Pole mounted distribution transformers insulation condition assessment

Hydro-Québec (HQ) Distribution grid has close to 600,000 overhead distribution transformers in service. On average, every year 10,000 units are taken out of service for various reasons such as failures, replacement due to load rise and network reconfiguration. In 2006/07, HQ Distribution started a refurbishment program to give them a second life on the distribution grid. It is neither acceptable nor efficient to proceed with the refurbishment of a transformer having a degraded solid insulation and consequently an expected low residual life. In 2010, a project was started to assess the condition of the insulation of different service-aged transformers. 121 distributions transformers were randomly sampled from the units sent for refurbishment. Samples were taken from the following manufacturing year ranges: before 1985, 1985-1989, 19901994, 1995-1999, 2000-2004 and 20052010. In this evaluation, oil samples were taken from each transformer to assess oil quality and transformer condition. Paper sampled from the coils, were also taken in specific cases amongst the faulty transformers. The preliminary conclusions that can be drawn from this study are that transformers manufactured after 1985 do not seem to experience any major aging with respect to their insulation. It therefore appears that if no faults are present, the transformers should be refurbished since the insulation basically in a good condition.