Breaker coordination with multiple parallel generators 1

[rockman7892]

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?

 

[davidbeach]

It's only money. With the right equipment you can have total selectivity, at least for the primary protection. Fully selective backup protection is a bit more difficult.

 

[rockman7892]

Here is a system I am looking at and was looking for some additional insight

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? As long as both protect cables I am thinking they can be set the same.

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? I am saying that all three should have the same settings and be coordianted with the feeder breakers. This may require shifting of the curves for downstream fault current since it will be split between generators.

4) Should the (2) generator feeder breakers GF1 and GF2 have the same settings? I am saying that they should but wanted to confirm.

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?

 

[dpc]

If these are legally required emergency generators, the rules will be different.

But keep in mind that if multiple generators are on-line, the downstream breaker closest to the fault will see more fault current than the individual generator breakers. This improves the chances for coordination. The worst-case is when only one generator is on. Also, you must consider that the fault current out of the generator is not constant and generally decays dramatically over time. If the machine has some type of current boost or excitation support system, it may be able to sustain up to 300% of the rated current into a fault for a brief period of time. Without current boost, the fault current will generally decay to less than the full-load current (based on the synchronous reactance).

Then you also have to consider coordination if the fault is in one of the generators....

It gets complicated.

David Castor

http://www.cvoes.com

 

[odlanor]

rockman7892,
it wiil be more clear if you show an unifilar diagram and fault location you need seletivity.

 

[rockman7892]

Thanks dpc

Generator is a PM generator and capable of sustaining 3x fault current for up to 10s.

I agree that downstream breaker closest to fault sees full fault current while generator breaker only see's portion of fault current(1/3 in this case). Existing genrator breaker settings mis-coordinate with the downstream generator feeder breaker GF1 however for all 3 generators operating I believe you have to shift generator breaker curve to the right by 3x? So do you shift the generator pickup by 3x to view generator breaker coordintion for all 3 generators operating?

As you point out I still have to account for the issue of coordination with single generator in operation. However with only single generator running generator breaker G1 generator feeder breaker GF1 and generator breaker at main switchboard GM1 are all in series and coordination is not an issue correct? If any one of those breaker trips power is lost on the system? I would think here you just need to make sure that the largest feeder breaker on the main bus coordinates with the (3) generator breakers?

You also bring up a good point of protecting for fault in one of the generators. Although you mention it is complicated can you briefly mention what is typically done here.

I will try to post one-line of bus configuration.

Thanks for the help.

 

[waross]

I would consider the generator breakers to be for the protection of the generators and set them accordingly. I would set the other breakers so that the one closest to the fault clears first. There may be times when, with only one generator on-line, the generator breaker clears first. If so, it has done its job and protected the generator. The fault current has been interrupted. The mis-coordination may make trouble shooting a little more difficult if a generator breaker trips on a downstream fault but that is better than damaging a generator for the sake of perfect coordination.

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

 

[ScottyUK]

Normally generator overcurrent protection is by a 51VC or 51VR relay which adapts the trip setting current dependent upon the machine terminal voltage. They don't coordinate particularly well with standard IDMT relays for faults close to the machine. Unit protection - either REF or stator diff - is a common choice for protecting generator stator windings.


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

 

[ruggedscot]

This is something that Ive witnessed a long time ago and it is a nightmare fault as the generator current falls away very quickly. The whole set up of protection depends on a sizable current being provided to a fault to enable the protection to operate as intended. Having a utility supply available generally ensures that this operates as intended and the fault is quickly cleared from the network through the discrimination of the breaker protection. With the set up being discussed you need to look at some way of picking up the fact that the generators are being bogged down by the load - we had all the sets shut down on an overload fault. left us in darkness and of course the fault involved the utility aswell so that tripped out. We were left powerless and blacked out.
The generator panel board breakers didnt see the fault and this was a major failing. If the affected feeder breaker had opened then we would have survived this incident. Using a fast acting would have helped but this would bring in spurious issues as any sudden load application would cause this to go - the only way this could be resolved would be to employ zone protection of sorts to identify a fault within the zone protection and initiate a trip of the respective feeder breaker. It can be done - pricey and can be complicated but you can provide the required zone protection within your local network. Trouble would be a fault outwith the area causing the generators to trip on an undervoltage alarm. more so if you have only one set running - as said sometimes they can sustain the required level of current to provide a clean trip.