Electromagnetic interference and solid state protective relays

This paper presents new information on the nature of the electromagnetic environment in an electric power station which can influence the performance of modern solid state protective relays and other sensitive electronic control and data processing equipments. Present industry standard tests do not simulate the full range of this environment and may need to be revised. The frequency spectrum of the electromagnetic interference is shown to be significantly different from the spectrum of the voltage and current signals used for relaying. Therefore, filtering techniques can be used as a primary remedy for E.M.I. problems. The nature of transients generated in low voltage control circuits is shown in high speed oscillograms. Data are presented which show that these transients can damage avalanche type semiconductors which are usually considered to be self protecting. Finally, portable radio transmitters are shown to be capable of causing mis-operation of sensitive solid state equipments if operated nearby. Remedies for this type of interference are discussed.