Utility Generator Export Issue(Imbalance) 4

[Energyfit24381]

Hey all,

Trying to export to the grid through a step up transfer but gens are tripping on a kVar imbalance. I believe there isIs anyone aware of any equipment i can install at the generator to minimize or eliminate phase imbalance?

 

[waross]

We need a little more information.
KVAR are generally drawn by the grid/load.
A KVAR imbalance may be an unbalance on the grid.
This may be caused by failed capacitors in one phase of a capacitor bank.
This may be caused by failure to transpose a long transmission line.
This may be caused by unbalanced single phase loading.
You probably can't correct the grid.
Running unbalanced KVARs above the manufacturer's recommendations may eventually lead to rotor heat damage.
Check with the manufacturer as to the maximum imbalance that you may set the protection.
Check with the grid operator as to the possibility of failed capacitors or other grid conditions.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Energyfit24381]

@waross this was a great response and much appreciated as we suspected everything noted. We were able to trace the issue back to the grid but due to a heavy amount of red tape they are not willing to make the necessary corrections on their side. The application is very small. Currently there is (1) 480V, 3ph, 600kVA, 450 kW Synchronous generator exporting but we keep tripping on a kVar imbalance after a few seconds. With the utility not willing to make corrections to their equipment im looking for the most efficient way to prevent the kVar imbalance on the gen side.

 

[wcaseyharman]

I’ve never heard of a “kVAR Imbalance” in the 16 years of power plant operations and engineering in the US. Perhaps it’s called something else here.

There’s excessive leading or lagging VARs (ANSI #55 relay)
There’s Voltage or Current imbalance ( ANSI #47 or #46 relays)

If it’s current imbalance (just tripped a small gen off on this a few weeks ago), you might check the machine relay settings and see if you can safely increase the setting. Our issue stemmed from the imbalanced load - one of the reclosers in the circuit had 40 amps on two phases and 5 amps on the other. Since we are the utility we will try to get the line side to re-balance the circuit, but in the mean time we’re able to bring the setting up per the general ratings in NEMA MG1.

 

[Energyfit24381]

@wcasey thank you so much thats very helpful information. Yes this is an excessive VARs issue. Im leaning towards bumping up the settings but waiting on the manufacturer to get back to me on risk to the unit. The issue you reference is very very similar to what we are seeing as well. Any feedback on the excessive VAR issue you’ve seen on your side?

 

[wcaseyharman]

We also see an issue on the same machine with excessive leading VARs - it’s not bad enough to trip but close. This is due to the high voltage on the distribution system in the area. If you’re seeing this on your machine, I’d investigate whether your transformer has multiple taps. If you have taps you can adjust the turns ratio and reduce the VARs. What was the generator voltage when you tripped?

In our case, because changing taps isn’t an option, we are adding a reactive load bank this summer which should fix our excessive VAR issue.

 

[waross]

We had an unbalance issue on our small diesel utility.
Due to the economic unbalance of our user base we were unable to balance our phases.
The less fortunate user laod peaked at meal times.
The more affluent user load followed the A/C usage.
Unbalance causes rotor heating.
After several years, one of our rotors showed less than optimum insulation resistance ans was rewound.
I would suggest that you have a choice of addressing the phase(s) with the lower PF or addressing the phases with the higher PF.
Without more information, it is difficult to say conclusively, but as an approach to consider:
Consider raising the power factor of the lower Pf phase(s).
I would investigate using three power factor correction controllers and three capacitor banks, one on each phase.
That will present your machines with a controlled power factor and will track and correct imbalances on the grid.
If you want to go the reactor route, remember that your correction must be on individual phases.
I would investigate a technique that was popular before I was born. You will see "wire-less reactors" in texts a hundred or so years ago.
A wire-less reactor would raise the impedance of a transformer by a couple of percent and were used to parallel transformers with different impedances.
The wire-less reactor was basically a stack of laminations through which one of the transformer secondary leads was passed.
Imagine a window type CT with no secondary winding.
I would try an experiment. Take two different rated CTs and connect the maximum burden to each one. Make arrangement to short out the secondary of each one so you may toggle between a shorted secondary and maximum burden on the secondary.
Put am ammeter on the line and see if switching the secondary resistance in and out makes enough different to the current that the scheme may be used to control VARS on individual phases.
That said, you will probably go for the capacitor scheme.
Your generator should be looking into the delta side of a wye-delta transformer.
Any imbalance of one phase of either KW or KVAR will be spread across two phases of the generator and the imbalance will be lessened.
By the way, it was a 1600 KVA, 480 Volt diesel generator that we had the rotor issue.
I wasn't present and always suspected that the shop that did the testing was looking for more work.
We had two 600 KVA 480 Volt sets that supported the same grid for years without a rotor failure.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Generator is wound 277/480 and wye connected, correct?

Were you at any point able to collect vectorial traces so as to confirm the magnitude and flow of lagging reactive power flow each phase?

If you have that, it shouldn't be rocket science to connect either a cap or some manner of reactor a la Bill's suggestion to the offending phase.

