Results of a standardized survey about the reliability of power transformers

There are limited data available describing the failure behaviour of transformers. For this reason, a working group "Transformer Reliability Survey" was established by the Study Committee A2 (Power Transformers) of Cigré, which is to collect and evaluate existing methods for the statistical recording and evaluation of transformer failures. Based on these statistics, a standardized form of failure collection was developed and used for a worldwide collection of failure data. This contribution presents the methodology for a standardized failure data acquisition. 964 major failures were collected which occurred in the period between 1996 and 2010. 58 utilities from 21 countries contributed to the survey. The overall failure rates of transmission and generator step-up unit transformers were all within 1%. Windings, tap changer and bushing related failures were the major contributors, followed by lead exit related failures, irrespective of application or manufacturing period. Dielectric mode failures were the most prominent, followed by mechanical and electrical type failures, for substation transformers, whereas GSU transformers had higher contributions of thermal and dielectric mode failures.