Independent report on ACRs

Professor Grahame Holmes,
Electrical and Computer
Building 10, Level 7, Room 1
376-392 Swanston Street
GPO Box 2476
Melbourne VIC 3001
15th September 2011
Tel. +61 3 9925 3874
Fax +61 3 9925 3242
Powerline BushFire Safety Taskforce
Independent Expert Report on Automatic Circuit Reclosers (ACR) for Single Wire Earth
Return (SWER) distribution lines
Circuit Reclosers are part of the primary protection equipment used on distribution feeders to
manage overcurrent conditions caused by fault occurrences. Their principle of operation is to
disconnect the feeder for a few seconds when a fault current is detected, and then to proceed through
a staged sequence of reconnection attempts that will reconnect the feeder if the fault condition is
only transient. For Single Wire Earth Return (SWER) systems, existing recloser technology is an Oil
Circuit Recloser (OCR), which has limited flexibility and does not limit the arc energy injected by a
fault condition to a value that reduces the risk of bushfire ignition to an acceptably low level.
Recently, a more advanced Automatic Circuit Recloser (ACR) has been developed by SPAusNET in
conjunction with recloser manufacturers. This new product provides greater flexibility of recloser
sensitivity to trip currents and reconnection sequences, faster circuit interruption compared to an
OCR, and remote reprogramming capability via SCADA systems to change both the trip current
limits and the reclose sequences as may be required. These advances offer the potential for the new
ACR to dramatically reduce the injected arc energy under SWER fault conditions, to levels that can
significantly reduce the probability of bushfire ignition even under high risk conditions.
Existing Oil Circuit Recloser (OCR)
Circuit Reclosers are one of the primary equipments used on distribution feeders to manage
overcurrents caused by distribution line faults with a minimum level of supply distribution to
consumers. Their principle is simple – detect the overcurrent in a distribution feeder caused by a
fault condition, disconnect the feeder for a short period of time, and reconnect to see if the fault is
transient or permanent. If the fault is permanent, the fault current will re-establish when the circuit
recloses, and the recloser then will detect this condition and re-open the connection to the faulted
feeder. Typically a number of reclose attempts are made before the recloser permanently locks out
the connection to the feeder.
In practice a large number of faults (around 70%) on a distribution feeder are transient, caused by
lightning strikes, small branches hitting the line, animal short circuits, etc and disappear within
seconds when the line is disconnected. Hence when the line is reconnected (a “reclose” event),
supply is fully restored, and consumers see a momentary supply disruption for only a few seconds.
However, it is important to ensure that reclosers differentiate between a short term load overload,
caused for example by a large motor starting current, and an overcurrent created by a real fault
To facilitate this differentiation, reclosers time their operation using Time Current Curves, which
implement a variable time delay before the recloser opens the feeder connection, depending on the
level of fault overcurrent. This means that a substantial overcurrent will “trip” the circuit very
quickly, while a marginal overcurrent may not cause a trip for several hundred milliseconds.
Reclosers typically incorporate several response curves (often grouped as “fast” and “slow” settings)
to further improve their sophistication of response to an overcurrent fault condition. For example, a
recloser may initially trip with a rapid “fast” response, which disconnects a long length of feeder
very quickly, in anticipation that the fault is transient and will rapidly clear. However, if the fault is
permanent, the fault current will re-initiate when the circuit re-closes. If this happens, the recloser
will now implement a “slow” overcurrent fault response, which can take up to hundreds of
milliseconds to respond. This gives time for other protective equipment (such as in-line fuses) that
are downstream of the recloser, to trip and isolate a much smaller section of the feeder permanently.
Hence only those customers that are connected very near to the fault will be permanently
disconnected, with the majority of customers remaining on supply after the short term transient
For Single Wire Earth Return (SWER) feeders, the recloser that has been in common use for many
years is an Oil Circuit Recloser (OCR). This recloser has a relatively limited capability, with only
two Time Current Curves as shown in Fig 1, essentially a fast trip (curve A) and a slow trip (curve
B). OCR’s can be set to implement several trip-reclose sequences, typically Fast-Fast-Slow-Slow,
but these must be manually setup at the time of commissioning or field service, and cannot be
readily varied to suit changing operating or weather conditions. Also, by virtue of their mechanical
construction, OCR’s have a minimum trip time of over 80 milliseconds, and even a fast trip can be
delayed for up to 500 milliseconds for a moderate overcurrent condition. The result of these
limitations is that while OCR’s are very effective in managing fault overcurrents and avoiding
significant customer off-supply time for transient faults, they cannot minimise the energy injected
into a fault arc sufficiently to avoid a reasonable probability of fire start if the fault occurs on high
fire risk days in a high risk area.
