transformer inrush for distributed transformers 1

[csparks]

I work for municipal that distributes electrical power. Voltage is 12,470 phase-to-phase. Understanding/calculating transformer inrush for a single transformer is pretty straightforward -- 12X rated load for 0.1 second.

We have fused sections of distribution where the connected transformer kVA varies, but, for example lets assume we have 480 kVA connected over a section of 2 miles. We would probably use 4/0 aluminum primary. Lets say the 480 kVA is 8-50 kVAs and 1-80 kVA and is distributed across the miles.

Would the inrush still be 12X 480kVA at 0.1 second? If not, how much less would it be? And, why would it be less?

I also believe inrush at .01 second is about 25X rated load.

 

[wareagle]

12X is a rule of thumb. It does not mean every transformer will take 12X inrush. I have seen utilities use 150% of the FLA as a fuse size. It depends on the fuse type your are using. You can look at the fuse curves and determine if the 150% will work for your situation. I would not think you would be fusing a 4/0 feeder. If you are, you may need some assistance in the sectionalizing procedure for your system.
Generally a 4/0 ckt would be under the sub breaker or a line OCB.

 

[csparks]

True, we would not typically fuse the 4/0. For the 480 kVA we would use #2 aluminum. Our guidelines call for an 80K fuse. It seems to some that this may be extremely conservative for the example I posed.

We are thinking we could put a relay on a section that is likely to operate soon and take a real meaasurement. However, maybe someone has already some knowledge regarding this issue.

 

[wareagle]

I may have confused you. When I said utilities use 150% of FLA, I meant that is for the individual transformers. Your line fuses are determined by a coordination study. You use the max size fuse that will coordinate with the sub breaker. How were your guide lines determined?

 

[csparks]

Coordination is a different concern.

Our Max KVA for a fuse has been based upon the assumption that a distributed set of transformers has the same inrush current as one equivalent big transformer. Intuitively, this does not seem to be true. The concern is that if the fuse is not sized for the inrush current, when you close it back in, it will operate again on inrush and we would be "up the proverbial creek without a paddle."

Actually, we would have to take time to sectionalize the section and close it back in in pieces.

 

[davidbeach]

Multiple transformers undergoing inrush at the same time exhibit the same inrush characteristics. What inrush looks like depends on where on the wave the voltage opened, and where on the wave the reenergization occurs. I've seen wind farm collector circuit inrush events and you can't tell that it is multiple transformers by the shape of the current waveform. So I wouldn't take much, if any, of a reduction in inrush compared to rating for a collection of transformers.