Absorption index of insulation at end of service life – Part I
Old-aged transformers equipped with silica gel air breather
Transformers are the most important and most expensive equipment at a substation, and their failure leads to severe problems in the power supply. The purpose of preventive testing is to determine a runaway for the condition of the complex system of the transformer insulation from the initial state (at the factory and installation site), and upfront detection of the defects occurred. Preventive measures are based on the systematic inspections, and they allow to keep the equipment in operation state by means of cost-effective way, reducing the risk of permanent damage and potential failure, assisting the companies in avoiding major and costly repairs, and extending the service life of old-aged transformers.
Transformer fleets in all countries are ageing. With insufficient maintenance, a transformer older than 40 years can accelerate failure due to deterioration of the insulation condition (primarily due to its moisture and pollution). A middle-aged transformer may fail earlier, because its safety margin is substantially less, due to the fact that its design has been optimised for the limited-service life of 25 years, according to GOST , Russian technical standard.
A significant number of old-aged unsealed USSR-made power and distribution transformers (in accordance with GOST) are in operation in the power systems and electric networks of Eastern Europe and a number of countries in Asia, Africa, and South America. Excessive moistening and contamination of these transformers are their main problems, since moisture and dirt at operating voltage, and even more so in case of the voltage increase, accelerate the insulation ageing. The combination of electrical voltage and insulation deterioration over time leads to the fatality of the transformer.
Measurement of insulation resistance (IR) at 15 and 60 seconds (IR15 and IR60) is the routine factory test, according to GOST. These values are specified in the certificate of each transformer. The operating manuals require regular (once per year) measurements of these values. During establishing these requirements, it was believed that using the trend of IR15 and IR60 decrease, it is possible to evaluate the degree of insulation moisture content of the transformer in operation. In addition to moistening, the experience has shown that these values are also affected by insulation contamination.
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