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DARFIELD TO DUNSANDEL AND BEYOND – THE DEVELOPMENT OF THE

GROUND FAULT NEUTRALIZER IN NEW ZEALAND AND LESSONS LEARNED.

Author: Dennis Keen, (Connetics Limited);

Co-Author: Steve Macdonald (Orion NZ Limited)

Presenter: Dennis Keen, (Connetics Limited)

EEA Conference & Exhibition 2013, 19 – 21 June, Auckland

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DARFIELD TO DUNSANDEL AND BEYOND – THE DEVELOPMENT OF THE

GROUND FAULT NEUTRALIZER IN NEW ZEALAND AND LESSONS LEARNED.

Dennis Keen* 1, Steve Macdonald 2

1 Connetics Limited 2 Orion New Zealand Limited In 1916 Waldemar Petersen developed the Arc Suppression Coil and in 1918 the Wattmetric Earth Leakage Relay (ELR). The original Petersen coil is still on display in the German Museum in Munich. The Petersen coil was developed initially as a protection device used on a generator stator, it was later used on transformer neutrals in power transmission and distribution networks to limit earth fault currents. This paper develops from the Papers presented at the EEA Conferences in 2006 by Stephen Hirsch – “Resonant Earthing - Advantages & Disadvantages. Why do we not use it in New Zealand?” which posed the question and discussed why we should use Neutral Earthing in New Zealand and goes onto briefly mention the first Ground Fault Neutralizer system installed at Darfield by Orion, presented at the EEA conference in 2007. By Tas Scott. – “The First Application of Resonant Earthing with Residual Current Compensation to a NZ Distribution Network” The paper considers the developments and learning’s that Orion have had since then with further installations at their own network substations and at other Network companies in New Zealand and Australia. The paper examines the technology development of different types of arc suppression coils, including dry and oil filled types. The tuning methods used including switched capacitances or reactance and moving coil types. How this method of earth fault protection compares with other methods and what options can be added to the arc suppression coil to improve the performance as an earth fault minimisation system. This paper also considers the wider electricity industry’s attitude to Arc Suppression Coils or Rapid Earth Fault Current Limiters (REFCL) and applications to safety including Earth Potential Rise (EPR) and induction effects. Mitigation of bush fires and forest fire risks and the use of REFCL’s in transmission and distribution systems up to 110kV. Also their application for generators, wind turbine and network systems to allow continued use in fault conditions whilst still maintaining public safety and minimising asset damage, providing examples from other papers where relevant. The paper concludes with a summary of advantages and disadvantages of earth fault protection options available now and how these may develop in the future to minimise fault occurrences and assist network operators to maintain electricity supplies to consumers.

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1. Introduction

“Historia Est vitae Magistra” – History is the tutor of life (Latin proverb).

The first Petersen coil [1] was developed by Waldemar Petersen in 1916, this Petersen coil can still be found on display at the German museum in Munich with the title “Erdschlusslöschspule von Petersen“ figure 1[2].

The Petersen coil was first used at Pleidelsheim power station for generator neutral earth protection mounted in a cubicle, shown in figure 2. From this initial development the use of Petersen coils became more widespread and with the development of tuneable Petersen coils, were later used on transmission and distribution networks throughout Europe, Asia and America from 6.6kV (Figure 3) up to 230kV (Figure 4).

Figure 1 – First Petersen Coil 1916, now in Munich Museum, Germany.

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The Petersen coil, later referred to as the Arc Suppression Coil (ASC) has developed as a dry-type and an oil filled device. Orion’s Darfield Ground Fault Neutralizer (GFN) was equipped with a dry-type ASC designed and built by Swedish Neutral AB (SN). This is shown in the Darfield Schematic of Figure 5.

