I am experiencing power swings when I reconnect our facility back to the grid.

We are running around 150 MW, PF 0.8 and can experience +/- 200 MW and +/- 200 MVAR power swings within the first 10 seconds of reconnecting with the grid.

We have a gen-set running on natural gas. The utilities power system is very weak in our area, i.e. rural.

Any thoughts or guidance on where to beginning to tune the AVR, governor, Generator Controller? Tune what first? Major/minor loops?

Any thoughts would be greatly appreciated.

 

This sounds as if the weak grid connection is reducing the available synchronizing power. If this is the case, synchronizing as close as you can to the correct speed and phase shift will reduce the power and the associated swings.

 

Kevin... do you get the swings if synchronizing is done manually?

Regards,
Phil Corso ([email protected])

 

I was also thinking that is was the very weak system that was compounding the issue at the very least.

I have not manually synchronized to the grid. I am reluctant since I am new to the facility.

I was hoping to extend the automatic synchronizing parameters to ensure a smoother connection; the facility is in no hurry when connecting back onto the grid.

However, I still am thinking even if the grid is a great majority of the problem that the gen-set/synchronizer could behave a little better in reducing these power swings.

Hence, do you have any suggestions on tuning the AVR, governor, Generator Controller/synchronnizer? Am I leaving something else out?

 

Kevin... you may have to go back to basics!

Have you calculated the magnitude of Synchronizing-power and Damping-Torque?

 

Synchronizing pulls the rotor very quickly into the "right"position from wherever it is when you close the breaker. If you're a long way away from this position, the instantaneous power requirements can be very high, and this must come from the supply you're connecting to. Ideally, if you get the sweet spot, there is minimal effect - this usually means that the synchroscope is rotating about 1 rev in 5 or 10 seconds or less, and the breaker is closed at about 10 o'clock. Fine-tuning the synchronizer to get as close as you can to the sweet spot will probably be more beneficial than trying to adjust governor or voltage regulator settings - these tend to be too slow to cope with the rapid changes involved.

 

How about the most basic question: Does this happen every time the gen-set is synchronized to the grid?

What mode is the governor in during synchronizing; Isoch or Droop?

If the unit is in Isoch at the time of breaker closure when--and now--does it get switched to Droop (manually or automatically)?

What is the load on the gen-set during synchronizing?

What is the load on the gen-set after synchronization?

If this problem just started, what has changed to cause the problem? Is the synchronizing procedure the same or has it changed?

What causes the facility to be separated from the grid?

What is the grid frequency during synchronization (during speed- and voltage matching)?

When is the breaker close signal being initiated (10 o'clock? 11 o'clock? 11:30 o'clock?)?

Do you have any way of determining how many cycles it's taking for the breaker to actually close? 6? 8? More?

What is the generator terminal voltage with respect to the bus voltage just prior to breaker closure?

How fast is the synchroscope spinning just prior to breaker closure?

 

Hi,

First, I am asking if the wild load swings have always occurred when the unit was re-synchronized to the grid or if this is a relatively new problem.

Second, I'm asking if synchronization occurs with the governor in Isochronous or Droop Speed Control--and when and how the governor is switched to Droop after the generator breaker is closed.

Third, how old is this machine and how long has this "problem" existed--since originally commissioned? One month? Two? Six?

Finally, if there is something wrong or misleading or incomplete about the post(s) please be specific about the error or omission or add additional information as necessary, or state why you disagree with the information provided. But clicking on 'thumbs down' without asking for more information or pointing out the error or omission so the correction can be made does nothing to help improve the post or reply. When it's clear that a post is vague and too brief to be of any use with a lot of unnecessary back and forth on a forum then it's entirely appropriate--and warranted--to use the 'thumbs down' without any added clarification. But when a lot of thought and effort goes into a reply to try to elicit more information and provide clarity and background and a basis for developing a concise and helpful reply, to use the 'thumbs down' without stating the error or omission or why the post is not helpful.

 

Kevin...

What are the exact delta-speed, delta-voltage, and time-aperture, parameters, set into your synchro-scope?

Phil Corso ([email protected])

 

Phil Corso,

What basics of synchronization would you think it would be be helpful for Kevin to understand or review to help him with this problem?

Where can one find the formula for calculating this, or can you post it here, or give Kevin your email address and requirements for taking this post off-line (which benefits only Kevin)?

Keeping the post on-line and providing, or asking for, necessary information keeps the back-and-forth to a minimum and benefits many people on this public and open forum. Which should be our goal here--providing concise and complete information that is beneficial to many.

 

Kevin, CSA...

Contact me at [email protected]. But be advised that I don't respond to anonymity.

Regards,
Phil

 

I am definitely going to change parameters to make for a softer paralleling of the generator with the utility.

However, I am still thinking that the AVR, governor, Generator Controller/synchronizer could be better tuned. On occasion the generator syncs but the power swings don't seem to be dampening down and appear to be getting worse.

 

I don't have a standard synchroscope. The Generator Controller/synchronizer will allow the generator back onto the utility grid if it syncs for 0.6 seconds under the following conditions:

1) Sync Window: 6 degrees (allowed +/- phase shift between the generator and the utility);

2) Sync Window: 6 volts (allowed +/- voltage difference between the generator and the utility).

I am thinking of changing the parameters to allowing closing onto the utility grid if I get a phase shift of less than 5 degrees at less than 5 volt difference for 1 second. Any thoughts?

 

> How about the most basic question: Does this happen every time
> the gen-set is synchronized to the grid?
Yes, it appears to be unstable every time the gen-set is synchronized to the gird; at least +/- 50% power swings.

