Download Recloser Application Notes - Association of Municipal Electricity

DRAFT ONLY
Recloser Application Notes
Introduction
Automatic Circuit Reclosers (ACR) have been around in one form or another for over half a century.
Studies of faults on overhead distribution lines have shown that most are transient and can be cleared
without interrupting customer supply. This is especially true of areas that experience lightning activity or
mix power lines with trees.
At its most basic a recloser will detect a fault and then open for a preprogrammed time before closing
automatically. This automatic close is referred to as an auto-reclose and multiple open and close
operations can be utilised to clear transient faults. If the fault is transient and cleared when the line is deenergised the next auto-reclose will restore supply. If the fault is permanent the recloser will eventually
open and not attempt to close until instructed by an operator. This state is referred to as recloser lockout;
manufacturers have standardised on a maximum of four protection trip operations before lockout occurs.
Early reclosers simply detected a level of current, opened after a fixed time, closed and repeated the
sequence a programmable number of times. These devices were graded by number of operations; ie
downstream reclosers had fewer auto-reclose operations than upstream reclosers. This method of
grading subjects all the customers on the affected feeder to outages, but only the first recloser upstream
of the fault locks-out and isolates downstream supply.
The next advance was the introduction of current dependent time to trip. Terminology widely used to
describe current dependent operating speed is IDMT (Inverse Definite Minimum Time) curves. IDMT
application in the first reclosers used electric and hydraulic mechanisms to provide a variable period from
detection of fault current to opening the recloser. This time was inversely proportional to the magnitude of
the fault current and allowed devices in series to be graded on current through use of Current
Transformers (CT) with differing turns ratios. These reclosers also incorporated mechanical means to
provide a time multiplier to the curve, allowing better co-ordination of series devices with differing
operating characteristics.
Grading on current relies on downstream reclosers opening more quickly at a given fault level than
upstream reclosers. Only the recloser closest to the fault operates, upstream supply is not interrupted at
all. Hydraulic operating mechanisms are limited by a time-current characteristic governed by the
operating mechanism and the inaccuracy of the time to trip on any given fault current. This inhibits
accurate grading resulting in compromises having to be made in the accommodation of all fault
conditions.
Subsequent generations of reclosers benefited from the introduction of electronic relays to refine flexibility
and accuracy of time to trip. The advent of microprocessor control has led to the current generation of
reclosers with increasingly sophisticated protection capabilities.
Voltage detection has also become an integral part of distribution network protection as technology
advanced. Control logic employs voltage sensing to augment the current sensing capability of reclosers.
Traditional arrangements use external voltage transformers but reliability is poor due to the difficulty of
retaining impulse and potential rise immunity in a non-integrated system. It also makes for a very crowded
pole top.
The parallel evolution of Supervisory Control and Data Acquisition (SCADA) and its application in
Distribution networks has led to the concept of Intelligent Electronic Devices (IED) being discussed by
Distribution Utilities around the world. The idea evolved from papers and presentations discussing
Distribution System Automation (DSA) schemes and the associated infrastructure requirements.
In the context of a recloser, an IED is characterised by having the data logging and processing power to
be able to generate system information for subsequent analysis. It must also be able to communicate with
a central or distributed decision-maker to facilitate system reconfiguration. Ideally the decision and
reconfiguration is done automatically with the view of minimising customer minutes lost. Investment in
IED’s has become increasingly attractive in the global climate of regulation being experienced by the
Electricity Supply Industry.
Future developments in information technology will see the IED play a huge role in underpinning the
flexible Distribution Networks required to meet market expectation. Without these basic building blocks no
DSA scheme can adapt, as new technology becomes available.
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Nu-lec Pole Mounted Reclosers
Nu-lec equipment has evolved through field experience and intensive R&D effort to provide reliable
operation in the harshest environments.
In excess of 6000 Nu-lec reclosers are in service in a total of 56 countries throughout the world. Our
reclosers are protecting medium voltage Distribution Networks from the Mekong Delta region of Vietnam
to the misty green hills of Ireland. They have been integrated into basic and sophisticated control
schemes from Tarawa, Main Island of the Republic of Kiribati to downtown Rio de Janeiro, Brazil. The
same product is in service in climates ranging from the Australian Outback in summer to the snow of
Tianjin, China in mid-winter.
Our success in these diverse parts of the world is through providing a realisation of the IED concept.
Intelligent protection decisions are made by the Control and Protection Module (CAPM) based on
feedback from the Current Transformers (CTs) and Capacitive Voltage Transformers (CVTs) built into
each recloser. Information is relayed to a control system using the in-built RS232 or V23 FSK
communications. Support for industry standard protocols such as Conitel, DNP3 and IEC-870 allow
seamless integration into DSA and SCADA.
A comprehensive event log provides a detailed record of every protection operation to assist restoration
crews in the fault-finding process. Comprehensive demand history is logged to provide demand-profiling
capabilities and allow Design Engineers to optimise system performance. Duration of outages can be
monitored and the information retrieved for the calculation of Industry standard performance indices
SAIDI and SAIFI.
Integrated design of the controller and switch ensures that the equipment will withstand the extremes of
