An Attempt to Investigate the Transformer Failure by using DGA and SFRA Analysis

Abstract

A sudden increasing power demand leads to manufacture of large number of oil immersed power transformers and other electrical power equipments. Power transformers are the most vital equipment in power system. Any failure in transformer leads to malfunction of whole power system. Unfortunately, the failure rate of these transformers is very high in India, 25% per annum, which is not favorable as compared to international units of 1-2 %. Failures happen due to internal reasons or operational hazardless. Transformer insulation deteriorates as the function of temperature, moisture and time. The core and winding losses, stray losses in tank and metal support structures are the principle sources of heat which cause oil and winding temperature rise. There are multiple reasons for overheating such as improper cooling, excessive eddy currents, bad joints, blocked radiators, overloading, improper earthing and harmonic contents in power supply. This leads to accelerated aging of oil and cellulosic solid insulation, which generate the gases within transformer and further leads to permanent failure. To prevent such failures, effective analysis and diagnosis needs to be investigated. The type of gases generated and amount of gas concentrations in oil efficiently evaluated using Dissolved Gas analysis (DGA). Various other electrical diagnostic tests like winding resistance test, short circuit impedance, oil analysis and sweep frequency response analysis (SFRA) are also helpful for identification of abnormalities and probable fault area. SFRA technique is widely accepted and used for transformer mechanical condition assessment. Based on the type and concentration of gases generated in oil along with application of SFRA test on transformer can help to identify the abnormal areas prior to catastrophic failure. An attempt has made for the investigation on relation of DGA with SFRA response. Case studies are presented here for the transformers which have higher fault gas concentrations (DGA). Additional diagnostic tests and analysis, inspection and history data has found supportive in investigation.