Investigation of Diagnostic Technique for Thermal Fault on Natural Ester Oil-immersed Transformer by Dissolved Gas Analysis

摘要

Dissolved Gas Analysis (DGA) of insulating oil has been widely used as an essential diagnostic technique for condition assessment of oil-immersed transformer's. Many researches and investigations have been reported for the DGA of mineral oil-immersedtransformers and guidelines have been proposed to assess the condition of oil-immersed transformers in the world. Recently transformers using alternative insulating oil such as natural ester oil have been installed extensively because of its good environmental andfire safety aspect such as biodegradation, carbon-neutral and high flash point. Consequently there is an increasing demand for the diagnostic techniques and guidelines for DGA of the natural ester oil-immersed transformers.This paper describes the behaviour of gas generation obtained by heating test of natural ester oil simulating thermalfault of natural ester oil-immersed transformer. Natural ester oil were continuously heatedfor 10 minutes at the temperature from 100°Cup to 900°C with the step of 100°G After the heating tests, combustible gases such as hydrogen, methane, ethylene, acetylene and ethane were detected and the correlation was observed between heating temperature and concentration ratio of decomposed gases.Further improvement of the accuracy of estimation of overheating temperature, a diagnosis map using a combination of multiple ratios of gas concentrations is also commonly used for diagnosis of thermal fault. The diagnosis maps selectedfrom combination of gas including acetylene are most effective diagnosis map to diagnose overheating temperature of ester oil-immersed transformer. Even in case when acetylene is not detected or high concentration of hydrogen only detected such as Stray Gassing, the overheating temperature of natural ester oil-immersed transformer can be estimated by using the diagnosis map with another indicator gas not using acetylene or hydrogen.