Sniffing out an intermittent high-voltage fault

Application Update: Troubleshoot a high-voltage electrical fault and see recommended solutions.

02/24/2014


Figure 1. Sketch shows the distribution line in need of troubleshooting. Courtesy: Fischer Technical ServicesDetective work found the root cause for a high-voltage electrical fault after repeated breaker trips. Here's how.

Recently, I received a late evening call from the controls engineer at a local chemical plant. He was seeking help troubleshooting a high-voltage electrical problem. Blown fuses on one of its 12.5 kV distribution feeders had shut down part of the plant. Unfortunately, it happened around noon on two days in a row. Power had been restored without finding any problems on both days. Pressure was mounting to find and correct the problem before noon the following day. 

Outages, restarts

During the first outage, 100 amp high-voltage fuses were replaced, the line was re-energized, and all equipment restarted normally.

During the second outage, fuses were replaced, but the 480 V main circuit breaker on one of the two unit substations served by the feeder had to be reset before all the equipment would restart. The plant electrician resetting the breaker reported smelling an unusual odor at the substation. He discounted the observation, however, since it was not suggestive of overheated or burned electrical insulation.

We made plans to start a forensic investigation early the next morning. My initial theory was that there had been flashovers triggered by high-voltage spikes on the distribution system. Since both faults had happened at approximately the same time of day, I suspected transients might be coming from a timer-controlled power-factor-correction capacitor. However, the plant and electrical utility company both reported no capacitor banks or major loads had been switched at the time of either outage.

Arc flash locations

Disregarding the "trigger," we were certain the faults had exceeded the 100 amp current rating of the fuses. The faults had obviously been cleared each time the fuses opened. I reasoned that arcing within the interlocked-armor-cable distribution wiring would have left enough carbonized residue to prevent the system from being re-energized. That assumption narrowed the logical places to look for evidence of arcing. There were only four places where arc flashes could have taken place in open air: the fuse compartment, the junction box where the interlocked-armor-cable was split to feed the two substations, and the high-voltage termination compartments of the two transformer substations. See Figure 1.

Due to the lost production and operational hardships caused by the previous power outages, a short feeder outage to look for damage was easily scheduled.

Telltale sign

As the cover was being removed from one substation's high-voltage compartment, I noticed an extremely strong odor of ozone escaping. This was after the distribution line had been down for 10-15 minutes. I immediately associated the ozone with the electrician's report of a strange odor following one of the outages. Ozone is a common product of electrical discharges. Before the cover was fully removed, I pronounced we had found our culprit. As soon as it was off, we could see where current from the center phase conductor had been tracking over a dirty insulator and arcing to the metal cabinet. See Figures 2 and 3.

Figure 2. The high-voltage compartment with 12,470 V bus bars is shown. Courtesy: Fischer Technical ServicesFigure 3. This closer view shows ground fault evidence at the center phase insulator. Courtesy: Fischer Technical Services

Phase-to-phase arcing had also left bright areas of eroded copper on both of the outer buses in addition to the bus bar above the site of the ground fault. See Figure 4.

Arcing fault evidence

All the evidence suggested this was the arc location and a classic example of what one might expect to find in the aftermath of an arcing fault. The heat of arcing current between the center phase and ground generated an expanding cloud of hot, ionized gasses. Being lighter than the surrounding air, the gasses rose to the upper levels of the high-voltage compartment. There, they accumulated around the uninsulated bus bars, lowering the dielectric strength of the air between the energized conductors. The ionized gas concentrations climbed to levels that facilitated phase-to-phase flashovers.

Figure 4. Damage caused by phase-to-phase arcing is evident in this closer view. Courtesy: Fischer Technical ServicesWorthy of note is that this particular high-voltage distribution system uses a neutral grounding resistor. An important function of these resistors is to limit the damage done by ground faults. But, in this case, the resistance could have held ground fault currents below the trip setting of the upstream circuit breaker for extended periods of time. That could have contributed to longer-term, low-level arcing from the center phase to ground.

Root cause, recommended actions

Root cause of the outages: Dirty high-voltage insulators

Recommended actions:

  1. Clean all the insulators before restoring power.
  2. Replace the damaged insulator.
  3. Increase the frequency of inspection and cleaning.
  4. Train plant electricians to recognize the odor of ozone.

 

- Robert Fischer is an electrical, instrumentation, and reliability consultant with Fischer Technical Services. Edited by Mark T. Hoske, content manager, Control Engineering, mhoske(at)cfemedia.com.

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See also: Arc Flash University webcasts are on-demand here and links at the bottom of this posting.

Key concepts

  • Troubleshooting high-voltage electrical systems.
  • Phase-to-phase arcing causes damage.
  • Root cause of the outages: Dirty high-voltage insulators.

Consider this

If more electrical maintenance could avoid arc flash or downtime, would there be more budget available?

More about the author

Robert Fischer is a physicist and electrical engineer with over 40 years' experience in automated manufacturing, chemical plants, and refineries.