2. Arc Suppression coils & related technology developments

The construction & operation of the Petersen coil has remained largely unchanged from 1916 through to today and consists in most cases of two coils one within the other, one of which is static and the other that is moved either by hand or motor drive in a series of steps or step-less

Figure 3 – Adjustable coil ground fault neutralizer for

6,600V system

Figure 4 – Ground fault neutralizer for 230kV system

of Southern California

Figure 2 – The Petersen coil in Figure 1 shown in its cubicle at Pleidelsheim power station (1917)

Figure 5 – Darfield Schematic & Dry type Arc Suppression Coil

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to tune to the capacitive reactance of the network it is connected to. An example of such a system is shown in figure 6.

The dry-type arc suppression coil was supplied for Darfield by Swedish Neutral AB, They also supply smaller dry type (figure 7) and larger oil filled arc suppression coils as part of their GFN system (figure 8 & 9).

These differ from the arc suppression coil shown in figure 5 as these designs use switched capacitance to vary the reactance to re-tune for changes in the network, instead of using a moving coil, making them much faster at responding to changes in the network capacitive reactance. A comparison table is given in figure 10, which shows other advantages over a conventional arc suppression coil. Of particular note is the ability to stress the network during commissioning and normal operation to find weak spots in the network and so minimise the risk of these weak spots causing a fault during bad weather etc. at a later date. Also the GFN has the ability to compensate, (using the Residual Current Compensation (RCC) feature), for the effects due to harmonic currents during faults as well as the fundamental fault current.

Figure 6 – Motor driven adjustable Arc Suppression Coil

Figure 7 – 6-22kV Dry type. Figure 8 – 6-36kV Oil filled Figure 9 – 66-110kV oil filled

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Conventional Resonant Earthing Ground Fault Neutralizer

Design Arc suppression coil; oil filled 1950’s design.

Evolved design, oil filled.

Test Needs “real” earth fault to confirm operation.

Can stress test network before “real” earth fault.

Residual Current May be left with 2A+ & 400V+ at the point of fault (may be much higher).

Almost zero current <50mA & <150V at point of fault, (with RCC).

Tuning Method HV / Motor drive tuning LV / Capacitive tuning

Compensating

Harmonics 50Hz fault current. Fundamental, 3

rd

, 5th

and 7th

Harmonics.

Re-tuning Time Up to 120 seconds for network changes.

Up to 2s for network changes.

Stephen Hirsch’s Paper[1] includes the development of the three phase system and different methods of earthing and the advantages provided by Resonant earthing in terms of reducing EPR risk particularly in high soil resistivity areas such as Canterbury, New Zealand. It also highlights that “the concern is that there is an increasing likelihood that someone is going to suffer severe injury or death due to an EPR event, either by fire or electrocution”. The paper goes onto state that “the only feasible way of meeting the requirements of IEC 60479 shock standard…is with the use of resonant earthing”. In this context the Residual Current Compensation (RCC) device provided by Swedish Neutral in their version of resonant earthing adds additional safety by neutralizing the resistive component of the fault current to a minimum within 60ms of inception of the fault. The fault is detected via a residual CT connection or core balance using existing CTs on the substation feeders, any CT errors are cancelled out by comparing the admittance of the feeders before and after the fault inception and in this way is able to detect that a fault has occurred in most cases during the initial transient of the fault and determine on which feeder the fault has occurred. The paper also cites other advantages of resonant earthing as, a reduction in auto-reclose operations and ability to fault find with the distribution network alive. The paper[1] considers the disadvantages of resonant earthing which include, difficult fault location (due to reduction in fault current making it more difficult to see the damage done at the fault site), and notes that NERs also displace the neutral and that cross-country faults can “be cleared using standard earth fault protection”. Restriking cable faults are a particular problem with conventional resonant earthing but with the addition of the RCC as described above this risk is minimised. In the UK and other countries “..increasing numbers of new systems being installed”[1].

