Not our fault! - earth faults, past, present, and future, and their mitigation

Study Committee B3 & Study Committee D1 COLLOQUIUM

Brisbane Australia ©Cigre Australia 9–11 September 2013

of 10

223

Not Our Fault! –

Earth Faults, past, present and future and their mitigation

D. J. KEEN

Connetics Limited

New Zealand

ABSTRACT

Since the development of polyphase systems in the late 1800’s transmission and distribution systems have needed over-current and earth fault protection, initially fuses were used and this was augmented with electromagnetic or permanent magnet based relays. With the development of transistors and later microprocessors from the 1960’s through the 1980’s Intelligent Electronic Devices (IED) and numerical devices were developed into the multi-functional protection relays we have today. During the same period in 1916 Waldemar Petersen developed the Arc Suppression Coil and in 1918 he developed the Wattmetric Earth Leakage Relay (ELR). He developed the first theory of the electric fields and electrical breakdown and has written basic textbooks on these areas. 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 charts the development of the Petersen coil and other neutral earthing methods used on generators and transformers from their original installations through to the present and their potential for future use in integrating with smart grid solutions and self-healing networks. This 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.



of 10

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 so they can say that any supply interruption is genuinely “Not our Fault!”

KEYWORDS

Rapid Earth Fault Current Limiter, Bush Fire, Arc Suppression Coil, Petersen, Ground Fault Neutralizer

1. INTRODUCTION

High voltage power was of interest to early researchers working on the problem of transmission over distance. They know from elementary electricity principle that the same amount of power could be transferred on a cable by doubling the voltage and halving the current. Due to Joule's Law, they also knew that the capacity of a wire is proportionate to the square of the current traveling on it, regardless the voltage, and so by doubling the voltage, the same cable would be capable of transmitting the same amount of power four times the distance.

Electricity supply began in Australia’s colonial era – in about 1880[1]. The Federal Constitution of 1901 vested in the Commonwealth the right to make laws only in those matters named in the Constitution, being those considered important to the business of nationhood. The constitution was silent on the subject of electricity supply, which thus remained a legislative function of each individual state. The history of the electricity industry in Australia is therefore the story of six separate states.

New South Wales did not regulate the industry until 1919, up to that time from 1888 Sydney Council actively legislated against the introduction of electricity in Central Sydney, making it the last major municipalities to be electrified in Australia.

Figure 1 – Left; some of the first electric lights in Sydney in early 1900’s and Right; 1906 Electric Tram in Melbourne.

In Victoria, the first power stations were constructed by private companies in and around Melbourne from 1880. State-wide ownership and control of the industry developed progressively



of 10

from 1921 following the creation of a State Electricity Commission to develop the vast brown coal deposits of Latrobe Valley.

Electricity was used to illuminate the General Post Office in Sydney in 1878 and the first lighting company, the Victorian Electric Light Company, was formed in Melbourne in 1880 to light the Eastern Market and the Athenaeum Hall.

It is generally considered that the first supply of electricity to the public at large occurred in two small country towns in New South Wales. Tamworth, with a population of 3,000 switched on arc and incandescent street lighting on 9th November 1888. In April 1899, the smaller town of Young switched on its incandescent street lighting and shortly thereafter went on to connect shops, offices and homes within reach of its lines. The Tamworth plant generated at 240V DC but Young had the further distinction of being the first supply in the country using three phase alternating current.

By the end of the 19th century, it was obvious to the technico-scientific community that electric current was dangerous for man, and that AC was more dangerous than DC. Between 1880 and 1920, transmission and distribution of electrical power took place on unearthed neutral uninsulated lines placed out of reach, supported by insulators; no points of the network were deliberately earthed. In the 1920 earthing and neutral earthing began to appear on Transformer neutrals, transmission systems and LV systems. This paper considers the development of neutral earthing and the methods of fault mitigation available today and considers how this may develop in the future.

2. DEVELOPMENT OF NEUTRAL EARTHING

There are several methods of neutral earthing in use today initially the neutral of a power system was unearthed and this was standard practise as described in the introduction to this paper until the beginning of the 1920’s[2] Ungrounded systems. Ungrounded systems offered the obvious advantage that no unscheduled service interruption was required at the first incident of a phase-to-ground fault. Power system engineers used three star-connected and neutral-grounded incandescent lamps as ground-fault detectors. The three lamps, each glowing equally and dimly under normal conditions, would signal the occurrence of a ground fault by changing to one dark and two bright lights. Only in rare instances today is the ungrounded-neutral system still used.

