Lightning and HV electrical installations

Abstract

The goal of this «Cahier Technique» publication is to:

• present an overview of lightning phenomena and their effects on electrical installations,
• present currently available means to protect installations and limit detrimental effects,
• discuss problems concerning continuity of service,
• indicate the main steps in lightning studies on the basis of an example involving an EHV installation developed by the Network Research Department at Merlin Gerin.

This document deals in particular with the transmission and distribution of electricity over medium and highvoltage networks. When designing these networks, the effects of lightning must be taken into account for insulation coordination. Low-voltage aspects are also mentioned, but in no great detail.

Introduction to lightning

Lightning is a major source of disturbances for all electrical installations and can affect them in several manners:

• all power and voltage levels are concerned, ranging from EHV transmission systems to integrated circuits and including LV power supplies and data transmission circuits,
• it can cause transient disturbances to the continuity of service, thereby reducing the quality of the power supply system,
• it can damage equipment and result in long interruptions in installation operation,
• it can be dangerous for life (pace voltage, increased potential of exposed conductive parts and earthing circuits).

Lightning has always been a source of disturbances for users of electricity, yet the fairly recent and growing demand for quality electrical systems (reliability, availability, continuity of service, etc.) must be taken into account, as well as the permanent necessity to minimise the costs of the production and the use of electrical power. It may be said that in the efforts to improve the above factors, lightning has come to constitute an obstacle. That explains why it is now one of the major preoccupations of everyone in the sector, whether they are distributors (EDF, private companies), manufacturers (Merlin Gerin, etc.), designers (design offices, engineering firms) or installers.

A study on the effects of lightning comprises two steps, but first requires in-depth knowledge of the phenomenon. Starting in the 1970’s, major international research programs were initiated, notably by EDF in France, and today, sufficient knowledge on lightning mechanisms is available. The two steps are:

• anticipate what can happen in a given installation and recommend improvements. This is possible using dedicated software, validated by experience, that simulates installation behaviour.
• carry out an engineering and cost study on insulation coordination, taking into account the cost of installations, maintenance and disruptions in operation.

Note: insulation coordination consists in defining, on the basis of the voltage and overvoltage levels likely to occur in an installation, one or several levels of protection against overvoltages, then in selecting installation equipment and protection devices.

AUTHOR: Schneider Electric expert | Benoît de METZ-NOBLAT