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This presentation discusses how to improve personnel safety with high-impedance fault
(HIF) protection.
An HIF is a type of fault on medium-voltage distribution systems. These faults are
typically caused by tree branches touching the conductors, dirty insulators, and downed
conductors.
These faults have small fault currents that make them difficult to detect with traditional
overcurrent elements. Due to the small fault currents, these faults do not cause thermal
damage to power equipment, and they do not interrupt normal distribution operations.
Nevertheless, if HIFs from downed conductors are not corrected in a timely manner,
they pose a serious public hazard.
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SEL has tools and information available to help understand and detect an HIF,
specifically Arc Sense™ technology (AST).
When to use AST depends on the standing unbalance from loads.
Not all HIF events and downed conductors can be detected; therefore, it is important to
understand the detection performance for HIFs.
Along the way, this presentation discusses AST detection algorithms and a more
accurate way to describe the dependability of HIF detection.
Last, we present some HIF test results and the experiences of a utility applying AST.
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The application of AST closely relates to the type of distribution system in place. AST
is not always the best solution for detecting HIFs for every system.
For the ungrounded distribution system shown on this slide, all loads are connected
phase to phase. The residual current during normal system operations (the standing
unbalance) comes from the asymmetries of three-phase equipment and feeder
constructions and is typically quite small. A simple, sensitive ground overcurrent
element set above the standing unbalance can reliably detect faults with high fault
resistance. The SEL-351 Protection System has a 0.2-ampere sensitive input (IN) that
can detect 5 milliamperes secondary overcurrent.
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The picture on this slide shows a staged test on a 4.6 kV ungrounded distribution
network with a downed conductor on a concrete street. The fault current is anywhere
from 0.2 to 1.3 amperes primary (approximately 2 kilohms of fault resistance). A 10:1
core-balance (toroidal or donut) current transformer (CT) provides 20 to
130 milliamperes secondary, well above the standing unbalance for this system of under
1 milliampere. The sensitive residual overcurrent element of the SEL-351 is well-suited
to detect these faults on ungrounded systems.
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For the multigrounded distribution system shown on the slide, the single-phase loads
between a phase and the multigrounded neutral wire cause a large system standing
unbalance. The utility system operation engineers balance the single-phase loads among
three phases and achieve a small unbalance when all loads are in service as planned.
However, the worst possible system unbalance occurs when a long single-phase lateral
is out of service. This scenario makes the traditional overcurrent element ineffective in
detecting the HIF in multigrounded systems and must be considered when setting the
ground overcurrent element.
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On a 12.5 kV multigrounded distribution system, a downed conductor on a concrete test
pad caused a fault with a fault current of up to 25 amperes primary (approximately
300 ohms of fault resistance). This 25-ampere fault current is easily under the worst
system unbalance when a major single-phase lateral is out of service. Traditional ground
overcurrent elements also need to coordinate with downstream protection devices, cold
load pickup, and transformer inrushes, further reducing the capability of HIF detection.
A special algorithm such as AST has to be used to detect HIF on multigrounded
systems.
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Unigrounded distribution systems have only one grounding point at the substation
transformer neutral point. Large loads are typically connected between phases. Without
load-caused standing residual unbalance, HIFs can be easily detected using a sensitive
residual overcurrent element, such as in the ungrounded systems. However, some
utilities supply remote small loads on a single phase with the earth as a return path for
the cost consideration. These single-phase loads can produce residual current as large as
10 amperes and, therefore, reduce the capability of a residual overcurrent element in
detecting HIFs. AST can be used in this situation to detect additional HIFs.
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The graph on this slide shows the fault current, in primary amperes, produced by a
downed conductor on different types of ground surfaces in multigrounded distribution
systems.
When a conductor falls on a surface such as dry sand and asphalt, it generates no fault
currents. There is no signal back at the substation that can be used to detect such faults.
The substation-based detection device cannot detect all downed conductors and HIFs.
Note: If the downed conductor associates with considerable amounts of load loss, it is
possible to detect a broken conductor fault using a negative-sequence overcurrent
element.
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Some HIF detection device manufacturers like to use a single number to represent the
HIF detection performance. One manufacturer uses 80 percent for their HIF detection
function.
Utility customers typically want to get the detection rate as a simple answer.
