January 10, 2023

The SDIPF reliability curve of old EHV power transformers – Part II

Failure is one of the basic concepts in the theory of reliability.

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Failure is one of the basic concepts in the theory of reliability. In electrical engineering, the IEC 61508-4 definition applies (quote): «failure – the termination of the ability of a functional unit to perform a required function». CIGRE, a major failure, was defined as any situation which required the transformer to be removed from service for a period longer than 7 days for investigation, remedial work or replacement [1]. The authors of the cited works objectively could not adhere to any of these definitions since they wrote before the formulation of these definitions, but this is not fundamental for our review.

In the classical theory of reliability, the dependence of failures on the lifetime of an asset is described by the well-known bathtub curve (Fig. 1). It starts with Installation. Commissioning such complex equipment as the PT is fraught with dangers. First, the PT could very well have left the factory with an internal defect that didn’t show up until the transformer was stressed in transit. Cases of careless handling of cargo are not uncommon, especially on railways. Secondly, the installation process itself may include a sloppy moment or deviations from factory requirements. Finally, the PT is at great risk the moment it first turns on and experiences load conditions for the first time. The first part of the bathtub curve is called “Infant Mortality”. For PT, it has a duration of 3-5 years; failures during this period, in addition to those listed above, are also due to shortcomings in production technology and poor quality control.

The second part of the curve is called Normal Operation, which has random failures. This period for LPTs of advanced manufacturers lasts at least 25–35 years with good maintenance, that is, actions to detect, prevent or mitigate the degradation of the transformer.

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