Connection of a Star Generator to a Star/Delta Transformer

[nateinglis]

Hi There,

We have a 1500kVA Emergency Generator (star wound) onsite that is connected to the Essential Services Board (800kVA), with a synchronizer to allow to load test. The Essential Services Board is connected to the 11kV Site Switchboard via 2 x 11/0.415kV 1500kVA Dyn11 Transformers (6% imp and LV neutral solid earthed). I am trying to figure out if I run the generator at 100% (1500kVA) and backfeed onto the 11kV bus whether it will cause undue circulating currents in the transformers and generators and possibly trip the LV incoming Circuit Breakers. There is no directional protection on the CB's and the OL is set to 2250A for 12 sec; SC is 4000A for 0.3 sec (inverse curve) and Instantanious is set at 7500A.

Currently the Earth Leakage on the two transformer incomers is set at 50A for 1 sec (def time).

Cheers

Nate

 

[waross]

Feeding a wye:delta transformer or bank with a four wire connection is often a bad idea. If the voltages and phase angles are balanced and equal there may not be a problem.
Voltage adjustment is used to adjust and control reactive current.
If there is a voltage unbalance on the 11kV side the reactive current will be unbalanced and you will not be able to adjust the reactive currents to zero on all three phases simultaneously. This is not circulating current but unequal reactive currents.
If there is a phase to ground fault on the 415 Volt system, the un-faulted phases will back-feed through the delta and add to the fault current. This will be seen as a circulating current in the delta. If you are close to a limit on available fault current this added back-feed may become an issue.


Bill
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"Why not the best?"
Jimmy Carter