Unfortunately, the successful detection of HIFs depends on many factors. The foremost
factor is the type of ground surface that a conductor falls on. As shown in the graph on
the previous slide, substation-based devices cannot detect downed conductors on
asphalt. The detection rate for this type of HIF is zero.
Even for the same ground surface, the closer an HIF is to the substation, the easier it is
to detect because the HIF signature passes fewer power apparatus (like capacitors) and
becomes less attenuated. Compared with a dry season, a fault during the rainy season
has an increased fault current for the same ground surface at the same fault location and,
therefore, is easier to detect.
It is not accurate to describe the HIF detection performance with a single number.
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SEL uses a more scientific method to describe AST performance. We use a probability
to give customers a clear picture of what to expect from the SEL AST algorithm for
different types of ground surfaces.
This slide shows the likelihood of detection for downed conductor faults on different
ground surfaces, from the highest likelihood for earth to the lowest for wet sand.
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There are several technologies available on the market today to assist utilities in
detecting HIF and downed conductors.
Because high-impedance fault current magnitudes are below the system standing
unbalance, all HIF detection technologies utilize current quantities other its magnitude.
These quantities include even-, odd-, and interharmonics of phase currents.
This slide shows two different detection algorithms. These two algorithms complement
each other. The first algorithm uses a sum of the difference current (SDI) that represents
the total interharmonic content of a phase current. The algorithm derives the detection
threshold through an adaptive tuning process that automatically tunes to the background
or ambient noise level of a feeder. The second detection algorithm is a statistical
method. A special filter screens out the odd harmonic content of a phase current.
In an initial tuning process, the algorithm learns the statistics of the odd harmonic for
normal system operations and derives the detection threshold. The algorithm constantly
monitors and compares the statistics of the odd harmonic with those from the normal
situation. It declares a fault when there is a large statistical difference.
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Other HIF detection technologies include the expert system and neural networks and
wavelet, as shown on this slide.
The expert system uses a set of weighted factors to derive final decisions based on
several detection algorithms such as energy and randomness.
A neural network uses layers of interconnected nodes that are similar to human brain
cells (neurons). The interconnection factors are obtained through training the network
on sets of fault and unfaulted data.
Wavelet uses a special set of filters to decompose input signals into different frequency
bands. The results in higher frequency bands are effective to detect discontinuities of the
inputs, one of the main signatures of arcing faults.
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The slide shows an HIF event with a downed conductor on wet earth. The fault
generated an approximately 13-ampere fault current, as shown in the upper plot on this
slide. The middle plot is the current measurement back in the substation. Note that the
noise on the envelope of the substation current is not related to the HIF, because the
noise is also there without the fault.
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The graphs on this slide show one type of noisy load that impacts HIF detection. The
feeder loads include rail transportation, commercial and residential.
The current waveform on the top clearly shows the train acceleration, running, and
deceleration processes. The train load generates many harmonics and a lot of noise that
HIF detection algorithms are using (shown in the bottom plot).
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We have done many HIF field tests with different utilities to validate HIF detection
algorithms. This slide shows results from one of the HIF field tests. The test site is about
14 miles away from the substation where the relays under test are located. Eighteen total
tests are carried out on different ground surface materials. Relay 1 detected four of the
faults and Relay 2 detected three of the faults. The number difference may look small,
but the difference is large in terms of percentage. Both relays use the harmonics of the
phase currents as the HIF detection algorithms. However, Relay 1 uses more traditional
logic in making the detection decision, and Relay 2 uses one type of artificial
intelligence.
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This slide shows the test results for downed conductor faults staged 2 miles from the
substation.
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The table on this slide shows part of an HIF alarm log from a utility.
The first HIF alarm was associated with a feeder that burned down between poles
MA-6 and MA-7. (The circuit had to be manually opened after the circuit breaker
reclosed twice.)
In the second event, a service person found that an approximately 8-inch-round tree was
laying on the conductor while it was energized.
The third event is a case of two downed phase conductors, one laying on the ground
burning when the service person arrived. The fire lasted at least 20 minutes.
In the last event, a tree came down and broke a phase and the neutral wires. The wires
were not on the ground but were open on the source side. The HIF most likely occurred
when the tree was coming through the wires.
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