Recommendations for
Next Generation Auto
Circuit Reclosers to
Reduce Bushfire Risk on
SWER Networks
Introduction
In recent years the prevalence and consequence of faults on medium voltage
(MV) networks causing the ignition of destructive and fatal bush fires has
resulted in new approaches to risk minimisation for electricity distribution
companies. The findings of the 2009 Victorian Bushfires Royal Commission
provide a number of recommendations to electricity distributors to reduce bush
fire risk.
A number of these recommendations focus on the single wire earth return
(SWER) networks prevalent in rural Victoria and other parts of Australia.
Recommendation 27 of the royal commission findings includes the following
statement:
“the progressive replacement of all SWER (single-wire earth return)
power lines in Victoria with aerial bundled conductor, underground
cabling or other technology that delivers greatly reduced
bushfire risk.” (emphasis added)
There is a high cost and an implementation difficulty of installing thousands of
kilometres of aerial bundled conductor or undergrounding cabling of the
network. For this reason the focus of this white paper is on the ‘other
technology that delivers greatly reduced bushfire risk” being the next
generation Automatic Circuit Reclosers (ACR), proposing this technology as a cost
effective solution for use on all SWER networks. More specifically, this paper
describes to electricity distribution companies a means to greatly reduce bushfire
risk using ACRs on SWER networks and describes the performance and features
that may be prescribed in technical specifications.
Disclaimer
This document is provided for discussion purposes only. Siemens accepts no
liability for any damage or loss that may be suffered by any person or entity that
relied upon the information in this document.
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2. Bushfire Mitigation
Challenges on SWER Lines
2.1 SWER Line Overview
SWER (single-wire earth return) lines are a low cost means
to provide electrical power, to rural areas. SWER is a
single phase distribution method using a single wire
distribution with the earth (IE the ground) used as the
return path, eliminating the requirement for a second
wire. As shown in figure 1 below an isolating transformer
is used to create the SWER line from the three phase
feeder. Customers are supplied from a fuse protected
step down transformer as shown.
Figure 1. SWER Line Overview
• SWER lines are typically long with low customer density.
Typical load currents are reliable and range from
approximately 1 to 10A. Due to the relatively high
impedance of the earth return fault currents are typically
low in the range of 10’s to 100’s of Amps.
• Most faults on SWER lines are temporary in nature
meaning SWER lines are well served by reclosing
protective devices. However a broken or downed
conductor can be difficult to protect against as the fault
current may not be greater than the existing load current.
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2.2 Royal Commission Recommendations
The 2009 Victorian Royal Bushfire Commission - final report
summary [1] made a number of recommendations
applicable to SWER lines and these are shown below.
SWER is a single wire, so the proposal of aerial bundled
conductor (ABC) has no direct application.
The author contends that the intent of the recommendation
is to utilise insulated cable as a barrier to electrical faults.
There are still questions as to the robustness of this
approach due to concerns about:
·
Potential damage to the insulation layer of the
conductor due to branch strikes and other
mechanical mechanisms.
·
Electrical degradation of the insulation at clamping
points in particular
Informal feedback from electricity distributor companies is
that ABC is being used for three phase applications and
shorter line lengths.
Due to long lengths and remote location of most SWER
lines, re-conductoring with aerial bundled (Insulated)
conductor or undergrounding these networks is prohibitive
due to the cost associated with such a program.
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Due to the large areas covered by SWER networks the
practical implication is that all SWER Automatic Circuit
Reclosers (ACR) must have SCADA communications. The
SCADA communications enable change to the protection
trip and reclose sequence (via broadcast) of the recloser on
high fire risk days. This requires the removal of all old style
reclosing technologies such as hydraulic reclosers, and the
application of a “technology that delivers greatly
reduced bushfire risk”
Typically the modern microprocessor
controlled single phase recloser carries a
significant cost burden for application on
SWER lines which generally supply only a
few customers.
When read together the above recommendations
demonstrate the cost/benefit to be considered by electricity
distribution companies. i.e if it was viable to install aerial
bundled conductor or to underground SWER network then
recommendations relating to altering reclose sequences
would not be necessary. Certainly changes to the protection
trip and reclose sequence (via broadcast) represent a first
step to reducing bush fire risk.
Figure 2 is significant for SWER line application where fault
levels are within the applicable range of these experiments.
