Shunt circuit-breaker installed in ungrounded medium voltage systems

The "Entreprises Electriques Fribourgeoises" (EEF), a Swiss utility, operate their medium voltage system with isolated neutral. In order to comply with the legal requirements for the security of people, the protection relays trip the lines also in the event of an earth fault. This kind of operation is not appreciated by our customers who have to accept too frequent interruptions of supply, mainly due to faults on the overhead lines. A recent study suggests using a shunt circuit-breaker in the HV/MV substations combined with the existing isolated neutral system. The improvement aimed at is to operate continuously our electrical network in case of an earth fault. Our studies show a potential reduction of supply interruptions of over 50 %. The solution with isolated neutral and a shunt circuit-breaker, consisting in grounding the earth- faulted phase to the ground at the HV/MV substation, seems to have all the advantages of a compensated neutral point (Petersen coil) as well as a lower cost (approx. ratio 1:4).