Analytical prediction of abnormal temporary overvoltages due to ground faults in MV networks

The paper deals with the temporary overvoltages that build up in radial MV distribution networks following the inception of a 1-phase-to-ground fault. It will be demonstrated that, for extended cable/overhead MV distribution networks with ungrounded neutral, in case of low resistance faults at critical locations along overhead lines, the neutral voltage can reach very large values, significantly higher than 1 p.u. (up to 2.5 p.u.). The attendant temporary overvoltages on healthy phases can be very large, much higher than √3 p.u. (up to 3.5 p.u.). Fault currents are also affected, reaching twice the value calculated with simplified methods, i.e. neglecting series impedances. Quick yet accurate analytical formulas for the prediction of maximum overvoltages, fault currents and critical fault distances are presented, together with their validation by detailed ATP-EMTP simulations. The results of a study on an existing Italian 15 kV–50 Hz extended mixed cable/overhead distribution network are finally presented and discussed.