TABLE OF CONTENTS The current that flows through an element of a power system is a parameter which can be used to detect faults, given the large increase in current flow when a short-circuit occurs. For this reason a review of the concepts and procedures for calculating fault currents will be made in this chapter, together with some calculations illustrating the methods used. While the use of these short-circuit calculations in relation to protection settings will be considered in detail, it is important to bear in mind that these calculations are also required for other applications, for example calculating the substation earthing grid, the selection of conductor sizes and for the specifications of equipment such as power circuit breakers.
Electrical faults are characterised by a variation in the magnitude of the short-circuit current due to the effect of the equivalent system impedance at the fault point, which produces a decaying DC component, and the performance of the rotating machinery, which results in a decaying AC component.
System currents cannot change instantaneously when a fault occurs due to the equivalent system resistances and reactances at the fault point, which result in a decaying DC component. The rate of decay depends on the instantaneous value of the voltage when the fault occurs and the power factor of the system at the fault point. To perform the corresponding calculations, the treatment of electrical faults should be carried out as a function of time,...
