Table 3 Winding configurations of interface transformers and associated protection challenges
Configuration | Advantages | Disadvantages | |
|---|---|---|---|
HV (utility-side) | LV (DER-side) | ||
YG | YG | Low ferroresonance sensibility in cable-fed applications System voltages in HV and LV sides are in-phase Mitigates TOV after ground faults on both sides | Permits circulation of triplen-harmonics (zero-sequence) on both sides Has an effect on relay coordination Permits the DER to feed ground faults in the utility zone and vice versa, which increases damage |
Δ | YG | Prevents the DER to feed ground faults in the utility zone and vice versa. Blocks triplen-harmonics at DER side to flow in utility | Ungrounded side expose utility to TOV during ground faults Permits circulation of triplen-harmonics (zero-sequence) at DER side Highly sensitive to ferroresonance in cable-fed applications |
YG | Δ | Blocks triplen-harmonics at DER side to flow in utility Prevents the utility to feed ground faults in the DER zone No TOV at utility side due to ground faults | Permits circulation of triplen-harmonics from utility which heating-up the transformer Participates in utility ground faults increasing the damage level Increases possibility of sympathetic tripping towards adjacent ground faults due to the transformer contribution Ground-relays settings are dependent on the existence of the transformer in service to keep proper coordination and sensitivity of relays |