WHITEPAPER: A New Approach To Transformer Bushing Monitoring
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 spillage. 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 will not lead to a catastrophic failure can harm people due to burst porcelain insulators, catapulted through the air by the force of the arc of the breakdown.
Today, the experience is that a transformer during his lifetime will have two sets of bushings. As transformer today expected to last for 50 years, bushings expected to have a lifetime of 25 years. The past experience showed, that there are two major periods where bushings are going to fail compared to other bushing ages. The production/ quality related failures are happening once the bushings are reaching an age of 10 to 13 years. The second wave of bushing failures is happening between 20 to 30 years of age, which is considered as the normal life time. Nevertheless it is also true, that bushings fail in an earlier age the 10 years but is also true, that there are bushings installed on transformers with an age of more than 50 years. Bushing monitoring is aiming to detect incipient faults and give an early warning as well as using the bushings till 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. For example, even if a quite relevant amount of moisture impregnated into the bushing core, the lost angle will only show a slight change at ambient temperature. Bushing monitoring systems today are not able to capture these slight, but important changes. The problem is that no standard capacitors as measuring references are available in field for online monitoring. The sister bushings are used as a reference source to assess the condition of a bushing (like the balanced current method). Voltage and angle differences between phases as well different temperatures aging rate are not considered with this methods.
New approaches are using stable voltage sources as reference signals, preferable from the same phase as the monitored bushing. The phase shift between the leakage current signal from the bushing and the voltage from the voltage transformer will be measured, corrected by the phase shift offset and the loss factor can be calculated directly. By using for example a voltage transformer (VT) as reference source, accuracies up to 0.1mrad in terms of measuring the phase shift can be achieved and small, but relevant changes can be detected. The two main health indicators for a bushing are the loss factor (tan δ/ power factor) and the capacitance. While the loss factor is sensitive to almost all bushing faults, the capacitance is an important factor to detect partial breakdowns between capacitive layers and to detect, in combination with the loss factor, contact problems inside the bushing.