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Can one synchronize a COGEN with utility without parallelling ?Helpful Member!(4) 

hmchi (Electrical) (OP)
23 Oct 09 7:53
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 (Electrical)
23 Oct 09 8:02
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

hmchi (Electrical) (OP)
23 Oct 09 9:45
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 (Electrical)
23 Oct 09 10:18
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

edison123 (Electrical)
23 Oct 09 10:58
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 ?


Hoxton (Electrical)
23 Oct 09 14:49
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 (Electrical) (OP)
23 Oct 09 14:50
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 (Electrical)
23 Oct 09 14:55
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

dpc (Electrical)
23 Oct 09 14:56
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 (Mechanical)
23 Oct 09 15:07
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 (Electrical) (OP)
23 Oct 09 15:45
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 ..
Helpful Member!(2)  davidbeach (Electrical)
23 Oct 09 15:55
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 (Electrical) (OP)
23 Oct 09 15:59
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 (Electrical)
23 Oct 09 16:06
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 (Electrical)
23 Oct 09 18:35
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.

"Why not the best?"
Jimmy Carter

alehman (Electrical)
23 Oct 09 20:20
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.

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

hmchi (Electrical) (OP)
23 Oct 09 21:26
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 (Electrical)
23 Oct 09 21:47
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.  
Helpful Member!  ScottyUK (Electrical)
23 Oct 09 21:59
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 (Electrical)
24 Oct 09 1:44
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.
Helpful Member!  PHovnanian (Electrical)
24 Oct 09 14:42
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 (Electrical) (OP)
24 Oct 09 15:22
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 (Electrical)
24 Oct 09 20:42
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.  

"Why not the best?"
Jimmy Carter

ScottyUK (Electrical)
25 Oct 09 2:46
Hi hmchi,

Sorry for the mis-understanding!

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

rhatcher (Electrical)
25 Oct 09 8:24

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 (Electrical)
25 Oct 09 8:53
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.  


waross (Electrical)
25 Oct 09 9:40
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.

"Why not the best?"
Jimmy Carter

davidbeach (Electrical)
25 Oct 09 10:58
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 (Electrical)
25 Oct 09 14:16
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.

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

itsmoked (Electrical)
25 Oct 09 14:54
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 -

byrdj (Mechanical)
25 Oct 09 14:55
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 (Electrical)
25 Oct 09 16:57
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.

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

hmchi (Electrical) (OP)
25 Oct 09 16:58
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 (Electrical)
25 Oct 09 19:23
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 (Electrical)
25 Oct 09 19:46
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?
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.

"Why not the best?"
Jimmy Carter

rhatcher (Electrical)
25 Oct 09 20:04
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 (Electrical)
25 Oct 09 20:09
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 (Electrical)
25 Oct 09 20:25
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.


sibeen (Electrical)
25 Oct 09 21:43
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 (Electrical)
25 Oct 09 23:45
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.
slavag (Electrical)
26 Oct 09 7:23
Im still not sure if it's possible.
Yep. David right, we do transfer scheme too, but it's for two synchronasing systems with big AC motors.
Here trip of generator are possible from few reasons, as Bill pointed, for example from underfrequency protection or loss of execitation, or from fault on busbur....
Isn't simple issue.

Best Regards.
rhatcher (Electrical)
26 Oct 09 18:15

You brought up a good point that led me to think of why the OP's utility is concerned about running in parallel. That being said, your actual point differs from the point I see.

I can imagine a scenario where the contributiun of fault current to the system from a cogen would be a concern beyond the point of interconnection and would involve costs not normally directly associated with construction and connection fees.

Imagine a power plant with a radial feeder system. A distribution substation at the end of a feeder has enough excess load capacity to feed a proposed new small manufacturing facility. The short circuit current from the utility at that substation is relatively low because of the radial distance, let's say 30 miles.

Let's also say that the manufacturing faciltiy is cogen with a generating capacity equal to it's power consumption (1:1) and that it is only 1 mile from the existing substation.

