A Worldwide Transformer Reliability Survey

There is limited literature available in the public domain discussing failure statistics of transformers. This contribution presents the methodology for a standardized failure data acquisition developed by the Working Group A2.37. The working group has collected 964 major failures which occurred in the period 1996 to 2010, within a total population of 167,459 transformer-years, contributed by 58 utilities from 21 countries. The overall failure rates of substation and GSU transformer were all within 1%. For three groups of substation transformers detailed population data were collected en-abling the calculation of hazard curves. All populations show a low hazard rate and no distinct bathtub curve character. An increasing probability of failure after a particular age, which would justify an exchange of the transformer, cannot be derived from the available data. 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 me-chanical and electrical type failures, for substation transformers, whereas GSU transformers had higher contributions of thermal and dielectric mode failures.