Overvoltages and Insulation Coordination in MV and HV Electrical Networks

Insulation coordination is a discipline aiming at achieving the best possible technico-economic compromise for protection of persons and equipment against overvoltages, whether caused by the network or lightning, occurring on electrical installations. It helps ensure a high degree of availability of electrical power. Its value is doubled by the fact that it concerns high voltage networks. To control insulation coordination:

·        in the level of the possible overvoltages occurring on the network must be known;

·        n the right protective devices must be used when necessary;

·        n the correct overvoltage withstand level must be chosen for the various network components from among the insulating voltages satisfying the particular constraints.

The purpose of this "Cahier Technique" is to further knowledge of voltage disturbances, how they can be limited and the standards to ensure safe, optimised distribution of electrical power by means of insulation coordination. It deals mainly with MV and HV.

1. Overvoltages

Power frequency overvoltages

Switching overvoltages

Lightning overvoltages

2. Insulation coordination

 Definition

Clearance and voltage withstand

Withstand voltage

Insulation coordination principle

3. Overvoltage protective devices

Dischargers

Surge arresters

4. Standards and insulation

 HV insulation coordination coordination as in IEC 71

5. Coordination applied to

Breakdown consequences

Reduction of overvoltage risks and level

6. Conclusion

Appendix 1: propagation of overvoltage

Appendix 2: installing a surge arrester

Maximum safety clearance

Cabling the surge arresters

Appendix 3: electricity standards

Appendix 4: bibliography

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