Theoretically, the cogen facility could supply much more contribution to a fault current at that substation than the distant utility. Unless this type of contribution was considered in the original design of the feeder substation and everything downstream of it, it can be imagined that everything in that substation and downstream of it would need to be upgraded to a higher MVA interrupting rating if parallel operation was allowed.

Of course, if this is true, then how/why would the utility allow an interconnection at that substation in the first place? They are protected from the fault current contribution of the cogen by having a contract that prevents parallel operation.

Perhaps this is how and why the OP is in this situation.  
rhatcher (Electrical)
26 Oct 09 18:31
davidbeach, I'm not picking on you but I also have some questions about the utility-plant phase relations you describe.

I would think that a wye-wye transformer would have the utility-plant phase relationship at 0 degrees ("I'd connect that reactor through a Y-Y transformer connected to put the plant 60 degrees ahead of the utility...").

I would think that a standard delta-wye step down transformer would have a difference of +30 degrees ("We have a plant where the off-site aux power source was 30 degrees ahead of the plant...").

I would think that a wye-delta step down would have a phase difference of -30 degrees (" After reconnecting the aux transformer so that it lagged the plant by 30 degrees...").

Finally, I would think that it is possible to connect a wye-wye or a delta-delta for 180 degrees but I do not understand how you could get a phase relation of 60 degrees with a single transformer nor do I see how you could get any "odd multiples of 30 degrees" such as 90 or 150 degrees.

Of course, they may be something really obvious (dumb on my part) that I am overlooking at the moment. If so, I hope that someone points it out and, if so, I apologize for the mistake.  
davidbeach (Electrical)
26 Oct 09 18:55
When living in the ANSI world, it is very easy to think that the standard that low side lag high side by 30 degrees is a hard and fast rule.  It is useful, and for probably 99.99% of all applications it is all that is necessary.  But it ain't the whole picture.

The IEC world gets a bit closer with their clock notation, but even there there can be a temptation to assume that a Dy1 transformer is always a Dy1 transformer.

First off, break all preconceived notations about phases and terminals of the transformers.  The internal wiring of the transformer tells how H1, H2, H3 relate to X1, X2, X3 but doesn't necessarily say how A, B, C have to relate to a, b, c.

First set of transformer transformations - take your a, b, c leads and roll them one position either way.  You now have the possibility of steps of 120 degrees.

Second set of transformer transformations - take two phases on the high side and swap them and at the same time swap the same two phases on the low side, that makes a 60 degree shift.  If the low side lagged by 30 degrees, it now leads by 30 degrees and if it lead it now lags.

Between those two transformations you now have 60 degree steps.  For all transformers with a delta and a wye, those 60 degree steps are all odd multiples of 30 degrees (30, 90, 150, ...) and for transformers with two wyes or two deltas, those 60 degree steps are all even multiples of 30 degrees (0, 60, 120, 180, ...).

At the plant in my previous example, we'll take the 500kV bulk power system as the reference at 0 degrees.  The plant source for the motor bus is separated from the 500kV system by two delta-wye transformers and therefore is at -60 degrees.  The off-site aux power was separated from the 500kV system by one delta-wye transformer and therefore was at -30 degrees.  -30 degrees is 30 degrees ahead of -60 degrees.  Through a swap on the high side and a swap and a roll on the low side the transformer for the off-site aux power moved forward 60 degrees (the swaps) and then back 120 degrees (the roll) and wound up at -90 degrees.  -90 degrees lags -60 degrees by 30 degrees.
rhatcher (Electrical)
26 Oct 09 19:23
Good explanation of your answer. If I follow you correctly, you start out of phase 120 degrees, reverse phase rotation 180 degrees, and the result is a net 60. I get that, although I will sleep on it tonight to make sure...That is clever.  

You are right, although I have looked at zig-zag connections before, for the most part I was stuck in the conventional thinking of transformer phase relationships between delta and wye and had not considered the possibilities of phase roll and reversal. Thanks for the clear and detailed answer and for not making me sound like a dummy.
rmw (Mechanical)
26 Oct 09 19:35

Thanks from me too.

davidbeach (Electrical)
26 Oct 09 19:56
I missed the rhatcher (Electrical) 26 Oct 09 18:15 post in my previous response.  In the world I live in, we can't say no to an interconnection request, and that is probably true for most utilities in the US.  But what we can do, and do do, is study all of the impacts of the proposed interconnection and tell them the cost of mitigating all of their impacts.  That can range from nearly nothing to extensive rebuilds, sometimes well away from the proposed point of interconnection; but there are no insurmountable technical obstacles.  Money can cure anything, but the need for too much money can kill most any project.

