Transformer bushings are one of the most critical components of a transformer. Up to 20% of major failures on high voltage transformers today can be related to bushings. Almost half of these failures result in catastrophic failures like explosions, fire or oil spill. The cost of these damages and the lost opportunity to deliver energy could be several hundred times higher than the price of a bushing. Even a failing bushing which does not lead to a catastrophic failure can harm people due to burst porcelain insulators, catapulted through the air by the force of the breakdown arc. Today, experience shows that a transformer during his lifetime will have two sets of bushings. As transformer today are expected to last 50 years, bushings are expected to have a lifetime of 25 years. Past experience showed, that there are two major periods where bushings fail. The production/quality related failures take place once the bushings reach an age of 10 to 13 years. The second wave of bushing failures takes place between 20 to 30 years of age, which is considered as their normal life time. Nevertheless, it is true, that bushings fail earlier than 10 years of age but is also true, that there are bushings installed on transformers with an age of more than 50 years. The two main health indicators for a bushing are the loss factor (tan 8/ power factor) and the capacitance. The loss factor is sensitive to almost all bushing faults, but the capacitance is an important factor to detect partial breakdowns between capacitive layers and also to detect, in combination with the loss factor, contact problems inside the bushing. On-line monitoring of loss factor and capacitance aims to detect incipient faults and give an early warning as well as using the bushings until its real end of life. In order to have a reliable monitoring system, the accuracy of the acquisition of the monitored parameters needs to be very high. The lost angle, at a quite relevant amount of moisture impregnated into the bushing core, will only show a slight change at ambient temperature. Some bushing monitoring systems today are not able to capture these slight, but important changes. The problem is that standard capacitors as measuring references are no available in the field for online monitoring. Instead sister bushings are used as a reference source to assess the condition of a bushing (balanced current method), but voltage and angle differences between phases as well as different temperatures aging rates are not considered with these methods. New approaches use stable voltage sources as reference signals, mainly from the same phase of the monitored bushing. The phase shift between the leakage current signal from the bushing and the voltage source is measured, corrected by the phase shift offset and the loss factor calculated directly. Using for example a voltage transformer (VT) as reference source, accuracies up to 0.1 mrad in terms of measuring the phase shift can be achieved and small, but relevant changes can be detected.
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