Requirements for rating of MV zinc oxide surge arresters on EDF distribution networks

To improve the quality of service on MV networks, EDF has decided to replace its spark gaps with ZnO surge arresters. The author essentially presents the rating of the zinc oxide varistors, which is based on a statistical survey of lightning strokes. The energy levels likely to be dissipated in the ZnO surge arresters are calculated on the basis of the nonlinear characteristics of the surge arresters and the lightning current which actually flows through them. The choice of a failure rate of about 1.10-3 per year, led to specification of withstanding 4/10 surges of 65 kA for the French network. In addition, the rises to the phase-phase voltage caused by single-phase line faults are frequent. An original test consisting of repetitive voltage rise cycles was defined to guarantee the lifetime of the varistors