Pl. help me in sizing the grounding transformer for delta side of Gen. Trf. (525/23kV, 720MVA, YNd1). We have a Generator circuit breaker connected between Gen. Trf. and Generator. There is also a tap-off to Unit Transformer connected between Gen. Ckt. Breaker and Gen. Trf. Generator is 776 MVA, 23kV and grounded through an NGT with resistor. When Generator Circuit breaker is open (Off, the delta side of Gen. Trf. is ungrounded. Our Client wants us to provide a grounding transformer with isolator which will open when Gen. Ckt. Breaker is closed. It will be required only during back-charging from the grid when GCB is open. How to size the grounding transformer, isolator and what protection is to be provided for this grounding transformer.