System short-circuit currents proposed new calculating procedure for application of interrupting devices and relays

THE current flowing in a short circuit in a simple a-c system can be expressed as a function of time in the form of three exponentially decaying components and a term which is substantially constant. These components are termed the subtransient, the transient, the d-c components, and the steady-state component. In actual systems all these terms are affected to some extent by the number of machines and the phase angles between them and also by their excitations. The phase relations between machines may change during the period of short circuit and thus introduce further complications. Automatic voltage regulators also introduce another factor which tends to increase the short-circuit current, since their action causes increase in machine excitation. Consequently, the accurate determination of short circuit current at any instant may be and usually is quite difficult.