The influence of rectifier harmonics in a railway system on the dielectric stability of 33-kV cables

The electrified sections of the Southern Region of British Railways outside the London suburban area derive their traction current requirements from mercury-arc-rectifier substations, which, in turn, receive high-voltage power supplies from the British Grid by means of a 33-kV railway-cable network. The earliest installation was that between Croydon and Three Bridges, which was brought into operation in July 1932; satisfactory operation was experienced until 1944, when it became apparent that the incidence of 33-kV cable failures had risen. The paper shows that surges were not responsible for these failures, but that the harmonics generated by the rectifier equipment, together with the system resonance characteristics, were such that the waveforms of the system voltage were considerably distorted. The conditions were aggravated by irregular operation of the rectifiers. It is deduced that this waveform distortion is mainly responsible for shortening the life of the dielectric material of the impregnated supply cable. An unusual feature of the cable damage is the presence of a number of uncarbonized pinholes in the paper tapes in the vicinity of the fault; it is concluded that these pinholes are caused by internal discharges, accentuated by the presence of the relatively high-frequency and highamplitude harmonic voltages in the system. An explanation is offered to show why the greater proportion of the cable failures occurred in sections where the cable was either in the ground or in a tunnel. The paper concludes with a short discussion of the practical methods of reducing or avoiding similar troubles.