> What mode is the governor in during synchronizing; Isoch or Droop?
I wish I knew the answer to that; I will try to find out.

> If the unit is in Isoch at the time of breaker closure when--and
> now--does it get switched to Droop (manually or automatically)?
I will attempt to look into that also.

> What is the load on the gen-set during synchronizing?
Immediately before synchronizing the facility (hence the gen-set) load is around ~170 kVA, ~160 kW, ~50 kVAR.

> What is the load on the gen-set after synchronization?
The load from the facility does not change after synchronization. However, the gen-set load varies drastically during the time period when the gen-set is parallel with the grid; after around 10 seconds the gen-set breaker will open and the facility will only be on the utility grid.

> If this problem just started, what has changed to cause the
> problem? Is the synchronizing procedure the same or has it changed?
Looking at old alarm parameters and talking with operators, I believed the problem has existed since the facility was commissioned a decade or so ago. The power swings appears to have gotten worst lately.

> What causes the facility to be separated from the grid?
Loss of utility power; and testing the gen-set to make sure it works during loss of utility power.

> What is the grid frequency during synchronization
> (during speed- and voltage matching)?
I haven't chart recorded it yet but nominal is 60 Hz.

> When is the breaker close signal being initiated (10 o'clock?
> 11 o'clock? 11:30 o'clock?)?
I don't know since I don't have a normal style synchroscope.

> Do you have any way of determining how many cycles it's taking
> for the breaker to actually close? 6? 8? More?
I haven't chart recorded it yet.

> What is the generator terminal voltage with respect to the bus
> voltage just prior to breaker closure?
I was wondering the same thing; I know it needs to be a maximum of 5 V difference to close and parallel but I sure would like to verify that reading.

> How fast is the synchroscope spinning just prior to breaker closure?
Again, I don't know since I don't have a normal style synchroscope.

I have a lot of homework to do...

 

> Have you calculated the magnitude of Synchronizing-power and Damping-Torque?

Now you've stumped me! I had to get out my old school books. I'm working on it.

Now I'm readings books by Steinmetz from 1917 and IEEE (AIEE) articles from the 1940's. Luckily I have access to this literature since I'm taking a Materials class at school.

Next you'll have me modeling the system in Matlab/Simulink. You're killing me!

But, I'm up to the challenge; however, it may take me a little time to get caught up.

 

Bruce, I agree. It is the reason I sugggested "return to basics"!

Regards,
Phil

 

Kevin...

Hogwash (I'm beginning to sound like Chief Hornblower) on computer simulation. Can you operate an Oscilloscope?

Phil

 

Kevin... Actually it was "Marmaduke", a character out of <i>Power Magazine</i>.

Phil

 

Kevin,

Please don't be in such a rush to start changing things; it could be a relatively simple fix.

First, I'm confused because your original post seemed (to me) to indicate that the instability lasted for ten seconds--but the unit remained synchronized after the instability. Now you say the breaker opens after about ten seconds of instability. Can you please clarify?

Second, there's something about a stand-by generator? Can you clarify that bit please? Are you synchronizing the "big" machine or some smaller one when the instability occurs?

If the "plant" (and I'm referring to only a single generator-set--the "big" machine) is operating independently of the grid (in what's typically called "island mode") it is most likely operating in Isochronous speed control to maintain a stable frequency. Unless there is some kind of external frequency control that's adjusting the machine speed reference while in Droop mode. If a machine operating in Isochronous mode is paralleled with a grid it usually behaves just as you described--with wild load swings, which usually results in tripping--of the main generator breaker on reverse power or loss of excitation. Because we don't--at least I don't--understand the situation for the reasons cited above, I am surmising the unit isn't switching to Isochronous from Droop immediately after the breaker connecting the plant to the grid is closed.

As an aside, many plants manually switch from Isochronous to Droop before synchronizing just to prevent the issue you are describing.

But the more I writ the more I realize the information provided and my understanding of the plant and the situation is very minimal. So, without a better understanding of the plant and the situation I can't be of much more help.

How does the "big" machine normally get synchronized to the grid?

Is the plant operated independently of the grid?

What's this about a stand-by generator? (By the way, stand-by generators--almost by definition--operate in Isochronous mode and are not generally synchronized to a grid....) I hope you can understand my frustration in trying to offer help.

 

> First, I'm confused because your original post seemed (to me) to
> indicate that the instability lasted for ten seconds--but the unit
> remained synchronized after the instability. Now you say the breaker
> opens after about ten seconds of instability. Can you please clarify?

After paralleling with the grid the gen-set supposedly ramps down its output to zero in roughly 10 seconds; at which time the gen-set breaker automatically opens.

> Second, there's something about a stand-by generator? Can you clarify
> that bit please? Are you synchronizing the "big" machine or some
> smaller one when the instability occurs?

There is only one gen-set on the facility site. It is referred to as a stand-by generator.

> Because we don't--at least I don't--understand the situation for
> the reasons cited above, I am surmising the unit isn't switching to
> Isochronous from Droop immediately after the breaker connecting the
> plant to the grid is closed.

I am trying to verify at what point the generator control switches from Isochronous to Droop.

> How does the "big" machine normally get synchronized to the grid?

GenCon II Pro

> Is the plant operated independently of the grid?

If we lose utility power the facility can and does continue to run indefinitely independent from utility power. If the utility power comes back up, we sync up, parallel for about 10 seconds, and then go back to only utility power.

I realize that I am keeping some of the details vague. I am hoping that by keeping the discussion generic that I and the readers of forum can advance our knowledge of generators and synchronizing with the grid irrespective of the particulars at my specific facility.