MV Generator Breaker

Status
Not open for further replies.

Sajid khan

Senior Member
Location
Pakistan
Dear Forum Members,

In my previous thread with a general question about MV & LV generator, i have faced a lot of critic on asking general question from some folks while some replies in a very professional manner and thank to all of them.

Now coming up with a specific question, the question is,

I have seven MV generators 3150kVA, 13.8kV just delivered to site (photo attached).The breaker in the synchronization panel for generator is 200A from which MV cables will be connected to generator terminal box. The question is, Do we need a circuit breaker at the termination point in generator to protect the cable length from synchronization panel upto generator terminal?. The generator delivered on site is with a terminal box mounted on the genset but without a circuit breaker. Our Specification call for a circuit breaker at generator terminal and during material submital review the supplier in compliance statement put the breaker option in MEP contractor scope. The external MV breaker if brought by the contractor cannot fit in the terminal box or else it can be placed in a separate assembly nearby the generator terminal box.

waiting for the valuable feedback.

Thanks
 

Attachments

  • generator.jpg
    generator.jpg
    99.1 KB · Views: 0
Last edited:

ron

Senior Member
Great question because it is something that I struggle with too. MV and LV.

Cummins has been making a big thing over the last few years about their listed protective relay they have at the generator set itself.
http://power.cummins.com/sites/default/files/20170124_PowerHour_NEC 2017 Requirements .pdf


This provides local generator relay the protection required by Section 445. 12 and the recent addition of 445.13(B) in 2017.

There is no magic distance saying how far can you go if you don't have listed protection at the generator terminals. Many use the feeder tap rules of 240.21 to determine how far.

I had been compromising in the past and providing cable differential protection with CTs at the gen terminals and at the first breaker (when it is greater than 10' of conductor away) to protect the cables to that point.

Although a regular generator has proteciton built into the gen controller, it is not listed as overcurrent protection.
 

big john

Senior Member
Location
Portland, ME
The way I always see it done is through differential at the terminal boxes.

I'm not a protection expect, but I'd be hard pressed to imagine a current fault that would not be inadequately covered by that. And you have essentially no overload risk because that is covered by the CTs at the switchgear.

If somebody was concerned, why not use the CTs in the peckerhead for overload pickup?
 

Sajid khan

Senior Member
Location
Pakistan
Great question because it is something that I struggle with too. MV and LV.

Cummins has been making a big thing over the last few years about their listed protective relay they have at the generator set itself.
http://power.cummins.com/sites/default/files/20170124_PowerHour_NEC 2017 Requirements .pdf


This provides local generator relay the protection required by Section 445. 12 and the recent addition of 445.13(B) in 2017.

There is no magic distance saying how far can you go if you don't have listed protection at the generator terminals. Many use the feeder tap rules of 240.21 to determine how far.

I had been compromising in the past and providing cable differential protection with CTs at the gen terminals and at the first breaker (when it is greater than 10' of conductor away) to protect the cables to that point.

Although a regular generator has proteciton built into the gen controller, it is not listed as overcurrent protection.

Thanks very useful information..

Is this applicable for both Generator type MV & LV?
 
Great question because it is something that I struggle with too. MV and LV.

Cummins has been making a big thing over the last few years about their listed protective relay they have at the generator set itself.
http://power.cummins.com/sites/default/files/20170124_PowerHour_NEC 2017 Requirements .pdf


This provides local generator relay the protection required by Section 445. 12 and the recent addition of 445.13(B) in 2017.

There is no magic distance saying how far can you go if you don't have listed protection at the generator terminals. Many use the feeder tap rules of 240.21 to determine how far.

I had been compromising in the past and providing cable differential protection with CTs at the gen terminals and at the first breaker (when it is greater than 10' of conductor away) to protect the cables to that point.

Although a regular generator has protection built into the gen controller, it is not listed as overcurrent protection.

Is this type of Gen protection at short circuit conditions is capable of Immediately prevent the gen to inject current to the network/fault point between the gen terminal and synchronization panel.
 

ron

Senior Member
Is this type of Gen protection at short circuit conditions is capable of Immediately prevent the gen to inject current to the network/fault point between the gen terminal and synchronization panel.
I think you asking whether differential protection for this application stops flow of current from the generator. If so, I have the 87 shunt trip the downstream breaker and remote activate the EPO on the generator controller. I think that causes the voltage regulator to drop to zero. I think that will address additional flow from the gen. I'm not sure, but it is the best that can be done in that situation.
 

Sajid khan

Senior Member
Location
Pakistan
Is this type of Gen protection at short circuit conditions is capable of Immediately prevent the gen to inject current to the network/fault point between the gen terminal and synchronization panel.

Not an expert, others can correct me if wrong. IMO, the permanent magnet of the generator (PMG) of the generator set has the capability to hold the short circuit for few seconds till the AVR of the generator coordinate with downstream breaker to clear the fault. In this case the short circuit will remain in between gen. terminal and synchronization panel for a few seconds until the downstream breaker will clear the fault.
 
I think you asking whether differential protection for this application stops flow of current from the generator. If so, I have the 87 shunt trip the downstream breaker and remote activate the EPO on the generator controller. I think that causes the voltage regulator to drop to zero. I think that will address additional flow from the gen. I'm not sure, but it is the best that can be done in that situation.
OK, As per your explanation the cable from generator to the panel it will be protected or it will not be effected during this short time until clear the fault.
 

Sajid khan

Senior Member
Location
Pakistan
in my experience, i will not place a breaker in the gen terminal..


Sent from my iPad using Tapatalk
What brand of generator was that?? If u have any refrence from manufacturer catalogue recommending not to install circuit breaker please share..
My gensets are Cummins MV genset 2500KW

Sent from my GT-I9300I using Tapatalk
 

rian0201

Senior Member
Location
N/A
What brand of generator was that?? If u have any refrence from manufacturer catalogue recommending not to install circuit breaker please share..
My gensets are Cummins MV genset 2500KW

Sent from my GT-I9300I using Tapatalk

I think the brand has nothing to do with it. It is a 14MW generator. Just like others have said, it matters on the installed protection and the philosophy that has been applied to it.

If the concern is the scope of work and specification, then ask whom the scope is referred to. ( I think this is the main concern, not the protection of generator).
 

Sajid khan

Senior Member
Location
Pakistan
I think the brand has nothing to do with it. It is a 14MW generator. Just like others have said, it matters on the installed protection and the philosophy that has been applied to it.

If the concern is the scope of work and specification, then ask whom the scope is referred to. ( I think this is the main concern, not the protection of generator).

Our generator specification calls for the generator circuit breaker but is a debatable issue, whether it is required or not , as the MV generator breaker is an expensive item.
 
Status
Not open for further replies.
Top