Overvoltages in HV DC Transmission Systems Caused by Transformer Magnetizing Inrush Currents

HV DC transmission schemes can have system conditions in which the ac system is in resonance with the ac harmonic filters at particular harmonics of the supply frequency, and these resonances may be excited by transformer magnetizing inrush currents, generating overvoltages at the ac busbars of the HV DC convertor station. In the case of a rectifier station, supplied directly by generators with no local load, these overvoltages may be as high as twice the rated voltage and may exist for many cycles of the supply frequency. This effect is examined in this paper and the results of many thousands of model tests are included.