Wye-Delta reverse power flow

[Power0020]

in a simple cycle PP, the GSU is YNd1 and UAT is Dyn11, during initial start of the station, the GSU will feed the UAT to enable generator starting (SFC/Cranking motor), the reverse power flow will result in two deltas connected back to back with the GCB open. see attached SLD.

The question here is how can we control the voltage in a two delta back to back connection during starting? - this may last upto 6 minutes.

 

[davidbeach]

What do you mean by "control the voltage"? The phase-phase voltage shouldn't have any issues; the phase-ground voltage may be different than they are when the generator is running.

 

[Power0020]

Sorry about the confusion, I meant the Line-to-ground voltage, since we have two deltas with no earth reference, I doubt that any delta leg voltage may float.

Any clue?

 

[davidbeach]

Absent any actual ground fault, the phase-ground voltages will be a function of the capacitance to ground of the individual phases. Any 3-phase connections to the energized bus may also influence the voltages.

 

[rcw retired EE]

Typcial designs usually have wye-wye connected voltage transformers on the GSU - UAT- Generator Breaker bus. Both wyes are grounded. In addition to the usual secondary windings needed for synchronization, metering or protection, a broken delta winding with a loading resistor is included. Usually the ressistor and VT can be sized to handle the capacitive charging current and stabilize the phase-to-ground voltage on the bus and teh delta transformer windings.

If your VT's are open delta connected phase-phase, this doesn't work. Look at putting surge arrestors phase-ground.

 

[Power0020]

There are already surge arrestors to ground on the IPBD, however I wonder if the very small leakage current of the SA during normal (or slightly higher!) phase to ground voltages will be sufficient to balance the system and avoid floating terminal voltages?.

Connecting VTs as a star seems very reasonable, why should I worry about sizing them to match the capacitive current?