A common approach to test cable and determine insulation integrity is to use a Hi-pot test.
In a hi-pot test, a DC voltage is applied for 5 to 15 min. IEEE-400
specifies that the hi-pot voltage for a 15-kV class cable is 56 kV for an
acceptance test and 46 kV for a maintenance test
(ANSI/IEEE Std. 400-1980). Other industry standard tests are given in
(AEIC CS5-94, 1994; AEIC CS6-96, 1996; ICEA S-66-524, 1988). High-pot
testing is a brute-force test; imminent failures are detected, but the
amount of deterioration due to aging is not quantified (go/no-go test).
The DC test is controversial
– some evidence has shown that hi-pot testing may damage XLPE cable
(Mercier and Ticker, 1998). EPRI work has shown that dc testing
accelerates treeing (EPRI TR-101245, 1993; EPRI TR-101245-V2, 1995).
For Hi-pot testing of 15-kV, 100% insulation (175-mil, 4.445-mm) XLPE cable, EPRI recommended:
- Do not do testing at 40 kV (228 V/mil) on cables that are aged (especially those that failed once in service and then are spliced). Above 300 V/mil, deterioration was predominant.
- New cable can be tested at the factory at 70 kV. No effect on cable life was observed for testing of new cable.
- New cable can be tested at 55 kV in the field prior to energization if aged cable has not been spliced in.
- Testing at lower dc voltages (such as 200 V/mil) will not pick out bad sections of cable.
Another option for testing cable integrity: ac testing does not
degrade solid dielectric insulation (or at least degrades it more
slowly). The use of very low frequency AC testing (at about 0.1 Hz) may
cause less damage to aged cable than DC testing (Eager et al., 1997)
(but utilities have reported that it is not totally benign, and ac
testing has not gained widespread usage).
The low frequency has the advantage that the equipment is much smaller than 60-Hz AC testing equipment.
Fault Location
Utilities use a variety of tools and techniques to locate underground faults. Several are described in the next few paragraphs [see also EPRI TR-105502 (1995)].
Divide and conquer
On
a radial tap where the fuse has blown, crews narrow down the faulted
section by opening the cable at locations. Crews start by opening the
cable near the center, then they replace the fuse. If the fuse blows,
the fault is upstream; if it doesn’t blow, the fault is downstream.
Crews
then open the cable near the center of the remaining portion and
continue bisecting the circuit at appropriate sectionalizing points
(usually padmounted transformers). Of course, each time the cable
faults, more dam-age is done at the fault location, and the rest of the
system has the stress of carrying the fault currents. Using
current-limiting fuses reduces the fault-current stress but increases
the cost.
Fault indicators
Faulted circuit indicators (FCIs)
are small devices clamped around a cable that measure current and
signal the passage of fault current. Normally, these are applied at
padmounted transformers. Faulted circuit indicators do not pinpoint the
fault; they identify the fault to a cable section.
After identifying the failed section, crews must use another method
such as the thumper to precisely identify the fault. If the entire
section is in conduit, crews don’t need to pinpoint the location; they
can just pull the cable and replace it (or repair it if the faulted
portion is visible from the outside). Cables in conduit require less
precise fault location; a crew only needs to identify the fault to a
given conduit section.
Utilities’ main justification for faulted
circuit indicators is reducing the length of customer interruptions.
Faulted circuit indicators can significantly decrease the fault-finding
stage relative to the divide-and-conquer method. Models that make an
audible noise or have an external indicator decrease the time needed to
open cabinets. Utilities use most fault indicators on URD loops. With
one fault indicator per transformer (see Figure 1),
a crew can identify the failed section and immediately reconfigure the
loop to restore power to all customers. The crew can then proceed to
pinpoint the fault and repair it (or even delay the repair for a more
convenient time).
For larger residential subdivisions or for
circuits through commercial areas, location is more complicated. In
addition to trans-formers, fault indicators should be placed at each
sectionalizing or junction box. On three-phase circuits, either a
three-phase fault indicator or three single-phase indicators are
available; single-phase indicators identify the faulted phase (a
significant advantage). Other useful locations for fault indicators are
on either end of cable sections of overhead circuits, which are common
at river crossings or under major highways. These sections are not
fused, but fault indicators will show patrolling crews whether the cable
section has failed.