It seems very odd to me that such a pronounced imbalance should be imposed by the grid onto your generator . . .

Seriously, for a utility to swap around the phasing for some of their area customers so as to ameliorate such a phase imbalance shouldn't be rocket science, either, and I take a very dim view of any utility that begs off correcting a technical issue because of the paperwork involved; clearly they need to buy a fork lift so they can deal with the heavy red tape! ;o)

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Is the grid imbalance stable or variable?
And, the grid imbalance may by causing overheating of the recommended delta winding.
It is recommended to float the neutral to mitigate circulating currents.
You may have to correct on the primary side.

Note, It may be difficult to correct the worst grid phase.
In that event, consider using either capacitors or reactors to degrade the better phases in order to improve the phase balance.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

Without being on-site and being fully aware of local conditions, take this as a matter for consideration rather than a firm suggestion:
Consider high resistance grounding of the generator neutral rather than a bolted ground connection.
If possible, it may be the easiest and cheapest mitigation.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Energyfit24381]

@crshears correct it is a 277/480 wye connected. And agreed on the heavy lifting. At the moment we are stuck with providing a solution after the meter. Great post!

@waross thanks so much for all the great info. It may be challenging to do the work on the primary side of the transformer so we are left with providing a solution on the secondary side. We only have a minimal amount of data to pull from ourselves since the gens are tripping so quickly when we synch with the grid. With that we believe the imbalance is stable. Im leaning towards your original option above to provide a 3phase cap solution so all phases are seeing a balanced PF.

 

[wcaseyharman]

Do you know what the voltage is on each phase on the system side of the generator breaker with the generator offline? This may provide good data to determine what mitigation strategies are needed.

 

[Energyfit24381]

@wcasey great question. Yes and No, We do have historical meter data that shows overall power characteristics for the facility but its in 15min intervals. We are needing something more granular to gather more data. In a perfect world i would request a power study but that option is low in probability.

 

[waross]

Is your transformer (bank) wye-wye, wye-delta, delta-wye, delta-delta?
Are your transformer taps set correctly.
I once had an issue with one transformer in a three phase bank set on the wrong tap.

When the power failed, the standby generator would come on-line. When the grid returned, the voltage on the low phase was not high enough to trip the ATS back to the grid.)​

If the tap is set wrong on one phase, that will cause unbalanced VARs.
For a three phase transformer with a three phase tap setting, the tap changing mechanism may be damaged, leaving one phase on the wrong tap.
Can you safely check the 480 Volts incoming from the grid with the generator off line?
It is a kludgy fix, but if one phase is consistently high or low, you may be able to go up or down one tap setting for that phase.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

You should be able to measure it with a decent meter like a Fluke 87 (or another meter rated CAT IIII at 600V). Ideally you have potential transformers that feed the relays and meters. I’d measure each phase to ground on the PT secondaries. You would need to know your PT ratio (probably 480:120 or 277:120 if you have them)

Measure all three phases to ground and then measure phase-phase. You want to look at whether all 3 phases are the same magnitude and what the overall magnitude of the voltage is. Keep in mind this requires people familiar with electrical safety protocols and work practices, which hopefully you have or you have access to those that do.

The real and reactive power flows from your machine are proportional to the relative voltages (and phase angles) between the machine and the grid, so knowing what the grid voltage is at your machine terminals will go a long way in determining what the issue is and how it fix it.

 

[davidbeach]

Remember, AC voltage is a phasor, not just a magnitude. Weird flows from generators can be driven entirely by weird angles and apparently normal phase-neutral voltages. Phase-phase magnitude will help much more than phase neutral magnitude but a phasor snapshot would be the best.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Energyfit24381]

@wcasey, thanks for the awesome feedback. No worries on safety i am fully qualified in lockout/tagout and energy isolation procedures. Thank you for putting safety first, im really big on that. I was recently assigned to look into this so just getting my ducks in a row before visiting site. I am an electrical engineer, started out on power for several years but have been focused primarily on controls lately and just making sure i run through as many scenarios as possible in preparation. Excellent idea on checking at the PT!

@waross i will confirm transformer configuration on Monday but most of our configurations are delta delta. Ive definitely made notes on the transformer and will be looking to troubleshoot once ive gathered all info and head to site. Im really hoping to check the low hanging fruits first like configuration, settings etc. before moving on to a deeper dive into the system. Thanks so much for bringing up and detailing the scenario you listed above.

 

[crshears]

Remember, AC voltage is a phasor, not just a magnitude. Weird flows from generators can be driven entirely by weird angles and apparently normal phase-neutral voltages. Phase-phase magnitude will help much more than phase neutral magnitude but a phasor snapshot would be the best.

Thanks davidbeach, you got at what I was trying to say but did it more accurately.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Energyfit24381]

@waross was doing some more digging today and fouund the transformer is actually Delta-Wye configuration FYI. Just wanted to make that correction to above.

“If my critics seen me walk on water they would say its because i cant swim”

- M.T.-

 

[jghrist]

Is the problem a phase unbalance or excess var export?