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.
Sand 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.
1. Seat of the arc fault. Courtesy: Austin Energy
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 325-A RMS 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 A RMS), 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 (ICE) shop removed the damaged segment but didn’t reinstall 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.
2. Cable three weeks later. Courtesy: Austin Energy
Eventually, demand for the equipment 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 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.
3. Cable damage as found. Courtesy: Austin Energy
The jackets on the cables had a discoloration (Figure 3), which suggested that the cables had previously experienced either partial or corona discharge and required immediate replacement (Figure 4). The unit outage was extended by two weeks to accommodate this.
4. Cable damage from below. Courtesy: Austin Energy
There were 12 fittings in the top plate of the breaker enclosure, each containing four cables. That is four cables per phase, three phases, a run between generator and breaker plus a run from breaker to transformer, and this arrangement exists for each of the two units. Looking from underneath the fittings, the cable damage was even more severe than first thought.
Repair was not that straightforward. The first hurdle was in getting the replacement cable itself, which was 750 MCM (1,000s of circular mils) and estimated at about 5,000 feet of length. Austin Energy does not keep cable of this gauge on hand. The only manufacturer that responded to a request for quote said that the lead time would be 40 to 44 weeks. Fortunately, a local distributor did have this size and quantity available for immediate delivery.
The next hurdle was that the cable trays for these cables had braces every few feet to keep the cables evenly spaced through the tray. This arrangement is called a cable bus. In this case, the original braces were made of wood, but several of them had been damaged by weather exposure over time. The manufacturer of the braces was still in business and able to supply new ones made of high-density polyethylene with a short turnaround. After that, Austin Energy’s network construction department came to the site to cut, terminate, and install all the new cable.
Across the industry, cables are frequently not considered after they are installed. They don’t have moving parts or pieces that wear out. They carry currents back and forth, and they just function. Unless cables are mechanically damaged during some sort of work, they are generally thought of as permanent.
This set of experiences has shown that, as with everything else, time tends to wear down even something as simple as a cable. Austin Energy is now planning inspections and testing for the higher-powered cables and will be vigilant for future defects.
—Matt Kuffler, PE is the plant engineer for Sand Hill Energy Center with Austin Energy.