TABLE OF CONTENTS T. Irwin and H.M. Ryan
The international standard covering insulation coordination is IEC publication 60071. According to the IEC, insulation co-ordination comprises the selection of the electric strength of equipment and its application, in relation to the voltages which can appear on the system for which the equipment is intended and taking into account the characteristics of available protective devices, so as to reduce to an economically and operationally acceptable level the probability that the resulting voltage stresses imposed on the equipment will cause damage to equipment insulation or effect continuity of service.
The operation of a transmission system will present the system insulation with various types of voltage stress from the continuous voltage at which it must operate to various forms of overvoltage.
Overvoltages can be classified into two groups. The first group is externally generated overvoltages from lightning which manifests itself in two ways: by direct strikes to the lines or substations and by strikes to the towers or overhead earth wires from which back flashover voltages from the towers to the conductors can occur. The second group comprises internally generated overvoltages due to switching. These include the switching of capacitive and electromagnetic loads and travelling waves due to the energising of transmission lines. Also taken into consideration are power frequency overvoltages which are caused either by load rejection or by voltage changes on the two healthy phases when a single phase fault occurs.
Up to about 300 kV experience indicates that the highest voltage stress...
