A direct current instrument transformer using reed relays

Abstract

This paper describes a simple direct current transformer which utilizes the switching action of reed relays which have their coils fed from a separate alternating current supply. The transformer is shown to be particularly suitable for applications where a low cost device is required for an existing heavy current or high voltage installation where interruption of, or electrical contact with, the main load circuit is undesirable. Operation of the new transformer is fundamentally based on Ampere's law for the closed path integration of magnetizing force around a current carrying conductor, and its theoretical response is shown to be polarity sensitive and independent of the auxiliary supply voltage, but dependent on supply frequency. In practice, the transformer is shown to be substantially unaffected by the limited variations of frequency and voltage which are associated with a normal a.c. supply. A complete range of test results, which includes temperature effects, is given for a 5000 A transformer and methods of range extension are indicated. An attractive feature of the new transformer is that the analysis of its basic response is extremely simple and leads to a theoretical performance which is closely followed in practice.