Overvoltages & Insulation Coordination - engineering site

Lif L < 25 m: a surge limiter placed on the pole is sufficientif L 25 m: a second limiter must be placed at the transformer terminals, ,,,,,,,, fig. 33: position of surge limiters on a substation supplied by an overhead-undergroundnetwork.A voltage drop, mainly inductive andwhich may be high, then occurs in theearthing circuit.Example:■ current wave: 1 kA/ µs;■ earth down-cable inductance:1µH/m;■ hence U L = 1 kV/m.If this voltage is not to be added toresidual voltage, the proposedequipment must be bypassed at thearrester terminals as regards the«lightning phenomenon».In practice this consists in connectingas shown in figure 33. If the HV/equipment link is not made on thearrester, conductor length must be asshort as possible (see fig. 34).arrestertransformerwrong cablingright cablingfig. 34: arrester cabling principle: load-arrester connections must be as short as possibletransformer.appendix 3: electricity standardsThere are three levels of standards:The following three organisations act atinternational, European and Frenchlevel respectively:■ the IEC (InternationalElectrotechnical Commission) producesthe «IEC» standards with theparticipation of 60 countries. Standardsare accepted only if opposition is lessthan 20 %. Its application may give riseto waivers in certain countries.■ the CENELEC (EuropeanElectrotechnical StandardisationCommittee) produces the «EN»standards and groups 18 countries,mostly European. Application of thestandards it votes is mandatory.■ the UTE (Union Technique del’Electricité) produces the French «NF»standards.Cahier Technique Merlin Gerin n° 151 / p.23