100kw asynchronous generator power factor
100kw asynchronous generator power factor
(OP)
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
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





RE: 100kw asynchronous generator power factor
Bill
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"Why not the best?"
Jimmy Carter
RE: 100kw asynchronous generator power factor
I'm not that familiar with induction generators. This one has a field circuit, I guess that means it's doubly fed?
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
My question is wheter the wiring scheme within the generator have a pronounced effect on power factor.
Thanks - Walt
RE: 100kw asynchronous generator power factor
Now your question:
So it's an asynchonous machine with a wound rotor and no external excitation to the rotor, right? [WRONG, SEE NEXT POST]
Assuming that is the case:
1 - I don't think you can reconfigure the rotor without a rewind.
2 - Even if you rewind, I don't think rotor winding configuration has much to do with excitation current at all. Consider the magnetizing circuit reactive requirement which is a large part of the total reactive requirement. The magnetizing portion is constant from no-load (where no rotor current) to full load and depends on stator configuration and iron configuration… independent of rotor winding configuration. The only reactive draw you could hope to influence would be the load-dependent vars consumed by the rotor leakage reactance (which is a relatively small portion of total vars consumed in the machine).
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
The unit was not operational when we purchased the plant due to a short caused by water infiltration, 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.
Thanks - Walt
RE: 100kw asynchronous generator power factor
RE: 100kw asynchronous generator power factor
Since it is squirrel cage machine (vs wound rotor), now we know you're talking about the stator winding. Stator changes could have more influence on reactive requirements than the rotor changes I was envisioining when I thought it was wound rotor.
I assume you are not talking about a rewind but just a change in connection. I don't work with induction generators, so I may not be the one to answer your questions, but some relevant details will include:
What is the nameplate voltage?
What voltage does the machine operate at? How is the voltage controlled?
What value of power factor correction capacitors are attached?
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
Where was the power factor measured? Power factor on the generator leads and power factor on the grid leads would be helpful.
KVARs of the capacitor bank may be useful.
A note about generator power factor ratings:
A name-plated power factor of 80% means that the generator is rated for 100 kW/0.80 PF = 125 KVA. This means that the load limit on the generator is 125 KVA.
KVA is voltage dependent so divide KVA x 1000 by the rated voltage to get the rated current.
This is the bottom line. This current should never be exceeded at any voltage.
Why a PF of 80%? Many loads operate at a reduced power factor. It is common to apply a prime mover of 100 kW capacity to a generator of 125 KVA. This is to allow low power factor loads (down to 80%) to be powered without overheating the generator. If the load power factor drops below 80% you should limit the current to the rated current.
The minimum capacity of a capacitor bank should be enough capacity to supply the reactive curren demands of the generator and to improve the power factor of the connected load to 80%.
Generators are typically wound with 12 leads to facilitate operation at two voltage classes and to allow re-connection for single phase use.
If a machine is being rewound for a known application it is common to make permanent connections internally and to only bring out 6 or 4 leads.
If the machine has been correctly rewound and connected then the internal connections should not make a difference.
Bill
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"Why not the best?"
Jimmy Carter
RE: 100kw asynchronous generator power factor
Operating voltage is 480 +/- and is determined by grid voltage
Current on generator leads are 225 amps
Current on grid leads are 128 amps
Current on capacitor leads are 167 amps
Power factor on generator leaks is .57
Power factor on grid leads is .98
Rated output is 100kw
Actual output is 100 to 105 kw
So rated current is 125*1000/480 = 260. Appreciate all the input as I'm still climbing the learning curve.
Walt
RE: 100kw asynchronous generator power factor
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
I = P / (sqrt3*V*pf) = 100*1000/(sqrt3*480*0.8)=150A
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
Your numbers add up better if your grid power out is at 0.98 leading than if you are at 0.98 lagging. Was that the intent? Can you confirm whether it is 0.98 leading or lagging.Excess capacitors may tend to drive up voltage at generator terminals for a given system voltage. Do you know actual generator terminal voltage?=====================================
(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
you might want to check my math on the leading or lagging
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
Walt
RE: 100kw asynchronous generator power factor
Sorry, I was wrong on that piece. When I rechecked (attached) it looks like your current to system is 0.98 lagging as expected. So disregard the quoted comment.It depends if there is an impedance between generator and grid (example transformer, possibly transmission line). Particularly var flow through reactive impedance elements creates voltage drops. I was wrong on the thought process that led me to that point (leading or lagging), but still the voltage at generator terminals is more relevant than a remote measurement, particularly if there's a transformer or long line between.
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
Dang, reversed myself back to original conclusion. I had Ic with the wrong polarity in my last attachment.
Attached is 2nd version which suggests you are over-corrected (your caps are sending vars to the system).
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
Attached document3 is a clarification of previous attachment (document2) where I have now corrected the comments labeled "hypothesis" to match the label of the variables (conclusion is unchanged that I_system is 0.98 leadingAttached is final... "document3.pdf"
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
It shows your numbers are roughly consistent with I_system "leading" (means the system is providing a portion of the reactive requirements of the generator).
It also provides comparison of nameplate and actual conditions (which are discussed a few posts later in this thread).
[this post has been edited from its original version.]
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
Keith Cress
kcress - http://www.flaminsystems.com
RE: 100kw asynchronous generator power factor
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
RE: 100kw asynchronous generator power factor
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)' ?
RE: 100kw asynchronous generator power factor
Walt
RE: 100kw asynchronous generator power factor
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)' ?
RE: 100kw asynchronous generator power factor
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
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(2B)+(2B)' ?
RE: 100kw asynchronous generator power factor
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