A new method for on-line monitoring of bushings and partial discharges of power transformers

This article describes a new on-line method for capacitance and dissipation factor measurements of high voltage bushings, for recording grid transients and for partial discharge monitoring at power transformers. Failure statistics identify HV bushings as one of the major reasons for transformer failures, which motivates operators for continuous on-line monitoring of their insulation systems health. The needed accuracy for reliable detection of failures is in the range of 5 pF for capacitance and 0.1 % for dissipation factor. For reaching this goal, a relative comparison of the bushing currents of the three phases, measured at the bushing taps, does not provide sufficient accuracy due to imbalances in the network. Additionally, simultaneous ageing, as it can be anticipated for a three phase transformer, will not be detected. To overcome these shortcomings, this paper introduces an absolute method for measuring capacitance and dissipation factor of HV bushings. Even under field conditions a measurement accuracy similar to accurate off-line power factor tests was reached. Partial discharges are an early breakdown indicator for HV dielectrics. With the measurement on the HV bushing tap, not only failure of the bushings dielectric but also of the transformer insulation system can be detected. As for off-line measurements, the discrimination between noise and true internal discharges is the key for successful diagnosis. Strategies for sensitive partial discharge measurements under disturbed on-line conditions are described, also involving measurements in the UHF range.