Measurement of switching transients in 735 kV substations and assessment of their severity for transformer insulation

An automatic measuring system that digitally records transients generated by EHV-transmission-system switching operations and lightning strokes has been developed. The severity of these transients was assessed by comparing them with test voltages applied to the transformers during acceptance tests. In order to quantify in-service transients with respect to the impulse-test voltages, the frequency spectra of the time-domain records were calculated and expressed as a percentage of the test level. The presence of high-frequency winding oscillations was revealed by simultaneous measurements of the bushing voltage and neutral current. The results of these measurements are presented in the form of a transfer function, which can also be used to detect winding displacements in service