On-line Monitoring and Diagnostics For Power Transformers Application to On-load Tap-changers

Transformers are a critical part of an electrical utility's asset base. Loss of a transformer in a utility, generation plant or process can cost many millions of dollars, depending on how long it is out-of-service. On-line monitoring and diagnostics is a useful tool to help operators to manage their assets and make decisions on continuing operation, maintenance or replacement. This paper illustrates how Dissolved Gas Analysis (DGA) is the heart of on-line monitoring as it is a well-established method of transformer diagnosis. Experience has shown that DGA can detect and give warning of 70% of the most common failures in power transformers. Some aspects of ongoing developments in DGA and its diagnosis are described. Most of the knowledge built up over the last 40 years on DGA has been based on mineral oil/paper insulation systems in transformers and other static equipment such as bushings and instrument transformers. In these types of equipment, the causes and mechanisms of gassing have been established and the diagnosis of faults is well documented. The other transformer-related equipment that has largely been ignored in the past is on-load tap-changer diverters or combined diverter-selectors. In oil-filled diverters, it was generally assumed that because arcing was taking place in the oil, large quantities of fault gases were being produced and therefore conventional DGA diagnosis could not be applied. In recent years, various parties have investigated tap-changer oil analysis. Several organizations have also been investigating onload tap-changer analysis based on ratios rather than discrete gas concentrations. This paper describes an example of on-load tap-changer condition assessment based on DGA measurements. The paper goes on to describe some of the recent developments in other on-line monitoring and diagnostic techniques, which help transformer operators to know the condition of their transformers and to make useful decisions on continuing operation, maintenance or replacement. The monitoring systems can be used, not only to monitor the parameters of the transformer, but also to carry out in-depth analysis of the condition of the transformer insulation and accessories.