Generator Problem

[BIGEYE]

The Setup
11 kV comes in from DNO onto a common busbar via our 11 kV CB.
On the same busbar I have 2 CHP alternators. They supply 11 kV onto bus by way of 415/11000 V step up transformers. Normal operation is for either or both CHP's running in parallel with DNO.

History
One of the CHP's recently had a catastrophic failure of it's 11 kV circuit breaker. Prior to the breaker failing, the alternator was allowed to run as a motor for some 8 minutes.
All repairs have now been carried out and the alternator tested and put back into service.

Problem
I was in the sub station the other day and noticed that the power factor meter on our incomer from the DNO was bouncing all over the place, from about 0.5 lagging to 0.5 leading and was very unstable and erratic. This also caused our power factor to trip off on unstable power factor. At the time, both CHP's were running in parallel with the mains.
I shut down the CHP that had the circuit breaker problem as described earlier, and the power factor stabalised at 0.7 lagging. I reset my power factor correction and my PF rose to 0.9 lagging and was stable.
Restarted and brought on load the CHP alternator, and again my PF went crazy. Eventually, shut down the machine and it stabilised again.

Everything is OK when running on mains only or mains in parallel with the other CHP.

Can anyone advise what may be the problem, how do I diagnose, and how do I fix.

TIA

[ajay]

Since, the problem occurs only when parallel with the DNO, the probable cause for this fluctuation could be the mode in which the Excitation control for the alternator is functioning.

When parallel with the utility, the excitation control is generally in the pf mode, wherein the pf of the machine is kept constant at a set point. This requires a feedback to be connected to the excitation control saying that the machine is running in parellel with the utility. this feedback is generally taken from the breaker of the utility.

Should this fail, the machine may not know that it is running in parallel with the utility and may not go into the power factor mode.

Ajay
www.electrotechnik.net