Y to Y transformer connection
Y to Y transformer connection
(OP)
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
Steve






RE: Y to Y transformer connection
I do not know what you mean by 120 volt delta power.
120 volts is measured phase to neutral.
"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."
You only need to wire the primary 480 volt delta with a 120 /208 volt wye secondary. No neutral required on the primary. You do need an EGC (equipment grounding conductor) on the primary.
"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."
Your phase to phase voltage should be 208 volts. 120 volts phase to neutral.
" 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)."
No only the salesman. I don't know what you mean by a 120
volt delta.
There is no such animal.
RE: Y to Y transformer connection
You could also take three 480-120V single phase transformers and connect them in delta to produce a true 120V delta system rather than the 120Y/69V system you presently have. You could run the 120V delta ungrounded or you could corner ground it. If you are subject to the NEC, the corner grounding will be required unless you can meet certain exceptions.
RE: Y to Y transformer connection
Steve
RE: Y to Y transformer connection
The X0 secondary neutral is currently grounded. Is this OK? It may even be hard wired this way. