DFA Case Studies
Event from week commencing March 28th 2016: USA. A substation circuit breaker tripped once and reclosed successfully, during a storm. SCADA told the utility this, but told them nothing more. This was a routine fault that ordinarily would not be investigated. From DFA, however, it was clear that the fault resulted from the catastrophic failure of an MOV-type arrester. When these fail, the body of the arrester may blow apart physically. In such scenarios it might be that part of the arrester body remains connected to an energized phase conductor and dangles in the air, another common scenario being that the arrestor blows apart and can eject hot porcelain and internal material posing a fire and safety risk in either case. Based upon knowing from DFA that this was an arrester failure, the utility investigated and indeed found the failed arrester which had suffered that latter failure mode on this occasion. In addition, DFA supplied a fault current level, which the utility put the DFA-generated fault current into their single line model to narrow the search to a small area. (A relay might have given them a fault current level, too, but this point is moot if they do not have a compelling reason to investigate the event.) Therefore the crew was dispatched knowing (1) where to look and (2) what to look for. If the utility had not investigated, it is impossible to have known either how long the fractured arrester body may have remained in the broken, dangling condition and a continued possible safety, bushfire, and customer service delivery/reliability risk, or, in the second and actual scenario in this case, whether there had been a fire start or injury to a passing person as a result. The prompt location was valuable in either scenario.
BC Hydro and other utilities have used DFA technology detect and locate conductor-slap events on overhead lines, enabling proactive repairs, to prevent future outages and stresses to the system. A conductor-slap event occurs when an initial fault induces magnetic forces that cause a second fault, closer to the substation, often leading to breakers tripping entire feeders.
Pickwick Electric Cooperative (PEC, Selmer, TN) used DFA technology to avoid an impending outage from tree encroachment in an overhead line. The DFA system discovered the problem, based solely on substation-based current and voltage waveforms, coupled with intelligent, on-line algorithms. It provided location information that enabled PEC to find and eliminate the encroachment before it caused a permanent outage. PEC had no other indication that a problem existed.
Cracked and burned bushings and insulators are well-known causes of interruptions and outages. DFA technology enables utilities to avert sustained outages resulting from apparatus failures, including compromised bushings. By applying intelligent, on-line algorithms to substation-based current and voltage waveforms, the DFA system detects electrical precursors and alerts utilities of developing problems. In the case of the burned bushing shown below, the utility used the DFA to discover the problem existed, locate the burned bushing, and make repairs that preempted a likely future outage. At no point did the utility receive any customer complaints related to this issue; the DFA system provided their first and only notice.