Breaker coordination of parallel generators

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mull982

Senior Member
Can coordination be achieved for multiple generators feeding a common bus when there is a fault downstream of the common bus. The way I see it a fault downstream will most likely trip all the generator breakers and thus kill all the generator power to the system. With a main breaker on the output of the common bus tripping it would also produce the same result of removing all generators and all power to the system.

Is there a way to design/coordiante breakers so that a fault downstream of the common generator bus will selectively trip breakers and not remove all generators from the system? Can this be done with multiple breakers feeding into one common bus or must a mutiple bus scheme be deisgned?

Are there any references on such design and coordination?
 

anbm

Senior Member
Can coordination be achieved for multiple generators feeding a common bus when there is a fault downstream of the common bus. The way I see it a fault downstream will most likely trip all the generator breakers and thus kill all the generator power to the system. With a main breaker on the output of the common bus tripping it would also produce the same result of removing all generators and all power to the system.

Is there a way to design/coordiante breakers so that a fault downstream of the common generator bus will selectively trip breakers and not remove all generators from the system? Can this be done with multiple breakers feeding into one common bus or must a mutiple bus scheme be deisgned?

Are there any references on such design and coordination?


You better do the coordination study right the first time so the fault of downstream CB won't trip generator breakers :cry:
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Not completely understanding your set-up, but It sounds like you have already answered your own question.

The load downstream will need to be divided up onto separate buses, with each separate bus having a main breaker on the common generation bus in order to remove the faulted bus without taking out the entire generation. A coordination study will be required to develop the appropriate settings.
 

mull982

Senior Member
Not completely understanding your set-up, but It sounds like you have already answered your own question.

The load downstream will need to be divided up onto separate buses, with each separate bus having a main breaker on the common generation bus in order to remove the faulted bus without taking out the entire generation. A coordination study will be required to develop the appropriate settings.

Thanks! Here is a system I am looking at similar to what you described.

There are (3) 1000 kW generators connected to a emergency bus via (3) generator breakers G1, G2, G3. Off of this emergency bus there are two feeder breakers GF1 and GF2 with each feeder breaker feeding a section of switchboard BusA and BusB. At these switchboards there two breakers GM1 and GM2 at which the generator feeder circuits are connected to in order to tie into these main switchboards. These (2) buses (A&B) have a tie breaker between them which is normally open. Both Buses A&B have a utility source feeding them, however my focus right now is on the generator system. I am trying to coordinate all these breakers on the generator system and wanted to verify a few things.

1) For the GM1 and GM2 breakers located on the switchboards I am settings these to coordinate with the largest feeder breaker on these switchboards. Is this correct

2) Is there any reason why generator breaker at the switchboard GM1 should not have the same exact settings at the generator feeder breaker GF1 at the emergency switchboard since these breakers are in series?

3) Should the (3) generator breakers all have the same settings in order to protect the generators? Should these settings be coordinated with the GM1 and GF1 breaker settings which we are saying are the same?

4) Should the (2) generator feeder breakers GF1 and GF2 have the same settings?

5) I also am making sure that the tie breaker is coordinated so that it coordinates with the largest load on each section of switchgear and so that is falls below and coordinates with the GM1 and GM2 breakers at the main switchboards.

Thanks for your input on this.

Are there any references you know of discussing coordination settings for multiple generators in parallel?
 

Mike01

Senior Member
Location
MidWest
Generator Breakers

Generator Breakers

Why use generator output breakers? Typically the generator control panel will provide overcurrent protection (50/51) along with other relays to put the generator into shut-down mode. Ideally the paralleling equipment would be switchgear construction there for you can turn off the instantenous region of the low voltage power circuit breakers that serve the generators, and leave the instantenous on for the feeder breakers allowing for selective coodination, typically the paralleling switchgear manufacturer will also provide some generator protective functions in the control section of the gear usually a multi-function relay of sorts connect to the generator breaker, one function to ask for is a 32 directional / reverse power, if there is a fault in the generator or the cables from the generator to the gear the gen. control panel will shut down the generator, due to the fault the directional / reverse power relay will open the generator breaker taking that generator out of the system leaving the other units up and running providing power to the emergency power bus (provided the c/t for the 32 relay is facing the proper direction). The key is switchgear type construction. If you use switchboard and molded / insulated case breakers you will not be able to turn off the instantenous setting making coordination more difficult but not impossible there are aslo techniques such as zone selective interlocking that can assist in coordination, and be layered into a switchgear to provide better equipment protection. Just a thought...
 

