Can one synchronize a COGEN with utility without parallelling ? 4

[hmchi]

The utilities in the world usually do not like co-generation systems of their customers to be parallelled to the utility own system. Many simply forbid it. However, when there is a problem and the customer paid to have a 'hot standby' source from the utility, they will allow a source transfer operation.

Obviously we need to switch when the 2 sources are in sync. So is there a way to keep the COGEN in sync with the utility source without actually connect in parallel ?

 

[rbulsara]

Yes.
Synchronizer only cares for the voltage reference signal for the 'sync to' source. No need to bring phase buses together or close the breaker.

You always synchronize the two sources before paralleling, so all those controls remain, except for closing the breaker.

Rafiq Bulsara

http://www.srengineersct.com

 

[hmchi]

Thanks. However, in general, can cogen steam T-G control be good enough to keep up with this constant fine-tuning control ? The utility source may be very stable, but loads may change from time to time.

 

[rbulsara]

As long as the T-G control has a synchronizer, I can't see why not.

There is more to the design though, depending up on how you want to accomplish the source transfer, when needed.



Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

If you're asking that whether your cogen can stay synchronized to the grid and yet still not deliver power, I don't think so. Your reverse power may act if for some reason your cogen speed falls. How long does it take to synch a running machine to the grid really ? A minute tops ?

Muthu

http://www.edison.co.in

 

[Hoxton]

Please post a single line diagram.

Why not use the synchroniser to synchronise, but inhibit the connection breaker closure. Then remove the inhibit when you want to connect.

Make sure that your synchroniser is rated for continuous operation.

 

[hmchi]

The purpose of the exercise is to transfer the loads quickly to the utility source if there is a problem that trips the COGEN Turbine Generator --- without losing the loads.

If there is no synchronism between COGEN and utility, fast transfer may not be possible and loads will be lost even if subsequent dead bus transfer is made.

so, my question really is --- if the 2 systems can be in sync most of the time, say, 80% of the time, then the chances are the turbine trip would have a 80% chance of occurring during synchronism and fast bus transfer can be successful. If the COGEN is constantly chasing synchronism and succeeds only 20% of the time, then chance of saving the loads not too good then ..

What do you guys think the chance of success, based on your experiences ..

 

[rbulsara]

hmchi:
No offense, but you need to hire consultants experienced in this area. There is no such thing as a fast transfer on co-gen, nor would I attempt one.



Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

I think it will be somewhat difficult to maintain exact phase angle match between the two systems consistently over a long period of time. With a steam turbine with a very good governor, you have some hope, but still it will be a challenge. There will be some inevitable drift over time and you will be constantly correcting.

An autosynchronizer is really not designed to handle this job on a continuous basis. You will need to have a good isochronous governor that can accept very small incremental corrections based on the slight phase angle differences that will occur.

You might want to look into synchronous phasor measurements and see if this might help you to bias the isochronous governor.

 

[byrdj]

If your transfer is going to be by contacts, there WILL be a few cycles with out power so the phasing of the closure to the utility will not matter.

If you NEED bumpless transfer, a uninteruptable power supply could be used if the requirements can be met.

I did not visit many industrial stations, but I recall one which had a large meter that indicated whether they were buying or selling power. the plant's generator was loaded to keep that meter NULL with alarms if there was a deviation.

the next question would be is your utility feed capable of providing power for your plant

 

[hmchi]

Thank you dpc, I share your opinion that it is difficult to remain in sync most of the time.

byrdi, if the initiation of turbine trip and fast transfer occurs at synchronism, the Cogen spin-down and phase angle slip you referred to can be managed by fast transfer actions [Siemens and ABB swears by them]. The trouble is if we start out not in sync and say, 120 degrees apart .. there is no adjustment of the rotation possible .. loads will be lost ..

 

[davidbeach]

Actually you don't ever want to be in synch. What you want is to be running 30-60 degrees ahead of your alternate (utility) source so that your fast transfer system can have a bit of play to work with. The fast transfer system itself isn't a concern, yes they do exist, and yes they work well; another possible source is Beckwith. Your big problem will be the control loops necessary to maintain the desired relationship.

 

[hmchi]

Your big problem will be the control loops necessary to maintain the desired relationship --- I agree.

This is why I came here to see if someone might say .. Hey, here's what we did and was successful .. Hoping against hope, I guess ..

 

[dpc]

From an economic point of view, your serving utility will likely be charging you a hefty standby charge every month even if you never take actual kWh from them. If you want the utility to be available to instantly take up your entire facility load at any moment, you will certainly be asked to pay for the privilege.

 

[waross]

Load changes will cause frequency changes. The controls we have been discussing will then correct the frequency.
You may be able to speed up the correction time with a "feed forward control loop". Use a coarse speed control loop based on load. That is monitor the kW load and open the throttle to give the amount of fuel or steam required to produce that amount of power output. This system will be difficult to set up. That is probably why it is not often used. Use a conventional control loop to fine tune this loop.
That may be able to keep your parameters closer than a frequency based control scheme.
I like David's suggestion. I would add a suggestion, 60 degrees is fine but inhibit closing until you are within 30 degrees. Running at 60 degrees from true sync, you will have 30 degrees of margin that will help you cope with smal load and frequency swings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[alehman]

Speaking from experience, a typical synchronizer won't even come close when there is a load step that is a significant fraction of the generator kW rating. It will eventually resynchronize, but it may take a few seconds.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[hmchi]

My big question is this --- there must be at least 10,000 CONGENs in this world, and we conceivably share this problem, really caused by the utility's unwillingness to allow us to sync and parallel the system .. Surely we all want to save the loads without any of them dropping dead .. There should be someone out there solved the problem already ..

dpc --- yes, hot standby utility contract is expensive. But the business case of a Cogen is the economy of using the surplus process steam to generate electricity. This savings is 24x7. I guess one must be balanced against the other ..

We would have gladly pay more if the utility will allow us to sync and parallel. But they rejected it outright, not even a discussion ..

alehman --- I agree with you. Good thing our load changes is not that that big a step ..

 

[davidbeach]

Not all utilities are the same. Follow the rules and many utilities are fine with parallel operation. I know we are. Really tough luck if your utility flat out refuses.

 

[ScottyUK]

Unless your cogen is an order of magnitude larger than the load it is very unlikely that it will accept a block load from a fast transfer. If it is a single-shaft industrial gas turbine prime mover then load steps greater than about 20% of set rating will likely cause problems, especially the initial step from idle to (say) 20%. We looked at this as part of a black start recovery plan for a big generating station and the block loads which the utility wanted us to accept would have stalled the engine. I'm not sure how the aero machines fare on a step load but gut feel is that a two-shaft machine will probably be more tolerant overall but that the generator frequency excursion will be more severe. The frequency will of the cogen machine will be falling as soon as it loads up so maintaining synchronism is of limited benefit.

A more realistic solution will be to accept that there is a definite break in continuity of supply and then implementing a sequenced restart. Protect your critical loads using some form of UPS.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hi.
Intresting thread.
Never heard about such application.
It seems a very problematic solution, but maybe is possible.
Possible think about David's solution with high speed busbur transfer.

But, from practic point of view, Im second with Scotty. Break loads with local standbay ( rotate UPS, UPS )of critical loads.

I think, politic of your utilities is very wrong, co-generatiton is a good future.