3. The Bigger picture

In Australia, the Powerline Bushfire Safety Task force was established to review all options to reduce the risk of catastrophic bushfires from the electricity supply. The Taskforce prepared a report, which was presented to the Victorian Government on 30 September 2011[3]. In a report presented to Energy Safe Victoria in September 2011 Parsons Brinkerhoff

Figure 10 - Table of comparison between conventional Petersen coil and GFN

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provide a risk assessment of options for the relative risk reduction and costs for possible bushfire mitigation options, a summary of which is shown in the following table[3].

Installation Type Risk reduced by[3] Relative cost

Underground Cable 99% Highest

Aerial Bundled Conductor (ABC) 90% High

Bare Conductor 60% Low

Covered Conductor 90% High

ABC with Ground Mount Switchgear 99% Higher

CCT with Ground Mount Switchgear 99% Higher

Hendrix Overhead Cable 90% High

REFCL / Ground Fault Neutralizer 70% Low

Resonant Earthing can be installed for a number of reasons as follows;

• Reduction of Earth fault current to minimal levels[1, 2]

• Reduction of EPR levels [1, 5, 7, 9]

• Reduction of SAIDI and MAIFI [5, 6, 8]

• Minimising bush-fire risk [3, 4]

• Maintaining customer supply during a fault [6, 12]

4. The Orion Timeline

The sequence of events for the development and use of the Ground Fault Neutralizer technology in New Zealand from 2005 to 2012 is as follows;

• In 2005 Tas Scott while on holiday in England, came across an installation of the Swedish Neutral Ground Fault Neutralizer (GFN), while visiting a friend who worked

for EdF (Electricitie de France) in England.

• In April/May of 2006, Tas Scott and Stephen Hirsch then carried out a due diligence visit to Europe and existing Swedish Neutral GFN customers

• An order was placed with Swedish Neutral in late 2006 for delivery in April of 2007

• Installation and commissioning of our trial site at Darfield was completed in late April 2007

• Orion entered a Distribution Agreement and GFN supply agreement for the Orion Network with Swedish Neutral in August of 2007

• Orion discovered harmonic issue in November of 2007

• Orion continued the installation of GFN from 2008 to 2010, while harmonic solution is developed until 2010 when earthquakes delayed our programme

• Orion resumed installation of GFN in 2012.

• Orion transferred Distribution Agreement to Connetics in Aug 2012

Figure 11 – Relative risk reduction and costs of bush fire risk mitigation options

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5. Recap on Darfield

A Business case was initially presented to Orion Management[5], that showed a cost per customer per year of $18 and a projected cost benefit ratio of 1.56 based upon a calculated “Value of Lost Load” (VoLL). During the installation and commissioning at Darfield, corona scanning was carried out on the lines and switchgear, both before and after installation of the GFN. The switchgear showed no significant difference which was expected, but Orion did replace a further 33 insulators that did not show heightened levels of corona discharge at system voltage (Figures 12 and 13).

As the Swedish Neutral GFN can operate reliably utilising the residual connection of the 3 phase CTs with a ratio of 400 or less, Orion did not install core balance CTs instead Orion broke into the existing residual connection of each feeder. Orion modified their 3 phase VT to provide an open delta voltage and at the time Orion specified it incorrectly and ended up with an open delta of 190V when what was required was 110V. So this required them to install a small 190V / 110V VT as well. Tree trimming was completed on all feeders immediately

Figure 12 – Corona camera image of an Insulator on 11kV line operating at normal voltage (6.35kV to earth)

Figure 13 – Corona camera image of the same Insulator on the 11kV line operating at elevated voltage (9.5kV to earth)

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before commissioning. Orion installed an 11kV manual bypass switch (figure 5) to allow them to revert to solid earthing or to carry out maintenance on the ASC. The low voltage supply was assessed and the GFN required 50kVA and their local service transformer was 50kVA, this was assessed as adequate as 50kVA would only be required by the GFN for a very high impedance fault and only when the fault is present. Primary fault testing where Orion connected an 11kV line to earth via a remote controlled circuit breaker, Orion also connected a section of cable with a fault in it to test the effects of not utilising the RCC and having only the ASC in circuit and this confirmed that damage to the cable would occur in a short period of time. This primary fault testing also allowed Orion to confirm fault finding tools such as 5th harmonic handheld detectors and “distance to fault” functions within the GFN software.

