Distorted current waveform on one phase

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vectormechanic

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We are having an issue with current unbalance on one our generators that I am thoroughly baffled by at this point.

The generator where we are having the problem is a 600V unit rated at 1200kW. It is connected to a liquid filled 1500kVA, 24.9kV/600V ungrounded wye-wye transformer, and goes to the local utility?s substation from there (only about 1/3 of a mile). The problem is a current unbalance that is proportional to generator load. At 1200kW load, a current unbalance of 42% is exhibited from C to A phase. (we are currently running the unit at 700kW, which results in a 10-15% unbalance). After going through the generator itself, we found that it had a PI of only 0.82, so the generator was changed with a used/reconditioned spare. (The original generator that was changed out had also been overhauled/reconditioned) When the unit was put back in service, the exact same unbalance appeared. I put a power recorder on the unit and found the A phase current waveform to have clipping with a notch in the center of the clipping. We thought it was possible (though unlikely) that each generator had the same problem, so the leads were rolled at the generator breaker (A phase generator cables to B phase lug on the breaker, B to C, C to A, etc.) with the thought that if the generator was the problem, the clipped waveform would follow the generator leads.

After restarting the unit, I found that the clipping stayed on A phase. My next thought is that there is a problem with the transformer. We had the transformer tested several months ago while the plant was shut down and being overhauled. However, the tests that were performed were somewhat limited since H0/X0 are internally connected inside the transformer tank and could not be separated to check individual phase windings; it did pass the tests that were conducted. I?ve attached screen shots of the waveforms and phasors of the 600V side of the system.

I have gone through the generator excitation system and have not found any anomalies.

At about 700kW load, the 24.9kV side is exhibiting about a 1kV voltage imbalance. I am unable to look at current on the high side due to limitations of my test equipment.



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Any ideas?
 
A little background information

A little background information

Sorry about that - we are a geothermal power producer and our generator is driven by a binary cycle turbine. We sell our power to the local utility.:smile:
 
A couple of thoughts/observations.
Is the star point (neutral point) of the 600V wye winding of transformer connected to the star point of the generator - if it has one? If not, the voltages from the line to star point may well be significantly out of balance.
The A phase voltage, just from visual observation, has voltage distortion and this is reflected in the current in that line.
I don't know the answer to your problem. Starting with a clean sheet, I'd have gone for a delta winding on the 600V input side.
 
Delta would have been nice... This plant is 20 years old and I inherited recommissioning it after about a one year shutdown (but didn't have any say in equipment) :rolleyes:

The generator neutral is not connected to the neutral of the transformer. The gen neutral is connected to earth ground through a standard grounding transformer scheme - a 600/120V transformer with the secondary of the grounding transformer connected to a resistor bank which is monitored by a ground fault relay for voltage build up. The transformer neutral is floating (both sides), so the only way I imagine any ground fault current would flow would be through earth to/from the utility's substation.

I measured transformer phase voltages directly to the transformer neutral, and got the same results shown in the phasor screen. One thing I neglected to do was measure voltage from X0 to ground. I'll give that a shot later today.
 
I think the problem here is the Wye/Wye connection. Having a Wye primary with the neutral point floating means that the system is not capable of supplying an unbalanced load.

Consider the ultimate unblanced load - only one phase of the secondary loaded - lets take phase A. In order to supply current from the secondary A phase, a current must be drawn by the A phase primary. This primary current flows through A phase primary winding to the star point. If the star point where connected to the generator star point then there would be a return path and all would be well, but in this scenario there is no direct path so it must flow through the other two phases to return to the generator. The problem here is that there is no load on the other phases, so they are acting like a choke against the A phase current due to lack of balancing ampere turns. This is what is happening in your scenario - the end result is that the potential of the star point is moving with respect to earth as the system attempts to feed the unbalanced load offered by the utility.

As to the clipped waveform that you see - this is a reult of harmonics. The depressing of the peak of the sine wave is classic of third order hamonic currents. (ie. 180Hz in the US) These are also called triplen harnomics. I suggest you do a google search if you wish to know more. My point being that this waveform is not the result of your equipment, more likely the result of the load being offered by the utility you are supplying. Third order harmonics can also be generated by the magnetising currents of transformers. It is possible that some of this current is a result of the unequal voltages applied to the transformer due to the floating star point configuration in use.

I would suggest that you investigate connecting the genrator star point & the transformer primary star point together & to ground.

Also, is the star point of the secondary side earthed? If not, then you will be experencing similar problems on the secondary side.
 
No ground reference

No ground reference

I do not see where there is any ground reference. The generator is resistively grounded. The transformer is ungrounded. Any load impedance unbalance will force the neutral away from ground, causing voltage distortion.
 
why c phase ?

why c phase ?