electrical disturbance generated by fault and storm activity. Extremes in climatic conditions are catered for
in the use of non-corrosive materials and ventilation/heating to suit the operating conditions.
Minimal maintenance is an integral design concept. Only the batteries need to be replaced on a 3 to 5
year cycle, depending on operating conditions.
All the features inherent in the design of Nulec Pole Mounted Reclosers ensure that the logic, control and
communications circuits of the CAPM are able to carry out their prime function – clearing a fault and
restoring customer supply as quickly as possible.
Where do you need a recloser?
Nu-lec reclosers are utilised in over head networks where any of the following apply
1. High lightning activity, exposed OH supply lines
2. Fuses protecting downstream spur lines
3. Large concentration of customers on a spur or junction feeder
4. Radial Feeders
5. Open Ring or meshed network systems
6. Remote site, difficult access
7. Bush fire prone areas, exposed lines
8. Co-generation interchange sites
1. High Lightning activity
An electrical storm can generate extremely large faults with enormous potential for damage. A fast
tripping time reduces equipment risk and danger to public safety. Time between initiation and clearing
of a fault can be achieved from 75ms utilising an instantaneous element.
Use of instantaneous operation can allow fuse saving strategies to be employed to prevent fuses on
an upstream spur or transformer from blowing. This strategy is generally only successful with large
fuses, repeated near - misses can eventually cause smaller fuses to fail under load.
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2. Downstream fuses.
Alternately, a fuse burning strategy can be adopted; this allows a downstream fuse to clear a fault
without the recloser operating. Most successful in rural area with fuse protected three phase spurs.
An instantaneous multiplier with a minimum time can be selected as a modifying element to an IDMT
curve to also provide fast operation to clear high faults generated by lightning activity.
3. Large concentrations of customers
Using reclosers on feeder junctions or spurs allows the smallest possible area to be left without HV
supply. A fault causes the first upstream recloser to operate to isolate the faulted segment. Rearrangement of normally open points together with SCADA capability allows system reconfiguration to
restore supply to the largest number of customers as quickly as possible, reducing customer minutes
lost.
4. Radial Feeders
Long radial feeders require multiple protective devices to provide over-lapping zones of protection.
This is achieved through co-ordination of multiple reclosers in series to minimise the possibility of low
level faults not being ‘seen’ by the nearest upstream device.
5. Open Ring Systems
Open ring or meshed networks can benefit enormously from Nu-lec’s Loop Automation scheme. This
utilises voltage sensing on all six bushings to allow a network to be automatically reconfigured in
response to a fault on any segment. The scheme acts to minimise loss of supply to the shortest
possible network segment while restoring supply to all unaffected segments. The scheme will also
automatically reconfigure the system when normal operating conditions are detected if the AutoRestore feature is used.
Loop Automation requires no communications between devices since the network itself is used as the
communications medium. Once the appropriate recloser has cleared a fault the CAPMs determine
which section is faulted based on Source and Load side bushing status. Reclosers have different
responses depending on whether they are designated a Feeder, Midpoint or Tie recloser and act to
automatically restore supply to all unfaulted segments.
6. Remote Site, difficult access
Remotely controlled reclosers allow fast switching and network re-configuration to minimise customer
minutes lost. Where travelling times are significant this can provide immediate benefits in customer
satisfaction and reduced call out rates.
Nu-lec reclosers can be remotely controlled very cost effectively using our Windows Switchgear
Operating System (WSOS). Telephone landlines, or digital / analog mobile ‘phone networks provide
a medium for cost effective initial remote control where no SCADA capability is available. The call
back features of the CAPM and optional WSOS capability allows pre-programmed messages to be
sent by email, text message or pre-recorded voice message to alert duty officers of a problem.
Dialing the recloser and using DTMF tones to affect a remote close provides limited control. If this is
unsuccessful the operator can upload the full event log by going on-line with WSOS using his laptop
and a modem.
WSOS can also be used over a VHF or UHF radio network.
7. Bush fire prone areas
Bush fires are a great danger in many parts of the world. Studies in Australia have determined that in
order to minimise risk of a fire the primary protection must operate in less than one second.
Nu-lec reclosers are able to be closely co-ordinated to allow the maximum number of protection
devices in series without exceeding this 1-second rule. Some authorities set all their reclosers to only
operate once to lockout during periods of high fire danger, relying on current co-ordination to
correctly isolate the fault. Remote control is then used to quickly restore supply once the fault has
been located and cleared. The recloser that operated to clear the fault does not have to be attended
by field personnel.
Unlike an oil filled recloser, SF6 and solid epoxy reclosers will not create a fire hazard in the unlikely
event of an internal arc fault failure.
8. Co-generation interchange sites
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Co-generation plants can use a recloser at the interchange site. Our Synch Check functions allows
connection of separate supplies once they are synchronised and prevents closing onto
unsynchronised supplies. Under and over frequency tripping allow load to be progressively shed and
restored without operator intervention.
Together in the one package these features provide a vital protection device for co-generation
facilities. Nu-lec remote control capabilities facilitate full automation where available.
Recloser Protection
Reclosers are used to protect power lines, transformers and other Distribution equipment from being
exposed to damaging levels of current. They also detect downed power lines to reduce risk to the public.
This is achieved through monitoring three-phase current and voltage to detect a fault situation.

