Switching in Electrical Transmission and Distribution Systems

2 Faults in Power Systems

2.1 Introduction

With the increasing dependence of our society on electricity supplies, the need to achieve an acceptable level of reliability, quality and safety at an economic price becomes important to customers. Mostly, the power system is well designed and adequately maintained to minimize the number of faults that can occur.

In normal operating conditions, a three-phase power system can be treated as a single-phase system when the loads, voltages, and currents are balanced. In practice, one can successfully use a single-line lumped-element representation of the three-phase power system for calculation.

A fault brings the system to an abnormal condition. Short-circuit faults are especially of concern because they result in a switching action, which often results in transient overvoltages. Both the high current and the switching overvoltages stress the equipment in the system.

Line-to-earth faults can result from direct contact of an overhead line to earthed objects, such as trees, falling limbs of trees, cranes and so on. Also, reduced isolation can lead to a short-circuit, for example in the case of a faulty component or initiated by ionized air above a bush-fire or pollution of insulators. Also, erroneous switching operations or human errors, for example, during maintenance or repair, can induce short-circuit.

The main failure mode, however, is the lightning back-flashover [1], in which a lightning stroke in an overhead shield wire or tower ...