A new method for online condition monitoring of high voltage bushings

Elimination of temperature dependences (Double-Reference Method) Abstract Standard online monitoring systems for bushings must, in particular, consider the dissipation factor and the main capacity (C1)...

byDr.-Ing. Junliang Wu, M.Sc. Tobias Gruber


Elimination of temperature dependences (Double-Reference Method)

Abstract

Standard online monitoring systems for bushings must, in particular, consider the dissipation factor and the main capacity (C1) of the bushing. Typically, the known principles have problems due to network imbalances and temperature influences caused by the environment and load.

A new method, described in this article, Double-Reference Method, monitors the changes in capacitance and dissipation factor in a three-phase system so that the temperature dependence is eliminated, since the algorithm continuously incorporates all three bushings into the mutual monitoring. Furthermore, the signals of the respective voltage transformers are used as a reference for detecting the symmetry of the three-phase mains voltage. Only if the grid symmetry is within a defined bandwidth, the measurements are taken into account. So, this method ensures that the influence of temperature and voltage fluctuations on a bushing monitoring system is effectively limited by the cross-checking of values.

Keywords: online monitoring, power transformers, high-voltage bushings, condition monitoring

1. Introduction

According to the Transformer Reliability Survey conducted by Cigré working group A2.37, 17% of all transformer failures worldwide can be traced to defective bushings – and they are also one of the main causes of transformer fires and explosions. A particular problem associated with failures of high-voltage bushings is that, in a significant number of cases, such failures are catastrophic – a failed bushing explodes and starts a fire which spreads over the transformer.

Therefore, it is of increasing interest for transformer operators to evaluate the online conditions of the transformer bushings, especially the conditions of their insulation systems.

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