Study Committee B3 & D1 Colloquium Earth Faults

Study Committee B3 & Study Committee D1
COLLOQUIUM
Brisbane Australia
©Cigre Australia 9–11 September 2013
Page 1 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.

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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

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Brisbane Australia
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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.

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Brisbane Australia
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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.

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Brisbane Australia
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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 I2
t
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.

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Figure 6 – First Petersen coil 1916, now in German Museum.
Figure 7 – Tuneable Arc Suppression Coils 6.6kV on left and 230kV on right.

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Brisbane Australia
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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

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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.

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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.

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(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.

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