Shunt Trip to limit Gen Peak/Continuous Output

[SteveO NE]

Hi,

I have the following scenario:

Existing gear has a 600A breaker part of a kk interlock scheme for emergency whole building power. The 600A would not be adequate to provide for the entire facility but the generator is for a very specific set of loads to avoid losing process heat in a power outage and spoiling an entire batch of materials. The load is purely resistive so essentially we will be close to unity power factor. The generator they were able to get in a pinch is a 400kW/500kVA generator. Their loads only require half or less of that. The generator cannot be dialed back. The electrician wired everything in and the inspector then asked him to have an engineer look at it, here is what they did:

Gen with Integrated 800A CB --> Conductor --> 600A CB interlocked breaker in switchgear.

The issue is that they need this up and running yesterday, and everything these days has ridiculous lead times. The 800A CB in the generator has a shunt trip relay, I had a thought of a resolution, but not sure it is feasible. Seeing at the loads they intend to connect were previously behind a 225kVA gen without issue, and we have a list of possible load sources that could be connected at a single time (per 702.4(B), and 702.4(1)), is there anything that would specifically indicate that I could or could not use utility grade relaying with CTS on all 3 legs to manage provided current to not exceed 80% continuous of 600A (400kVA) / 100% non continuous (498kVA)? The only reference I see specifically to nameplate ratings are load related, not generator related, so my logic seems to tell me that if I am limiting full load current with a fail-safe device (if the device is inop the gen won't work). However, logic also tells me I am missing something . Another side note is that in all reality here the breaker is 100% rated and everything ahead of it is as well so we are talking 1A over at 80% power-factor highly improbable) that still causes the trouble; we are actually in the process of trying to figure out if everything is 100% rated while we work through solutions though don't know what that really does for us anyway.

In another time when breakers or generators weren't a 6 month + lead time, I would've just said do it right, but here they are really in a pinch, ever day could mean big $$$ lost if they have a power outage at the wrong time (which is not terribly uncommon).

Thanks in advance for your thoughts on this! Sorry if this seems like a whacky suggestion, open to other suggestions as well.

 

[petersonra]

I see any requirement to account for PF. It is all about current.

 

[hillbilly1]

Is the 600 amp in the gear, an actual circuit breaker? Or just a molded case switch with no ocp? If an actual breaker, you would just use the tap rule, and not worry about it. Since it is a manual transfer, all non essential loads can be marked, and turned off before transferring to generator as required by code to prevent overloading.

 

[SteveO NE]

I see any requirement to account for PF. It is all about current.

Yes, I just mention PF because the generator is 400kW which is 480A x 1.25 = 600A OCPD, so no problem. That means at unity PF the generator actually fits behind the breaker at full capacity. The problem is it is capable of 500kVA at 80% PF which puts us at 601A continuous so we need an 800A bkr or a 601A fuse.

Is the 600 amp in the gear, an actual circuit breaker? Or just a molded case switch with no ocp? If an actual breaker, you would just use the tap rule, and not worry about it. Since it is a manual transfer, all non essential loads can be marked, and turned off before transferring to generator as required by code to prevent overloading.

Yes, its an OCPD with a key'd interlock to force disconnect from main OCPD.

So you are saying because its a manual transfer we can say we are protecting against overload and overheating of the OCPD itself vs. just the generator by identifying what loads can be utilized? My issue is the generator is capable of providing enough current continuously if asked for it to load the breaker more than 80%.

 

[d0nut]

You are looking at the problem backwards. The generator could not care less about the downstream overcurrent protection. It will not force extra power through the system that exceeds the load. It will provide what the load requires.

What is the calculated load that you are trying to operate during a power outage? If that number, including demand factors, continuous loads, etc. is less than or equal to 600A, your system will be fine. If that number is greater than 600A, you need to replace the 600A breaker.

 

[gadfly56]

You are looking at the problem backwards. The generator could not care less about the downstream overcurrent protection. It will not force extra power through the system that exceeds the load. It will provide what the load requires.

What is the calculated load that you are trying to operate during a power outage? If that number, including demand factors, continuous loads, etc. is less than or equal to 600A, your system will be fine. If that number is greater than 600A, you need to replace the 600A breaker.

He already mentioned that the loads were previously served by a 225 kVA generator. If the loads are the same, the conductors are the same, I can't see the issue with putting in a 500 kVA generator in its place. As you say, the generator isn't pumping out any more than the loads require. It seems to me the OP is overthinking this.

 

[sgtsparky]

Yes, I just mention PF because the generator is 400kW which is 480A x 1.25 = 600A OCPD, so no problem. That means at unity PF the generator actually fits behind the breaker at full capacity. The problem is it is capable of 500kVA at 80% PF which puts us at 601A continuous so we need an 800A bkr or a 601A fuse.


Yes, its an OCPD with a key'd interlock to force disconnect from main OCPD.

So you are saying because its a manual transfer we can say we are protecting against overload and overheating of the OCPD itself vs. just the generator by identifying what loads can be utilized? My issue is the generator is capable of providing enough current continuously if asked for it to load the breaker more than 80%.

Your breaker size is based on the load value not what the generator can supply, if we had to put breakers for the supply every home would need much larger breakers… what brand and model of breaker are you using some commercial gear is 100% rated but that’s not the real issue.

 

[kwired]

Yes, I just mention PF because the generator is 400kW which is 480A x 1.25 = 600A OCPD, so no problem. That means at unity PF the generator actually fits behind the breaker at full capacity. The problem is it is capable of 500kVA at 80% PF which puts us at 601A continuous so we need an 800A bkr or a 601A fuse.


Yes, its an OCPD with a key'd interlock to force disconnect from main OCPD.

So you are saying because its a manual transfer we can say we are protecting against overload and overheating of the OCPD itself vs. just the generator by identifying what loads can be utilized? My issue is the generator is capable of providing enough current continuously if asked for it to load the breaker more than 80%.

And likely the utility source is capable of that as well, I don't see a problem.

 

[Hv&Lv]

Yes, I just mention PF because the generator is 400kW which is 480A x 1.25 = 600A OCPD, so no problem. That means at unity PF the generator actually fits behind the breaker at full capacity. The problem is it is capable of 500kVA at 80% PF which puts us at 601A continuous so we need an 800A bkr or a 601A fuse.

That isn’t an issue at all.

If it were you would be required to put 1000 (or higher) amp breakers in where the utility connects on every service installed

As many others have said, there is no issue