Fluctuating power generation in Hydro generator
Fluctuating power generation in Hydro generator
(OP)
Weird situation. Antique hydroelectric plant, three 750KW 40Hz, 120RPM generators driving a 2.5MW 40Hz. motor, shaft coupled to a 60Hz. generator. Yes, 40Hz. generators.
Power from ONE generator fluctuates 200 - 300KW when paralleled with one or both other generators. Stable when only the one generator is driving the motor. The other two are reasonably stable. Variation is fairly rapid, as fast as can be observed on the GPS relay. Increasing field (over exciting it) seems to stablize power, but increases stator current. Motor power does not fluctuate outside of what is expected.
This is a relatively new condition, did not exist a year ago.
Power from ONE generator fluctuates 200 - 300KW when paralleled with one or both other generators. Stable when only the one generator is driving the motor. The other two are reasonably stable. Variation is fairly rapid, as fast as can be observed on the GPS relay. Increasing field (over exciting it) seems to stablize power, but increases stator current. Motor power does not fluctuate outside of what is expected.
This is a relatively new condition, did not exist a year ago.





RE: Fluctuating power generation in Hydro generator
Bill
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"Why not the best?"
Jimmy Carter
RE: Fluctuating power generation in Hydro generator
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If we learn from our mistakes I'm getting a great education!
RE: Fluctuating power generation in Hydro generator
Does the gateshaft position vary on the unit or is it stable? If it is varying, check out the governor: Air in the oil or dashpot, belt drive slipping, loose linkages in the position feedback...are these the original governors or have they been replaced?
What about the DC excitaion? Is the original AVR still in use? Does the DC excitation current vary? If the governor is acting in an unstable manner this can also cause momentary fluctuations in the field current, and large fluctuations in the stator current. Can you change over to manual excitation and see if that stabilizes the unit?
If it is an excitation problem, check thr PTs for loose connections on the HV and LV sides as well as for bad fuses.
Is the original metering still in use, with the old analog meters one can see what is varying easily. If these were replaced with some kind of digital system, it may be difficult to see what is going on on an instantaneous basis.
just some ideas...
rasevskii
RE: Fluctuating power generation in Hydro generator
rasevskii
RE: Fluctuating power generation in Hydro generator
Also that the exciters are a static system. Has this been checked out, especially regarding PT and CT connections?
rasevskii
RE: Fluctuating power generation in Hydro generator
Gates are not moving.
Excitation varies a fraction of an amp, out of 90ADC. Probably due to brush loss. Drive to analog output control module is stable, fluctuates slightly to regulate KVAR's. I should force it to a constant value, see if I still get power fluctuation.
Generation is stable when problem generator is the only unit driving the 40Hz. motor.
Francis wheels are not in the best condition, but should cause equal trouble when operating as only unit.
Agreed, fluctuating horsepower applied to the shaft should be the only thing that can affect output KW, unless excitation is too low for the load.
RE: Fluctuating power generation in Hydro generator
The turbines, being old, will have likely different cavitation areas.
The regulating system should have a lower load limit that prevents this happening, set differently for each unit, activated only when the unit CB is closed, overridden by a protection trip of course.
BTW can the frequency changer be started from the 40hz side or only from the 60hz side? Does the syn motor have fixed excitation or an AVR?
rasevskii
RE: Fluctuating power generation in Hydro generator
rasevskii
RE: Fluctuating power generation in Hydro generator
Frequency changer is started using one of the units on the 40Hz. side. Has same excitation system as the units, operated to maintain a constant voltage. Generators cannot swing the bus voltage very much. We do not see the power variation in the 40Hz. motor. By the way, these generators are ungrounded wye, motor is grounded wye.
Problem happens regardless of weather. We can tell when the units are sucking air due to low intake water level- output power drops and air is discharged downstream. Power fluctuation due to low level is slower than what we observe on the problem units. Intake & turbine configuration hasn't changed in 100 years. Hmm, I'll take that back, trash racks were changed 3 years ago. Problem was first observed some months after that.
RE: Fluctuating power generation in Hydro generator
Does the stator current as seen on the actual ammeter and the kw as seen on the actual kw meter fluctuate? What is the frequency of this fluctuation? It can be that the old meters are too heavily damped to see any fast oscillation.
It could also be turbulance in the turbine pit due the new trashracks, but nobody noticed it before.
rasevskii
RE: Fluctuating power generation in Hydro generator
IOW the excitation can go too far into the underexcited region at times.
Solution would be to run all the units at unity PF including the FC motor. No sense in having a lot or VARS circulating around.
Are there any VAR or PF meters on the panel...
rasevskii
RE: Fluctuating power generation in Hydro generator
Generators are 12,000V nominal. Make 36A at rated power. Yes, we do try to run at 0 KVAR.
Metering is a Basler GPS100.
RE: Fluctuating power generation in Hydro generator
I still suggest to look at the analog meters (if still in use) to verify any oscillation or hunting in kw or stator amps.
rasevskii
RE: Fluctuating power generation in Hydro generator
First time it was ever noticed unit B was fluctuating, unit A was stable. Either by itself was stable. Put C on with B, no problem. Put C on with A, problem. Turned out unit A had a bad waterwheel.
Yes, units have been inspected.
RE: Fluctuating power generation in Hydro generator
i hope i could help, from my poor point of view.
RE: Fluctuating power generation in Hydro generator
RE: Fluctuating power generation in Hydro generator
The sample rate and filtering of the digital metering may be just that unfavorable so that he readout goes to extreme values.
rasevskii
RE: Fluctuating power generation in Hydro generator
RE: Fluctuating power generation in Hydro generator
Thanks.
Bill
--------------------
"Why not the best?"
Jimmy Carter