Figure 2 ‐ Unearthed arrangement with earth fault & related phasor diagram.



of 10

The investigations established that all so-called ungrounded systems in fact are weakly and reactively grounded through the capacitive impedances to ground attributable to the insulation of the system’s energized phase conductors (refer to figure 2). Solidly Grounded Systems

Figure 3‐ Solid Earth arrangement with earth fault & related phasor diagram.

Solid (also called direct) grounding where the transformer neutral is bridged to ground by a solid copper bar. With this grounding system the fault currents are in the order of tens of kA. With regards to the protection time this concept enables relatively fast tripping, down to around 100 msec. However, if downstream breakers are introduced to improve selectivity this will slow down protection, as upstream breakers will have to wait for downstream breakers to operate. It will be a trade-off to improve selectivity. With regards to fault detection requirements, 10 kA is significantly more than necessary, this massive fault current is not needed to detect an earth fault. Low Resistance Grounding Typically this method of grounding consists of a Neutral Earthing Resistor (NER) or is provided via a single phase transformer and resistor (grounding transformer) to limit the neutral current to a few hundred Amps. These have been used in the U.K. and many other countries. This current level is still high enough to allow simple over current protection to detect an earth fault. The speed of protection is the same as for solid grounding with the same problem when down-stream breakers are introduced to improve selectivity. As the number of NERs used in the network increases, this can reduce the sensitivity of this method of grounding to a level that does not permit the protection system to detect the fault.



of 10

Figure 4 ‐ NER arrangement with earth fault and related phasor diagram.

High Resistance Grounding Presently, high-resistance grounding is in common use in plants where process continuity is an overriding consideration. To successfully apply this mode of grounding there must be a management commitment to locate and remove the first ground fault at once to preclude its potential escalation to a phase-to-phase fault, especially for faults occurring in motor and generator windings. The probability of such escalation is to a large extent influenced by the so-called I2t energy (in amperes2-seconds) released at the point of fault. The low fault current that results from this form of neutral earthing allows the operator time to find the cause of the fault before damage to equipment becomes irreversible. Resonant Grounding This form of grounding began with the invention of the Petersen coil in 1916 and was developed initially as a protection device used on a generator stator at Peidelsham power station in Germany. Since then it has been widely used in Europe and around the world at voltages from 6kV up to over 200kV. The next section explains in more detail the development of the Petersen or Arc Suppression Coil (ASC) and the development of available tuning methods.

Figure 5 ‐ Resonant Earthing arrangement with earth fault and related phasor diagram.

3. DEVELOPMENT OF ARC SUPPRESSION COILS (ASC)

From the initial development of the Petersen coil (which was a fixed reactance coil, figure 6). Whilst it could be easily tuned to match the fixed reactance of a generator stator, it did not provide complete protection of the stator winding and was not suitable for use as a protection device in its fixed form on electricity transmission and distribution networks. This led to the development of the tuneable arc suppression coils of the 1950s.



of 10

Figure 6 – First Petersen coil 1916, now in German Museum.

Figure 7 – Tuneable Arc Suppression Coils 6.6kV on left and 230kV on right.



of 10

These were soon developed upon and have led to the arc suppression coils available from Trench see photo below on left, Figure 8) and other suppliers that we have today. These have developed initially from a dry type coil similar to Petersen’s first coil into an oil filled coil as well as the dry type. Another type of Arc suppression coil uses switched reactances to tune the coil (this is marketed by Swedish Neutral as part of their Ground Fault Neutralizer system in both oil filled and dry types) figure 8 below right photo, shows an example of the oil filled type.

As well as the coil types that have been developed, the method of tuning has also developed from the initial fixed type Petersen coils. These include the methods of tuning shown in Figure 9. These include the step adjustable coil where the tuning can be manual changed off-load and the plunger coil design, where the tuning can be adjusted on-load. The motor driven design has the advantage of tuning to the exact reactance of the circuit but has the disadvantage of taking around one to two minutes to complete the tuning process and has many moving parts and HV contacts. The Swedish Neutral method of using LV capacitors and if required, a number of switched coil reactances to achieve a similar tuning accuracy but within one to two seconds of a change in circuit reactance and with no moving parts and maintenance requirements are considerably reduced. The speed of tuning can be an important factor[3] as the part of the network that is switched in may have an existing fault on it, so in effect the network operator may be bringing a fault into the stable network.

