Generator Load Unbalance settings

[ECpower]

The factory default on an Easygen 3000 for generator load unbalance is 15%. Our generator is set up to load share with the utility. We have a fused capacitor bank on the load side of the utility recloser. Gen set is 2mw and the load runs below 700kw. Cap bank has(3)200kvar units, one per phase and wired in a wye configuration. During system startup we had issues with one of the CTs which was open circuited. When the CT failed the capacitor fuse blew as well. Since then, the fuse has been replaced and the system is back up and running. My question is what the maximum recommended %current unbalance for a gen set should be. The default generator protection will not allow the system to run beyond 15% imbalance. Can this be set higher so that a blown capacitor fuse won't shut us down when the utility goes down?

I guess part of the answer will depend how much the current imbalance effects the bus voltage. We haven't tested yet but let's assume it won't change over 3-5%.

 

[ScottyUK]

The biggest problem is heating of the generator rotor caused by the imbalance, not bus voltage itself although imbalance there would cause problems on rotating loads elsewhere.

On larger sets we describe this as negative phase sequence heating, but the problem is the same: currents circulate at 2x line frequency, i.e. 100Hz or 120Hz, in the rotor forging causing it to heat up. 15% is quite a high value, which reflects the small size of the set involved; large utility machines have much lower values. The ability of the rotor to tolerate NPS heating is highly dependent on the machine design, so the manufacturer is really the only place to go for the information.

Can you change the protective device to a ganged type so that one pole tripping disconnects all three poles?

 

[waross]

Voltage unbalances are caused by current unbalances (absent fault conditions). Generators under steady state excitation conditions may drop the voltage as much as 50% under load. Normally the Automatic Voltage Regulator corrects this very well. The implication is that a small current unbalance may cause a larger PU voltage drop on the loaded phase. Or it may hold the voltage on set point but drive the voltage up on the lighter loaded phases.
The issue is not only utilization voltages but rotor heating. Unbalanced currents may result in circulating current in the rotor and eventual rotor damage.
Capacitor fuses don't blow very often.
I would leave the settings as is.
If the set trips on over 15% unbalance, let it shut down and locate and repair the root cause.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ECpower]

Thanks, it sounds like its not a good idea to go any higher. The gen set is actually a 1.2mw instead of a 2mw. It's an old Brown Bavari 1200rpm. I left out some information in that there is a step up transformer Wye-wye between the gen set and the capacitor bank. The capacitor bank is at 25kv l-l and the gen set is only 480v. We'll probably leave the settings alone and just have the operators pull the two good fuses with a hot stick if it ever happens again. Hopefully this was just start up pains.