Generator voltage output mismatch
Generator voltage output mismatch
(OP)
Dear all, on site we have a 170MVA generator with 15kV voltage output, Y connection, earthed through transformer and resistor.
On this generator we notice from the first day of operation a consistent difference on the terminal voltage, meaning that on normal operation we measure phase A 14.99kV, phase B 15.15kV, phase C 15.18KV.
This is noticed during full speed no load (when the generator circuit breaker is open) and when the generator is synchronized to the grid via the step-up transformer.
Can you give me some points of searching or testing?
Voltage is measured on the terminal box of the generator (before the star connection) and on the brekaer side.
On this generator we notice from the first day of operation a consistent difference on the terminal voltage, meaning that on normal operation we measure phase A 14.99kV, phase B 15.15kV, phase C 15.18KV.
This is noticed during full speed no load (when the generator circuit breaker is open) and when the generator is synchronized to the grid via the step-up transformer.
Can you give me some points of searching or testing?
Voltage is measured on the terminal box of the generator (before the star connection) and on the brekaer side.






RE: Generator voltage output mismatch
The PTs ration can be checked using a TTR and the meter can be checked with a relay test set. I think you'll find that errors in one of the two components account for your issue.
As far as I'm concerned your readings seem within the normally expected error band.
Best Regards,
JB
RE: Generator voltage output mismatch
RE: Generator voltage output mismatch
respectfully
RE: Generator voltage output mismatch
JB
RE: Generator voltage output mismatch
respectfully
RE: Generator voltage output mismatch
Are you measuring at the VT secondaries or reading data from a DCS screen? Measure the VT secondaries directly using the same meter for all measurements. After that the suggestions to 'roll the VTs' is a good one.
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RE: Generator voltage output mismatch
JB
RE: Generator voltage output mismatch
So... is it real or just a transducer error?
RE: Generator voltage output mismatch
RE: Generator voltage output mismatch
I must say your answer surprises me. Not the discussion of concerns related to negative sequence currents, but rather the magnitude of voltage imbalance being a concern to you.
I'm used to seeing a negative sequence current relay/function set in the 10 % range. Granted, I usually deal with gens in the 5MVA range. Would you expect a 1% voltage mismatch to cause negative sequence currents around 10% of rated curent, or are the larger gens just protected more tightly? I had a gen manufacturer report that their unit could handle 70% negative sequence current a couple of years ago!
Of course, if 1% is too much error, the next logical question is what is the threshold that is tollerable? Then there's the question of PT accuracy. A pair of .5% accuracy PTs can't be trusted if they show a mismatch of 1% becuase one could be .5% high and the other .5% low at precisely the same voltage.
Looking forward to your response.
JB
RE: Generator voltage output mismatch
On the subject of VT accuracy, I would expect the VTs to be a set of identical transformers. In my experience wound transformers of the same design, and very probably from the same batch, are usually very well matched even if their absolute accuracy might be Class 0.5. The chances of getting one at the top of the tolerance and another at the bottom is very small. Of course if the VTs are of different designs then it is quite likely that the matching will be poorer. I've not seen a generator supplied with different designs of VT from original build but I guess it could happen somewhere. Assuming that the VTs are of identical design, a significant mismatch in secondary voltage would suggest that the primary voltage has a similar mismatch.
The NPS current relay is usually set around 10% or so with a definite minimum time function. The UK Grid Code has a requirement continuous operation with a defined level of negative sequence voltage / current present. I'll have to check what our is (at home right now). We keep a close watch on anything unusual which could lead to generator damage simply because a big generator is such an expensive asset. Voltage imbalance is an anomaly which would get looked at because we do not typically see an imbalance greater than a few tens of volts.
We normally see a very well balanced set of voltages on the UK grid, at least where I've worked and where the network is robust. If I was seeing an imbalance where previously none had existed then it would set alarm bells ringing that something had changed on that unit. One probability with a lot of scope for causing huge amounts of trouble for maintenance would be high current joint on a bushing or at the neutral bar going 'high' resistance. With perhaps 10kA passing through the LV joints it does not take much increase before things become troublesome.
