7.1 Introduction

In this chapter we review some aspects of insulation coordination and overvoltages on transmission networks. The purpose of insulation coordination is to ensure that the probability of insulation breakdown is limited to an acceptable value and that any breakdown is restricted to self-restoring insulation. It is based on computing the most severe overvoltages occurring on the network and relating these to the breakdown characteristics of the insulation through appropriate margins to obtain withstand voltages for the network components together with the statistical risk of insulation failure. The procedure is summarised in Figure 7.1.

Figure 7.1: Insulation coordination

From the network configuration and component data, an appropriate simulation of the network is formulated and used to compute the overvoltages, taking into account the effect of control and protective devices. The breakdown voltage characteristics are based on the type and configuration of the insulation. The coordination procedures relate these two components to give withstand voltages and to quantify the risk of insulation failure.

Coordination is an integral part of the design process, giving criteria that are satisfied through insulation design and through overvoltage control and protection. Although simple in concept, involving comparisons between maximum network overvoltages and minimum insulation breakdown voltages, in practice the process is complex.

Network overvoltages are usually the result of some sudden change in conditions, typically switching operations, lightning strokes or faults. They have a very wide range of waveshapes and durations, the most important being:

• temporary overvoltages - lightly damped...