Symmetrical and Asymmetrical Fault Current

•Symmetrical fault current: If in a circuit mainly containing reactance, a short circuit occurs at the peak of the voltage wave, the short-circuit current would start at zero and trace a sine wave which would be symmetrical about the zero axis.  This is known as a symmetrical short circuit current

•Asymmetrical fault current: Right after a fault occurs, the current waveform is no longer a sine wave. Instead, it can be represented by the sum of a sine wave and a decaying exponential. Figure below illustrates this phenomenon

•Please note that the decaying exponential added to the sine wave causes the current to reach a much larger value than that of the sine wave alone.

•The waveform that equals the sum of the sine wave and the decaying exponential is called the asymmetrical current because the waveform does not have symmetry above and below the time axis

•The sine wave alone is called the symmetrical current because it does have symmetry above and below the time axis.

The above figure shows the sine wave, decaying exponential, and their sum.

•Therefore, we can define  a symmetrical fault current  in the following manner: If, in a circuit, containing only reactance, the short circuit occurs at any point at the peak of the voltage wave, there will be some offset of the current. This is known as asymmetrical short circuit current.

•Maximum asymmetry occurs when short circuit takes place and when voltage is zero .

•Asymmetrical fault remains only for few cycles after which it becomes symmetrical fault. Decay of asymmetrical component depends on the value of X/R. More the value of R, faster is the decay of asymmetrical fault current

•Magnitude of asymmetrical fault current is more than that of symmetrical fault current.

•If the short circuit current does not include DC component, it is called symmetrical short circuit current. If the short circuit current contains DC component it is called asymmetrical component.

•The previous figure represents the short circuit current with and without DC component.