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 (1- -to-Gr). For extended cable/overhead MV distribution networks with ungrounded neutral, in case of low resistance faults at critical stretch of overhead lines, in [1] it has been evidenced that the temporary overvoltages on healthy phases can be very large, much higher than 3 p.u. (up to 3.5 p.u.). Fault currents can reach twice the value calculated with simplified methods, i.e. neglecting series impedances. In this paper the study is extended to MV networks with neutral grounded by both Petersen coil and compensating impedance (Petersen coil with a resistance in parallel), in normal operation and under contingency, i. e. in case of whole or partial loss of the compensating impedance. It is demonstrated that the presence of Petersen coil, stand alone or in parallel with a grounding resistance, drastically reduces the above temporary overvoltages at values not greater than 1.7-1.8 p.u. Application of simple derived formulas to the case of partial loss of the compensating neutral impedance show that overvoltages can be reduced at 1.82.2 p.u., also in case of MV network having very high capacitive fault current (e.g. 300 A) and long overhead lines. An ATP case study on an existing 20kV large Enel-Distribuzione network reported in the paper confirm that the theorical predicted overvoltages are in the above mentioned range, and that the technical solutions adopted by Enel- Distribuzione [9-15] are able to limit in most cases the overvoltages at values not greater than 1.85 p.u.