Protection of MV networks including zig-zag grounding transformers

Zig-zag grounding transformers are often used in Medium Voltage (MV) networks to provide a return path for ground fault current. The protection of these MV networks is challenging. On the one hand, transformer differential unit does not have enough sensibility to detect internal ground faults on the delta side of a YD transformer with the zig-zag transformer in the protection zone, due to the zig-zag current limitation, the power transformer ratio and the presence of capacitive current on the MV feeders. Restricted Earth Fault (REF) unit can be used to increase the sensibility, however special care must be taken for external phase-phase-ground faults with CT saturation. The common directional comparison unit that compares the angle between the ground and neutral currents cannot be used. Instead, the use of the external fault detector that supervises the transformer differential unit is proposed. On the other hand, the neutral directional unit based on the angle between neutral voltage and current wrongly activates the forward fault condition on the healthy feeders. A directional unit based on the magnitude of the zero-sequence impedance instead of on its angle is proposed. Several RTDS simulations are done to study the existence of the mentioned problems with the conventional protection units and to confirm their solution using the new units.