Analysis of dielectric measurements on switchgear bushings in British Rail 25 kV electrification switching stations

Diagnostic measurements have been carried out in the 25 kV electrification trackside switching stations on British Railways since 1972, following the discovery of audible discharge in the synthetic resin-bonded paper bushings. Laboratory measurements of discharge magnitude, capacitance, dielectric loss and ultrasonic noise were correlated with physical damage at the bushing foils and criteria for bushing rejection were established. A batch of inferior-quality bushings was identified which were spread between 16 switching stations. Further service experience, including bushing punctures, led to a revision of the rejection criteria and prompted diagnostic testing of 2500 bushings in all 92 switching stations. The results are discussed in terms of the rate of deterioration, critical discharge levels for stability and the merits of various diagnostic techniques.