LV CB coordination with multiple parallel transformers

[living2learn]

I am coordinating some circuit breakers and I am utilizing SKM. I have (3) 300kVA transformers fed from a 480V distribution board with a main circuit breaker that I am trying to coordinate. All three of these transformers will energize at the same time. Each transformer is 5% impedance and has a 480V ampacity of 370A. Transformers inrush can be as high as 12x the fla and as little as 8x.
My question is when I energize this system and the transformers begin to energize will their currents sum? if each transformer pulls 3700A x 4 then I would need to set the instantaneous portion of my MCB at 14800A? SKM only shows each individual transformer not the sum of their inrush currents.

Thanks

 

[Bramb]

Hi Living2learn,
in my opinion, as a first approach you'd better consider the sum of the 3 inrush currents.

The reality will be probably slightly different:
- the inrush current value you've indicated is the maximum one and it depends by the instantaneous voltage at the CB closing instant;
- the contemporary energization of the 3 transformers will probably cause a busbar voltage drop which will influence (reducing) the inrush currents values.

I'm afraid that, in order to exactly evaluate what above, you'll need to study the electromagnetic circuit (a software to analize the transient behavior could be needed).

I hope this will help.
Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[living2learn]

I was afraid of that. Thanks for your input. I think I am going to put a transient recorder on this system and see what exactly the inrush is.

To further explain the situation - The transformers are fed via a fast open transistion transfer switch that drops the load for a few 100ms. During this transition the circuit breaker downstream of the ATS opens up. Hence the study. I know the transformer inrush is based upon where the voltage is in the circuit in respect to the 0 axis, but this transition is pretty fast and I don't think the voltage has decayed completly.

Any additoinal thoughts - and thanks for your time

 

[VTer]

Xfmr inrush will always vary depending on points at which the supply voltage was removed, the residual flux in the core, and the point of the wave the voltage is applied to the transformer again. Now consider this for all three phases and its very evident that the inrush is very hard to predict.

In addition to individual inrush on transformers which you may sum up for the purpose of the study as the possible worst case scenario, I would also consider any inrush for motors that may be connected to the same bus.

Also please note that SKM may blindly take 8 to 12 times the xfmr FLA but your inrush will also be limited by the stiffness of your system as ErmBra pointed out. Your inrush magnitude cannot be more than your available short circuit current. You may manipulate SKM to give you the numbers you want based on your system study. Hope that helps until some of the experts on here chime in.

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[jghrist]

If all of the transformers are de-energized by the same device, then the residual flux will be at the same point on all transformers. If all of the transformers are energized by the same device, then the instantaneous voltage at energization will be the same. If one transformer experiences maximum inrush, then all of them will. The 8X or 12X FLA maximum for specific times are conservative estimates and there will be a variability among transformers. I would not expect that all of the transformers would have the maximum inrush characteristics, but if you don't have anything else to go by, adding the maximum inrush would be the conservative thing to do.

There has to be some limit. When a utility considers closing a breaker to energize a distribution feeder, they don't add up all of the connected transformer kVA to estimate inrush. They use typical cold-load inrush characteristics based on studies. The most I've seen considered is 9 time maximum feeder load. Maximum feeder load is usually a lot less than the sum of connected transformer kVA because of diversity.

To get a better answer to your specific scenario of a fast transfer, you would have to perform a transient analysis, using EMTP or similar software.

 

[burntcoil]

I think for a transformer size of 300kVA no need to consider Inrush of 8~12 times of FLA. It should be much lower. Better to ask the Vendor for the Inrush Current value or if not available, take it as 6 times. After choosing the value, go ahead with your strategy of summing the inrush currents.