Utility Distribution Circuit Inrush

[saladhawks]

I am working on a 4.36 kV, 4-wire distribution circuit from a solidly-grounded delta-wye substation transformer with a connected load of 3,000 KVA.

The connected load of 3,000 KVA is comprised of multiple 50 KVA 1-phase distribution transformers connected 2.4 kV L-to-G balanced across 3 phases.

Circuit protection is based on a vacuum interrupter with a SEL-351R relay with reclosing enabled.

Reclosing logic is programmed with 2 fast curve and 2 delay curve operations. For the fast curve operations, recloser is implemented in a fuse saving approach whereby recloser will operate before downstream 50A Type 200(N) overhead lateral fuses.

My question is what the realistic inrush current the vacuum interrupter will simultaneously see from the multiple 50 KVA, 1-phase distribution transformers upon circuit energization?

Someone has recommended modeling the multiple 1-phase distribution transformers as a single 3,000 KVA 3-phase transformer. If this approach is followed, the recloser fast curve does not coordinate with the 3-phase transformer 12x @ 0.1sec and 25x @ 0.01sec damage curve points for inrush current.

What is the realistic 12x @ 0.1sec and 25x @ 0.01sec damage curve points for inrush current I should use for recloser fast curve coordination during circuit energization?

 

[bacon4life]

We have just started capturing inrush transients on our 12 kV feeders, so I surely haven't seen the worst case. In the couple of waveforms I have, the inrush was about 14x @0.01sec and 5x @ 0.1 sec. How long is your distribution line? The amount of reduction will in part depend on how much additional impedance the line adds between transformers.