New Automatic Circuit Recloser (ACR)
In the last two years, SP AusNet has been working with recloser manufacturers to create a modern
Automatic Circuit Recloser (ACR) that provides a significantly more sensitive response to moderate
overcurrents, and also provides substantially more flexibility and adaptability in terms of the
sequence of Time Current Curves that can be implemented for any given operating conditions. The
new ACR also includes remote programming capability so that both the sequence of Time Current
Curves, and the operating limits of these curves, can be rapidly and conveniently varied from a
remote control room to suit daily weather conditions, load conditions, etc.
To date, Cooper Power Systems have produced a new generation ACR in response to the
requirements specified by SpAusNet, with the following capability.
Three Time Current Curves are provided – curves 101, 104 and 161. Fig 1 shows the sensitivity of
these curves, and compares them to the Time Current Curves of the older OCR systems.
 Curve 101 provides an essentially instantaneous trip, shown in Fig 1 to respond to a 14A
minimum overcurrent, and with a minimum trip time of about 40 milliseconds for a fault current
above about 35A (This trip time is made up of an approximately 16 millisecond minimum fault
detect time, and a circuit breaker mechanism response time of about 23 milliseconds1). This is an
extremely rapid fault response that dramatically limits the energy injected into the fault.
 Curve 104 is a less sensitive response, that takes substantially longer to trip for a moderate
overcurrent condition. This response reduces the number of spurious trips for the ACR caused by
short term overloads such as Direct On Line Motor start inrush current. From Fig 1, it can be seen
that this curve has a similar sensitivity to the existing OCR curve “A” for a moderate overcurrent,
but responds more quickly for a higher overcurrent. Note in particular how this response curve
Cooper Power Systems Recloser Specification: maximum 7000A arc extinction time for 3 phase interrupter
is 35millisecond. Estimated maximum 50A arc extinction time for single phase interrupt: 23 millisecond.
trips quite slowly for moderate fault currents (< ~40A) because of the increased delay response
provided by this curve.
 Curve 161 is a much slower response curve, similar to the existing OCR Curve “B” which is
intended to provide the opportunity for a graded downstream response to a permanent fault
condition to separate only the faulted section of the distribution feeder.
These three Time Current Curves can be combined to operate in one of nine alternative setting
groups, as follows:
Fast(104) – Fast(104) – Slow(161) – Slow(161)
Instantaneous(101) – Instantaneous(101) – Slow(161) – Slow(161)
Fast(104) only
Instantaneous(101) only
Slow(161) only
Fast(104) – Slow(161)
Instantaneous(101) – Slow(161)
Instantaneous(101) – Instantaneous(101), 55 sec delay before reclose
Instantaneous(101) – Slow(161), 55 sec delay before reclose
Note that other setting combinations can be programmed by the ACR manufacturer, but these are the
combinations currently provided for the systems that are currently being evaluated.
It should also be noted that the current trip levels shown in Fig 1 are the total current seen by the
ACR, i.e. the total of the load current and any fault current. The typical load current for a SWER
feeder with a 14A minimal trip current setting is between 5A and 10A, so that the effective fault
current is reduced by the contribution to the ACR trip current made by the load current (this
reduction is approximately linear, since for a SWER line, the high resistance of the feeder wire
makes the fault current have a much less lagging power factor than is the case for a three phase
feeder system).
Probability of Fire Ignition with ACR protection of SWER feeder
The fire ignition testing conducted separately by the bushfire taskforce has provided new insights
into the requirements for protection systems to reduce the probability of fire ignition from an
electrical fault to an acceptably low level. In particular, this testing has established a quantifiable
probability relationship between fault (arc) duration, fault current, and fire ignition (Fig 1, Interim
Report, Probability of Bushfire Ignition from Electrical Arc Faults). From this figure, a fault
clearance time of 40 milliseconds reduces the probability of bushfire ignition to about 6% for a 200A
fault current, 3% for a 50A fault current, and essentially to zero for a 4.2A fault current.
Fig 1 in this report shows the Time Current Curves for the new ACR manufactured by Cooper
Power Systems. Curve 101 for this ACR shows a relay response time that reduces to a minimum of
about 16 milliseconds for fault currents above about 50A. When the estimated mechanism/arc
extinction time of 23 milliseconds is added to this relay response, the total fault clearance time for a
50A fault current becomes about 40 milliseconds (this is consistent with the dotted asymptotic
clearance time for curve 101 of about 60 milliseconds, which is assumed to include the three phase
arc extinction time of 35 milliseconds). Hence the new ACR will reduce the probability of fire
ignition for a 50A fault current to about 3%, compared with over 80% with the fault clearance time
of approximately 85 milliseconds for a 50 A fault current that is achieved by an existing OCR
operating with Curve A. This is a dramatic improvement in performance.
