sizing breaker for gas compressor

[jtran91]

this is a NEC job, sizing a breaker for a 3-phase 13.8 kV, 10,000 HP motor

i am coming up with a 1200A breaker, is this correct?

 

[davidbeach]

Sounds like you may be a bit out of your depth. At those voltages the breaker is just an interrupting device that trips when told to by an external relay; the breaker does not make the trip decision. 1200A is probably the smallest breaker that can be purchased, but a larger breaker would not be a problem. The relay settings will determine when/under what conditions the trip occurs.

 

[jraef]

The motor FLA is likely around 450A, but davidbeach is right, a 1200A breaker is probably the smallest you can get in 15kV class. Are you using a motorized breaker and a protection relay as the motor controller? Are you aware of the pitfalls in that?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[jtran91]

i see, since the motor FLA is around 450A, do you multiply FLA by 6, for the starter inrush current. if thats the case then 3000A breaker would be ideal?

yes, we are using a protection relay (SEL)

 

[davidbeach]

Inrush doesn't matter. That 1200A is a continuous, thermal, rating. It won't trip on inrush unless the relay tells it to. Look at the short time rating(s) of the breaker, they'll be far more than 1200A.

 

[jtran91]

What do you recommend?

 

[davidbeach]

It's hard to recommend anything with the information available. I'd use the same size breaker for all positions in the gear, that makes spares easier to deal with.

 

[melspuds]

Without much info Dave Beach is correct. I don't know the type of gas being compressed, but have worked on a lot of natural gas compressor rebuilts. The swgr is normally far away from the compressor. Some of the compressors use dry compressed air for controls and some are electric controls. All wiring must per NEC including the RTDs. There is also a lot cooling systems for both the compressor and the gas after the compressor, so the is a lot of other smaller motors (10-150 hp).
Good Luck
Dave

 

[oldfieldguy]

In recent years my employer installed several electrically driven gas compressors. Below 10,000 HP we chose induction motors driven by variable frequency drives and above that, synchronous motors driving variable speed transmissions, all driving centrifugal compressors.

We put in quite a few 15,000 and 22,000 HP synchronous motors in the 12.47-13.8 kV range. The motor breakers were 1200-amp VCB's with protection taken care of by microprocessor-based motor protection relays and the motor manufacturer's excitation controls. The electrical control and protection was pretty conventional, as one might imagine, but it was tied into compressor control and station control systems.

Previous problems with breakers failing to open caused us to use a breaker failure backup system. If a motor breaker was called upon to trip and failed to do so, we trip the bus incoming main. I find that prudent.

At another facility I work with, we have a couple dozen 4160-volt motors controlled by 5 kV circuit breakers. Again, motor protection relays provide the protection functions.

old field guy