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Power System capacity vs. Motor Starting KVA

Power System capacity vs. Motor Starting KVA

Power System capacity vs. Motor Starting KVA

(OP)
The largest loads on our 13.8kV - 480V, 2500 kVA, Z=5.6% transformer: are two 350 Hp motors, with Code Letter G. Based on that Code Letter, the starting kVA is around 2700kVA...The motors are started across the line.

One of the options mentioned in reducing the arc flash level is to change out the 2500kVA transformer to a smaller one...since other than these two motors, it's hardly loaded at all, and the transformer is old and may need replacing sometime in the near future anyway.

My question is this: Since the starting kVA for one of the motors is 2700kVA or so...what exactly does that mean? CAN we change the transformer out to something like a 1500kVA? Or do you have to have the system capacity to match the the starting kVA?

Is there an equivalent circuit that shows this issue?

RE: Power System capacity vs. Motor Starting KVA

The starting kVA value assumes rated voltage at the motor, which is almost never the case during locked rotor conditions.  At locked rotor, the motor looks like a constant impedance load, so as the motor terminal voltage goes down, the starting current and kVA go down.  This is why reduced voltage starting lowers the motor starting current.  

You may be able to use a smaller transformer as long as the motors can successfully accelerate their driven equipment at a reduced voltage.  This can be calculated if you have data on the motor and the load.  

The transformer can generally handle the very short-term overloads associated with motor starting - assuming the motors are not starting 20 times a day.   

David Castor
www.cvoes.com

RE: Power System capacity vs. Motor Starting KVA

Firstly, I would not let arc flash analysis dictate the design like this.  

Rafiq Bulsara
http://www.srengineersct.com

RE: Power System capacity vs. Motor Starting KVA

You could probably significantly reduce the size of the 2500kVA transfomer while still adequately serving the two motors. Transformer sizing for motor starting has been addressed here before. I believe that the consensus was that you need transformer kVA equal to 150% of motor kVA to start without excessive voltage drop. If this is correct, you may be able to get away with as little as 750-1000kVA. Hopefully some other regulars will post to clarify this number or to provide the correct value.  

However, you stated that the transformer only feeds two motors and perhaps a few other small incidental loads. So for the cost of a new transformer you are reducing the arc flash potential on two starters and however many other controllers comprise the additional load (1,2,3 more?).

On a per device basis, replacing the transformer is a very expensive way to address this issue, especially if the only load is the two starters.   

Couldn't you just buy arc flash protection at the level required for the 2700kVA transformer? This seems to be a much more practical and cost effective solution.  

RE: Power System capacity vs. Motor Starting KVA

(OP)
The most favored solution to the arc flash issue is to change out the secondary main bolted pressure switch with a new circuit breaker, remote trip etc. But there is some concerns about this as well:

1) Due to its outdoor location, and its dusty, the worry is the cb won't work after years of use. They've had issues in the past.

2) Its been pointed out that the energy level will remain high AT this breaker due to the large size of the transformer, so though we've solved our downstream issues -we still have the issue at the breaker.

I've gotten quotes on the breaker, which is a drawout type with the works, rework the bus to get in there....And I've received a quote on a 1500kVA transformer - there is not a whole lot of difference in price, that's why I ask. And it would seem this would leave us with no issues at all.

We are doing the study right now to see if we can change out the primary fuses...which would be the easiest of all.




 

RE: Power System capacity vs. Motor Starting KVA

bdn2004,

Wow, it is surprising that the cost of the new breaker (4000amp?) plus installation is equal to the cost of a new 1500kVA transformer plus installation. I believe you, I just would not have predicted this to be the case.

However, won't you need to still replace the breaker either way since the existing unit is in such poor condition and may not work?

If so, then it is the cost of the 4000 amp breaker plus installation versus the cost of the 1500kva transformer plus installation and a 2000 amp breaker plus installation.

Also, I have proven that I am not a good predictor of price but I would suggest checking the price of a fixed breaker of the insulated case type. You may find this to be cheaper, especially in modifying the existing structure and bus. I would think the only disadvantage would be if you require breaker draw-out as a safety procedure for lock-out/tag-out and if this is a frequqntly worked on circuit.

I hope this helps.   

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