Phase protection generates a trip operation if the current on any phase exceeds a pre-programmed
limit known as the Phase Trip Current.

The vector sum of the current in each of the three phases is monitored by the CAPM. If the resultant
is in excess of the Earth Trip Current, Earth Fault protection will operate.

In radial systems with a floating neutral, a feeder earth fault generates a flow of earth current towards
the common bus on any unfaulted feeders connected to it. Directional Blocking allows reliable Earth
Fault protection to be implemented by only operating when the direction of the perceived fault current
is away from the bus.

Automatic Protection Group Selection (APGS) determines and activates a pre-programmed
protection group based on direction of Power Flow in an open ring or meshed network.

Loss of Phase (LOP) protection will detect loss of one or two phases and operate to protect
downstream customers with three phase loads.

Under and Over Frequency protection allow progressive load shedding by in-series reclosers without
operator intervention. The restoration facility allows progressive automatic restoration of each
segment when normal supply frequency is maintained.
Grading Series Reclosers on Current
To achieve current co-ordination on series reclosers the operating time of each recloser must be faster
than any upstream device and slower than any downstream device. A safe margin between operating
times of successive devices must be maintained for all fault levels on the segment of network being
protected. For Nu-lec devices in series a 200 ms interval between devices is the minimum safe grading
margin.
Nu-lec reclosers provide Protection Engineers with a high degree of flexibility in catering for alternate
network configurations. Up to 10 protection groups can be stored within the CAPMs non-volatile memory
and each group has independently programmable protection characteristics.
Pre-programmed IDMT curves or definite time to trip can be utilised for Phase and Earth Overcurrent
protection. Provision of curves to IEC and IEEE standards allows close grading with Substation protection
relays. Custom curves provide a means to grade with hydraulically operated reclosers. For those
impossible-to-grade scenarios upto five user configurable curves can be stored in the CAPM. The curves
are constructed using WSOS and allow characteristics to be tailored to the exact requirements of any
application.
The IDMT curves are augmented by a wide range of curve modifying options, these include:
Insert figures from appropriate section of CAPM4 manual

Inrush Multiplier – caters for system inrush currents (eg transformer magnetising current) without
compromising steady state protection.

Instantaneous Multiplier – lightning generated power frequency follow through and ‘bolted’ faults (ie
minimal fault impedance) are both characterised by high fault currents. An instantaneous multiplier
allows very fast tripping to reduce system exposure to damaging levels of fault current.

Threshold Multiplier – can be applied to phase or earth protection to prevent a trip operation before a
pre-programmed threshold is exceeded. A pickup event is logged on detection of current in excess of
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the Trip Current even if the threshold is not exceeded. The logged peak current and duration provide
advance warning of system load encroaching on protection levels.

Maximum time to trip – All phase and earth IDMT curves can be pre-set for maximum time from
pickup to trip.

Additional Time – series reclosers can be easily graded by applying using the same Trip Current,
IDMT curve and time multiplier. This is achieved by applying an Additional time that is longer on each
upstream device.

