Dear users,

We are currently commissioning a diesel power plant. We are testing different scenarios of synchronization etc. This plant is designed for Grid Parallel / Infinite mode as we don't have much load of our own plant. But in case of Island operation the generators should keep on running and supplying power to its own auxiliaries.

I am facing a problem in Island mode when more than one generators are connected on the same bus bar and I shift them "all" to frequency control mode some of the generators circuit breaker opens due to reverse power measured by the protection relay.

We increased the protection relay setting to 12sec and measured 1.2MW flow towards the geno side. Can anybody give some tips of how to overcome this problem.

 

Dear Iceman,

Unless the diesels have some kind of Isochronous Load Sharing system, or the Isoch Load Sharing system is not tuned properly, precisely what you say happens will happen when you try to operate more than one unit in frequency control.

On any grid, there should only be one unit operating in "frequency" (Isochronous) control, unless there is some kind of Isoch Load Sharing scheme, which must also be adjusted or tuned for proper operation.

When more than one unit is operated in Isoch ("frequency") control on a grid, those two units will typically "fight" each other for control of grid frequency and all manner of load swings will occur.

Please go back and reset your reverse power relays; they are there to protect the diesels.

Please go back and rethink what you are doing and how you are doing it. If there is some kind of Isoch Load Sharing scheme, make sure it's adjusted or tuned properly.

If there is no Iscoch Load Sharing scheme, then what you are trying to do can't be done in most all circumstances. (I have seen this done when the load is very stable, but anything more than a minor change in load would send the plant into a blackout, which is why I always carry a torch when I'm in a power plant now. The emergency lighting hadn't been commissioned and the only lighting was from the alarm annunciator panel which, fortunately, was lit up like a sky full of fireworks!)

 

Hi,

I would like to know one thing from you. At such low plant loads (Auxiliaries) in isolated conditions, why do you want to keep all the machines producing power? May be it could be sufficient to have one machine, for instance, online or is it due to the fact that you would like to keep the machines available for the next opportunity to synchronize with the system following restoration.

Regards, Ramu

 

Dear Users,
Thanks for your replies. Actually we need to run the Genos with CB close so that as soon as we have the opportunity to sync again with the grid then we could do it on our HV breaker instead of doing it on individual MV breakers which could cost us a dear time.

The governors which we are using is Woodward 723 Electronic Control. If there is no provision of hardware load sharing than how can we do software based load sharing ? is any solution possible?

Thanks,
Iceman

 

You can't will generators to operate in parallel in "frequency" control, though many try. From the description in the original post, it sounds like this desire to operate in "island mode" is something that may not have been considered during the design phase of the project and so the equipment isn't available.

Do you have to switch all generators to "frequency control" when you are separated from the grid?

All you need do is put one of the generators in frequency control when separated from the grid. That generator will respond to any load changes, keeping the frequency of the "island" (all the generators operating in parallel) stable and at nominal.

It's as simple as that.

You also have to consider how you're going to resynchronize to the grid no matter how you operate the units. One method is get the "island" load stable and then switch the single unit which is in "frequency control" to droop and then use that unit to adjust frequency and then close the HV breaker at the appropriate time. Then, when the group of generators goes on the grid, all the generators are in droop mode, just as they should be.

You will have to look at your governor manuals to see that they are capable of, and also, how they were configured. I sincerely doubt that unless some kind of Isochronous load sharing module was used (and Woodward does make them) that all generators would be switched to "frequency control" mode at the same time; that just wouldn't make any sense. Unless the designer thought that only one unit would be used in "island mode" at a time and just wired all the governors to switch to frequency control thinking that only one would be in service anyway. We don't have enough information to know that; you'll need to review your plant drawings and schematics to determine that.

There are all manner of hardware and software control schemes which could be put in place. But NOTHING takes the place of understanding how a such a system would operate automatically, and from the description in the original post that understanding is lacking.

Again, a system can't be forced to operate the way you think it should, without some serious thought and configuration. And that requires a basic understanding of how synchronous electrical generators operate in parallel with each other, in island mode and when connected to an "infinite" grid. Unfortunately, there isn't a lot of practical written information available about that, and a lot of what is available is very misleading and written purely from a theoretical point of view and not from a practical, operational standpoint.

 

Place all generators in droop mode when isolated from the grid. When paralleled to the grid let the utility act as the swing generator, that is absorbing the load changes, that might work for you and the generators will run smoother. And yes with the Woodward 723 you may tie all governors together via the load share line. You may also modulate this voltage with an analog output in voltage mode and a PLC so you may adjust the desired output level of the whole generator group if you want. This way you may prevent excessive exporting to the utility grid, or control how much you import from it.

Search for document 26260 from Woodward, there is a lot of useful concepts there.

Romulo

 

Hello,

What KW size and Voltage are your generators and how many are there?

When you say you have 1.2MW flow towards the geno side, do you mean from another generator or from another source ?

Do you have load sharing lines installed between the generator Governors ?

Should the Grid breaker open is there not an automatic system to switch the generators to load share mod? This may be triggered by monitoring of circuit breaker positions.

In the event of a loss off Grid, while in parallel with the generators, I would expect the System to brake parallel automatically.