Induction Generator Negative Sequence Current Trip

[kmh1]

We have a 256 kW, 600 V induction generator coupled to a hydro turbine. We also have a 120 kVAR capacitor bank which switches in step after the generator in online. The unit is connected to a relatively small residential utility distribution system via a 600/4160 volt step up transformer (HV grounded wye - LV ungrounded delta).

Recently, after the unit was briefly shutdown for some mechanical work, the generator began tripping on negative sequence timed overcurrent (pick up set to 40 amps)at higher loads. LV system ph-ph voltages range from 601 to 608 when the unit is online. We are seeing a 12% current imbalance from the generator to the grid (capacitor currents only have a 4% unbalance).

Can any one give me a typical unbalance trip setting for an induction machine such as this? Could this possibly be a problem with the windings in the generator?

 

[alehman]

Sounds like your load is unbalanced. That wouldn't be unusual for a small residential system. Can you provide more description of your system. Is the generator operating in parallel with another source?

Consult the turbine-generator manufacturer for maximum tolerable unbalance and recommended trip settings.

 

[alehman]

You said induction generator. What is the other source? Have you measured the load currents when operating from that source alone?

 

[kmh1]

The other source is a utility distribution transformer with I believe a 25 kV secondary (I am not sure of the size or loading). The generator step up transformer is connected to the secondary of a 4160 volt distribution transformer which services a small apartment building. It is likely that due to primarily single phase loads which vary seasonaly that the loading unbalance (and voltage unbalance) will vary. I will consult with the generator manufacturer as to a recommended tolerance for unbalanced loading.