So, I'll stand behind my statement that there are no technical reasons to prohibit interconnection, only economic realities.  Maybe the co-gen in question would have required more system upgrades than the cost of the plant; possible, and a good reason not to design for parallel operation, but not a technical obstacle.

Connection through reactors would reduce the reduce the fault impact but allow a phase relationship to be maintained.  There would be some load flow back and forth through the reactors as the plant load changes due to load-generation mismatches that presently show up as frequency changes.
rhatcher (Electrical)
28 Oct 09 18:53
For anyone else who is still thinking about the transformer phase shifts that davidbeach described but who are still not fully understanding it, this paper explains it and makes the explanation clear with phasor diagrams and connection diagrams. The phase shifts are not the point of the paper, differential relay protection is. However, in order to explain his point, the writer gives us all we need to know about the possibilities of reconnection of transformers to get non-standard phase shifts as davidbeach describes. It definitely helped me.

for davidbeach: Any time I see someone say; "I'll stand behind my statement" in response to one of my posts, I get the impression that they think that I have disagreed with what they said. That is not the case here.   

A few days ago, I was stuck on the question of why would a utility that is capable of supplying load current to a plant not want to allow parallel operation with the same plant having a 1:1 cogen capability. Your post of 25 Oct 23:45 gave me the idea that answered my question; the fault current contribution of the cogen to the utility system beyond the point of interconnect.

In the scenario that I offered, there is no technical reason not to parallel if you can afford to pay the economic cost of upgrading the utility's distribution substation(s) to a higher MVA interrupting rating. It's just a matter of cost. We agree on that point.

That being said, the OP has stated several times that his utility does not allow parallel operation and he has stated that he is in Asia, not the US. I'll bet he wishes that he had a utility that was from 'your world' where this would not be a question or a problem. However, 'his world' is obviously different. I was offering an idea of why that may be true.  

Moving on...You have offered a good, cost effective, solution for the OP to eliminate the utility's concern about the cogen's contribution to utlity system fault current. The use of a significant, high impedance, reactor when operating in parallel with the utility would definitely allow sychronized operation when there is no power flow while protecting against increased fault current contribution to the utility from the plant (or visa versa).

However, it seems to me that the reactor would have to be dropped out of the circuit once there was power flow through the interconnect point or else there would be a significant voltage drop across the reactor proportional to the plant load. A simple way to do this would be a breaker in parallel with the reactor that closes and shorts the reactor when the cogen generator breaker opens.

Does this agree with your knowledge and experience or am I missing something (again)?
rbulsara (Electrical)
28 Oct 09 19:01
I also know of at least one utility co in New England that would not allow paralleling with them in certain parts of their network, especially those in downtown area with a lot of spot networks, without giving a reason. Too much liability perhaps.

Rafiq Bulsara

rhatcher (Electrical)
28 Oct 09 19:41
I hate to be long-winded in my answers, I am working on that. Also, perhaps I am too concerned about upsetting other forum members.

I should have simply replied that I believe that my response was appropriate to the OP's situation and then pointed to this post from the OP on 25 OCT 09 16:58 where he said:

" 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 ? (I added the underline)

I think my answer to the OP of why this may be true was a good possibility.
davidbeach (Electrical)
28 Oct 09 19:43
rhatcher - Yep, I was thinking reactor only in when generator is paralleled; standby connection through a different path that doesn't have the reactor.

rbulsara - Secondary network systems were never designed to be anything other than radial.  Any generation introduces complications that are difficult to overcome.  IEEE P1547.6 will address these issues and has been rather contentious over the 4 plus years we have been working on it.
rhatcher (Electrical)
28 Oct 09 19:54
Can you guys explain what the terms 'spot network' and 'secondary network system' refer to? Thse are terms are new to me.  
davidbeach (Electrical)
28 Oct 09 20:32
Network systems - the short story.