Traditional reclosers have a fastest possible clearing time in
the range of 30 to 60ms. This means that for fault levels in
the range of 50A there is still a significant risk (>20%) that a
fault may cause initiation of a fire.
When arc durations are very short (below
20ms) the probability of fire ignition from
an electrical arc approaches zero for fault
levels associated with SWER lines.
The report by HRL Technology Pty Ltd [2] also confirmed
that when reclosing onto a permanent fault and tripping
again that if the dead time is short (5s) that this
significantly increases the probability of ignition. HRL
Technology Pty Ltd [3] conducted a further study to
determine the extent of additional reclose delay and the
impact on arc ignition. This study found that dead times in
the range 8-10s appear adequate to not increase the
probability of fire ignition on the second shot in the reclose
sequence. However, both reports [2] and [3] provide clear
evidence that the longer the dead time the further reduced
the probability of fire ignition on the second shot is.
2.3 Electrical Faults Causing Fire Ignition
Following the Black Saturday bush fires in Victoria on 7th
February 2009 significant investment has been made, and
research conducted, to better understand the mechanisms
and parameters associated with electrical faults causing
ignition of fire. In a study conducted for Energy Safe
Victoria by HRL Technology Pty Ltd [2] found that arc
duration was the dominant variable in the probability of an
electrical fault causing ignition of a fire. Figure 2 below is
reproduced from page 10 of [2].
Figure 2.
Ignition probability against arc duration for 4.2, 50 and 200
amp arcs at 45oC and 10 kph wind speed for hay/straw at 5%
moisture.
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3. Technology to
reduce SWER fire risk
Based upon the above considerations the author proposes
that the best approach to making cost effective reductions
to SWER lines igniting bushfires is to improve the capability
of Automatic Circuit Reclosers (ACR) to reduce ignitions.
This proposal is supported by the exposure draft of the
“Electrical Safety (Bushfire Mitigation) Further Amendment
Regulations” section 7, “Prescribed particulars for bushfire
mitigation plans – major electricity companies” subsection
(hd) that:
The author contends that the above prescription whilst reasserting the well understood function of an ACR fails to
explicitly address a number of functional requirements that
would further enhance capability to reduce bushfire risk on
SWER lines. The following additional performance may be
prescribed in technical specifications, and contribute to the
reduction of bushfire risk:
3.1 Electricity Company ACR Ratings Appropriate for SWER
“details of the processes and procedures
by which the major electricity company will
ensure that, on and from 1 January 2023,
the major electricity company has installed
an Automatic Circuit Recloser in relation to
each SWER line in its supply network;”
Electricity distribution companies are well practiced in
specifying and purchasing ACRs for conventional reliability
and protection improvement programs. The technical
specifications prepared typically provide blanket coverage of
network needs from three phase feeder applications down
to SWER line applications.
Table 1 below compares the specified current ratings
typically found in electricity distribution company ACR
technical specifications to those actually required for SWER
line application.
This over specification of ratings results in higher
manufacturing costs for these products however do not
address specifically a reduction of bushfire risk.
This exposure draft also provides a definition of an
automatic circuit recloser as follows:
Automatic Circuit Recloser means a device in relation to a
SWER line that—
(a)
(b)
(i)
(ii)
(iii)
(iv)
may be remotely controlled; and
is able automatically to interrupt and reclose an
electric circuit by means of a programmed
sequence that involves—
opening and reclosing the electric circuit; and
resetting the electric circuit; and
holding the electric circuit closed; and
permanent interruption of the electric circuit;
PARAMETER
The author proposes that electricity distribution companies
ought to consider a standalone technical specification for
SWER Automatic Circuit Reclosers (ACR) with ratings
appropriate for SWER application, describing features and
performance contributing specifically to the reduction of
bushfire risk.
TYPICAL ACR
SPEC.
PROPOSED
SWER ACR
SPEC.
4
2
Increased dead time has greater impact on
clearing a transient fault than multiple
recloses. Additional recloses increases
electronics cost.
Rated Current
630A
20A
Higher rated currents significantly increase
size and cost of primary conductor path and
therefore overall product size and cost.
Breaking Capacity
630A
20A
Fault Make Capacity (RMS)
12.5kA
1.0kA
Fault Break Capacity (RMS)
12.5kA
1.0kA
Higher ratings increase requirements for
vacuum interrupter and drive mechanism
significantly increasing size and cost of
ACR.
