Overvoltages & Insulation Coordination - engineering site

6. conclusionInsulation coordination aims at findingthe right balance between equipmentreliability from a dielectric standpoint,on the one hand, and their sizing andthus cost, on the other.The presentation made in thisdocument shows the complexity of theparameters involved in such ananalysis.Moreover, the statistical aspect ofbehaviour to transient overvoltagesrules out the possibility of absolutesolutions.Although the modellings chosen mayappear somewhat arbitrary at first sight,they have been confirmed byexperience.More detailed information can be foundin the publications quoted for readerswishing to examine the subject ingreater depth. The progress made inknowledge of phenomena now ensuresincreased installation reliabilityalongside optimisation of economy andelectrical operating stresses.Increasing use of arresters, partly dueto improvement of their characteristicsand reliability, contributes to greatercontrol of protection levels.Consideration of this aspect byinternational standard committees, bothgenerally and as regards productrecommendations, is proof of theimportance of the subject and of itsassociated advantages.appendix 1: propagation of overvoltageWhatever the origin of the overvoltage,it will propagate along the line or cablemaking up the network.This propagation support can bemodelled by using values per lengthunit of inductance and resistance inlongitudinal and of capacitance andconductance in transverse (see fig. 30).The impedance, in sinusoidal state, isthen given by:L ω + RZ =C ω + GAt the high frequencies generallyassociated with overvoltages, theinductive and capacitive terms becomepreponderant. The impedance knownas "characteristic impedance" thenequals:Lfig. 30: modelling of a propagation support.evolutionin timeRCV600 kv1/G2 µsPropagation speed approaches that ofthe velocity of light, i.e. approximately3 x 10 8 m/s. This speed can also besaid to be equal to 300 metres permicrosecond, thus providing anestimation of the distribution along theconductor of a very short term wavefront (see fig. 31).The theory of guided propagationmakes it possible to establish that,when a wave propagating along afront: 300 kv/µsLZc =CThe resistive and conductive termscorrespond to losses causing waveattenuation during its propagation.The magnitudes of the characteristicimpedances are:■ EHV lines: 300 to 500 ohms;■ HVA lines (overhead); approximately1,000 ohms,distributionin spaceVfig. 31: representation in time and space of a lightning wave.t (at constant x)front: 1 kv/m600 mx (at constant t)Cahier Technique Merlin Gerin n° 151 / p.21