A survey on the reliability of protective systems for power transformers

This paper presents the results of a survey on the reliability of protective systems for power transformers that serves at distribution substations of the CHEC system. The protective systems of seventeen 115/33 kV power transformers in the range of 20 to 60 MVA and twenty-six 33/13.2 kV power transformers in the range of 3 to 30 MVA were surveyed using operating records that covers 7 years. Reliability indexes were computed for the protective systems as a whole and for their components and the models of arrival of false openings and their corresponding repairs were obtained. Results show that: i. Security is the weakest aspect of the studied protective systems, being false openings the more frequent problem. ii. Failure to close is the second more frequent problem of the studied protective systems. iii. Circuit breakers are the protective system component that require more attention because they cause most of false openings and failures to close and have the highest repair time. iv. Arrival of false openings and their times to repair are stationary what means reliability of studied protective systems is not deteriorating. v. Reliability indexes of the protective systems for 115/33 kV power transformers is higher than the ones of the 33/13.2 kV power transformers.