Figure 8 – Oil filled types of Arc Suppression Coils from Trench on the left to Swedish Neutral on the right.

One method used by some suppliers for the control of the (Petersen) or arc suppression coil is to use Control by Injecting Frequency (CIF) algorithm which has given promising results[4] for Petersen coil control.

An option that can be added to the arc suppression coil to improve the performance as an earth fault minimisation system includes using an auxiliary coil input to inject a equal and opposite current into the arc suppression coil to cancel out the resistive component of the fault current. This option has been successfully employed by Swedish Neutral in their Ground Fault Neutralizer (GFN) system[5]. Other arc suppression coil providers have employed a Qu2 algorithm in their fault detection relays to improve the fault detection ability of their systems[6]. The advantages of this



of 10

method of fault detection include; a sample rate of 2kHz can be used, The integration and evaluation can be done over half a period, the integration of i0 over a larger range before the trigger-point enables the detection also of high ohmic earth faults up to about 5kOhms. The disadvantages include, Sensitive to phase-splitting (problem for all relays and especially cos(phi) relays) and sensitive to cross-talk from parallel systems.

Figure 6 – Left diagram shows Trench type stepped core design, right diagram shows motor driven plunger core

design.

4. RESONANT EARTHING AND APPLICATIONS

Ground Fault Neutralizers (GFN) or Rapid Earth Fault Current Limiters (REFCL) as they are known in Australia have many applications including their use in network safety and Earth Potential Rise (EPR) minimisation, step and touch potential minimisation and mitigation of bush fires and forest fire risks. The GFN or REFCL’s can be used in transmission and distribution systems up to 110kV and can minimise the fault current down to 50mA in less than 60msecs[5]. The Petersen coil was originally developed for use on generator systems for earth fault protection, this method has fallen into disuse in recent years but there are a number of papers that have explored this method of earth fault protection on generators[7, 8, 9], as well as for wind turbines[10]. These papers indicate that use of the ground fault neutralizer (or REFCL) on stators of generators can contribute to minimal generator damage from fault currents and 100% generator stator protection and can permit the generator operator the time needed to bring the generator off load in a controlled way, without damage to the windings.

Use on network systems whilst still allowing continued use in fault conditions and whilst still maintaining public safety[11] and minimising asset damage, refer to the Grudie Bridge & Brora Network System study paper by Swedish Neutral on the Scottish & Southern Energy network[12], a study by University of Auckland for Northpower in 2012[13] which discusses benefits which include a reduction and SAIDI (by 61%) and EPR levels and the paper on the GFN installation by AES Sul of Brazil[14] which describes the benefits the installation has provided on their network, as examples.

The use of the resonant earthing with residual current compensation (RCC) and in particular the Swedish Neutral GFN can provide fault detection of low and high impedance faults up to around 20kOhms, as well as, through the use of the RCC, provide selective compensation of harmonic fault currents up to the 7th harmonic level and allow the use of digital 5th harmonic detectors on the network for use on radial feeders to accurately locate permanent earth faults for repair teams.



of 10

In Australia since the bush fires of Ash Wednesday in 1983 and Black Saturday in 2009, concern over the electrical system and bush fire mitigation led to the establishment of the Powerline Bushfire Safety task force which resulted in a number of recommendations including an Independent Expert report on Rapid Earth Fault Current Limiters[15] by RMIT University in September 2011 and a further study by HRL Technology for Energy Safe Victoria on the Probability of Bushfire Ignition from Electric Arc Faults[16] in December 2011. Which indicated that REFCL’s can provide an excellent way to virtually eliminate the risk of bush-fires on electricity networks in Australia.

There is some concern over the use of multiple Ground Fault Neutralizers in a network and how they may interact. This is touched upon in a paper[17] which states that “..The more neutralizers there are at different points in the system, the less will be the charging current at any one location, and the tendency for ground relays to mis-operate on charging current will be reduced.”

Line fault indicators may be used in conjunction with the Ground Fault Neutralizer[18] and this shows improved fault location and reduction in frequency of outages with improved supply quality for customers.