6. Evaluation & the next phase

Orion committed to purchase a further 14 units of 100A capacity over 3 years. Orion entered a distribution agreement with Swedish Neutral AB, for New Zealand and parts of Australia. As a result installations by Orion have now been completed for the following companies in New Zealand and Australia.

Network Company GFNs Purchased Year First Ordered

PowerCo 3 2008

United Energy 1 2008

Northpower 2 2009

Electronet 1 2008

Top Energy 3 2009

Unison 2 2010

WEL Networks 1 2012

7. Trouble ahead

Following a successful evaluation at Darfield Orion intended to roll out 5 per year, but in the summer of 2007 while carrying out primary testing in front of another network owner it appeared that the GFN was acting as an arc suppression coil only. In that it allowed cable re-strike to occur. At this stage Orion turned off the GFN and consulted with Swedish Neutral over the reasons. Although suspecting the worst and blaming the supplier Orion learned that the issues were on their own network and were being caused by a high level of harmonics with the dominant frequency being the 5th. As GFN’s and traditional arc suppression coil systems are tuned to match only the 50 Hz component any high levels of other frequency would have the same negative effects in terms of damage and safety. Orion analysed the power quality information from Darfield 11kV and their GXP 33kV and found that while in winter when Orion commissioned the GFN Orion were seeing levels of 1.7% of THD, in summer this rose to above 7% THD. From calculation it can be seen that potentially instead of;

6,350V * 1.7% = 108V of 250Hz voltage being across the point of fault,

Figure 14 – Network companies who have purchased GFNs.

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Orion was effectively seeing; 6,350V * 7% = 445V of 250Hz voltage across the point of fault.

Orion made a decision that they could still operate the GFN system during the months of April to October as the THD was at an acceptable level from a safety and damage perspective (figure 15). Concurrent to this Orion entered discussions with Swedish Neutral for them to deliver a solution to also compensate the 5th Harmonic when a fault occurs. Orion agreed that they would reduce the harmonic level to around 4% and Swedish Neutral would be responsible for removing the rest. Orion entered a commercial agreement with Swedish Neutral for them to develop and deliver hardware and software for the monitoring and compensation of the 5th harmonic in the zero sequence path.

8. Proceeding with the Plan.

Orion installed their second unit at Killinchy with some modifications made to their installation plan. While Orion intended to leave earth faults on permanently they did not see the need in the trial to implement feeder tripping. But as the whole basis for Orion leaving the faults on safely was Swedish Neutral’s innovative RCC system, Orion realised that there would be some situations where they would want to trip the feeder, these are;

• A loss of inverter power,

• The GFN being in the manual state where the inverter is not available,

• Or a loss of inverter (RCC) communication with the Neutral Manager (HMI Panel) Orion also implemented an earth fault protection blocking scheme whereby Orion monitor the neutral voltage and if it is elevated then Orion block their conventional earth fault protection. Orion treated the arc suppression coil the same as a transformer where they

Figure 15 – Variation of THD at Darfield Jan-2008 to Jan-2010

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monitor the temperature via SCADA. Orion have tested the distance to fault measurement available in the GFN and confirmed that it is reasonably accurate but made the decision that they would not utilise it in their network due to the resource required to set up pre-defined parameters and the relatively short loops Orion would be able to create. In November 2009 Swedish Neutral demonstrated a prototype of the harmonic software and hardware to prove the concept. Then in March 2010 SN delivered a functional product which was installed, commissioned and tested at Killinchy zone substation. Primary Fault applied and voltage at point of fault was measured as shown below, figure 16.

