Y to Y transformer connection

[steve96]

I frequently need 120 volt delta power. It's always temporary for about a week and loads very from 1 to 30 kW. I usually simulate this by re-wiring a 480 volt delta to 208Y/120 transformer so that the primary is Y with the neutral ungrounded. I end up with 69 volts to ground on my grounded Y neutral conductor in the secondary, but the 120 phase-to-phase is right. A Square D engineer once told me this was OK (I ended up with him because I was trying to buy a 120 volt delta transformer and no one at the sales level even understood what that was). After reading posts to this site, I thought I should ask for a second opinion. If this sounds dumb, notice the 'mechanical' next to my name. Your comments appreciated.
Steve

 

[waross]

Hi davidbeach;
I don't have my codes with me but doesn't the code require any system to be grounded if by so doing the maximum voltage to ground will be 150 Volts or less? That would require a ground on the 69/120 volt system.
I'm 3000 miles away from my NEC and I may be remembering the Canadian code.
Connecting X0 and H0 will be connecting a ground to a neutral. A ground fault on the secondary will flow back on the primary neutral.
A YY system requires a primary neutral for functionality.
A neutral conductor is required and a seperate grounding conductor is required for the equipment. The grounding conductor may be sized to serve as both the equipment ground and the system ground.
respectfully

 

[steve96]

While I've got you all on a roll I'd like to pose a slightly different question. What if I cut the hard wired wye secondary and reconfiqure as delta (primary and secondary). Could I then run the transformer as 480 delta primary to 120 delta secondary? It seems like I get the full KVA rating of the transformer this way. I don't think I should ground anything with this setup?

 

[waross]

You should still have your transformer case grounded. This is the law. If you ever had an accident in that area and OSHA or an injured worker's lawyer found out the a piece of electrical equipment was being used without an equipment ground, your position would not be good.
respectfully

 

[davidbeach]

waross, you are correct about grounding such that the maximum voltage is 150V or less. With a wye-wye transformer you can connect H0 to the system neutral and then jumper H0 to X0 (and not ground X0). In this configuration the transformer does not become a separately derived source and does not require separate grounding. Your statement about ground fault on the secondary flowing back on the primary neutral would be true if X0 is grounded and connected to H0.

A wye-wye transformer is a different beast than a delta-wye transformer when it comes to how ground faults on one side show up on the other and it all depends on how the neutral points are connected. I agree that H0 needs to be tied to the system neutral for voltage stability.

steve96, you could reconnect your secondary as a delta and use the transformer as a delta-delta transformer. To meet the requirements of the NEC, you would then also need to ground one corner of the delta since it unlikely that you would be able to get at the mid-point of one winding to ground it.

 

[pgcusa]

I would like to see this illustrated.