Acceptable gas-in-oil levels in generation and transmission power transformers

Dissolved-gas analysis (DGA) was used to evaluate acceptable and dangerous gas levels in transformers, taking the equipment type (generation or transmission transformers) into account. The results are based on 6046 gas-in-oil analyses performed on Hydro-Quebec transformers over a period of 20 years and incorporated in a central computer data bank. The transformers are high-voltage conservator-type power transformers without on-load tap changers communicating with the main tank. The probability of failure in service as a function of the dissolved-gas concentration increases rapidly above a given gas concentration level to an extent considered unacceptable or dangerous in service. It appears that the acceptable levels of H₂, C₂ H₄, and CH₄ are lower and those of C₂ H₂ higher for the generation transformers than for the transmission transformers