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Generator problem?

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

Reply to
BIGEYE
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Unfamiliar acronyms CHP=? DNO=?

Machine is what frequency and number of poles? Rating 415V, 3PH, x KVA @ PF?

Was it motored or inadvertently energized? (If it was on line and lost prime mover power, it was motored.)

What are the meters on this machine doing? Also, the meters on the other machine?

How erratic was the pf meter? Fast, slow, oscillating (periodic motion)?

My first suspect is the voltage regulator of the motored machine; either a failed component or improperly connected PT or CT. If the droop compensation circuit was connected wrong, this machine would fight the var sharing controls of the other machine.

Is the voltage stable when the machine is isolated?

The problem occurs at what loading?

Were the instrument transformer circuits checked after the failure?

Suspect #2 is the field circuit of the affected machine (a long shot).

Is the field resistance unchanged?

What kind of excitation system does this machine have?

What was repaired following the motoring incident?

Reply to
Fred Lotte

How does the unit of concern behave when it is the only tied to the system? Was its voltage regulator checked under load conditions?

Reply to
Don Kelly

Guess it all depends on just how complex the regulator design is. But a first test would be to monitor the field current when you put the unit on-line. If the field current swings with the pf meter, then its definitely the regulator.

If the voltage sensing portion itself were oscillating, you would have seen this before connecting the unit to the bus.

But a related possibility is the voltage droop circuits of the regulator (or line compensation if it has them, they are basically reverse droop circuits). If there is a cross-current connection to the other CHP, that would also be worth looking at.

Or, if the regulator has some sort of field current limiting circuit. If it senses a overload and suddenly kicks in it would reduce field current for a few moments and then drop out allowing field current to rise again. An overexcitation circuit that is misadjusted might do this too.

daestrom

Reply to
daestrom

Bearing in mind the earlier circuit-breaker failure are you absolutely sure that you have not got an open circuit somewhere in your circuitry on one phase ? I can not quite see how, but it sounds the sort of effect that can be caused by ferro-resonance. The classic form of this involves creating a series circuit with cable or powerfactor capacitance feeding a transformer because a fuse has blown ib a three phase circuit. The result is overvoltages. If somhow this is occuring on your generator between the power factor bank and say the voltage transformer you could get violent oscillations on the output from the voltage transformer, and may be generate a high enough voltage to explain the circuit-breaker failure that occurred

John

Reply to
John Rye

Nice to see there are others around apart from those who inhabit the c.s.a newsgroups :-)

Stuart

Kinetic on 4.39, SA RPC about to be 4.39 and VARPC adjust.

Reply to
Stuart

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