Recognition of overheating in different temperature ranges, which is accompanied by discharges of different intensity, based on the dissolved gas analysis

Abstract

In wartime and post-war reconstruction, it is the responsibility of a rather ageing fleet of electricity generation equipment to provide a reliable supply of electricity to the households and industry. The challenge is to ensure operational reliability and to extend the service life of this equipment. Non-destructive testing methods are the most practical in terms of operation. For non-destructive testing of high-voltage power transformers, the dissolved-oil gas analysis is the most common and widely used method. This method, using various diagnostic criteria (value of gas ratios, percentage of gases, etc.), allows most of the defects (both electrical and thermal type) occurring in a transformer to be detected. However, during operation, so-called combined defects can occur, i.e. discharges turning into overheating, or overheating accompanied by discharges. In addition, two or more defects of different types can occur in the same transformer. The values of the criteria used for the recognition of the combined defects differ significantly from the values of the criteria typical for thermal or electrical defects, which significantly complicates the recognition process. The purpose of this article is to analyse the values of diagnostic criteria as well as the recognition reliability of overheating in different temperature ranges, accompanied by discharges with different intensity. For a comprehensive analysis, the results of dissolved gas analysis were used for 471 high-voltage power transformers in which overheating in different temperature ranges, accompanied by discharges of different intensity, were detected. The 471 analysed values were split into 38 arrays with similar gas percentages, gas ratios and defect nomograms. Based on an analysis of the recognition reliability of overheating in different temperature ranges, using the norms and criteria regulated by the best known standards and methods, it was found that the highest recognition reliability is provided by the values of gas ratios, which are regulated by the ETRA Square and also by Nomogram method.