A method of fault analysis and suppression of fault current in six-phase power transmission systems

The case of infinitely large tertiary impedances for fault suppression in EHV and UHV transmission systems is discussed. Although the voltage rise of the sound phase in fault condition is considerably high, the almost-null fault control gives great advantages to the important (low-voltage) distribution lines. It is shown that: the fault analysis of a six-phase transmission line can be performed by the combined use of two- and three-phase symmetrical coordinate methods; the ground fault and short-circuited fault currents can be suppressed to the magnitude of the same order as the exciting current of the transformer; the induced electromotive force of the transformer winding of faulted phase is reduced to zero and therefore the arc ground fault current could be extinguished by itself; and the symmetric two-phase ground fault current reaches a considerably large magnitude (about one-half the single-phase line-to-ground fault current of conventional double-circuit three-phase transmission line) but its earth return component is almost zero if the ground faults on the two lines occur at the same point.