Rapid earth fault current limiters

CONTROLING EARTH FAULT CURRENTS USING
RAPID EARTH FAULT CURRENT LIMITERS
Mark Mills

BLACK SATURDAY BUSHFIRES
Black Saturday bushfires were a series of bushfires on the 7th February 2009,
which are rated are among the worst ever seen in Australia.
 Melbourne broke temperature records by experiencing three
consecutive days above 43 °C, peaking over 45 °C
 400 individual fires.
 358 Fire Fighters deployed.
 3500 structures destroyed.
 11,800 head of livestock lost.
Ref: Wikipedia

VICTORIA BUSHFIRE ROYAL COMMISSION
Royal commission convened to investigate causes of fires and provide
recommendations.
 Arson, some fires were deliberately lit.
 Alleged, man operating a power tool, sparks caused ignition.
 An incorrectly-rigged SWER line was ripped down at Kilmore East.
Ref: Wikipedia

ELECTRCIAL CAUSE OF FIRE AT KILMORE EAST
 As wind speeds were reaching their peak, an incorrectly-rigged SWER line
was ripped down at Kilmore East.
 This sparked a bushfire that would become the deadliest and most intense
firestorm ever experienced in Australia's post-1788 history.
 It was estimated that the amount of energy released during the firestorm in
the Kinglake-Marysville area was equivalent to the amount of energy that
would be released by 1,500 Hiroshima-sized atomic bombs.

 The accepted recommendations require the implementation of a range of
initiatives over a ten year period that will reduce the risk of bushfire ignition by
distribution networks by ‘nearly two thirds’.
 The outworking of these recommendations is through the Victorian
Government’s Powerline Bushfire Safety Program (PBSP) taskforce.
 Installation of Rapid Earth Fault Limiters (REFCL) in 22 zone substations by
December 2022.
 New network bushfire design standards for specified line replacement works
within defined areas.
ROYAL COMMISSION RECOMENDATIONS

The installation of REFCLs must comply with the following, in the event of a phase-
to-ground fault:
 Reduce the voltage on the faulted conductor in relation to the station earth
when measured at the corresponding zone substation for high impedance faults
to 250 volts within 2 seconds.
 Reduce the voltage on the faulted conductor in relation to the station earth
when measured at the corresponding zone substation for low impedance faults:
 1,900 volts within 85 milliseconds.
 750 volts within 500 milliseconds.
 250 volts within 2 seconds.
 During diagnostic tests for high impedance faults, limit
 Fault current to 0.5 amps or less; and
 The thermal energy on the electric line to a maximum I2t value of 0.1 A2s

 A REFCL is a protection device that reduces the risk of fires caused by
powerlines and is installed on a zone substation transformer neutral connection
to earth.
 It does this by rapidly limiting the energy that is released in certain types of
powerline faults, known as earth faults. As such the REFCL device trialed
protects the 22kV network from phase to ground earth faults. Other voltages in
the future.
 There are various types of technology that fall under the REFCL umbrella.
 The Ground Fault Neutraliser (GFN).
 Arc Suppression Coil (ASC).
 Solid State Fault Current Limiters (SSFCL) coupled with faulted phase
earthing (FPE).
 Other combinations are available
WHAT ARE RAPID EARTH FAULT CURRENT LIMITERS

ARC SUPPRESSION COIL - GROUND FAULT NEUTRALISER - GFN

 An Arc Suppression Coil (ASC) is an inductor that connects the neutral of the
source substation transformer to earth. Ground Fault Neutraliser (GFN) is a
proprietary design.
 The inductance value is tuned to achieve near-resonance with the total network
capacitance to earth.
 In this state the connection between the neutral and earth has very high
impedance at 50Hz, so earth fault currents are greatly reduced – typically to less
than ten amps.
 Test system comprised a 505mH inductor (80 amps of inductive current) in
parallel with a variable capacitor which ranged from 0 to 20uF, network tuned.
 The test network tuned at a setting around ten amps, i.e. the parallel capacitor
provided about 70 amps of capacitive current at full neutral displacement,
which meant it had a value around 17.5 microfarads.
ARC SUPRESSION COIL - GROUND FAULT NEUTRALISER

GROUND FAULT NEUTRALISER - GFN

 High voltage transistor switch in the neutral to earth connection of the
substation.
 The SSFCL switch comprised a ladder of six insulated gate bipolar transistor
switches, with a series RC over-voltage damping circuit.
 Over voltage protection via 24 kilovolt metal-oxide varistor across the switch for
power frequency over-voltage protection.
 This switch could be set to open when neutral current exceeded a set threshold.
By opening this connection, the earth fault current path was interrupted and
the earth fault current was limited to the capacitive current of the network (test
case 200 amps).
 The SSFCL operated at solid state speeds, i.e. in microseconds.
 A current transformer provided an analogue current feedback.
SOLID STATE FAULT CURRENT LIMITERS

SOLID STATE FAULT CURRENT LIMITERS
GROUND SENSOR
6 STAGE BIPOLAR
TRANSISTOR
RC
NETWORK
VARISTOR

STANDARD POWERLINE INSTALLATION – TN-S SYSTEM
Ref: REFCL Information Forum- CitiPower/PowerCor

NER POWERLINE INSTALLATION

GFN POWERLINE INSTALLATION

REFCL OPERATIONS

TESTING
 Typical 3m wire contact length of downed conductor.
 Fires are unlikely to occur if lineal soil current intensity is less than
0.15A/m (amps per metre) of fallen conductor on the ground .
 Equivalent to about 0.5 amps fault current in a worst-case ‘wire down’
earth fault (3m contact length).
 Grass fires often took many tens of seconds to ignite.
POWERLINE WIRE DOWN FAULT
Ref: Marxsen Consulting, REFCL technologies final
report 151204

