Station Service Upgrade

[UtilityGy]

Hello,

I truly beleive in" If you dont use it you loose it".
For the last one year, I have started working for a utility and working on 27.6 kV and onwards, I completely forgot how to set a molded case circuit breaker, which I used to know everything about. 120/208 V panels are the last thing on my mind.

But recently during a station service upgrade project, I have bumped in to these ones. Transformers were replaces few years back and who so ever was involved did not do justice to it. A 500 kVA transformer with secondary 208/120 V. It is approx. 1400 A main secondary current, this is a unusally high current for this voltage. Anyways my job starts after this boundary. To replace the transfer scheme, which includes three breakers probably 60 yrs old rated 1600 A at 208 V and a 800 A panel feeding the loads.

Here is what I am plannig, the new footprint will be very small compared to the old one. I plan to use 1600A frame molded case breakers two main one tie with interlocking arrangement with a new panel with 1000 A bus.

Trip unit I will be selecting for two main incoming breakers will be a 800 A trip unit with LSIG, all adjustable. Technically my main breaker CB1 and CB2 settings will depend upon the panel bus amperage as it is the lowest rated bus. Is this correct assumption, this is some how not getting in to my head.

COuld you please advise, if I have this correct.

 

[alehman]

I'm confused - so you are feeding both mains from the same transformer? Main-tie-main configurations are most commonly used where there are two sources. We need to know the loads to determine if the sizing is reasonable. Again, it is common to size the mains so that either may support the entire load. Otherwise the benefit of redundant sources is lost.

Verify the trip settings comply with applicable codes for protecting the transformer.

Molded case circuit breakers are not the best for transfer schemes. Better to use power circuit breakers which are more rugged and equipped with motor operators.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[UtilityGy]

Thanks Alehman. I apologize for lacking clarity in my question. Yes there are two sources, which will feed same panel in case of transfer.

1. Use of Molded Case CB- Eaton has MCCB up to 2500 A, Based on your experince a Power circuit breaker should be a the way to go. Would that not be more expensive and need more real estate ?

2. if my panel is rated at 1000 A then my mccb time short time or long time setting has to be less than 1000 A ?
As far as I can recall a Long time delay starts from like 400 secs typically which we can certainly change but is that not a lot for a panel which is rated at 1000 A to handle for a long period of time.

I would appreciate your comments.

 

[alehman]

1. Is the Eaton breaker equipped with motor operators? You get what you pay for. MCCB's are not intended for switching applications. Also, power circuit breakers have much greater flexibility as to time-current settings which improve the probability of being able to coordinate with downstream devices. Could MCCB's be made to work? Yes, probably.

2. Normally the long-time pickup for the circuit breaker would be set to protect the panel (and feeder), which would be 1000A. Why is your panel rated only 1000A if the transformer is 500kVA? Seems like you are stranding some of your transformer capacity in that case. Or am I misunderstanding?

No offense, but it seems like you need to hire a qualified engineer.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[UtilityGy]

Thanks Alehman. I just have to pick up pieces of some body who did not do a diligent job picking up the XFRM size in the past. I am sure, studying trip units again will give me a better handle on the situation. I was probably cutting corner by asking trip question. I have done a quite a bit of work like this in the past but just takes time to recollect.

Anyways one more aspect to the issue, This switchboard will be fed from a 500 kVA 13.8 kv. /208 V-120 V as I mentioned earlier. IEEE standard 1584- Vol. less than 240 V and 125 kVA transformer arc flash issue argument does not fit in here so arc flash could be a concern here as well?

Specially, even operating a 3 Pole 100 A MCB in the panel can be a Hazard?

Do you side my opinion ?

Thanks

 

[alehman]

Yes, arc flash is still a concern.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.