600v farm distribution

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artherd

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Have a farm service (240v high-leg delta 3-phase.) and need to run up about 2000' to a new building site. Thinking of (240->600)distro triplex(600->240) with a pair of transformers. Should they be delta wound or Wye (on pri/sec)? Why? My current thinking is to use Delta-Delta transformers, as 120v loads will be essentially nonexistent.

I'd like to go higher (e.g. 4160) but MV gear is way out of the budget for this project, and we need maybe 70A @240v at the destination in 3-phase tops. Do need 3-phase for machine tools.

Note: Most loads of any size will be 240v, a few lighting loads (negligible due to modern LED lighting) on 110v can be served by an existing center-tapped 240/120v autotransformer.
 
If you have a corner grounded delta output you can use the autotransformer to get ONE 120V to ground supply for lighting, office equipment, phone chargers etc.
A wye output, 208Y/120, might be the most flexible if the motors will operate on 208. Or 240Y/139 and use a buck transformer on the 139 to get 120.
 
What would it cost to have POCO run a service to that building?

Similar thread here which might be of some use:

http://forums.mikeholt.com/showthread.php?t=172543


The transformer in the remote building will have to be delta no matter what. I recommend a wye secondary on the transformer in the old building otherwise you will need a ground detector. Even then I am hesitant about any system being truly ungrounded. An intermittent or arcing fault may produce serve transient over voltages.
 
The POCO service is 3-phase high-leg delta, but is actually 240/120 with a center-tap in one of the pole-pigs providing neutral. That neutral is grounded at the main panel.

I haven't designed a system like this before, so would appreciate guidance on where and weather to use Wye transformers vs delta, and if I can corner-ground the 600v side of things.
 
mbrooke:

So a 240 to 600 delta-wye and then 600-240 delta-delta at the new site?

Note: I'll get a small 120v feed at the new site with a 240/120v center-tapped transformer across A & B phases. Loads will be negligible.
 
Per NEC Article 100 (definitions)


Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.

Does this mean I can actually use 1200volts phase-to-phase for the elevated voltage portion of this install, if I run a ground conductor? (or does that put me in MV territory even under the new NEC cap of 1000v? If so, can I use 1000v or 500v phase-to-phase?)
 
I have a hard time seeing the transformers and hassle beating out just using more aluminum wire for this situation. If you really want to use small wire without the cost of mv, use 2 kv pv wire. You will only need #12 and wont need mv terminations. 2400 volt transformers are pretty common, but obviously will put you up over 2k, so you will have to get creative. I don't think adjusting the taps will quite get you there. A 480 to 4160 is about the perfect ratio.
 
Any *Safety* reason not to use a pair of fairly standard 600v-208/120 Delta/Wye transformers, one in reverse?

Ie: 240v feeding into 600v (using the lower taps to avoid 630+v) and then 600v feeding back to 240v(and what would be a 240/139v wye) at the termination? Ground the center-point of the Wye and be done with it? (no line-neutural loads would be present in this system, instead 120v is created for a single sub-panel via an autotransformer that is center-tapped and grounded as it's own SDS.)

The delta-delta on the 600v side, should it remain open or corner-grounded? Or should a 600v/240v Wye-delta be employed at the end?
 
artherd said:
mbrooke:

So a 240 to 600 delta-wye and then 600-240 delta-delta at the new site?

Note: I'll get a small 120v feed at the new site with a 240/120v center-tapped transformer across A & B phases. Loads will be negligible.
Click to expand...

Yes, that is what I would do.
 
artherd said:
Any *Safety* reason not to use a pair of fairly standard 600v-208/120 Delta/Wye transformers, one in reverse?

Ie: 240v feeding into 600v (using the lower taps to avoid 630+v) and then 600v feeding back to 240v(and what would be a 240/139v wye) at the termination? Ground the center-point of the Wye and be done with it? (no line-neutural loads would be present in this system, instead 120v is created for a single sub-panel via an autotransformer that is center-tapped and grounded as it's own SDS.)

The delta-delta on the 600v side, should it remain open or corner-grounded? Or should a 600v/240v Wye-delta be employed at the end?
Click to expand...

In the 2014 NEC you unit must be listed for back feed.
 
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