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 ?

 

[PHovnanian]

hmchi, if I understand your problem coirrrectly, it appears that you want to keep your loads on your local cogen source and transfer to the utility at such time as that local source trips. Maintaining synchronization will aid in making a fast transfer so as not to disrupt loads by changing V phase angle suddenly. Correct?

Think about what sorts of trips your cogen plant might experience that will initiate such a transfer and consider whether a synchronizing scheme will even operate correctly under these conditions.

Some trip conditions that do not affect the operation of the prime mover's throttle _and_ do not require an instantaneous transfer could be delayed until the two sources are brought into sync. The same would occur for manually commanded transfers. Such a system would be easier to implement, as the prime mover only needs to be brought close to the system frequency and the transfer command blocked until the phases get 'close enough'.

 

[hmchi]

PHovnanian --- Your understood the question correctly, and your suggestion to look into the turbine/generator trip necessity is a good one. Thanks.

ScottyUK --- Hi, you have been a long term valuable contributor, thanks. However, we are not trying to transfer to Cogen, we are trying to transfer from Cogen after trip to utility --- and not to have a long dead time that will lose loads ..

 

[waross]

Most trips are related to faults or failures. I was wondering also about the points mentioned by PHovnanian in his first two paragraphs. Any event that causes you to lose your co-gen source may also cause you drop out of synchronism.

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

 

[ScottyUK]

Hi hmchi,

Sorry for the mis-understanding!


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

 

[rhatcher]

hmchi,

A lot of good points have been brought up in this thread but the one that stands out to me at the moment is that mentioned by PHovnanian and added to by waross. I am not an expert, I am just thinking this through, trying to contribute some thoughts, and hoping to learn from the responses.

It would seem that there are two basics types of transfers, make-before-break and break-before-make. The 'normal' case (assuming no fault) would be make-before-break in which you would synchronize the cogen to the utility, close the utility tie breaker, then open the cogen generator breaker. Even if the time between closing the utility tie and opening the generator breaker is short, the two sources must be in synch, voltage and frequency, for the transfer to take place this way. It would seem by definition (?) to be impossible to intiate this type of transfer in the case of a tripping fault on the cogen side. Even if you could hold (force) the cogen on line long enough to perform a make-before-break transfer, it is unlikely that you could maintain synchronization to do this because the fault would most likely affect cogen frequency or voltage (or both). I believe this would be true whether the fault was mechanical (prime mover) or electrical.

The second case, break-before-make, is much more forgiving. In this case, a trip on the cogen side would initiate closing of the utility breaker once the cogen breaker is open. This type of transfer can be very fast, but it is not guaranteed fast enough to prevent some loads from dropping. Of course, if your loads include synchronous motors, you will be guaranteed to drop those since they will fall out of sync the moment power is lost.

Finally, there is a worst case scenario that applies to both types of transfer. If the original fault was electrical and it has not been cleared, it seems that all you would accomplish is connecting to the utility long enough to trip the utility breaker. During this time, your fault is being fed power from a much larger source, resulting in much greater damage. That being said, I am not sure if I would want to automatically intitiate a transfer unless I could block transfer when the cogen trip is from an overcurrent relay.

What do you guys think?

 

[edison123]

Good point rhatcher

" If the original fault was electrical and it has not been cleared, it seems that all you would accomplish is connecting to the utility long enough to trip the utility breaker. During this time, your fault is being fed power from a much larger source, resulting in much greater damage."

This "Can one synchronize a COGEN with utility without parallelling ?" sounds weird to me. My 2 cents.

Muthu

http://www.edison.co.in

 

[waross]

Actually, during the first few seconds after loss of power, the synchronous motors will "lock" together and act as generators, backfeeding the plant. They will also slow down in "lock step". Those motors with high inertia will help those with little inertia. This will continue until either some protection trips the motors off line or the speed is to low to generate enough voltage to hold in the control relays.
The plant will be fed from the generators at a frequency determined by the speed of the slowing synchronous motors.

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

 

[davidbeach]

And that is why I suggested running the co-gen ahead of the utility, so that when there is a trip the motor bus phase angle will fall back and come into synch with the utility source. The only real problem is maintaining the relationship between the co-gen and the utility. I'd work on overcoming the objections to parallel operation. The is no techinical basis for prohibiting parallel operation and there are standards that can be the basis of agreement.

 

[ScottyUK]

You guys are expecting a heck of a performance from a governor, asking it to hold a constant phase relationship with the utility, which will itself move slightly in frequency, and all with a machine load which probably isn't constant. We had pretty good governors - our servo controllers could repeatably achieve 0.1% accuracy after a year in service and 0.02% over the short term - and I doubt we could achieve what you're asking over a prolonged period. The controller is fighting against too many variables: steam temperature and pressure, machine load, auxiliary plant load, and so on. Maybe the governor on a diesel set could do it - I have doubts but I honestly don't know - but a gas or steam turbine will struggle to lock a constant phase relationship.


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

 

[itsmoked]

I think davidbeach has the best solution. Just run paralleled with your cogen exporting some token amount but being held in synchronism by the utility and no technical schemes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[byrdj]

I think the expectation for the transfer relay is pretty great too.

I've been envision some type of dual sychronus condensor, one side utility and the other station.

 

[alehman]

I may be wrong, but I just doubt there is any speed control that will maintain synchronization on a consistent basis. It would be an interesting experiment however.

