Diagnostic testing of transformers on British Railways

There are some 5000 power transformers on BR electrification schemes, many of which are about 30 years old. These vary from 30 MVA single phase feeder transformers on 25 kV overhead line schemes to 1-5 MVA transformers on EMU (electrical multiple unit) and locomotive 25 kV rolling stock and to incoming feeders at 45 MVA 3 phase for the 33 kV network supplying the DC network on Southern Region. There have been few transformer failures on trackside fixed equipment, but rolling stock transformers suffer a higher failure rate. Some estimate of the overall condition of transformers was required to aid planning of replacements due to life expiry and also to isolate those transformers most likely to fail. Dissolved gas analysis (DGA) is now well established as a diagnostic technique (BS5800: 1979) and can determine from a small oil sample taken on-load what deterioration processes are occurring in the transformer. No previous DGA measurements had been taken on BR transformers and this exercise would also establish a datum base against which changes in DGA values could be monitored in the future. All types of fixed equipment transformers were sampled, but only one class of locomotive and EMU transformer