Whenever I hear about using a delta-wye generator step up transformer in reverse to feed plant service, I think of the Detroit Dam Powerhouse fire that occured in 2007. Detroit Dam Powerhouse is a 100MW plant in Oregon, USA, owned and operated by the US Army Corps of Engineers.
The plant has two 40MW generators and two 10MW generators 3 miles downstream at a re-regulating dam. All are 13.8kV, with two step up delta-wye transformers to 230kV at the upstream powerhouse. Each transformer has a bus with one 10MW and one 40MW generator. Station service is from two 13.kV:480V delta-wye transformers, one off each bus. Each station transformer feeds 1/2 the station service, with the ability to transfer all to one transformer. There is no interconnection between the two 13.8kV busses.
Each generator has an independent neutral grounding resistor. There is ground fault relaying triggered by each generators' neutral resistor and bus differential, either one tripping both the 230kV breaker and both generator breakers. There is also ground fault monitoring on the bus to alarm on a ground fault if all generators are offline. There are 14.4kV (L-N) rated MOV arresters at multiple locations on the 13.8kV system.
They experienced a broken insulator on the connection to the downstream power house, and the bus tripped on ground fault and differential. The operators of the plant decided to re-energize the 13.8kV bus to restore 1/2 of the station service, rather than transferring it all to the other bus. They thought that since the system is floating, it would be OK to leave it energized with a persistent ground. The problem was that it wasn't a solid ground fault, but a restriking arc. (A restriking arc is where there is an air gap in the ground fault. System capacitance will attempt to keep the system centered with respect to ground, but as the phase to earth voltage builds, it will arc over. The arc drops the phase to near zero abruptly. Without current to sustain the arc, the arc stops and then the voltage will build again. It is quite common for restriking arcs to arc at twice the line frequency.) The restriking arc created over-voltages on the other two phases, and after a while they overwhelmed the MOV arresters and those arresters failed violently. (Re-striking arcs create over-voltages on the other phases because the transformer leakage inductance and phase stray capacitance create a resonant circuit that rings every time the arc strikes.) When the arrestor failed, it created a phase to phase fault before the differential picked up. The fault managed to start a fire in the switchgear that spread throughout the cabling in the powerhouse.
It created $9 million in damage, created chaos as they had to rush to bring in generators to restore power to the spillway gates to prevent over-topping the dam, and took more than a year to recover.
All of that has led me to the opinion that if you are going to back feed a delta-wye generator step up transformer as a regular practice, put a grounding transformer on the bus.
