Prolonged inrush currents with parallel transformers affect differential relaying

It has long been known that transient magnetizing inrush currents, sometimes reaching magnitudes as high as eight times full-load current, may flow in a transformer winding for a period following the moment when it is energized by connecting it to an electric power circuit.
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It has not been generally appreciated, however, that other transformers already connected to the power circuit near the transformer being switched may also have transient magnetizing currents of considerable magnitude at the same time, although they themselves are not switched but remain continuously connected to the power circuit, carrying load. It has not been appreciated, moreover, that with this arrangement the transient periods of the inrush currents may be very long, the currents dying away at a much slower rate than would the inrush current of the transformer being switched if the others were not connected. This paper discusses the cause of these phenomena and describes tests made to investigate their occurrence. The results of a mathematical analysis for the currents in the circuit under various conditions, supporting and extending the test results, are given in the appendix. The amount of resistance in the transmission line circuit connecting the parallel transformers with the generating source is shown both by tests and mathematics to be a determining factor; the magnitude of the inrush current in the already connected transformers increasing to values of over twice full load current as the line resistance increases. Magnetizing inrush currents tend to operate the transformer differential relays, causing the circuit breakers to trip and remove the transformer from service when no fault exists within it. Various means have been devised to prevent the operation of the relays on magnetizing inrush current. This paper discusses the effectiveness of these various means when subjected to the inrush currents occurring with parallel-connected transformers, with particular reference to the harmonic restraint relay described in a recent paper.
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