The OP said the imbalance had existed since Day 1, so it may not be a symptom of a progressive failure but equally could be a symptom of a problem from construction. Do the revenue metering VTs on the HV side show a similar imbalance? If not then it points to a problem local to the generator. Is there a sister unit in the plant of similar construction which can be used for comparison? Does it display an imbalance?
This discussion is proving interesting.
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Sometimes I only open my mouth to swap feet...
RE: Generator voltage output mismatch
Are the VTs equally loaded?
respectfully
RE: Generator voltage output mismatch
About the VT's used their are from the same batch at each generator. Fir the ST generator, 15kV raing, the same difference in measurements apply for both the VT's located at the start point and those located at line side of the generator circuit breaker. Finally i want to mention that the ST generator has assymetry at the currents as well 4238A-4072A-4106A.
RE: Generator voltage output mismatch
I've not seen a three winding transformer used as shared GSU for two large generators before. I assume it is a ddYN* design with the two generators each having a dedicated high resistance earthing arrangement? Is the 324MVA set the STG you refer to or is it the 170MVA set? Are you saying that one machine has a load current imbalance while the other machine on a separate winding of the same transformer does not have (significant) imbalance?
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Sometimes I only open my mouth to swap feet...
RE: Generator voltage output mismatch
The 2 secondaries are connected to each generator output, via generator circuit breakers. The 17.78 is a tyoe fault, stand for 15,78kV. Also both generators are earthed via trasnformers and recistance in order to limit stator earth fault. The generator with the voltage unbalance is from ST generator 15kV nominal rating 170MVA nominal.
RE: Generator voltage output mismatch
Regarding the issue at hand:
1. The voltages listed from the second generator have about a .4% error / mismatch.
2. The voltages listed from the grid have about a 1% error / mismatch.
3. The currents listed for the generator in question have about a 4.2% error / mismatch.
I can believe that an actual error (not a measurement error) of 1% in the no load voltage could result in a 4% error in phase currents as VARs are drawn into the machine from the grid due to the low excitation in the phase that exhibited the lower voltage. The only thing that doesn't add up for me is that the OP states that the voltage error is observed when gen is isolated and when it is synched to the grid. I would have expected the voltages to balance and the VARs (and current) to show the imbalance after synched to the grid. There cannot be an actual voltage mismatch between the gen and grid while in parallel without a current flowing throught the bus, breaker, and XFMR impedances trying to balance the two voltages. Of course, now we know the grid has a 1% voltage imbalance at the point of common coupling. Any thoughts here?
Stratford, do you have any additional indications you can provide? Per phase kW, VARs, KVA, and pf would be nice. Also, positive, negative, and zero sequence currents and voltages. I would think these values would be available on a system the size of yours. By having this discussion on the basis of voltages and currents alone, we are only able to guess what changes in VARs and sequence voltages / currents are driving the indications.
On to the side issues:
1. I Fully agree with ScottyUK that any change in an indication that has significant history must be explained. Since the OP stated this was a problem from day one, this maxim is not applicable.
2. I also agree that VT (usually referet to as potential XFMRs - PTs in US) arrays are almost always comprised of identical VTs. Exceptions to this rule are very few and are usually due to in service failures (rare) or cheap contractors making bad decisions to save a few pennies (more common).
3. I think my experience with VT accuracies tells a different tale than ScottyUKs, and I'm interested in finding out why... I expect it's because my experience is with the cheap end of the market.
I've ratio tested hundreds of VTs in service in industrial plants. Typical applications are gens under 5MVA, and services under 50 MVA. It would probably be worth me reviewing the last year's data sheets, but off the top of my head I'd say errors in the 1% range are pretty common.
Regards,
JB
RE: Generator voltage output mismatch
In general, VTs loaded with the same ratings (regardless of manufacturer) that are loaded with the same burdens would not exhibit that much of a difference between units.
For example, if a VT is rated 0.3% accuracy up to a burden of 75 VA, then most VT manufacturers will place the 0 burden point as low (positive error) as they can in the accuracy class and place the 75 VA point as high (negative error) as they can.
As was said above, it is not likely to have VTs that have the same burden being reading at opposite ends of the accuracy range...in fact it's pretty near impossible.
