Introduction

The sequence circuits and the sequence networks developed in the previous chapter will now be used for finding out fault current during unsymmetrical faults.

Three Types of Faults

Calculation of fault currents

Let us make the following assumptions:

• The power system is balanced before the fault occurs such that of the three sequence networks only the positive sequence network is active. Also as the fault occurs, the sequence networks are connected only through the fault location.
  
• The fault current is negligible such that the pre-fault positive sequence voltages are same at all nodes and at the fault location.
  
• All the network resistances and line charging capacitances are negligible.
  
• All loads are passive except the rotating loads which are represented by synchronous machines.

Based on the assumptions stated above, the faulted network will be as shown in Fig. 8.1 where the voltage at the faulted point will be denoted by V_f and current in the three faulted phases are I_fa , I _fb and I _fc .

We shall now discuss how the three sequence networks are connected when the three types of faults discussed above occur.

Fig. 8.1 Representation of a faulted segment.