Arcing video: https://www.youtube.com/watch?v=Q1MNBV48x0Q
POWERLINE WIRE DOWN FAULT
Ref: Marxsen Consulting, REFCL technologies final report 151204

LOW IMPEDANCE FAULT
 Within 85 milliseconds of fault occurrence the REFCL must limit the voltage
on the faulted conductor to less than 1,900 volts
 Within 500ms of fault occurrence the REFCL must further limit the voltage
on the faulted conductor to less than 750 volts.
 Within two seconds of fault occurrence the REFCL must further limit the
voltage on the faulted conductor to less than 250 volts except during
diagnostic tests.
Definition of a Low impedance fault: a resistance from any high voltage powerline
conductor to earth of value equal in ohms to the nominal phase-to-earth voltage in
volts divided by 31.75 (400 Ohms in a 22kV network).
REFCL PERFORMANCE STANDARD

From tests, it was concluded that to extinguish bounce arcs, the REFCL must reduce
the conductor voltage below 1,900 volts.
FALLEN POWERLINE BOUNCE ARCS

0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 2 4 6 8 10 12
VOLTAGE
TIME sec
REFCL VOLTAGE
LOW IMPEDANCE REFCL PERFORMANCE STANDARD
Time Voltage
0.085 1900
0.5 750
2 250
AS3007 Table F3
Time Voltage
10 50
5 80
1 120
0.5 150
0.4 180
0.1 300
0.05 420
0.03 550
REFCLTouch Voltage AS3007 V REFCL Operational Performance

HIGH IMPEDANCE FAULT
 The REFCL must detect the fault within 1.5 seconds of its occurrence.
 Within two seconds of fault occurrence, the REFCL must limit the voltage
on the faulted conductor to less than 250 volts except during diagnostic
tests.
 During diagnostic tests to confirm if the fault is sustained or not or to
identify which powerline it is on, the REFCL must:
 Limit the fault current to less than 0.5 amps.
 Limit the I2t to less than 0.1 A2s.
Definitions of high-impedance fault: 25,400 Ohms in a 22kV network
REFCL PERFORMANCE STANDARD

REFCL PERFORMANCE IGNITION ENERGY CURVE
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2 2.5
Current
Time
IGNITION ENERGY
I^2t = 0.1A/sec
0.4sec
Curve Limit the I2t to less than 0.1 A2s

The Kilmore South GFN was tested at less than 0.5 amps fault current level and the
results are shown in the table
FAULT DETECTION TEST RESULTS 25kΩ FROM KILMORE SOUTH
REFCL must detect the fault within 1.5 seconds of its occurrence 0.4sec
For Ignition energy curve

The Kilmore South GFN with 15,600 ohm fault
FAULT DETECTION TEST RESULTS 15kΩ FROM KILMORE SOUTH
The Kilmore South Solid State FCL with 15,600 ohm fault

SOLID STATE FCL LINE DIAGRAM
Ground Fault
BLOCKED
Limited: 800mA in <5ms

BODY DAMAGE CURVES AS60479
GFN High Impedance
SSFCL – 5ms – NOT ON GRAPH
Figure 20 – Conventional time/current zones of effects of a.c. currents (15 Hz to
100 Hz) on persons for a current path corresponding to left hand to feet

REMINDER - REFCL PERFORMANCE STANDARD
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2 2.5
Current
Time
IGNITION ENERGY
I^2t = 0.1A/sec
0.4sec
Standard Earth Leakage AS2081
500mA/500ms: Definite Time
Standard earth leakage settings DON’T
clear fast enough to match REFCL on Low
impedance Faults

HIGH VOLTAGE MINING INSTALLATION
22kV REFCL supply
CB through
feed and E/L
Outcome:
1. REFCL operates
2. NO E/L detection at site sub
3. You don’t even know about it, until 22kV supply trips

The insulation levels within the 22kV HV circuit breakers and transformers have to
be selected above the expected ground potential rise of 22kV with the REFCL
installed
ASSET EVALUATION - EQUIPMENT SELECTIONS

The insulation levels within Voltage Transformers have to be selected above the
expected phase to ground fault voltage of ~22kV.
EQUIPEMENT SELECTIONS
Additionally surge arrestor selection needs to be set to the higher value of 24kV

The insulation levels for
Cables have to be selected
above the expected phase to
ground voltage of ~22kV.
CABLE CATEGORY SELECTIONS AS1429.1

 The REFCL systems have rapid clearance times.
 Ignition energy/current density curve will generally be the driver for REFCL
operations.
 No guarantee site protection will detect and operate, has to be faster than
REFCL. Review protection schemes.
 Ground fault (summation of CT in circuit breaker or protection relay)
 Earth leakage (RCD type or CT on MEN link)
 Transfer voltages still need to be controlled under current protection schemes
(AS/NZS3007, AS/NZS2067 etc.).
 Equipment selections will be based on IT earth requirements.
 Isolation transformers – isolate supply authority from site network?
 When will REFCL legislation move into mining with our extensive networks?
WHAT DOES THIS MEAN FOR US?

End
Thanks for Listening
Contact Details
Mark Mills
MIEAust CPENG NER, RPEQ
Office: 02 49368700
Mobile: 0419 614 675
Email: mark.mills@mrm.net.au
Web: www.mrm.net.au

Rapid earth fault current limiters

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