Time

Upon acceptance this has been incorporated into all Orion GFN installations[10]. While Orion is now able to fully compensate the 5th harmonic it has introduced some problems with using the 5th harmonic to find faults as these have now been eliminated. Swedish Neutral have been challenged to provide a solution that involves only partially compensating the 5th harmonic and this remains outstanding, (due for delivery May 2012). To date Orion has 14 fully commissioned systems, 4 systems partially installed and 5 designs being completed, including Dunsandel which is currently being designed for installation planned in this financial year (figures 17 & 18).

Figure 16 – Killinchy Testing 2010 – Voltage at point of fault

Volts

Figure 17 – Dunsandel Substation aerial view

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9. Benefits

In Tas Scott’s paper[5] to the EEA he claimed that Orion would see an improvement in safety, reliability and quality of supply. From a number of years operating the GFN Orion can see that these have been achieved. The benefits are; Safety: Both public and worker is improved as Orion know from the extensive testing that they have carried out that Orion can safely reduce the step and touch voltage to what would be considered a safe level as per IEC/TS 60479. From the Connetics Killinchy test report[10] , Orion could see that a typical touch voltage of 50-70V was achieved with full residual current compensation engaged. In order to Fault find “live” Orion was granted an exemption from the Electricity Safety Regulations 1997 which state in Regulation 62 item (5); “High voltage conductors of overhead electric lines must have earth fault protection fittings that interrupt fault currents to earth in 5 seconds or less”. However in 2003 this regulation was substituted by regulation 27 of the Electricity Amendment regulations 2002, which were again revoked on 1 April 2010, pursuant to regulation 120 of the Electricity (Safety) Regulations 2010 (SR 2010/36). Regulation 62 is now covered in Regulation 34 & 39 to 46 “..as if references in those regulations to works were references to high voltage installations.”[16,17] Reliability: Orion has, in the past year seen a saving of SAIDI across the 26,000 rural customers of approximately 22 minutes and conservatively valued the VOLL savings at $1,500,000 for the year March 2013. Orion are confident through monthly reporting of GFN operations that it is reducing the number of outages that customers would see.

Figure 18 – Dunsandel Substation Single line diagram with GFN

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Quality of Supply: Orion can say from modelling of the GFN operation using Mathcad that there is an improvement in the quality of supply and other New Zealand users of the GFN technology have also reported improvements. This is due to the way resonant earthing works on the zero sequence voltages and does not alter positive or negative sequence at all, (figures 19 & 20).

As all distribution transformers are connected phase to phase, when an earth fault occurs customer supply is maintained with no “brown out”.

Bush fire reduction: While involved in New Zealand and Australia as Swedish Neutral’s distribution partner Orion presented to the Victoria Bush fire task force following the black Saturday Fires of 2009. This was followed up with Orion test results and an Australian utility also supplied GFN fault data from their installation this was independently verified for its ability to reduce the network owner’s exposure to bush fire risk. The report found that a fire could not be initiated until conditions were twice worst case scenario. Although not an intended benefit Orion included the GFN as part of its mitigation of bush fire risk during extreme conditions in Canterbury during the 2012/13 summer.[3,4]

Cost benefit: The total assessed annual cost for Darfield project was $32k / year / substation based upon a project cost of $200,000 with a total assessed annual benefit of $50k / year / substation, giving a benefit / cost ratio of 1.56 with 1,800 customer connections to Darfield, this gave a cost per customer of $18 / customer / year.[5] The actual typical project cost now due to capital and labour inflation is approximately $300,000.

10. Conclusions & Lessons Learned

It is clear that Resonant earthing offers a number of benefits over the Petersen coil alone as a method of earth fault mitigation. In the conclusions of Orion’s paper[5], Orion stated that it “..has employed neutral earthing resistors (20 ohm) on its 11kV networks in the past to assist in mitigating some of these safety issues.” Then went on to state that, “The introduction of resonant earthing is seen to be a logical extension of these practises, particularly now that the problem of cable fault voltage restriking has been eliminated by the adoption of the residual current compensation.” Orion experienced some cost over-runs in their Darfield trail due to an under-estimation of how much should be allowed to introduce the new GFN technology and methodology into the company in terms of training and other related issues.

