testing

methods at levels above normal operating voltages, thus

enabling systematic fore-checking strategies for the early

detection of defective components. This is done without

affecting the power supply to end-users.

Distribution network owners and operators looking for new

and cost efficient means to monitor the condition of their

aging cable grids may benefit from this novel tool to

support their ongoing CBM activities. The GFN enhanced

monitoring works on all type of plant - rotating machines as

well as transformers, switchgears and cable feeders. The

method can also be used before, during and after

commissioning of new plant to verify the quality of

workmanship on cable joints and terminations.




Figure 7 GFN PD monitoring and control - a novel pre-

fault protection and “Early Warning System”

CONCLUSIONS

Upgrading the traditional arc suppression coil to a full

scheme Ground Fault Neutralizer means that Waldemar

Petersen’s superior protection concept of resonance

grounding will survive the ongoing cabling in many MV

grids. (1) (2) (3) The Ground Fault Neutralizer also

provides for the first time truly fire and personal safe

operation of sustained faults (6).

As many injury files and bush fire investigations indicate,

speed is still the most essential aspect of protection. With a

total response time of less than 3 cycles (Figure 5) –

independent of the actual fault location the Ground Fault

Neutralizer is substantially faster than traditional protection

schemes (4) (6). A first Brazilian Pilot installation was

presented at the PAC World Conference in Florianopolis

/Brazil (8).

Finally, the possibility to improve already existing PD

online detection methods into true “Early Warning Systems”

adds another strong argument for GFN resonance grounding

as the superior grounding concept (5) (7). Enhanced GFN

PD online monitoring in the ageing cable grids of the large

cities in the word is considered to bring about a major step

in CBM – condition based maintenance.

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Reference list

(1) Wilheim R. and Waters M. “Neutral Grounding

in High Voltage Transmission Networks”,

Elsevier Publishing, New York 1956

(2) Winter K. “Swedish Distribution networks –

A new method for Earth-fault Protection in

Cable- and Overhead Systems” 5th International

Conference in Power System Protection, IEE

conference publication No 368, York/UK 1993

(3) Koetzold B., Gauger V. and Winter K.et al.

“Erdschluss-Vollschutzsystem mit Reststrom-

kompensation – ein Weg zur höheren Ver-

sorgungsqualität in erdschlusskompensierten

Verteilungsnetzen” ETG Fachbericht Nr 66,

ETG/PSE Summer Meeting, Berlin/Germany

1997

(4) Winter K. “The RCC Ground Fault Neutralizer –

a Novel Scheme for Fast Earth Fault Protection,

Proceedings of the 18th

Internat. Conference on

Electricity Distribution, CIRED, Turin/Italy

2005

(5) Winter K. et al “ The RCC Ground Fault

Neutralizer – a Novel Scheme for Pre- and

Post-Fault Protection”, Key note address at the

Australasian Power Engineering Conference

AUPEC, Melbourne/Victoria 2006

(6) Victorian Bushfires Royal Commission,

Powerline Bushfire Safety Taskforce, Final

Report Chapter 3.4.1. GFN Rapid Earth Fault

Current Limiter, Melbourne /Victoria 2011

(7) M. Seltzer-Grant, et al “On-line Partial

Discharge Detection on MV cable networks

with Ground Fault Neutralizer” CIRED

Stockholm/ Sweden, June 2013

(8) Mauro Silveira, et al “The RCC Ground Fault

Neutralizer – First Brazilian Pilot Installation”

PAC World Conference Latin America, Paper

No OP005, Florianopolis/Brazil, Nov 2012

(9) SHM Othman et al “The Pilot Project of a

Ground Fault Neutralizer at Sibu Sarawak”

CIRED South East Asia, Kuala Lumpur,

Malaysia, May 2014

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the rcc ground fault neutralizer – a novel smart grid protection