General - A system in which multiple primary circuits (generally 3-4, can be as few as 2) feed transformers paralleled on the secondary side. Transformer protection in the form of a network protector; little to no protection against forward faults, very sensitive to reverse current to the point of interrupting transformer excitation current. Main purpose of the network protector is to remove back feeds to faulted primary circuits. Secondary faults, particularly on area networks, allowed/expected to burn open.

Area Network - aka grid network, aka street network; generally 208V. Many transformers all paralled over a large area (10's to 100's of blocks) in city centers. Loads tapped at many locations. Very high to extremely high available fault currents (well over 200kA in portions of Manhatten).

Spot Networks - Generally 480V. All transformers and network protectors in a single vault (spot) to serve a single building or portion of a single building; could have multiple spots at different levels of a high rise building.   
rbulsara (Electrical)
28 Oct 09 21:18

Also look up NEC 450.6 and read the commentary in the NEC handbook for it. Pretty self explanatory. NEC refers them as just transformer networks with secondary ties.


Rafiq Bulsara

rbulsara (Electrical)
28 Oct 09 21:49
Spot network are issues only if someone wants to export to the utility. Not all paralleling are for exporting. In fact many are used merely for soft transferring the loads between  the utility and the gen plant. Even the intertie breaker has reverse power protection.


Rafiq Bulsara

davidbeach (Electrical)
28 Oct 09 23:10
Inverters on spot networks are probably not going to be too difficult, but synchronous machines are quite another issue.  The problem isn't steady state operation, but the response of the network protectors to high side faults.  Without the generator, only the network protector on the faulted circuit will trip.  With the generator, and all of the primary circuits tied at the source as is the usual case, the fault current can flow out through all of the network protectors, causing all of them to trip.  That is a significant problem.
rbulsara (Electrical)
29 Oct 09 7:26
That is a separate discussion..there is a trade off of risk/benefit depending upon how long a gen remain in parallel and how much fault current a customer gen can provide.

Directional over currents on inter-ite breakers can easily be coordinated with network protectors, for non-exporting paralleling.

In fact loss of all NP (IF that happens) will not affect the user ( and should not to the utility). The user has the gens already running.  

Many of the issues are created by utility co. themselves, (I know of one case), where two MV feeders are arranged as spot network at the customer end, to help utility feed the heavily loaded feeder from both ends..! This though is very beneficial to the user as they have the increased reliability.

Much of this depends on who you are dealing with and somewhat on the design, I have had a flat denial, even to discuss, for a 3MW, 480V gen plant for a few seconds, a very elaborate exchange of information, impact studies and meetings to get a 4-6MW,13.8kV plant approved and absolutely no real push back for a 20MW and 7.5MW gen plant paralleling.

Not all issues or their solutions are technical, political will and relations also affect many decisions.

The explosion in wind turbines is one example. The most notorious and problematic interconnection from a stability point of view is a WT farm, yet they are more easily accepted compared to some roadblocks thrown at proven interconnection for conventional DGs. This is because of the political environment for green and renewable resource initiatives.

You can make anything happen if you have the will, especially the political will!


Rafiq Bulsara

hmchi (Electrical) (OP)
29 Oct 09 7:41
rbulsara said --- The explosion in wind turbines is one example. The most notorious and problematic interconnection from a stability point of view is a WT farm, yet they are more easily accepted compared to some roadblocks thrown at proven interconnection for conventional DGs. This is because of the political environment for green and renewable resource initiatives.

Absolutely true !  WTGs are induction machines, no var compensations at all [except GE machines].  reactive compensation and utility line fault decoupling and voltage support even bigger issue .. but they are allowed to connect and parallel [sure, they export almost 100% and COGEN may not ..]

Just think if utilities in the West can be so dictatorial, how bad can the Asian government owned and operated utilities could be ..

That is why I came to think that there must be a lot of COGEN before our facility who had the same problem and may have already solved the problem ... and it appears that they do not read this website and come to enlighten us ..
rbulsara (Electrical)
29 Oct 09 8:19

How big is your Cogen? and what type of loads?

