Generator Startup and Feeder GF Trips

[farmape]

In the early stages of investigating an intermittent GF Trip on a 4160V CB, and was hoping to get some good feedback.
The installation is typical 4160V 3W 3P Metal Clad switchgear lineup, featuring utility main (normal), and a generator main (backup). These 2 mains are interlocked to prevent simultaneous closing. The VCBs have electromechanical protective relays (ABB CO type) The feeder breakers have a GF (51G) Relay (I'm not sure at this point if it is a zero-seq. CT or residual connected from the 3 phase CTs). The feeder breakers feed only DY Distribution Xfmrs.
During Emergency Generator energization, the 51G trips (sometimes). During utility power energization, the 51G never trips. Note this is an open transition transfer, no sync. of sources, few second delay.
So...my suspicion is the generator excitation may be producing a phase imbalance during start-up, large enough to produce zero-seq current and activate the 51G protective relay (which appears set very aggressive (0.25 time delay)).
Is this possible, could the generator voltage imbalance cause this? Any other ideas? Thanks.

 

[living2learn]

Transformer inrush causeing saturation on some of the CT's. Especially true with a residual connection. .25s delay might not be long enought to get the xfmr's energized.

 

[farmape]

Thanks for the reply. Could a voltage phase imbalance (from the generator) cause / magnify a "nuisance" current to the 51G relay? I understand that a 3P 3W residual/zero seq CT(s) will not be affected by phase current (load) imbalance on a 3W 3P system ... However ..... I'm just confused on why this anomaly only occurs during generator power power. When the circuit is powered via utility there is never a 51G trip.