LV CB coordination with multiple parallel transformers
LV CB coordination with multiple parallel transformers
(OP)
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
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






RE: LV CB coordination with multiple parallel transformers
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.
RE: LV CB coordination with multiple parallel transformers
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
RE: LV CB coordination with multiple parallel transformers
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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RE: LV CB coordination with multiple parallel transformers
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.
RE: LV CB coordination with multiple parallel transformers