I do not believe in fast transfers for magnetic loads. I would rather look in to rotary UPSs (of the right kind) with good flywheels, as suggested before.

I presume, you are not talking about transferring when the co-gen fails under a fault. If so consider Static transfer switches (STS) for those loads that can take it. They have their own challenges, though.

You also can segregate loads that can take momentary outage and those who can't and devise a solution accordingly.


Rafiq Bulsara

rbulsara (Electrical)
29 Oct 09 8:30
Also I would look to build redundancy in the co-gen plant itself for critical loads and not depend on the utility.  

Rafiq Bulsara

hmchi (Electrical) (OP)
29 Oct 09 8:54

The machine is 15MW and load is approaching 10MW.  It is a textile plant so the load is mostly VFD drives.  Yes, very sensitive to undervoltage.  Yes, we are looking into segregating loads and perhaps a medium voltage UPS backup for the VFDs [about 5MW], but it is really expensive .. so if we could handle this with sync and fast transfer to utility, it would be a big cost saving not to bother with the UPS.

Static transfer switches only work when you are in phase or nearly in phase [we have the same problem of consistency in sync], and they are also terribly expensive at 6.6kv ..

The plant owner said that they did not initially invest in personnel training thus had more turbine trips than usual, but not out of electrical short circuit faults, etc.
rbulsara (Electrical)
29 Oct 09 9:07
Thanks for the feedback. Cost all relative. A mishap in a fast transfer can cost dearly too.

Rafiq Bulsara

ScottyUK (Electrical)
29 Oct 09 11:39
UPS modules for that kind of rating: contact companies like Anton Piller, Eurodiesel, Hitec Power Protection. All can either do a direct MV solution or an LV solution with a step-up transformer. I think GE had a big rotary design too although they might have bought up someone else to acquire the technology. No doubt Rafiq could add a couple more from North America.

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

rbulsara (Electrical)
29 Oct 09 12:56
Piller and Hitec are the names here. Rotary UPSs are not very popular here more due to misconceptions than anything else and some cost issues.

Although I feel more comfortable with Piller style, which is  different than Eurodiesel or Hitec. Those two have similar technology (MG set, clutch and engine).

In this particular case any solution other than utility paralleling is going to cost a small fortune.   

Rafiq Bulsara

rhatcher (Electrical)
30 Oct 09 19:51

I am curious. In most cases when I think of cogeneration, it is when a manufacturer has a byproduct that can be used to produce power. A common example is that many paper mills use the excess steam from the paper making process to run steam turbine generators. What byproduct is used to produce 15MW power at a textile plant? Is it steam or is it something else?

Back to your question....I would guess that the contribution of power (or fault current) from a WTG would be much less that the 15MW generating capacity that your plant has. That may be the difference, from an engineering and an economic standpoint, on whether your utility allows parallel operation.

From a political standpoint, it is sad to say that today the normal engineering criteria of simplicity, economy, and effectiveness are often overlooked in favor of being 'green'.  
hmchi (Electrical) (OP)
30 Oct 09 20:18
It is not a garment factory.  The outputs are films, yarns and so forth that are the first phase of the textile industry.  It is basically a petrochemical plant using steam extensively in the process.  They up the pressure to generate before sent to the process.

As for WTG, you should know that Farms these days would aggregate 50 or more WTGs at 2 MW each .. interfacing with utility at 115kv.

The WTG farms are a lot less forgiving then steam turbines and undervoltage conditions can be quite disruptive to the WTGs .. Wind generation output can be quite varied from day to night, day to next day .. which normally utility would cite as 'unreliable' ..  utilities bend over backwards to cater to the wind farm people.  Yes, it is political..

COGEN could be green too ..  If the generation increases the efficient use of fuel to energy transformation, it is no less worthwhile than Wind Farms ..
slavag (Electrical)
31 Oct 09 0:59
hmchi, try explain to your utilities, that cogeneration good for them too. See, your case is classical case of cogeneration, becouse you need steam for process too.
But w/o connection to utilities you are very, very weak source.

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