Figure 19 – schematic of earth fault with Petersen Coil. Figure 20 – Phasor Diagram for Figure 19.

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Subsequent installations within Orion have proved successful by VoLL measures as shown by Orion’s internal monthly reporting of operations. A further study by Northpower[11] of their GFN installation found that “..91% of all earth-faults were compensated successfully..” and they saw “..a 61% improvement in SAIDI..” There are now an increasing number of installations of resonant earthing throughout the world one of the latest is in Brazil[12] where a total of three GFN installations have been installed to date on the AES Sul network. In Europe[13] recently EdF / France[6] and ENEL / Italy[15] have introduced resonance grounding as their standard grounding concept for medium voltage grids.” The paper[14] provides a good comparison of anticipated current levels for different compositions and confirms this on a real network in Spain. The paper also states that “single phase faults represent more than 80% of the faults in the system..”, further reinforcing the idea of the value the GFN can be in improving reliability measures. Further developments in the future may include better fault detection by harmonic detectors and selective harmonic compensation. References:

[1] Hirsch, Stephen; “Resonant Earthing – Advantages and Disadvantages Why do we

not use it in New Zealand? – EEA conference 2006 [2] Wikipedia http://de.wikipedia.org/wiki/Waldemar_Petersen [3] Further details and reports are available from the Energy safe Victoria website at the

following link; http://www.esv.vic.gov.au/About-ESV/Reports-and-publications/Victorian-Bushfires-Royal-Commission/Powerline-Bushfire-Safety-Taskforce

[4] Holmes, G; “Independent Expert Report on Rapid Earth Fault Current Limiters” – RMIT University for Powerline Bushfire Safety Taskforce.

[5] Scott, Tasman L and Winter, Klaus; “The first application of Resonant Earthing with residual compensation to a New Zealand distribution network” – EEA conference 2007

[6] Griffel, D, Leitloff, V, Harrand, Y and Bergeal, J; “A New Deal for safety and Quality on MV networks” – Electricitie de France.

[7] Griffiths, Rodger; “Distribution Earthing Case Study” [8] Yeddanapudi, Sree R. K.; “Distribution System Reliability Evaluation” – Iowa State

University [9] NZCCPTS; “Neutral Earthing Resistors or Reactors Application guide Issue 3, Sept.

2010 [10] Connetics report for Orion; Killinchy GFN Operation Testing, draft issue, Nov. 2010. [11] Rao, Mengyun; “Assessing Ground Fault Neutraliser (GFN) deployment and benefits

in 11kV Electricity Network” – Power Systems group, University of Auckland [12] Winter, K and Silvera, M; “The RCC Ground Fault Neutralizer – First Brazilian Pilot

Installation”, Paper No. OP005 – PAC World Conference Latin America, 21-23 Nov. 2012.

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[13] Winter, N and Winter K; “The RCC Ground Fault Neutralizer – A Novel Smart Grid Protection”, CEPSI 2012 Bali/Indonesia, Presentation No. 564

[14] Zamora, I, Mazon, A.J, Eguia, P, Valverde, V. and Vicente, R; “Influence of Resonant Coil Tuning in the Fault Current Magnitude”, University of the Basque Country, Bilbao, Spain.

[15] Amadei, F; “Continuity of Supply: The Experience of ENEL Distribuzione during the regulatory period 2000 – 2003”, CIRED Conference, Turin, June 2005.

[16] NZ Governor General; Electricity (Safety) Regulations 2010, doc. ref: 2010/36 [17] NZ Governor General; Electricity Safety Amendment Regulations 2011, doc. ref:

2011/370