5. CONCLUSIONS

As described earlier in this paper, there are a number of options available for earth fault detection and minimisation. Whilst solid earthing or NERs are an option in conjunction with the use of protection relays these methods can only ever isolate the fault from the supply provided the fault impedance is not too high to be detected by the protection relay and may leave customers with no supply until the cause of the fault can be cleared or repaired. Resonant earthing and detection systems allows the fault current to be minimised and with the addition of the RCC system almost eliminated for single phase to earth faults (which can represent up to 90% of total network faults). This allows customers to remain supplied whilst the fault location is found and only isolated from their supply whilst the fault is repaired (unless the fault is transient or the repair can be done using “live line” techniques). Disadvantages of resonant earthing include, inability to minimise phase to phase faults (these would be isolated by the usual protection relays and systems). Issues around open delta regulators[19] which can be eliminated by installing closed delta regulators. Also the appearance of full line voltage on the un-faulted phases necessitating that all connected equipment is rated to full line voltage.

It is clear that further work is necessary to take full advantage of the potential of Resonant earthing and use of the RCC system to mitigate earth faults, but the systems available now and in particular the Ground Fault Neutralizer (GFN) system, can be used to provide accurate information about the status of assets on a network for the network operators and improve reliability and safety for customers, so that in future when there is a loss of supply reported by a customer, the network operator can genuinely say that it is “Not our Fault!”

REFERENCES

(1) Brady, F. – A Dictionary on Electricity (Contribution on Australia). (2) Owen, E.L. - The Historical Development of Neutral-Grounding Practices. (3) Zamora, I – Influence of Resonant Coil Tuning in the Fault Current Magnitude. (4) Druml, G. et al. – Operational Experiences with the New Method to Control Petersen

Coils by Injection of Two Frequencies. (5) Winter, K.M. – The RCC Ground Fault Neutralizer – A novel Scheme for Fast Earth-

Fault Protection. (6) Druml, G – QU2 – Algorithm for detecting earth faults.



of 10

(7) Zielichowski, M & Fulczyk, M. – Influence of Neutralizer on Ground Fault Overvoltages in Generator Stator Windings.

(8) IEEE, Std, C37.101-1993 – IEEE Guide for Generator Ground Protection. (9) Yorukeren, N. & Ucan, M. – Computer aided analysis of phase to ground fault for

Generators operated in Parallel with Networks. (10) Aalborg University – Grounding for Offshore wind Farm Electrical Systems. (11) ENA – EG-0 Power System Earthing Guide (12) Swedish Neutral – Grudie Bridge and Brora Network System Study. (13) Rao, M. – Assessing Ground Fault Neutralizer (GFN) Deployment and Benefits in 11kV

Electricity Network. (14) Winter, K & Silvera, M. – The RCC Ground Fault Neutralizer – First Brazilian Pilot

Installation. (15) Holmes, G. Prof. – Independent Expert Report on Rapid Earth Fault Current Limiters. (16) HRL Technology Pty Ltd. – Probability of Bushfire Ignition from Electric Arc Faults. (17) Mason, C.R. – The Art & Science of Protective Relaying. (18) Pernica, D. – Contactless Earth Fault Indicator. (19) Burgess, R. & Ahfock, A. – The use of Voltage Regulators in Power Systems with Arc-

Suppression Coils.

Biographical Details of Authors:

Author 1 Dennis Keen CEng. MIET

Dennis graduated in 1990 with a B.Eng (Hons.) in Electromechanical Engineering from Aston University in Birmingham U.K. and was sponsored by the C.E.G.B. just before its privatisation, he went on to work for National Power plc. as a Project Engineer at West Burton 2000MW coal fired power station in the U.K. Dennis became a Chartered Engineer in September 1994 and was a member of the I.Mech.E. he is currently a member of the IET and an IET mentor for those wanting to become Chartered Engineers and is also Secretary for CIGRE NZNC.

Dennis is currently employed by Connetics Limited, as Sales and Technical Project Manager, responsible for the day to day Ground Fault Neutralizer business unit as well as other technically focused products.

Dennis is interested in continual professional development and helping younger engineers develop their knowledge and experience and industry contacts through CIGRE, the IET and the Australasian Electricity Industry.

Not our fault! - earth faults, past, present, and future, and their mitigation

Engineering

Transcript of Not our fault! - earth faults, past, present, and future, and their mitigation