POWER September 2023 - 26

ELECTRICAL MAINTENANCE
Austin Energy Power Production
Cable Defects
Between 2022 and 2023, Sand Hill Energy Center encountered two serious defects
in cabling, one of which resulted in an arc fault. These were surprises and have led
the team to consider inspections and testing that had not been considered previously.
Fortunately, no one was injured, but this was two near-misses too many.
Matt Kuffler, PE
S
and Hill Energy Center is an Austin
Energy power production facility
in Del Valle, Texas. The center
is an approximately 600-MW plant, with
half its capacity in a combined cycle unit
and the other half in six LM6000 gas
turbines. Of the latter six, four were part
of the original plant going back to 2000,
while the other two were added in 2010.
The First Problem
In August 2022, plant operations received
an unexpected set of alarms, indicating
the operation of protective relays. Upon
inspection, a 4,160-V feeder cable to a remote
building exhibited signs of an arcing
fault. There were significant indications
inside the cabinet that an arc event had
occurred (Figure 1). The most prominent
sign was a small opening in one cable
with partially slagged copper visible.
The cables for this feeder pass down
into a manhole and through conduit to the
remote building. The manhole was found
to be full of water, higher than the level
of the conduits. It was not customary to
pump the water out prior to this event,
but operations personnel pumped out the
manhole after the event. It is common
knowledge across the electrical industry
that below-grade manholes, duct banks,
and conduits frequently fill up with water;
this is not normally a problem.
The SEL 751 feeder protection relay
associated with this feeder is equipped
with optical fiber to detect light in the
cabinet. When the relay detects both
light and an overcurrent condition, it is
considered an arc flash event. The relay
will then act to isolate the upstream auxiliary
transformer to clear this fault. The
trip equation in the relay contains terms
for light detection (TOL) and high-speed
overcurrent detection (50PAF).
Examination of the event files from
the relay showed that there was a 325ARMS
current
with the presence of light
for an unknown amount of time prior to
the event. This suggests that there was
a lower-energy discharge occurring for
an unknown amount of time before the
event. At some point, the accumulated
damage from this discharge allowed a
conductive path for a much greater arc
(11,000 ARMS
), which was sufficient to trip
the feeder protection. Once reaching this
trip state, the arc cleared within two line
cycles or 33.3 milliseconds.
There was enough slack in the cable left
in the manhole to be able to remove three
feet or so for a new termination. Technicians
from the instrumentation, controls,
and electrical
damaged segment but didn't reinstall
(ICE) shop removed the
it.
The cable sat in that state for about three
weeks while the team considered options.
Throughout this idle time, the cable
continued to weep a small amount of
water from around the conductor. Additionally,
a white, chalky substance
formed on the cable, running most of the
way to the ground (Figure 2). It was also
present on the other phase cables in the
cabinet even before this event. This substance
has been identified by the cable
manufacturer as the result of a hydrolyzing
reaction of the oil sealant applied to
the cable jacket.
Eventually, demand for the equipment
1. Seat of the arc fault. Courtesy: Austin
Energy
26
fed by this circuit reached a point where
the feeder needed to be put back into
service. ICE technicians took insulation
resistance measurements of all three
phase cables while they were isolated
www.powermag.com
2. Cable three weeks later. Courtesy: Austin
Energy
from the equipment, and again while
connected to the downstream breakers.
Since there was no baseline to compare
to, they also tested the next cubicle over
in the same way, because it has an identical
load and service life. With the readings
being similar, they re-terminated
the cable, and the equipment went back
into service without incident. At present,
Austin Energy is defining when and how
to replace these cables, as well as how
to keep the manhole pumped dry.
The Second Problem
In March 2023, LM6000 Units 6 and 7
were in a routine spring outage with unusual
inspection work taking place on the
conductor ties between the generator,
breaker, and step-up transformer. With this
extra inspection work, one of the ICE technicians
spotted damage to the jackets of
the generator cables where they entered
the breaker cabinet inside the breaker
house. These cables passed through fittings
installed in an aluminum top plate.
This plate is not present in the original
four LM6000 installations (Units 1-4). The
cables pass through the fittings and then
immediately bend to reach the stabs of the
breaker, stressing the cables as they pass
over the edges of the fittings.
The jackets on the cables had a discoloration
(Figure 3), which suggested that
the cables had previously experienced
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POWER September 2023

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