Protection against overvoltages using gapless zinc oxide surge arresters is now widely adopted worldwide for electrical power networks. In particular, key assets such as transformers, cables and gas-insulated substations are vulnerable to transient surges emanating from lightning and switching surges. The performance of ZnO gapless arresters under these types of overvoltages is superior to previously adopted coordinating gaps and silicon carbide gapped arresters. However, the effectiveness of the surge protection and the resulting protection margin for the equipment depends highly on the physical distance between the arrester and the equipment to be protected.
Since overvoltages due to lightning and switching surges have significant impact on substation design and insulation coordination of systems, lightning and switching surge scenarios in and around a substation need to be investigated to quantify and mitigate their impact. This will allow determination of overvoltage levels and their probability throughout the substation, which then facilitates the applied insulation co-ordination procedure. The aim is to reduce the rate of failure, system outages and cost of restoration. In order to reduce system outages, the withstand voltage level is selected such that it may allow flashover to occur at selected points (e.g. co-ordinating gaps) but near expensive and difficult-to-repair equipment, e.g. power transformers. At high system voltages, it is more economical to use overvoltage protection compared with increasing the insulation withstand level of equipment.
Modern zinc-oxide surge arresters are now recognized as an effective means to protect against lightning and switching surges. They are characterized by fast action and superior energy absorption capability, in addition to suppressing follow-on AC current and hence allowing continuity of supply following operation. Current insulation co-ordination practice adopts a statistical approach. The procedure involves determination of the overvoltage distribution caused by lightning events and switching operations and then relate this to the electrical strength of the equipment. Such an exercise consists of ensuring that the dielectric strength of substation equipment is higher than the overvoltage stress level imposed on them. Where this is 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 the withstand level of the substation.
In the United Kingdom, 420 kV substation equipment have a standard lightning impulse withstand voltage (LIWV) of 1425 kV. In IEC 60071-2, it is recommended that the highest occurring voltage on the terminals of equipment with non-self-restoring insulation should be reduced by a factor of 1.15 (1239 kV). The minimum protective margin for switching overvoltages is 15% and, for lightning events, is 25%. Commonly adopted 400 kV surge arresters present a determined LIWV. This value is obtained from standardized tests in the laboratory. However, the terminals of the equipment in the substation, where surge arresters have been applied, can experience significantly higher voltage magnitudes due to location of the protecting arrester.
Attend the 2017 INMR WORLD CONGRESS in Barcelona-Sitges from November 5 to 8 to hear a paper by Professor Manu Haddad that explores various generic scenarios of substation layouts and equipment to determine the lightning and switching surge levels, with and without surge arrester protection. The location of the surge arrester on the afforded protection is also investigated.