Fault current calculations

Calculations of fault current at different busses of complex power system and contributions of fault current by different elements in the power system are done using computer programs. These computer program utilizes matrix models of power systems. Using this program we can have fault currents at various busses in power system (for 3-Ph and 1-Ph faults at that busses). However for radial network or very small network under consideration we can do the calculations manually (or using Excel) using network element modeling discussed above and by using normal network reduction techniques. To explain the method let us use same power system as discussed above. Let us find the fault MVA (and current) for 3-Ph fault at 132kV bus.

Formula for calculating 3-Ph fault current

Consider a 3 phase fault at one bus in a power system as shown in figure. Every network between source and fault point can be reduced to a single impedance connected between source and respective points by using network reduction technique. Thus for the power system shown; the pu network between fault point and source can be reduced to a single source having unit voltage (rated source voltage) and single pu reactance (Xs). In this case 3-Ph short circuit MVA supplied by source would be

It is customary to calculate short circuit MVA. Hence-

Once we know the short circuit MVA it can be converted to corresponding short circuit current simply by multiplying this value by a conversion factor for respective voltage. The conversion factor for most common voltage levels are as shown in flowing table

SrNo	Voltage Level	Ampere equivalent to 1 MVA
1.	400kV	1.44
2.	220kV	2.625
3.	132kV	4.375
4.	33kV	17.5

+Ve Sequence Impedance of the Grid Source.

 As discussed previously the fault currents for faults at busses in a complex grid are calculated using computer programs. Hence using reverse engineering the complete grid for which such fault currents already known can be replaced with a source in series with reactance having value

Here subscript 1 represents +Ve sequence reactance of the grid. More about sequence impedances is explained through respective sections.

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