Reasons for Transformer Failure
Contrary to popular belief, transformers do experience short circuits and
abnormal electrical conditions that result in their failure.
As transformers become older, the likelihood of failure increases as insulation begins to deteriorate.
An example of one such abnormal condition is overexcitation.
Many industry experts have concluded that overexcitation and through faults are more detrimental to transformer life than load-associated aging [1].
Through-fault failures were a major industry concern in the United States during the late 1970s and 1980s, when the industry experienced an unusually large number of through-fault failures due to design deficiencies. As a result, the IEEE Transformer Committee developed guidelines (C57.12.00–2000) for the duration and frequency of transformer through faults. The requirements for Category III (5–30 MVA) and Category IV (above 30 MVA) transformers are depicted in Figure 2. The smaller Category III transformer’s through-fault standards are defined by two sets of curves: one for frequent faults and one for infrequent faults. This was done because of the use of transformers of this size for utility-distribution substation applications, which subject these transformers to frequent through faults and multiple automatic reclosing attempts. The multiples of normal current in Figure 2 are based on the self-cooled rating of the transformer being a 1.0 base current. These curves should be used when developing transformer time overcurrent relay settings.
The through-fault effects on transformer failures are somewhat mitigated at medium-voltage industrial installations because most through faults are line-to-ground faults and fault current is limited through 200–400 A grounding resistors in neutral. A detailed analysis of transformer failures, conducted by a major electrical equipment insurer, breaks down the cause of failures based on the insured transformers.
Table 1 shows the insurer’s breakdown of the failure causes from 1990 to 2002, which includes approximately 8,000 insured transformers. One of the conclusions was that whatever the cause of failure might be, age compounds the problem. Therefore, the proper protection of aging transformers warrants careful attention from industrial facility engineers.