The Big Question: When Should One Recommend Taking A Transformer Out Of Service?

This paper examines the goals and difficulties of dissolved gas analysis (DGA) implementation to evaluate the condition of transformers. Although the technique is quite old and well-known in many utilities throughout the world, diagnosis is still complicated and lies in the domain of the state of the art. The published guides indicating the limits for concentration values, or correlating gas ratio and failures, are too broad to be applicable in practice. Gas ratio guides refer to a single failure only, while in practice faults build up one on top of the other until the critical failure appears. This paper reviews some parameters liable to affect normal concentration values. The authors offer a different approach: to develop a database including all failures and unusual cases versus the gases developed at each stage. From the case histories, which took place at the Israel Electric Corporation, we have drawn some recommendations which help to find the right time for scheduled outage of the transformers. (Note: Israel Electrical Corporation acquires transformers on the free market. Therefore, the "gassing" transformers are different manufacturers.).