unbalanced 3 phase generator load
- Thread starter PLR
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Phil Corso
Senior Member
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- Boca Raton, Fl, USA
PLR...
The Ampere-Imbalance-Factor (AIF) is about 70%. This factor results in Positive and Negative Sequence-currents of about 110A, and 80A, respectively. I'm sure the latter value far exceeds the generator's capability!
Can you provide phase-to-phase and phase-to-ground voltage measurements... loaded and unloaded (if possible)?
Regards, Phil Corso
The Ampere-Imbalance-Factor (AIF) is about 70%. This factor results in Positive and Negative Sequence-currents of about 110A, and 80A, respectively. I'm sure the latter value far exceeds the generator's capability!
Can you provide phase-to-phase and phase-to-ground voltage measurements... loaded and unloaded (if possible)?
Regards, Phil Corso
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texie
Senior Member
- Location
- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
A 130 KW genset is likely rated @.8PF so the line current would be 390 amps. I don't see a problem with the load you mention. Also, the major genset manufacturers make so called "long stack" generator ends when this is an issue. I've also seen using setups using a transformer to achieve better balance on the genset. This is one example of why these types of services are not so popular.
masterinbama
Senior Member
- Location
- Huntsville Alabama
Just about every 120/240 Delta service I've seen has about the same imbalance. It's the nature of the beast so to speak.
Tom Jones
Member
- Location
- Northern Ca
Do you have a place where one could read about this? Not terms I have heard of before.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Tom...
An excellent start is: http://users.encs.concordia.ca/~pillay/16.pdf
Regards, Phil Corso
An excellent start is: http://users.encs.concordia.ca/~pillay/16.pdf
Regards, Phil Corso
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Would I be correct in saying, with respect to the voltage imbalance discussed, that the negative sequence component of the voltage would try to rotate the motor in the opposite direction? That gives some physical meaning to the term.
FWIW, I recall very little about symmetrical component analysis and likely just enough to be wrong most of the time. :happyyes:
The paper linked to by Phil is regarding unbalanced voltage. That has yet to be established in the case of the OP. What the OP has is unbalanced currents... but as we all know, voltage and current are related by "law". Generally speaking though in cases such as the OP, current is an effect of the voltage, not the cause. For now, its best to discuss this from the perspective of unbalanced currents.
See if this paper makes a difference....
http://www.basler.com/downloads/negseqcurrent.pdf
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Smart $...
Following is additional information I had presented to another forum to emphasize the erffect of negative-sequence currents:
To analyse unsymmetrical faults, such as voltage unbalance in polyphase circuits, EE's resort to a powerful mathematical tool developed called "Symmetrical Components." Unbalanced voltage and current vectors are replaced with 3-sets of balanced components called: positive-sequence; negative-sequence, and zero-sequence.
The first two react to positive and negative rotating voltage vectors, respectively. All electrical components... generators, motors, transformers, cables, etc... are assigned impedance values for each sequence-type. (At this point, I will ignore zero-sequence because they are used only for unbalanced faults involving ground/earth.)
For analysis of unbalanced supply voltages, the equivalent circuit is that of a 3-phase normal size motor, rotating in the positive sense, coupled to a smaller motor, connected to rotate in a negative sense! The developed current and torque are mostly influenced by the motor's positive-sequence impedance values.
Now here is where the second motor come in. Smaller in size, its stator winding, characterized by negative-sequence impedances, is connected in series with the large motor, but in reverse (negative) phase-sequence. Then, the smaller motor produces a counter torque to that produced by the larger motor. Physically, think of it as being reverse-wired in series with nominal motor's stator windings whose forward torque is countered by the the torque produced by the smaller motor.
Regards, Phil
Following is additional information I had presented to another forum to emphasize the erffect of negative-sequence currents:
To analyse unsymmetrical faults, such as voltage unbalance in polyphase circuits, EE's resort to a powerful mathematical tool developed called "Symmetrical Components." Unbalanced voltage and current vectors are replaced with 3-sets of balanced components called: positive-sequence; negative-sequence, and zero-sequence.
The first two react to positive and negative rotating voltage vectors, respectively. All electrical components... generators, motors, transformers, cables, etc... are assigned impedance values for each sequence-type. (At this point, I will ignore zero-sequence because they are used only for unbalanced faults involving ground/earth.)
For analysis of unbalanced supply voltages, the equivalent circuit is that of a 3-phase normal size motor, rotating in the positive sense, coupled to a smaller motor, connected to rotate in a negative sense! The developed current and torque are mostly influenced by the motor's positive-sequence impedance values.
Now here is where the second motor come in. Smaller in size, its stator winding, characterized by negative-sequence impedances, is connected in series with the large motor, but in reverse (negative) phase-sequence. Then, the smaller motor produces a counter torque to that produced by the larger motor. Physically, think of it as being reverse-wired in series with nominal motor's stator windings whose forward torque is countered by the the torque produced by the smaller motor.
Regards, Phil
Phil...
As stated at the onset of my previous post, I am not armed with a great enough understanding of symmetrical components to challenge your assertions. It just seems to me that your approach to the OP case is that of fault analysis, mostly in regards to unbalanced voltage and current going hand in hand. The OP has not established the voltage is unbalanced. The unbalanced current may simply be a result of different nominal impedances, all at nominal voltage. I do concede however, that torque will be uneven, causing uneven wear on the shaft as well as extra heat byproduct compared to a balanced output of the same amount of power. Yet the current levels are well below rated output levels, so I question whether the effects will lead to premature failure compared to MTBF.
Yes. No damage to the stator winding of the generator. But damage will be done to the rotor winding due to excessive heat generated by -ve phase sequence current circulating in the rotor winding.
Wouldn't one like to get a direct answer from the generator manufacturer? It may not be an answer that one may not want to hear but wouldn't the manufacturer know what effect the unbalance would have on the product that they have designed an manufactured.
I'll agree to extra heat. How can you establish heat will be excessive if you don't know the design parameters???
Just a guess: the current at phase b is small approaching open circuit condition......
So you're saying a 3? generator cannot power a single phase load...???
More specifically a 120/240V 3? 4-W 130kW-rated generator cannot power a 40kVA (giving a benefit of a doubt) 240V 1? load.
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You can certainly say that a 3? generator cannot deliver its full rated output power into a single phase load. Whether it can deliver more than 1/3 of its rated power into a single phase load is a different question and probably will end up involving both voltage regulation and mechanical stresses in addition to purely resistive losses.
A lot will depend on whether the manufacturer has designed all the windings with equal size wire or assumed that there will always be some extra load on the center-tapped 120/240 winding. It is best not to assume but instead to find out from the manufacturer. Typical specifications will describe the allowed imbalance and its effect on output power.
You appear to be choosing words wisely
knowing the proposed single phase load is less than 1/3 the rated output. One thing many forget is that in a delta configuration, 1/3 of a single-phase load's current is handled by the two indirectly-connected windings.I agree with both you and templdl that the best armed people to answer the OP question is the manufacturer... if not the only ones.
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