ron

Senior Member
Why use generator output breakers? Typically the generator control panel will provide overcurrent protection (50/51) along with other relays to put the generator into shut-down mode. Ideally the paralleling equipment would be switchgear construction there for you can turn off the instantenous region of the low voltage power circuit breakers that serve the generators, and leave the instantenous on for the feeder breakers allowing for selective coodination, typically the paralleling switchgear manufacturer will also provide some generator protective functions in the control section of the gear usually a multi-function relay of sorts connect to the generator breaker, one function to ask for is a 32 directional / reverse power, if there is a fault in the generator or the cables from the generator to the gear the gen. control panel will shut down the generator, due to the fault the directional / reverse power relay will open the generator breaker taking that generator out of the system leaving the other units up and running providing power to the emergency power bus (provided the c/t for the 32 relay is facing the proper direction). The key is switchgear type construction. If you use switchboard and molded / insulated case breakers you will not be able to turn off the instantenous setting making coordination more difficult but not impossible there are aslo techniques such as zone selective interlocking that can assist in coordination, and be layered into a switchgear to provide better equipment protection. Just a thought...
Generally the controller on the gen control panel is not listed as overcurrent protection, so you put a gen output breaker to protect the output conductor.
 

mull982

Senior Member
So if the main bus and emergency bus are switchboards, which I believe they are we must leave the instantneous settings on the breakers correct? (This is being discussed in similar post) If they are actual switchgear however then the instantaneous can be turned off?

Would you agree that the generator breakers should all have identical settiings (to protect the generators)?

Would you also agree that the two generator feeder breakers should have the same settings and these settings should be the same as the two generator main breakers in the main switchboard buses?
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
1) For the GM1 and GM2 breakers located on the switchboards I am settings these to coordinate with the largest feeder breaker on these switchboards. Is this correct

If the intent is to close the tie and operate both switchboards through either GM1 or GM2, then you will need to make sure that GM1 and GM2 are coordinated with the tie breaker.

2) Is there any reason why generator breaker at the switchboard GM1 should not have the same exact settings at the generator feeder breaker GF1 at the emergency switchboard since these breakers are in series?
Correct, you really don't care which one trips first, unless one is a set of fuses, e.g. easier to reset a breaker than replace fuses.

3) Should the (3) generator breakers all have the same settings in order to protect the generators? Should these settings be coordinated with the GM1 and GF1 breaker settings which we are saying are the same?
Yes, all three set the same, because these breakers are set to protect the machine. GF1/GF2 need to be coordinated in the sense that you don't want nuisance tripping when the generators are putting out maximum power, i.e. LT pu.

4) Should the (2) generator feeder breakers GF1 and GF2 have the same settings?
Probably, unless there is some operating scenario that might dictate otherwise.

5) I also am making sure that the tie breaker is coordinated so that it coordinates with the largest load on each section of switchgear and so that is falls below and coordinates with the GM1 and GM2 breakers at the main switchboards.
The tie should be coordinated with the GM1 and GM2 mains not the largest load.

Are there any references you know of discussing coordination settings for multiple generators in parallel?
Caterpillar has some good info. Check there website.
 

mull982

Senior Member
Yes, all three set the same, because these breakers are set to protect the machine. GF1/GF2 need to be coordinated in the sense that you don't want nuisance tripping when the generators are putting out maximum power, i.e. LT pu.

I assume you are saying that GF1/GF2 need to be coordinated with the generator breakers G1, G2, G3 correct? But as you stated also GF1 and GF2 can have the same settings.

I believe that you are also saying that GF1 and GF2 should have a longer LT p.u. than the generators? I believe this will happen when you shift the generator curves by a factor of 3?



The tie should be coordinated with the GM1 and GM2 mains not the largest load.

Yes I agree that the tie should be coordinated with GM1 and GM2. But shouldn't it also be coordinated with downstream largest breaker? If one of the GM breakers is supplying both buses through tie-breakers you would want a fault downstream of the second bus to trip the tie breaker rather than the GM generator breaker which would kill both buses.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Q1:Yes

Q2: yes they also need to coordinate with the largest load as back-up but you won't probably set the inst. setting on the mains or tie, unless need for arc flash. But, GM1 will need to also coordinate with Bus B load and vice-verse since they are cross tied. If the loads are identical then fairly easy, but if they are different you have to look at multiple scenarios. If the mains GM1 and GM2 have multiple setting groups, then you could set-up one group for when the tie is open, and then it switches if the tie is closed. If Bus A and B can be fed from one main through the tie, most likely the rating of the bus is going to be significantly higher than the largest load, and back-up coordination to the load is probably going to be in the ST function.
 
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