Minimum Time – each individual curve can have a minimum time to trip applied. This allows
downstream devices with slower instantaneous tripping times to be catered for.
Earth Faults down to 1A can be cleared using Sensitive Earth Fault (SEF) protection to provide a definite
time to trip. The number of SEF operations to lockout can be programmed independently of the number of
Phase and Earth overcurrent operations to lockout. Frequencies above 60 Hz are filtered to prevent
tripping in response to system harmonics.
A Cold Load Multiplier can be applied to automatically cater for Loss of Load Diversity during a prolonged
outage. The CAPM automatically adjusts its Cold Load Multiplier (CLM) depending on the duration of the
outage. Once supply is restored the multiplier is reduced at a pre-programmed rate. No operator invention
is required so even downstream reclosers will automatically cater for there own loss of diversity inrush
levels.
Sequence Advance capability allows the CAPM to be programmed to increment its operation counter if a
fault is ‘seen’ to be cleared by downstream protection. This prevents a slow 2 nd operation by a
downstream protection element from causing an operation on the fast 1 st operation of the upstream
recloser.
Grading Reclosers on Voltage
Loss of Phase protection will detect a single downed conductor in a three-phase system. Series devices
are graded with upstream operation faster than downstream operation so the recloser nearest to the fault
locks out.
Under and Over frequency load shedding requires series devices to progressively reduce load until the
system stabilises and correct frequency is maintained. Upstream series devices have a longer operating
time than downstream devices. The restoration facility provides the ability to automatically restore load,
one segment at a time in a pre-programmed sequence. Upstream devices have shorter restore times
than downstream devices.
Tripping Sequence
Feeders with different electrical characteristics required different protection strategies. Distribution Utilities
worldwide have developed there own strategies to suit there particular system conditions. Independent
configuration of the time to trip for each protection operation allows the utilities preferred protection
strategy to be implemented.
Studies at an Australian Electricity Supply Utility, Great Southern Energy, have identified the main causes
of transient faults in their rural networks and this provides a useful example to illustrate selection of an
optimum tripping sequence.
The main sources of transient faults were determined to be:

lightning induced power follow through

Birds flying into power lines

Trees or branches falling on lines
Great Southern Energy has developed an optimum tripping sequence for protecting their long rural
feeders based on the probable causes of any given fault.
1. The fault is first assumed to be lightning related. A fast IDMT curve with a small time multiplier (eg
0.05x) and an instantaneous element is used to ensure short exposure to the initial fault current.
2. This operation is followed by a very short reclose time (eg 0.5 sec) to limit the effects on electronic
equipment such as PC’s, VCR’s, microwave ovens, etc.
3. If the fault is not cleared on the subsequent auto-reclose a slow IDMT curve with a long operating
time allows tree or animal induced faults on rural spur lines to be cleared by upstream fuses.
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4. The reclose time that follows is long (eg 5 sec) to allow the bird or branch to fall clear of the line if it is
on the feeder or did not cause fuse operation.
5. If the fault is still present after the auto-reclose it is assumed to be permanent and a fast IDMT curve
minimises exposure to fault currents.
6. A Cold Load multiplier is utilised to provide protection from increased levels of load caused by loss of
diversity over time.
7. Sequence Advance is used to allow grading of series reclosers with a fast first trip operation and a
slow second trip operation.
Insert graphical representation of fault levels and trip times from the IEEE presentation – will
require an acknowledgement.
The financial cost of faults
The same study by Great Southern Energy also determined contributing factors in the costs associated
with line faults.
Factors that were determined to influence costs are:

Line Length

Faults per km – temporary and permanent

Outages per km

Restoration Times
Of these it is the restoration time and outages per km that can be reduced through effective use of
reclosers.
Remotely operated reclosers allow faults to be quickly detected, and the fault information to be analysed.
This reduces the time involved in generating switching sheets and dispatching line crews. Distances in
rural Australia can be great and remotely controlled reclosers significantly reduce patrol and switching
times. Coupled with the fault information provided by the event log or transmitted back to the control
center the improvements in restoration time are substantial.
The study determined that for a representative rural feeder, initial outlay of $100,000 on recloser
protection would save around $120,000 annually. It was concluded that the reclosers would eventually
add $1,000,000 to shareholder value. This was based on existing practice of extensive use of fuses for
rural feeder protection and the assumption that 80% of all faults are temporary and would be cleared by
recloser operation.
Insert summary table of cost benefit analysis from the IEEE presentation – will require an
acknowledgement.
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