A power transformer outage has a dramatic financial consequence not only for electric power systems utilities but also for interconnected customers. The service reliability of this important asset largely depends upon the condition of the oil-paper insulation. Therefore, by keeping the qualities of oil-paper insulation system in pristine condition, the maintenance planners can reduce the decline rate of internal faults. Accurate diagnostic methods for analyzing the condition of transformers are therefore essential. Currently, there are various electrical and physicochemical diagnostic techniques available for insulation condition monitoring of power transformers. This paper is aimed at the description, analysis and interpretation of modern physicochemical diagnostics techniques for assessing insulation condition in aged transformers. Since fields and laboratory experiences have shown that transformer oil contains about 70% of diagnostic information, the physicochemical analyses of oil samples can therefore be extremely useful in monitoring the condition of power transformers.

The condition of the internal cellulosic paper and oil insulation are of concern for the performance of power transformers. Over the years, a number of methods have been developed to diagnose and monitor the degradation/aging of the transformer internal insulation system. Some of this degradation/aging can be assessed from electrical responses. Currently there are a variety of electrical-based diagnostic techniques available for insulation condition monitoring of power transformers. In most cases, the electrical signals being monitored are due to mechanical or electric changes caused by physical changes in resistivity, inductance or capacitance, moisture, contamination or aging by-products in the insulation. This paper presents a description of commonly used and modern electrical-based diagnostic techniques along with their interpretation schemes.

Oil is a vital part of the transformer body and (similarly to blood in a human being body) keeps responsibility for the condition of the entire organism. Oil is particularly responsible for functional serviceability of the entire insulation system. The insulating oil must be kept in pristine condition, since its condition can be a decisive factor, which determines the life span of the transformer. Fields and laboratory experiences have shown that transformer oil contains a vast amount of information. Oil analyses can be extremely useful in monitoring the condition of power transformers. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. To ensure long-term reliability of oil filled power transformers, it is important to identify early sign of degradation of the insulating oil. In this paper, oil degradation was monitored with various ASTM test methods. Investigations were performed on service-aged oil samples as well as on oil samples aged in laboratory conditions. Many key parameters actually used to monitor the condition of transformer oil relative to oxidation/degradation were investigated. From the obtained results, correlations were found between some of them. The results indicate that Dissolved Decay Products (DDP) and turbidity, which change with a higher rate than interfacial tension (IFT) and Acid Number (AN) values, can be possibly used as an effective index for insulating oil degradation assessment. Limits are suggested which provide a “picture” of the fluid condition.

Aging of a power transformer’s insulation system produces carboxylic acids. These acids—acetic, formic and levulinic—are absorbed by the paper insulating material, thus accelerating the degradation of the whole insulation system. In this contribution, the effect of these acids on the aging of oil-impregnated paper insulation used in power transformer is reported. A laboratory aging experiment considering different concentrations of these three acids was performed to assess their effect on the insulation system’s degradation. Each acid was individually mixed with virgin oil, and a mixture of acids was also blended with oil. The paper’s degradation was assessed by the degree of polymerization (DPv). It was found that the DPv of paper aged with formic acid decreased much faster in comparison to the other acids.

Frequency Domain Spectroscopy (FDS) is an effective tool allowing assessing the condition of oil-paper insulation system in power equipment. However, results from these measurements are known to be greatly influenced by various parameters, including insulation aging, moisture content, and insulation geometry/volume, together with environmental condition such as temperature. In this contribution, a series of experiments have been performed under controlled laboratory conditions. The dielectric response of the oil impregnated paper, along with the degree of polymerization and moisture content, were monitored. Since dielectric parameters are geometry dependent, poles (independent of the geometry) which depends on resistivity and permittivity, were considered to assess the condition of the insulation. From the investigations performed on new and aged samples, it is shown that poles (P) can be regarded as insulation aging indicator. It is also shown that a per unit value based on the Dielectric Dissipation Factor (DDF), measured in the frequency range from 1 to 1000 Hz can be correlated to moisture content in the insulation paper.

Transformer oils are vulnerable to moisture, which can have adverse effects including a dramatic reduction in dielectric strength, accelerating the aging of the solid insulation (cellulose depolymerization) and the formation of decompositional gasses at high temperatures. Moisture may be generated inside the transformer due to degradation of the oil-paper insulation or there may be ingress of moisture due to free-breathing arrangements. A moisture content exceeding 50% of oil saturation greatly reduces the breakdown voltage, resulting in catastrophic failures as well as the potential for transformer fires [1]. The detrimental effect of water within a transformer is not only limited to the breakdown voltage of the oil. The rate of degradation of paper insulation increases in direct proportion to the water content [2], [3]. Moisture also has a decisive effect on the maximum temperature that the transformer can run at before moisture-induced failure occurs. Consequently, transformer life expectancy can be calculated from water content and temperature [3]. A failure of transformer accessories such as condenser bushings sometimes leads to a transformer failure and long-term outage. Often, moisture is a cause of explosion for service-aged bushings [4].