DC component in Asymmetrical Short Circuit Current ?

In symmetrical current the envelops of the peaks of the current waves are symmetrical with respect to zero axis. In asymmetrical currents the envelops of the peaks of the current waves are not symmetrical about the zero axis. If we plot short circuit waveform in oscillogram it appears that the short circuit currents are nearly asymmetrical for the first few cycles after the short circuit occurs and this asymmetrical behavior is maximum at the instant of the short circuit and then current waveform gradually becomes symmetrical after the few cycles of short circuit.

These asymmetrical alternating currents when considered as a single wave is difficult to interpret for circuit breaker applications and relay settings purpose. Complex formulation is involved for calculations of the circuit breaker applications and relay settings purpose. Hence this asymmetrical component is resolved in to simple components consists of (1) symmetrical ac component and (2) dc component. The sum of these two components at any instant is equal to the magnitude of the total asymmetrical component of the ac wave.

DC component present is considered as offset and the magnitude of this component depends on the instant at which short circuit occur. It varies from zero to maximum value which is equal to peak of the asymmetrical component.

DC component have decaying nature with respect to time after the short circuit occur and dissipate as I²R loss in the circuit. The decrement or rate of decay of the dc component is proportional to the X/R ratio. If the ratio of reactance to resistance is infinite (zero resistance), dc component will never decay. On the other hand if the reactance of a line is zero, the decay takes place instantly.