Overvoltages due to lightning and switching surges have significant impact on substation design and insulation coordination of power systems. Quantifying and mitigating their impact in and around substations needs to be investigated to allow determination of overvoltage levels and their probability throughout a substation. This facilitates insulation co-ordination procedures with the aim of reducing rate of failure, system outages and restoration costs. To reduce system outages, withstand voltage level is selected such that it will allow flashovers to occur at selected points (e.g. co-ordinating gaps) away from expensive and difficult-to-repair equipment. At high system voltages, it is generally more economical to use overvoltage protection versus increasing the insulation withstand level of equipment.
Zinc oxide arresters are recognized as an effective means to protect against lightning and switching surges. They are characterized by faster action and superior energy absorption capability, in addition to suppressing follow-on AC current, allowing continuity of supply following operation. Current insulation co-ordination practice adopts a statistical approach that involves determination of overvoltage distribution caused by lightning events and switching operations and then relating this to electrical strength of equipment. Such an exercise ensures that dielectric strength of substation equipment remains higher than the level of overvoltage stress imposed on them. Where not met, overvoltage protection is needed. Usually, a protective margin is adopted to ensure reliability of the system. Introduction of overvoltage protection can be used to increase withstand level of a substation.