DFA Case Studies
Event from week commencing April 4th 2016 – USA. This event involved a long circuit with many miles of overhead exposure. A school and other customers at various locations on the circuit began complaining of flickering lights. The utility patrolled but was unsuccessful. They were considering running an infrared scan of the circuit, but this would be difficult because some portions of the circuit were not truck accessible. The flicker condition was severe and continued for six days. Early in the process DFA detected a failing switch on the circuit and estimated that the switch had 83 kVA of load past it, +/- 50%. The utility company became desperate enough that they were willing to switch off sections of the circuit, one at a time, to see when the flicker went away. Unfortunately they did not have any system, other than DFA, that would tell them when the problem went away. Power Solutions Inc, the makers of and prime technical support team for the DFA, worked with them for four hours at that time during which time they opened and closed multiple switches. At each switching operation, DFA real-time measurements made it obvious whether the problem was downstream of the just-operated switch. This is possible because DFA and PSI is aware which parameter to monitor as an indication of switch failure. Neither SCADA nor anything else available to the customer utility would provide this parameter or enable this real-time feedback to guide their search. In addition, dispatchers asked PSI "could it be tree contact?" and "could it be a failing arrester?" and many such questions. The clear answer, based on DFA, was that it was not any of these things but rather a failing switch or clamp, with greatest likelihood of switch. This enabled the utility to know what to look for. When they finally identified the problem, it was indeed a burned top of a fused cut-out serving a single heavily-loaded 50 kVA service transformer. The DFA-assisted search took four hours. A search without DFA would likely have taken days, during which time the utility would have expended many more man-hours and would have continued to provide degraded service to their customers.
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.