High unbalanced generator phase currents 3

[getshorty]

Has anyone seen a phase unbalance of the scale of 575 Amps, 380 A, 370 A (these currents are estimates but close to actual) at the output of an AC synchronous generator?

A methane gas (dairy digester) powered CAT engine running at 1200 rpm provides power to a 5 MVA generator. The generator step up transformer is a padmount 277VLN/480VLLY-12.47Y kV connected directly to utility 12.47 kV, 3 phase, 4 wire, distribution system via a dedicated (generator to utility feeder) 15 kV class, 1000 ft long, underground cable. The dairy farm is located miles away from the substation at the end of the (radial) 3-phase feeder with small size conductor. The utility's feeder serves other customers including a few small industrial, commerical and other farms.

 

[cranky108]

Not having vectors or anything else, I'd be concerned you have an open phase on the 12.47kv side.

That is assuming the stepup is a delta-wye.

 

[davidbeach]

The circuit is not evenly loaded, by the time it makes it to your site the phase voltage balance is not an even 120[°] between phases all the way around. I've seen similar, even to the point of current out on two phases and in on the third.

 

[jghrist]

One of the problems with a wye-wye connection to the utility is that the generator zero-sequence impedance is relatively low compared to its positive-sequence impedance. This makes the zero-sequence path through the generator low and the generator can end up powering a lot of the utility unbalanced load. There may also be unbalanced phase voltages on the utility system because of single phase voltage regulators that aren't set properly.