The fire ignition tests separately conducted have also identified that there is a significant increase in
probability of fire ignition if a recloser operates in less than about 30 seconds after the initial fault
trip. This is a substantial increase in reclose period from the current 2 second period implemented by
existing OCR technology, and the standard 8 second reclose period programmed for normal ACR
operation. However, as noted above, the new ACR has the flexibility to implement a variety of
alternative trip setting groups by remote command, including in particular setting 8, which
implements two instantaneous Curve 101 trips, separated by a 55 second delay. Implementing this
setting on a high bushfire risk day will maintain the probability of fire ignition below 5%, even with
a reclose attempt. Note however that a second slow reclose attempt (curve 161) has a clearance time
of more than 200 milliseconds, which has a virtually 100% probability of fire ignition for a
permanent fault. Hence setting groups that implement a Curve 161 reclose attempt are quite
unsuitable for SWER protection operation during high fire risk conditions.
For a minimum fault current of 14A, Curve 101 for the ACR provides fault clearance in about 150
milliseconds. Extrapolating from Fig 1 of the Interim Bushfire Ignition Report, this suggests the
probability of fire ignition could increase to about 40%. However, at these very low fault current
detection levels, the load current contribution means that the actual fault current is likely to be
substantially less than the detected fault current level. For example, a 5A load current will mean the
actual fault current is less than 10A, a 10A load current will mean an actual fault current of less than
5A, and so on. Furthermore, at such low fault current, there is a significant expectation of early arc
extinction caused by wind, increasing arc gap because of fault clearing, etc.
Hence estimating the probability of fire ignition for such a low fault current is more difficult, and a
wider tolerance of ignition probability is to be expected (say 10-30%). Furthermore, it is feasible to
reduce the minimum fault current for the ACR to even less than 14A, tracking for example to set this
limit at a constant ratio of perhaps 150% of the SWER load current as this current varies over the
daily load profile. It should be noted also that the fire ignition probabilities presented in Fig 1 of the
Interim Bushfire Ignition Report are an absolute worst case, and reduce substantially when even
moderate wind speeds are factored into the analysis.
While further work is required to precisely optimise the low fault current response of the ACR to
minimise the probability of fire ignition from this type of fault, from this analysis it seems feasible to
achieve an ignition probability of no more than 10-20% even under high impedance low current fault
conditions. For the more common lower impedance higher current faults, an ignition probability of
less than 5% seems readily achievable with this new technology.
Flexibility of ACR operation for a SWER system
The improved functionality of the new ACR offers two substantial benefits for fault protection of a
SWER feeder under high bushfire risk conditions.
Firstly, the increased variety of trip combinations allows for combinations that can dramatically
reduce the probability of fire ignition. However it does have the penalty of at least a more substantial
reclose delay, and more potential for a complete feeder disconnection in the event of a permanent
fault. Hence there is the potential to inconvenience many more customers than would be the case for
the more normal SWER protection response of a Fast-Fast-Slow-Slow reclose setting.
Secondly, and more significantly, the remote control capability of the ACR allows the setting group
selection to be changed essentially on command, and even automatically using a central control
system, depending on the identified level of bushfire risk for a particular feed on a particular day.
For example, on an Extreme Fire Risk day, SWER feeders in identified high risk areas could be
reprogrammed to use setting 4 for the day, which would substantially reduce their probability of
igniting a bushfire in the event of a fault. At the same time, SWER feeders in somewhat lesser risk
areas could be programmed to setting 8 for example, to attempt a reclose 55 seconds after the fault
trip, but to again trip on the most sensitive 101 Time Current Curves if the fault is sustained.
It should be noted that this report does not attempt to validate the specific setting combinations for
the new ACR as the best and most appropriate for bushfire ignition risk mitigation for SWER lines –
further work is required to precisely identify the best possible settings for different operating
contexts. Hence this new ACR technology offers potential for substantially reducing the probability
of a SWER feeder causing a bushfire start in high risk conditions.
Professor Grahame Holmes
RMIT University
Current (Amps)
OCR 10 amp A
Device: Kyle ACR A TX=1.00 IX=20.00
CT Primary=1A CT Secondary=1A In=1A
electronic 15 amp min op
OCR 10 amp B
Device: Kyle ACR B TX=1.00 IX=20.00
CT Primary=1A CT Secondary=1A In=1A
6k BA
Device: 22kV S&C `K' SPEED
Fuse or Conductor Size: 6
electronic 15 amp min op
Device: F4C ACR 161 TX=1.00 IX=14.00
CT Primary=1A CT Secondary=1A In=1A
min. op low set
Device: Sensitive Earth Fault Setting
Min Op=14A Definite Time=1.00 Sec
OCR 10 amp B
electronic 14 amp min op fast 101
Device: F4C ACR 101 TX=1.00 IX=14.00
CT Primary=1A CT Secondary=1A In=1A
MO=14A Discrimination Curve= 0.05 Sec
Time (sec)
electronic 14 amp min op fast 104
Device: F4C ACR 104 TX=1.00 IX=14.00
CT Primary=1A CT Secondary=1A In=1A
MO=14A Discrimination Curve= 0.05 Sec
min. op low set
electronic 14 amp min op fast 104
6k BA
electronic 14 amp min op fast 101
OCR 10 amp A
Study No:
Date Printed: 15 September 2011
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