Reclose Sequence
(no. of trips in sequence)
COMMENT
Table 1. ACR Specification Comparison
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3.2 Ultra-fast Fault Clearing Capability
Given the fault levels generally associated with SWER lines
have a correlation with the fire ignition experiments
described in section 0 there is support for a recloser with a
fault clearing time performance of less than 20ms to offer a
significant contribution to the reduction of bushfire risk.
Given devices with clearing times below 20ms exist in the
marketplace today, electricity distribution companies ought
to prescribe technical specifications seeking equipment with
less than 20ms clearing time for SWER recloser applications.
3.3 Function of ACR Remote Control
The proposed exposure draft definition of an Automatic
Circuit Recloser includes point (a) that states the recloser
“may” be remotely controlled. The need for remote control
is understated in terms of contribution to the reduction of
bushfire risk. Strategies contributing to the reduction of
bushfire risk in SWER lines using ACRs require the following
additional functionality to be activated via remote control
(i.e broadcast):
·
The ability to change the number of trips in the
reclosing sequence depending upon the level of
fire risk on a given day.
·
The ability to change the protection trip time
current curve (TCC) and to apply an ultra-fast
protection mode on code red or extreme risk days.
A pragmatic approach to the combination of the appropriate
number of protection trips in a sequence and the
application of a “NORMAL” or a “FAST” time current curve is
shown in table 2.
The remote control capability of the ACR enables the change
from one set of parameters to another, preferably via a
single control command which can be delivered via a SCADA
broadcast.
Electricity distribution companies ought to consider
prescribing into their SWER Automatic Circuit Recloser
technical specifications, that the remote control of the
SWER recloser is capable via a single command to change
both the protection sequence and time current curve in
force based upon the bushfire risk valid on the day.
PARAMETER
NO. OF TRIPS
IN SEQUENCE
2
TCC BY TRIP NO.
COMMENT
NORMAL-NORMAL
TOTAL FIRE BAN DAY
2
FAST-FAST
CODE RED DAY
1
FAST
Optimised for network reliability
consideration.
Uses FAST curve to minimise fire ignition
risk, but allows reclose for network
reliability.
Uses FAST curve i.e less than 20ms to
reduce fire ignition risk from fault and
does not reclose.
NORMAL DAY
Table 2. ACR function according to Fire Risk
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4. Conclusion
This white paper has explored the current recommendations
in relation to the reduction of bushfire risk on SWER
networks. It proposes the use of Automatic Circuit Reclosers
is the cost effective means for reducing bushfire risk on
SWER networks
This white paper describes the current prescriptions for
technical specifications used for the deployment of ACRs on
SWER networks do not prescribe the full benefit these
devices could offer in reducing bushfire risk on SWER
networks.
This white paper supports three (3) proposals for electricity
distribution companies to consider when specifying and
purchasing ACRs for SWER line deployment. The essential
elements of these are:
1.
Due to the low load and fault currents on SWER
lines ensure the ACR ratings in tender documents
ought not to be over specified resulting in higher
cost of equipment.
2.
That an ultra-fast clearing time of less than 20ms
has clear benefit in bushfire mitigation on SWER
lines and that equipment with this capability
should be specified.
3.
That the ability to remotely change via a single
command both reclose sequence and the time
current curve should be specified.
Next generation devices such as the Siemens Fusesaver
fulfil these recommendations and provide a significant
contribution to electricity distribution companies specifying
to reduce bushfire risk at a cost significantly lower than
conventional solutions.
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References
B. Teague, R. McLeod and S. Pascoe, "2009 Victorian
Bushfires Royal Commission – Final Report Summary,"
Government Printer for the State of Victoria, Jul. 2010.
D. Coldham. A. Czerwinski and T Marxsen, "Probability of
Bushfire Ignition from Electric Arc Faults," HRL
Technology Pty Ltd., Melbourne, VIC., Australia, Tech.
Rep. HRL/2010/195, 2011.
T Marxsen A.and Czerwinski, "Effect of Auto-reclose
delay on ignition probability from electric arcs in
powerline faults," HRL Technology Pty Ltd., Melbourne,
VIC., Australia, Tech. Rep. HRL/2013/066, 2013.
Revision Control
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