Microgrid DER ( Distributed Energy Resources ) Transformer

YGND–Δ Transformer or YGND–Y Transformer?

• In Microgrids, the configuration of the transformer in the Distributed Energy Resources (DER) is critical in a three-phase, four-wire system.

Problem of DER with YGND–Δ transformer, is not desirable due to its negative impact on:

• Feeder protection and control schemes 

• Overvoltages within the microgrid during the faults, 

• Challenges with open phase conditions

• Ferroresonance

Fact of DER with YGND–Δ transformer - Relay Desensitized

• When DER has YGND –Δ transformer configuration (YGND = substation feeder side – Δ = DER side), a fault happens at upstream between substation feeder and the point of interconnection (POI) recloser, the fault current observed by the substation feeder relay reduced as the size of the DER increased.

• This means, depending on the size of the DER and location of the fault between substation feeder and POC, the substation breaker may not trip for a Line-to-Ground (LG) fault. And in the case of an LG fault, the Δ side of YGND–Δ transformer traps zero-sequence fault current, causing not all fault currents to flow back to the upstream. 

• Hence, many U.S. electric utilities do not permit the YGND–Δ DER transformer configuration on their distribution feeders.

Solution with YGND–Y Transformer:

• Change the DER transformer configuration to YGND–Y, which is an open circuit for a zero-sequence current network and privides line to line voltage connection for DER inverters. Most DER inverters only accept line-to-line voltage at their AC inputs and are not built to accept the ground.

• Upon repeating the fault simulation with a YGND–Y DER transformer, the fault current as observed by the substation relay was consistently 1.0 p.u., regardless of the size of the DER.