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Module 4 : Overcurrent Protection

Lecture 14 : Fuse Protection

14.2	Sources of Fault Current
	This lecture deals with overcurrent protection for radial distribution systems. In a radial system with single source, the magnitude of fault current depends upon the following:
	Source contribution (Source voltage and impedance).
	Transformer impedance.
	Motor contribution (Back emf and impedance of induction and synchronous motors).
	Distance of fault from the source.
	Meshed system or a system with multiple sources require directional relays discussed in subsequent lectures. The first step in the overcurrent protection is estimation of the fault current. If the system is radial and fed from a single or equivalent source, this job is simplified. For conservative calculation, utility source impedance should be considered as zero (unlimited MVA supplying capacity). The reason for this assumption is that, changes in utility system, addition of generators, strengthening of transmission network etc. can reduce the source impedance. Consequently, the maximum fault current is limited by the impedance of the distribution transformer. In a single source radial system, typically fault current reduces as we move away from the source (an exception being a system with large motor loads at the remote ends). The transformer short circuit current can be calculated from this formula,
	Isc = [( Transformer Full Load current ) x 100 / (% z)]
	Any motor e.g. induction, synchronous condenser or motor etc. must be considered as a source for calculating fault current in the first few cycles. For the sake of simplicity and because of its large usage, we consider only induction motor load. Further, when calculating short circuit current it is a common practice to lump all motors that are grouped together. In other words, they are treated as if they are on a common bus. Assuming a group motor subtransient reactance (X") of 25 %, the typical contribution is 4 times full load current.