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

 

[hmchi]

davidbeach --- 60 degree ahead of synchronism may be good but is not that critical, as we can see from synchronism check relays --- that it will work as long as the phasor difference is within the allowable tolerance. But the real question is whether one can hold that sync or a few degrees ahead CONSISTENTLY. I was hoping that someone may have looked into this problem or have solved the same problem would come up and say that they have installed a device 25 and measure one of the sync relay outputs. For example, if the time chart shows the device 25 has 80% 'close state', representing sync state, perhaps he could tell us what he did to achieve all that ... such as his rating vs. load ratio, load step ration, and other control input and/or feedback controls ..

itsmoked --- Our problem would not have existed had the utility would allow us to parallel. davidbeach says not all utilities are alike. True indeed, except in Asia most utilities are government owned and operated .. enough said .. talk about arrogance and dictatorial ... They claim [somewhat justified] they are concerned about fault current contribution to line faults .. You said you guys are OK, are you not concerned about that ?

Again, I only feel that other COGENs must have the same issue .. Maybe they read this and tell us what how they have succeeded ..

 

[rhatcher]

waross, you are right. During the moments that there is no power source, cogen or utility, the synchronous motors will act as generators with rapidly decaying voltage and frequency output until they trip on there own.

But, if they do not trip first, the moment power is restored the synchronous motors will trip from one of two things (in my opinion). First, the stator frequency will be much greater than the rotor frequency so they will likely trip on 'out of sync' relay in the synchronous controller. If that doesn't do it, the voltage and phase angle difference between the synchronous 'motor-generator' and the restored line voltage and frequency will likely cause a stator overcurrent trip for the synchronous controller.

Of course, if your facility's main load is synchronous motors, the disturbance to the oncoming power source caused by a fast bus transfer could cause a trip by itself.

 

[waross]

I suggest we go back to the beginning and ask why the co-gen system may go down. If you lose co-gen because of a fault, neither you nor the utility want to transfer to the utility.
What else will put the co-gen down?
Loss of fuel or steam pressure?
Loss of lubrication?
Loss of heating?
Loss of a critical auxiliary system?
Overheating?
Excess vibration?
Indications of bearing problems?
If your load is steady, these conditions may allow enough time for a fast transfer to the utility, particularly if you use David's suggestion to stay 60 degrees or more ahead and catch your window of opportunity as the frequency drops slightly.
The utility may not be happy accepting your plant as an unexpected block load.
You may be better served to identify your critical loads and plan your response to a co-gen outage on a load by load basis.

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

 

[rhatcher]

waross said: 'If you lose co-gen because of a fault, neither you nor the utility want to transfer to the utility.'

I agree. After all of these points have been brought up, perhaps there is a reason that there is no 'standard' control scheme to do this.

 

[GTstartup]

I don't see this idea ever working faster or better than just shutting the cogen down and connecting to the Utility. I can't envisage any situation where the cogen plant personnel will know that there is going to be a problem and initiate a synch. I also can't see any way for the control system to automatically detect a problem in the cogen and synch. Anybody that has been in power plant when things go haywire will know that this borders on a crackpot scheme (no offence). Better to make your "hot standby" connection to the utility on the auxiliary power supply

 

[edison123]

I have seen in nuclear plants some critical reactor loads being double fed by an UPS (battery driven DC motors driving ac generators or straight-up static ups).

In a sulphuric acid making plant, I've seen the set-up is a diesel-flywheel-ac motor-ac generator feeding the critical loads. When the main ac power is lost, the diesel starts in something under 50 milliseconds IIRR without any speed drop because of the flywheel.

Muthu

http://www.edison.co.in

 

[sibeen]

They claim [somewhat justified] they are concerned about fault current contribution to line faults ..

I have just recently come across a case where the supply authority knocked back a 7 kVA grid connect solar plant as their fault tolerance was so close to the limit that even the small cuurent produced by this system would have tipped them over the limit.

The same provider is now looking very closely at a reasonable size cogen facility (20 MVA) and whether to let them connect to the grid. Again the fault current on the line is the issue.

 

[davidbeach]

Fault current isn't really that much of a concern. If the co-gen raises the available fault current to the point that anything needs to be replaced, that replacement is part of the cost of interconnection. Standard procedure. Like I say, no technical reason that you shouldn't be able to run parallel, but there may be economic reasons.

If you have no desire to export but merely wish to remain in synch, the interconnection should be through a significant reactor. That would significantly cut down on the fault current without affecting normal operations.

For those of you who aren't familiar with fast motor bus transfer systems, the technology is well established.

For this case, given no export/no import during normal operation, I'd connect that reactor through a Y-Y transformer connected to put the plant 60 degrees ahead of the utility, so that the fast bus transfer system has 60 degrees to play with.

We have a plant where the off-site aux power source was 30 degrees ahead of the plant and would loose critical motors waiting for the motor bus to fall back 330 degrees; around 270 degrees we'd start loosing critical fans. After reconnecting the aux transformer so that it lagged the plant by 30 degrees, the fast transfer works like a charm. The actual transfer happens at about 45 degrees after separation from the plant, but that is only 15 degrees out of phase with the aux source and not a problem (all induction motors). Given that I only had odd multiples of 30 degrees to work with we couldn't connect for a 60 degree difference. If you need equal performance both directions, I'd try for 180 degrees out of phase between the two sources so that the motor bus load falls back the same amount in both directions. In our case the transfer from plant to aux is always unplanned and the transfer from aux back to plant is almost always planned, it was much more important that the plant to aux transfer always succeed than that we have equal performance in both directions.