480 to 4160 Step up

[natfuelbill]

Hi Folks -
Scenario
Large apartment complex receiving primary voltage 4160 three phase. Voltage is stepped down by customer owned transformers to 240V delta (for motor loads) and to three 120/240 V single phase (each feeding a distribution board). Largerst billed demand is 180 kW.

The customer wants to investigate backup power with generator and transfer switch.

Customer has received quotes for 240 generator and transfer switches.


Possible alternative solution...

Install 4160 V transfer switch
Feed switch with utility and generated 4160.

4160 volt generator is big buck$$$.....so...

Here are the questions:
If 480 V generator, say 300kW with it's 480 V breaker is fed to the supply side primary of a 480-4160V ~300kVA transformer and then to the 4160 V transfer switch. The primary is the supply side (per Code).

What NEC table would be used to size overcurrent protection? 450.3(A) or 450.3(B)? Neither seems to apply for a stepup transformer for sure....

Would the transformer be delta-delta? and if so, does this allow 240.4(F) be used to protect the secondary conductors?

Any experience or caveats?

Bill

 

[kingpb]

The transformer would be a delta (LV side) and a grounded wye on the HV side. You will have to decide if you need to ground the generator and/or the transformer through a resistor.

I would agree that the 4160V machine would be expensive for a 300kW size.

Using the NEC outdated terminology (according to IEEE/ANSI standards anywway), in this case the primary is the low side, and the secondary is the high side. Don't get hung up on primary and secondary.

With that said, foregt about how the NEC says to size the protection. The correct way is to plot the transformer and cable damage curves, and select the protection based on those. The NEC is just a guide, not the absolute. You'll find to properly protect the transformer, the protective device rating will be lower then what the NEC would allow as a maximum.

 

[natfuelbill]

I saw in the Code that the primary is the supply side.

Any good tools, or help, on damage curves? Is it as simple as keeping to the left on the curves?

I learned that some some utilities don't allow their primary into a transfer switch. any notes on this?

Also, I learned that the generator would need to be carefully sized for transformer inrush. Any info here?

Thanks
Bill

 

[kingpb]

Onan, Cat, Kohler, etc, all have programs that can help with sizing, free to download. They also have in house engineers that the local reps can use to get the info you may need. Contact your favorite.

As far the curves, yes, in general, you stay to the left, and below. The settings do need to be above the inrush, or you will have a trip on inrush. That's where the adjustable trip devices come in handy. Curves are available from manufacturers, and are also built into the library of most coordination programs.

I can't say I've ever investigated a 4160V transfer switch, or had the need. I know you could easily use 2 x 4160V contactors, with electrical control and interlocking. But, why step back up to 4160V, then you end up with losses going back through the other transformer.

Why not use a 208Y/120V system, that gets you the three phase for motors, and you can still feed the 3-single phase distribution boards with 2-pole breakers.

Better yet, use 4160V -480V utility transformer (no neutral), run motors from 480V 3-phase, and feed generator directly into 480V bus. Then get a step-down transformer from 480V-120/240V to feed your distribution panels.

 

[dbuckley]

This arrangement works very well when you have a big site with MV distribution in place already. A hospital of my previous acquaintance on a 180 acre site had a 850KVA genset feeding a step-up tranny, and had a (open air switchyard!) MV transfer switch. The cable distances involved were just too large to make LV distribution sensible, and this was in the UK at 230/400.

I'd have though economics would be a big factor in doing this where you don't absolutely have to.