Fault indicators may be reset in a variety of
ways. On manual reset units, crews must reset the devices once they
trip. These units are less likely to reliably indicate faults.
Self-resetting devices are more likely to be accurate as they
automatically reset based on current, voltage, or time. Current-reset is
most common; after tripping, if the unit senses current above a
threshold, it resets [standard values are 3, 1.5, and 0.1 A (NRECA RER
Project 90-8, 1993)]. With current reset, the minimum circuit load at
that point must be above the threshold, or the unit will never reset. On
URD loops, when applying current-reset indicators, consider that the
open point might change.
This changes the current that the fault
indicator sees. Again, make sure the circuit load is enough to reset the
fault indicator. Voltage reset models pro-vide a voltage sensor; when
the voltage exceeds some value (the voltage sensor senses at secondary
voltage or at an elbow’s capacitive test point). Time-reset units simply
reset after a given length of time. Fault indicators should only
operate for faults – not for load, not for inrush, not for lightning,
and not for backfeed currents. False readings can send crews on wild
chases looking for faults. Reclose operations also cause loads and
transformers to draw inrush, which can falsely trip a fault indicator.
An inrush restraint feature disables tripping for up to one second
following energization. On single-phase taps, inrush restraint is really
only needed for manually-reset fault indicators (the faulted phase with
the blown fuse will not have inrush that affects downstream fault
indicators). Faults in adjacent cables can also falsely trip indicators;
the magnetic fields couple into the pickup coil. Shielding can help
prevent this.
Several scenarios cause backfeed that can trip fault
indicators. Downstream of a fault, the stored charge in the cable will
rush into the fault, possibly tripping fault indicators.
McNulty
(1994) reported that 2000 ft of 15-kV cable created an oscillatory
current transient that peaked at 100 A and decayed in 0.15 msec.
Nearby capacitor banks on the overhead system can make outrush worse.
Motors and other rotating equipment can also backfeed faults. To avoid
false trips, use a high set point. Equipment with filtering that reduces
the indicator’s sensitivity to transient currents also helps, but too
much filtering may leave the faulted-circuit indicator unable to detect
faults cleared rapidly by current-limiting fuses.
Self-resetting
fault indicators can also falsely reset. Backfeed currents and voltages
can reset fault indicators. On a three-phase circuit with one phase
tripped, the faulted phase can backfeed through three-phase transformer
connections, providing enough current or enough voltage to reset
faulted-circuit indicators. On single-phase circuits, these are not a
problem. In general, single-phase application is much easier; we do not
have backfeed problems or problems with indicators tripping from faults
on nearby cables.
For single-phase application guidelines, see
(IEEE Std 1216-2000). Fault indicators may have a threshold-type trip
characteristic like an instantaneous relay (any current above the set
point trips the flag), or they may have a time-overcurrent characteristic
which trips faster for higher currents. Those units with
time-overcurrent characteristics should be coor-dinated with minimum
clearing curves of current-limiting fuses to ensure that they operate.
Another type of fault indicator uses an adaptive setting that trips
based on a sudden increase in current followed by a loss of current.
Set
the trip level on fault indicators to less than 50% of the available
fault current or 500 A, whichever is less (IEEE P1610/D03, 2002). This
trip thresh-old should be at least two to three times the load on the
circuit to minimize false indications. These two conditions will almost
never conflict, only at the end of a very long feeder (low fault
currents) on a cable that is heavily loaded.
Normally, fault
indicators are fixed equipment, but they can be used for targeted fault
location. When crews arrive at a faulted and isolated section, they first
apply fault indicators between sections (normally at padmounted
transformers). Crews reenergize the failed portion and then check the
fault indicators to identify the faulted section. Only one extra fault
is applied to the circuit, not multiple faults as with the divide and
conquer method.
Section testing — Crews isolate a section of
cable and apply a dc hipot voltage. If the cable holds the hi-pot
voltage, crews proceed to the next section and repeat until finding a
cable that cannot hold the hi-pot voltage. Because the voltage is DC,
the cable must be isolated from the transformer.
In a faster
variation of this, high-voltage sticks are available that use the AC
line voltage to apply a dc voltage to the isolated cable section.
