Bushing Monitoring to protect Power Transformers against Bushing Failures

Summary

High voltage condenser bushings of power transformers, according to their construction and age, are amongst the most endangered components from all operating equipment used. In the past, off-line measurements like the measurement of bushing insulator capacitances and measurement of the dissipation factor were carried out for determining the operational state. Today, modern microprocessor and computer technology makes it possible to carry out the measurements and analysis continuous within a monitoring concept. The monitoring of the electrical measurement quantities is achieved by a voltage measurement consisting of a bushing adaptor, directly connected to the measurement tap of the bushing and connected voltage sensor. The output signal of one phase is compared with two remaining phases. Short and long term grid unbalances can be filtered by certain algorithms. The paper discusses the preconditions when modern bushing monitoring solutions are ready to protect bushing from failures caused by breakdown of layers. The thresholds criteria can be distinguished by the design of condenser bushings as the measured voltage jump caused by breakdown of capacitive layers is depending on the number of layers. As the resulting dielectric stress on the remaining layers increases, the operation of the bushing leads to increased risk for failure, i.e. at least a warning should be generated automatically. The breakdown of further layer might lead in a cascade breakdown ending with a catastrophic bushing failure, i.e. an alarm should be generated and the transformer should be de-energised.

Additional informations

Authors

Polcar Petr, Ceský Josef, Nixon Ken