Three phase pad mount

[THE KID]

Can you energize a 3 phases padmount 2000 KVA transformer with no load with just single phase fuses and no switch on a Delta connection on primary side?

[topgroove]

Really bad idea.

[THE KID]

Thanks Topgroove
This is the first one that we have came across that doesn't have a switch in front of it. We didn't think that we should. Appreciate the info!

[rcdallas]

I gotta ask. Why is it such a bad idea? Wye-delta I can see a problem with Ferro. Why not just because its delta highside? As long as the length of the cable doesn't exceed the KVA of the transformer I don't know of any other problem with no load on it?

Feed my sponge Buffalo Groove.

[topgroove]

the bank the Kid is refering to is a wye, Delta bank. In this case I believe the switch he's talking about is a switch that grounds down the primary float. Without that float grounded really bad things can happen when closing switches one at a time. Its possible to develope over two and a half times the normal voltage durring the switching process.

Sometimes you can get away with it , other times ... not so lucky. Having no load on the secondary only makes things worse.

[Pootnaigle]

Ummmm I bleve the original question said it wuz delta on the high side? There would be no floating buss.

[rcdallas]

Ummmm I bleve the original question said it wuz delta on the high side? There would be no floating buss.

That's the way I understood it. Delta on the primary.

[topgroove]

True.. If this Bank is fed delta (primary) and the secondary was a wye the Float would be grounded. I'm kinda thinking it might be a typo or misunderstanding though.


If no grounding switch is present in this case I would think its safe to assume this particular bank has a permanant ground installed on the float and can never be backfed.

Hey kid, can you give us a little more info on this 2000kva padmount. whats the voltage? Are the delta connections internal? On the wye side are the concentric nuetrals all permanantly bonded to the ground lugs and ground grid

[lewy]

We have started to get some big transformers like that as well & they like all of our new transformers come with primary switches on both the A side & the B side, so you could close your single phase switches 1 at a time & then close your transformer switch to pick up the transformer. The extra cost on a 2000 kva would be small.

[THE KID]

We are a Delta system 13,800 The transformer is a 2000 KVA 277/480. We are in the process of retiring a bank of three 500 KVA transformers and a 1500 KVA replacing with two 2000 KVA tx. Both transformer will be tied to a switch gear. They will be able to use either tx later on but right now they are used seperatly. The reason I ask is we have a 3 phase goab switch but when I open it it kills all the banks. We were wondering if we could just close each 2000 tx by itself without opening the switch. Didn't know if ferroresinece played a part? The reason we hate to kill everything is this is the water plant for the whole city.

[topgroove]

ferroresinece is kinda hard to predict. As far as the banks go, the 277/480 wye side is permanatly bonded and grounded. You should be able to tie the banks together and not take an outage.

[THE KID]

forgot to mention only has tap changer inside tx

[rcdallas]

I don't see or know of any problems that you'd have with ferro being that it's a delta-wye. Ferro occurs in wye-delta and also as I mentioned earlier if the length of the underground primary cable exceeds the KVA of the transformer you'll have a problem--meaning if the cable is 2500' in length you'll have problems, but I kind of doubt the length of the cable will exceed the KVA of the transformer...that's roughly 1/4 mile of cable BUT you never know-downtown environment, etc.

I would close in each fuse individually with no customer load.

[topgroove]

Ferro is a tough thing to wrap your head around. The best explaination I've had was think of the coils in a transformer as inductence, and the underground cables as capacitense. when those two values are equal or close to equal the circuit will start to resonate.

I like to use the old analogy of an army crossing a bridge marching in step. their footsteps all pounding at the same time sets up a rythem which brings the bridge down. Actually anything will self destruct when its natural frequency is reached. Kinda like how an singer can shatter a glass if she hits the right pitch.

But in your case I wouldn't worry about that 277/ 480 bank It'll be fine.