Well we agree a wye to wye the primary must be grounded as well as the secondary . transformer & generator connections but what i do not see is just one phase being effected why c phase alone ? comments best to ya
 
ohmhead said:
Well we agree a wye to wye the primary must be grounded as well as the secondary .
Click to expand...
Functionally, the system needs the two neutrals, generator and transformer primary. Grounding is a safety issue.
ohmhead said:
but what i do not see is just one phase being effected why c phase alone ? comments best to ya
Click to expand...
That's possibly a result of how the transformer is being loaded on the secondary.
 
Functionally, the system needs the two neutrals, generator and transformer primary to be connected.
Is what I meant to post.
 
Problem (mostly) solved

Problem (mostly) solved

Adrian, you pretty much nailed it.

Here's what we found: We did IR images of everything in our gear, transformer, etc., and found no problems. We decided to image the utility's metering rack also; it is on two poles outside of our plant boundary. As my partner was doing the scan, I was visually inspecting the rack and found a burned off phase conductor. From our vantage point inside the plant, we were unable to see the broken conductor. (a month ago when we first restarted the plant and noticed the problem, we called the utility who sent a lineman that claimed everything was fine on their side...)

We shut the unit down and the utility reterminated several connections on the pole.

After restart, the unit ran just about perfectly balanced, with only a 20A difference between the phases (at an average line current of 1300A).

I checked for distortion again, and found that there was still excessive third and fifth harmonics. We have yet to tie the transformer neutral to ground - I'm at another plant this week and hope to get back to that one shortly.

Thanks to everyone for the input.
 
One more thing...

One more thing...

I didn't see the missing phase when checking the 25kV metering PTs because it was B phase that was broken, and we are using an open delta PT arrangement, with B phase grounded.

The reason the distorted waveform appears on A phase in the screen shots is that the utility comes in ACB, while we are running ABC in our plant. The phases are swapped at the transformer primary for rotation reversal.

Cheers
 
Please bear with me here, I am a tech, not an engineer, so I have a few questions. As far as diagnosing and repairing the original problem, we don't get much in the way of support from our engineering department, so most things are left to techs such as myself.

With the exception of our demand start diesel generators (which have solidly grounded neutrals), all of our generators (we have over 50 units in the US, ranging from 800kW to 50MW) have high impedance grounding schemes, which in my understanding limits current during a ground fault event and provides the ability to detect the event through the use of a 59 relay. Granted, a 59 relay does not provide 100% stator protection (other functions within our generator management relays cover the rest). In our GMRs we have all the standard protections set up (81O/U, 87, 27, 59, 50/51, 32, 40, 46, 47, 24, etc.)

Are you saying that we should not be using the high impedance grounding, and should tie our generator neutral directly to our step up transformer neutrals? I have seen prints for nuke plants that use impedance grounding schemes. I can't imagine they are doing it wrong.

Through the grounding transformer, the generator neutrals are tied to the transformer neutrals via the plant grounding grid and bonding jumpers (plant in question at the beginning of this thread excepted). Is this what you are referring to?

I've attached an oversimplified one line of one of our more typical plants for reference.

Thanks for all your thoughts on the matter. I have some engineering education (2 years, didn't finish), lots of books, and lots of OJ experience. in addition to troubleshooting stuff like this, I also do control system design and implementation (PLCs, etc), instrumentation, generator control systems (excitation, governors, etc.) and the like.

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vectormechanic said:
Are you saying that we should not be using the high impedance grounding, and should tie our generator neutral directly to our step up transformer neutrals?
Click to expand...

Two seperate issues here:

As you have a wye transformer then the star point of the tranny should be connected to the generator neutral.

But - that doesn't impact your choice of using high impedance grounding or not, and I would suggest that your sort of installation is exactly the sort for which HRG provides greatest benefit.

As an aside, since the transformer is a wye/wye, the utility are providing a neutral connection to the transformer, yes? I live in different lands than you, but I cant say I've ever seen a utility provided transformer with anything other than delta on the utility side, so having a wye/wye seems distinctly odd...
 
dbuckley said:
Two seperate issues here:

As you have a wye transformer then the star point of the tranny should be connected to the generator neutral.
Click to expand...

Completely agree.
As I said in post #13, neutral and grounding are quite different matters.
 
I apologize for the confusion. In the one line I posted, yes, I did depict a wye/delta transformer. That is one of our more typical plants, not the one in question. I attached it because I am in disbelief that the generator neutral should be directly tied to the transformer neutral as Besoeker states (bypassing the HRG setup). It is not a representation of the plant that we were originally having the problem with. I have attached a one line of the plant in question.

As for why the B phase opened above the cutout, we don't know. I had to leave the site, and when the utility's lineman repaired it, he simply stated that it had "burned" off, and no one from our company asked for any more details.

Our utility does not provides us with a delta connection (i.e. no neutral). Why a wye/wye transformer was chosen is beyond me. To be frank, 20 years ago when the plant was built, the company probably got a smoking deal on the transformer and decided to use it.

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