100kw asynchronous generator power factor 2

[wpuryear]

I have a water turbine driven submerged 100kw induction generator that was rewound a couple of years ago. Power factor is running at .57 which we are correcting with capacitors, however the high current between generator and capacitors is causing heat problems.

The field connections are 12 pole, wye connected, adjacent pole, 4 circuit. Would some other connection scheme provide improved power factor or is this irrelevant?

Thanks,
Walt Puryear

 

[wpuryear]

The 480 measurement is before we go through two transformers - 480 to 4200 and then 4200 to 42k. Our synchronous 2mw generators runs at 4200 but is currently off line for lack of water.

I might add that field coil temperatures are in the 165 to 170 F. range. So, is the low power factor simply the nature of this particular beast or is there some way to improve it? I would certainly like to lower the temperature.

Regards - Walt

 

[electricpete]

You do realize you can now edit your posts in here? A record is even kept.

Aha! . No I didn't realize that. I think I can make good use of that feature! LPS to you. I will go back and clean those up when I get a chance.

So, is the low power factor simply the nature of this particular beast

No, not really according to the numbers in my attachment:
Actual conditions: Ig = 128.25 + 184.8701 i
Rated conditions: Ig_rated = 120 + 90 i

The real part indicates the generator is putting out only slightly more than rated real load. The imaginary part indicates more than double the reactive load of rated conditions. You are welcome to double check the math.


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(2B)+(2B)' ?

 

[wpuryear]

So what would be possible causes of the doubled reactive load?

Walt

 

[electricpete]

So what would be possible causes of the doubled reactive load?

Good question. I guess it's your original question.

If terminal voltage was high, that could cause increased reactive "load" (load referring here to internal var consumption in the generator). You seem to have ruled that out.

If rotor position shifted axially such that rotor iron is no longer centered on stator iron, that could cause increase reactive current drawn by the generator.

I can't think of much else off the top of my head. How long has the machine behaved this way? (was it ever different)?

It might be worthwhile to explore where your power factor number came from. What type instrumentation? Is it developed from three phases? Are they balanced? Does it include contribution from harmonics? (harmonic content can reduce total power factor). Is there any low frequency oscillation present? Got any waveforms?

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(2B)+(2B)' ?

 

[electricpete]

but as I recall, there were considerably more leads connected for each phase (only two per phase now) and thus my question about field connections affecting power factor.

Don't quite know what that's about. You gave machine ratings 100kw, 0.8 pf. Can you provide complete nameplate details including speed, voltage (possibly more than one speed or voltage) and any available information about connection for nameplate conditions. waross had good comments about common connections.

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(2B)+(2B)' ?

 

[electricpete]

I might add that field coil temperatures are in the 165 to 170 F. range

Just a point of information - I have many class B, F, H air-cooled machines that run hotter than that temperature near full loads indicated by RTD between coils in slot and does not concern me becuse it is far below insulation temperature limits (even adding hot spot allowance) and it is normal for many of my machines. Of course I don't know where your temperatures are monitored and what's normal for your machines.

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(2B)+(2B)' ?

 

[waross]

OK! 100 kW and 0.80 PF = 125 KVA
125000 VA / 480 V / 1.73 = 150 Amps. This is for a synchronous generator. See the rated current for the actual maximum current for an induction generator. There will be extra current for excitation.
Your actual current = 225 Amps
Forget kW. You may be running almost 50% over current and over KVA.
Throttle your water flow back until the current drops below 150 Amps or below the rated current.
Often the power available from the prime mover will keep a machine from overloading too badly. Yes there are exceptions.
However when the power factor drops below 80% you can exceed the current limit before you reach the kW limit of the prime mover.
The most important factor in generator loading is the current and the heat developed from the generator current. You can play with the numbers all you want and possibly convince yourself that you are in a safe operation range but if you exceed the rated current you are misleading yourself.

Bill
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"Why not the best?"
Jimmy Carter