4 - Fault calculations

Abstract

It is necessary to know the fault conditions for predicting the performance of a protection scheme. This chapter focuses on the process of determining the fault level during a short-circuit, which requires the knowledge of impedance of various components of the power system and the ability to calculate current in every part of the system. For fault calculations, it is usually sufficient to take into consideration only reactance rather than resistance. Typical impedance values can be attributed to all components of a power system in the absence of definite information. Transformer impedances are usually easy to determine as the value is marked on the rating plate. The impedance of generators is usually of secondary importance as most distribution systems have a much higher in-feed and fault contribution from the public electricity-supply system.