Thumper – The thumper applies a pulsed dc voltage to the cable. As its
name implies, at the fault the thumper discharges sound like a thumping
noise as the gap at the failure point repeatedly sparks over. The
thumper charges a capacitor and uses a triggered gap to discharge the
capacitor’s charge into the cable. Crews can find the fault by listening
for the thumping noise. Acoustic enhancement devices can help crews
locate weak thumping noises; antennas that pick up the radio-frequency
interference from the arc discharge also help pinpoint the fault.
Thumpers are good for finding the exact fault location so that crews can
start digging. On a 15-kV class system, utilities typically thump with
voltages from 10 to 15 kV, but utilities some-times use voltages to 25
kV.
While pulsed discharges are thought to be less damaging to
cable than a steady dc voltage, utilities have concern that thumping can
damage the unfailed sections of cable. When a thumper pulse breaks down
the cable, the incoming surge shoots past the fault. When it reaches
the open point, the voltage doubles, then the voltage pulse bounces back
and forth between the open point and the fault, switching from +2 to
–2E (where E is the thumper pulse voltage).
In tests, EPRI
research found that thumping can reduce the life of aged cable (EPRI
EL-6902, 1990; EPRI TR-108405-V1, 1997; Hartlein et al., 1989; Hartlein
et al., 1994). The thumping discharges at the failure point can also
increase the damage at the fault point. Most utilities try to limit the
voltage or discharge energy, and a few don’t use a thumper for fear of
additional damage to cables and components (Tyner, 1998). A few
utilities also disconnect transformers from the system during thumping
to protect the transformer and prevent surges from propagating through
the transformer (these surges should be small). If the fault has no gap,
and if the fault is a solid short circuit, then no arc forms, and the
thumper will not create its characteristic thump (fortunately, solid
short circuits are rare in cable faults). Some crews keep thumping in an
effort to burn the short circuit apart enough to start arcing.
With
cable in conduit, the thumping may be louder near the conduit ends than
at the fault location. Generally, crews should start with the voltage
low and increase as needed. A DC hi-pot voltage can help determine how
much voltage the thumper needs.
Radar
Also called Time-domain reflectometry (TDR),
a radar set injects a very short-duration current pulse into the cable.
At discontinuities, a por-tion of the pulse will reflect back to the
set; knowing the velocity of wave propagation along cable gives us an
estimate of the distance to the fault.
Depending on the test set
and settings, radar pulses can be from 5 ns to 5 µs wide. Narrower
pulses give higher resolution, so users can better differ-entiate
between faults and reflections from splices and other discontinuities (Banker et al., 1994).
Radar does not give pinpoint accuracy; its main use is to narrow the
fault to a certain section. Then, crews can use a thumper or other
pinpoint tech-nique to find the failure. Taking a radar pulse from either
end of a cable and averaging the results can lead to an improved
estimate of the location. Radar location on circuits with taps can be
complicated, especially those with multiple taps; the pulse will reflect
off the taps, and the reflection from the actual fault will be less than
it otherwise would be.
Technology has been developed to use above-ground antennas to sense and pinpoint faults based on the radar signals.
Radar and thumper
After a fuse or other circuit
interrupter clears a fault in a cable, the area around the fault point
recovers some insulation strength. Checking the cable with an ohm meter
would show an open circuit. Likewise, the radar pulse passes right by
the fault, so the radar set alone cannot detect the fault. Using radar
with a thumper solves this problem. A thumper pulse breaks down the gap,
and the radar superimposes a pulse that reflects off the fault arc. The
risetime of the thumper waveshape is on the order of a few microseconds;
the radar pulse total width may be less than 0.05 µsec.
Another
less attractive approach is to use a thumper to continually burn the
cable until the fault resistance becomes low enough to get a reading on a
radar set (this is less attractive because it subjects the cable to
many more thumps, especially if crews use high voltages).
Boucher
(1991) reported that fault indicators were the most popular fault
locating approach, but most utilities use a variety of techniques (see Figure 2). Depending on the type of circuit, the circuit layout, and the equipment available, different approaches are sometimes better.
When applying test voltages to cables, crews must be mindful that cables
can hold significant charge. Cables have significant capacitance, and
cables can maintain charge for days.
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