The application of current transformers to three-phase circuits

The performance of current transformers when their secondaries supply simple series loads, and the methods of test for determining the errors introduced by the transformer when used with known secondary connected loads have been considered in previous papers. The secondaries of two or three current transformers whose primaries are supplied from the lines of a three-phase circuit, are, however, frequently interconnected to save room, simplify wiring, and diminish cost. In this case the equivalent load carried by the current transformer secondary can not be determined in the simple manner that applies to an ordinary series connection, since the devices used in a series circuit may constitute a very different equivalent load when used in an interconnected circuit. An understanding of the equivalent load carried by each transformer is necessary in order to determine suitable limits of load from results of tests made in the ordinary manner. The term “equivalent load” is here used to indicate the load carried by the secondary of a current transformer where this may differ from that obtained by combining in series the resistances and reactances of the devices used.