Step Up Transformer Inrush Current Problem 1

[RobsVette]

Hi Guys,

Looking for some help with a situation going on at a project now.

We are installing some new cooling equipment at an older hospital here in NYC.

The services for the hospital are currently all 208V. So the original plan from the engineer, was to tie the new transformer into an existing 2500 bolted pressure switch (boltswitch model VLB 3411 - 3 phase 4 wire bolted pressure switch.)

The transformer we are going to use is a 1000kva step up transformer, with the 208V side as the delta and the 480V side as the grounded wye.

I called bolt switch and they told they would not recommend this service for there switch, due to closing in on the inrush current from the transformer. They advised that as the switch is closing and voltage makes it to the transformer the 12x inrush could potentially fuse the switch part way closed.

My ultimate solution to this is to use a GE HPC butt contact 2500 amp switch between the boltswitch 2500 amp switch and the transformer to act almost as a "starter."

As I understood it, motor starters are butt engagement contacts to handle the inrush from the motor. So this GE switch is going to act very similarly.

If anyone has any experience with this, I would appreciate some feedback and considerations on my designs here.

Thanks,
Rob

 

[dpc]

If you have enough room for a second switch, why not just put in a circuit breaker and get rid of both switches.

 

[living2learn]

Also the transformer inrush could be much higher depending on how they build it. When I energize transformers from the secondary for CxA reasons the inrush could reach 20X's!

I max = 2020*height of coil*area of coil*(B residual flux+2*B max operating flux- B sat)/(N excitation turns*Effective Area)

 

[RobsVette]

Wow, I didnt realize it could get that high. I knew the old fashioned rule of motor are 5x and transformers are 12. I guese that makes sense with what i know intuitively that since there is no field in the core its equivalent to a short circuit.

I would be interseted to know what is the 2020 comes from. I assume thats a simplification of some sort, but do you have a website with a derivation?

Thanks,
Rob

 

[living2learn]

I was actually going to ask that very question later today. I got the 2020 from Square D transformer impedance. I have also attached some other papers for your reading pleasure.

 

[living2learn]

forgot to attach the other papers