Short Circuit Current for Unsymmetrical Faults - General information

Experience suggest that most of the fault (nearly 70%) in power system are single line to ground faults. Other faults are line to line faults, double line to ground faults and three phase faults. Out of these except three phase faults all other are un-symmetrical faults. Theory behind methods to calculate fault current is included in respective text books here only necessary part discussed.

In 1918 C. L. Fortescue introduced a new system called as the method of symmetrical components. Which is used to simplify fault analysis by converting a three-phase unbalanced system into three sets of balanced phasors, or symmetrical components. These sets of phasors are called the positive-, negative-, and zero-sequence components. These components allow for the simple analysis of power systems under faulted or other unbalanced conditions. According to this theory every sequence component of the unsymmetrical current flowing through balanced series impedance produces voltage drop of that sequence component only if there is no mutual coupling exists (In power system line impedances and transformer impedances are balance also mutual coupling between them is negligible except zero sequence in case of long parallel lines which need separate discussion). Also power system elements response depends upon sequence currents. Thus under unsymmetrical fault condition power system network can be seen as combination of three separate networks called as +Ve sequence network, -Ve sequence network and Zero sequence network. Depending upon nature of fault these sequence networks can be interconnected and fault current can be worked out by using network reduction technique.

Pictorially sequence network with their interconnections for different types of fault can be shown as. 

Though at first glance it may feel something very abstract however the matter will be get clear after going through solved example

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