Theory of Abnormal Line-to-Neutral Transformer Voltages

The abnormal voltages considered here are due to the occurrence of a large harmonic of fractional, even, or odd frequency in the line-to-neutral voltages of certain Y-connected transformer circuits. These phenomena were first described before the Institute in 1915, and other papers on the same subject have appeared since. The results of a detailed study of the instantaneous currents, voltages, and transformer flux densities in the type of circuit involved are presented in this paper. From this study the origin and characteristics of the abnormal voltages have been deduced. In general the voltages are found to be self-excited by successive alternate saturations in the transformer cores. The first saturation, which starts the phenomena, occurs within one cycle after the voltage is applied, due to the starting conditions of voltage and residual core density. When coupled with the system capacitance in the circuit of this discussion, each saturation establishes the condition for producing a succeeding and alternate saturation so that the process is self-continuing. The frequency of the harmonic voltage generated by a succession of alternate saturations is dependent only upon the rate at which they occur. It is not surprising, therefore, that harmonics of unusual frequencies have been observed. The wide variations between the initial core conditions in the three transformers result in highly irregular saturations and correspondingly irregular voltages at the start. Such irregularities are of a transient nature.