Evaluation of underground submersible distribution transformers through oil analysis

The underground electrical distribution system of Hydro-Québec is mainly located in two cities: Montréal and Québec. Submersible transformers are an important part of underground assets and ensuring that they are in good condition is crucial for the reliability and continuity of electrical distribution service. Due to their location, accessibility issues and the condition of their sampling valve, oil analysis is not economically feasible on a regular basis as is routinely done for power transformers. Oil sampling is generally performed only when the transformer is deenergized. Sampling and analysis are performed only in critical cases, not as part of the regular preventive maintenance program for underground transformers. In 2000, a major program was initiated to evaluate transformers through oil sampling and analysis. It covered 3,400 submersible transformers ranging from single-phase 333-kVA to three-phase 1,000-kVA ones. Oil samples were collected from transformers during scheduled shutdowns and were analyzed by our analytical laboratory for their dissolved gas content and physical properties. The samples were taken following ASTM standards. This paper presents the results of a survey of transformers. Typical values for the dissolved gas content of oil and for other oil parameters were obtained by statistical analysis, as were fault types, severity and frequency. Given the large number of transformers analyzed, the results are statistically significant and representative of this type of electrical equipment. The results may be used to support a condition-based preventive maintenance program using dissolved gas analysis and monitoring.