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Synchronous Diesel Generator Voltage Fluctuation
Diesel generator is running parallel with utility grid and the line voltage starts to fluctuate.

Greetings,

We are running 2,5 MW Diesel generator (400 VAC) and synchronizing it to the utility power grid. There is nothing abnormal during ramping load up to 1 MW. When the load is increased to 2 MW, we are starting to see something abnormal behaviour in grid line voltage and therefore in generator voltage. Both voltages are starting to swing from 380 VAC to 420 VAC. When we reduce the load back to 1 MW the swinging stops and the voltages are stabilized. Could the 20 kV / 0,4 kV transformer affect the voltage fluctuations?

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Salutations,

How is the "AVR" being operated? Is it being operated in Automatic mode, or Manual mode? Or is there some kind of VAr or Power Factor control which is switched on when this problem is occurring?

What is the grid voltage when the diesel generator is first synchronized to the grid?

WHEN DID THIS PROBLEM START? (In other words, has it been happening since commissioning, or did it just start a few days or weeks ago?)

What is the power factor of the generator--and in which direction (Leading or Lagging)--when it is first synchronized to the grid?

How many VArs--and in which direction (Leading or Lagging)--are being produced when the generator is first synchronized to the grid?

What is the power factor of the generator--and in which direction (Leading or Lagging)--when the generator power output is 1.0 MW (as the load is increasing)?

How many VArs is the generator producing--and in which direction (Leading or Lagging)--when the generator power output is 1.0 MW (as the load is increasing)?

What is the power factor of the generator--and in which direction (Leading or Lagging)--when the generator power output is 1.5 MW (as the load is increasing)?

How many VArs is the generator producing when--and in which direction (Leading or Lagging)--the generator power output is 1.5 MW (as the load is increasing)?

Greetings CSA,

>How is the "AVR" being operated? Is it being operated in
>Automatic mode, or Manual mode? Or is there some kind of VAr
>or Power Factor control which is switched on when this
>problem is occurring?

- We have an automatic AVR controlling the generator voltage during synchronization and loading. We are running 1.0 pf whenever the generator is being synchronized to the utility grid.

>What is the grid voltage when the diesel generator is first
>synchronized to the grid?

- Grid voltage is 400 VAC during synchronization.

>WHEN DID THIS PROBLEM START? (In other words, has it been
>happening since commissioning, or did it just start a few
>days or weeks ago?)

- This started since comissioning. We haven't see any fluctuations before.

- No, we are running quite stable AVR settings which produce 1.0 power factor during loading.

>What is the power factor of the generator--and in which
>direction (Leading or Lagging)--when it is first
>synchronized to the grid?

- Power factor is 1.0. Voltage difference during synchronization is 0 - 5 VAC. Slip frequency is very small.

>How many VArs--and in which direction (Leading or
>Lagging)--are being produced when the generator is first
>synchronized to the grid?

- VArs are very small, only -10 kVAr to 20 kVAr Lagging.

>What is the power factor of the generator--and in which
>direction (Leading or Lagging)--when the generator power
>output is 1.0 MW (as the load is increasing)?

- When the generator output power is 1.0 MW the power factor is 0.99 to 1.0 lagging.

>How many VArs is the generator producing--and in which
>direction (Leading or Lagging)--when the generator power
>output is 1.0 MW (as the load is increasing)?

- Only -10 to 20 kVArs are produced, target pf is 1.0.

>What is the power factor of the generator--and in which
>direction (Leading or Lagging)--when the generator power
>output is 1.5 MW (as the load is increasing)?

- As load is increasing the power factor is 0.97 to 1.0 lagging. At 1.5 MW we do not see any voltage fluctuations.

>How many VArs is the generator producing when--and in which
>direction (Leading or Lagging)--the generator power output
>is 1.5 MW (as the load is increasing)?

- VArs are the same, from -10 to 20 kVArs. Power factor target is 1.0 as load is increasing. We have 30 seconds ramp rate during load ramping.

We should see only kVAr and power factor swinging if the AVR setup is messed up. We shouldn't see any voltage fluctuations in Grid side because the grid is very strong. Our 2.5 MW generator shouldn't affect the grid side voltage at all.

Even if the load is ramped up in 100 kW steps the voltage starts to swing when the generator is loaded to 2.0 MW. This is very strange because we haven't seen this kind of action before. I have synchronized and commissioned multiple diesel generators and the utility voltage has always stayed very stable. I wonder if there is something going on with the utility side transformer.

Best regards
Ahto

Ahto,

I'm missing something, because the power factor can't be 1.0 and the VArs be -10 to +20; that just doesn't compute.

And, you said this has been happening since commissioning--which usually means since the unit was initially installed and started-up and turned over to the Customer (owner/operator). How long ago was that?

Or, are you referring to commissioning as being after a maintenance outage--a re-start after a maintenance outage (planned or unplanned)?

Yes; power factor will be unity (1.0) prior to synchronization. But, after synchronization power factor is a function of VAr flow. (Research power triangle for more information on how VArs and power factor are directly related.)

I would suspect some issue with the AVR (exciter regulator). OR, some problem with the sensing devices for the AVR (in particular, PTs (Potential Transformers) and/or CTs (Current Transformers)).

It is possible for some generators at some points in a grid to have a very profound effect on grid voltage--especially if the generator is a long distance (physically) from the nearest generator on the grid. Or, sometimes there can be problems with "softness" of a grid due to loads and/or transformer issues.

Is this generator connected to the grid through a step-up transformer (so 400V to something like 4160 VAC or even higher, 11 KV, for example)? Transformers are passive devices, usually (some have tap-changers, and some have auto tap changers--which can operate automatically). So, while a single transformer isn't usually a problem, there can be problems with two or more transformers?

Also, if another generator and its prime mover is connected to the same bus upstream of a single step-up transformer the AVRs usually have to have some kind of droop compensation tuned for them to operate properly and to share reactive load (VArs) and prevent oscillations.

That's about all I can offer. Sometimes power system instabilities can be extremely difficult to solve--and require lots of power system studies and analyses and lots of really smart people (smarter than me, because I'm not good with vectors and phasors and such--and solving this problem may require that level of knowledge and experience and sophistication).

It would be nice if you could write back to let us know how this problem is ultimately solved.

Best of luck in resolving the problem!

Did the current swing together with the voltage?

Greetings,

And thank you very much for your answers and feedback. Our reserve power controller is measuring power factor and reactive power. I agree that the power factor cannot be 1.0 if the reactive power is swinging from -20 to 20 kVAr. However this reactive power swing is very small compared to the capacity of the generator. And therefore the power factor value in the controller panel display is approximately 1.0 most of the time (0.99 to 1.0).

The utility grid is very strong, our plant is operating in the middle of the capital city in North Europe. That is the reason why I'm little bit surprised that we managed to swing the transformer secondary voltage at this point (there are several transformers, nearest feeding transformer is 20kV/0,4kV).

Today we solved the problem while running the generator at 2.0 MW load. You were right that there might have been some problems with the AVR settings. AVR pf gain setting was too high and it caused the reactive power to swing. For some reason the transformer secondary voltage started to fluctuate too (I expected that only kVARs and pf would have been fluctuating, not the transformer secondary voltage). Utility voltages and the generator voltages are measured by the controller and the analog voltage meters.

We reduced the pf gain from the reserve power controller and this was the solution for our problem this time.

Br
Ahto

Ahto,

Thank YOU very much for the feedback! "Feedback is the most important contribution!"(c) here at control.com. It helps others who read these posts--now and in the future--know how problems were solved (or not) and if information provided was helpful (or not).

Glad to hear the problem is resolved!