If the VTs have a metering accuracy rating, are loaded with the same burdens, and are wired correctly, I would not expect to see a 1% difference between 2 of the outputs due strictly to VT errors.
RE: Generator voltage output mismatch
<aside>
While PT is common usage in the US, the IEEE standardized on the use of VT sometime in the late 70's. The use of PT was/remains so deeply ingrained that PT is still more commonly used than VT.
</aside>
Rolling the VT connections will easily allow determination if the problem is the VT or the generator. At no load (no current, therefore no voltage drop across the internal impedance of the generator) you will only get different voltages per phase if you have a different number of turns per phase or some geometry problem in the stator.
Differing voltages while current is flowing could point to differing impedances of the three windings, but with no current you have the internal voltage at the terminals if internal leakage currents are neglected.
Maybe, before rolling the phases, a TTR test of the VTs might provide an explanation. The possibly bad VT shows the possibility of too many primary windings or too few secondary windings. Fortunately that wouldn't suggest shorted turns on the primary, but could suggest shorted turns on the secondary.
But in any case, it is imperative that the problem be isolated and determined to be either a generator problem or a VT problem. Until then we are just speculating.
RE: Generator voltage output mismatch
Your answer (assuming I understand you correctly) implies that each individual VT is tested for accuracy at the two burden points, then modified as necessary to adjust the error in the desired direction prior to leaving the factory. I'll freely admit that I don't know, but I would be surprsed to learn that such a process takes place.
I figured that some significant effort went into developing the manufacturing process for a given model of VT, then the process is baselined to ensure that it produces XFMRs that vary radomly around the desired error / accuracy target within the acceptable range. Sure, Testig of ratio and insulation, etc take place before a unit is shipped out, but I would be surprised to learn that a manufacturer reworks each VT to get ratio accuracy closer to 0 error if the unit is within the accuracy band. We're talking about relatively cheap commodities here - especially at the low end of the market.
Anyone else have thoughts on this point?
Thanks, JB
RE: Generator voltage output mismatch
"About the VT's used their are from the same batch at each generator. Fir the ST generator, 15kV raing, the same difference in measurements apply for both the VT's located at the start point and those located at line side of the generator circuit breaker."
Doesn't this rule out the PT's as a problem since both sets of PT's show the same voltage difference ?
I must say I am surprised by the amount of current variation for such a small voltage deviation.
Would the 3 winding trafo have to do anything with this ?
*Nothing exists except atoms and empty space; everything else is just an opinion*
RE: Generator voltage output mismatch
As well as "rolling" the VT primaries, it may be well to roll the connections on the VT secondaries. This should pick up any difference in cable impedances and any difference in individual loading.
respectfully
RE: Generator voltage output mismatch
Maybe my wording wasn't clear.
VTs are accuracy tested at 2 points...0 burden and full burden rating. The accuracy changes linearly between these points as a function of applied burden. The 2 points are going to be near the extremities of the accuracy class because the longer the load line, the more cost-effective the unit is to produce (i.e. smaller core and/or less turns).
What this means is that while there is some variation from unit to unit, it is not possible to have 2 units (with the same connected burden) and have one unit giving -0.3% and one unint giving +0.3%. The difference between 2 units with the same connected burden is not going to be very much.
Waross-
Rolling the secondaries is a good idea, but only to see if there is some problem or inequality in the connected burdens. The lead impedance (or difference in lead impedance) is not going to be of any significance, in that the lead impedance of a VT circuit is normally negligable in terms of the VA burden presented to the VT.
RE: Generator voltage output mismatch
In a perfect world I agree with you.
and with one reading high the wiring is probably good, but with one indication low, as it is here, there may be a possibility of a connection developing high resistance, or some other unequality in the connection wiring. When trouble shooting, I sometimes sometimes try to look at each part off the circuit or system and ask myself,
"What can be expected from abnormal operation of this part?"
respectfully
RE: Generator voltage output mismatch
Agreed. Could be a high resistance connection issue.
RE: Generator voltage output mismatch
Yes. This is probably not the problem, but rolling the connections should detect any problems with the wiring and remove the wiring from further suspicion.
respectfully