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

 

[Don Kelly]

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

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

 

[daestrom]

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.

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

 

[John Rye]

Hello Big Eye

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

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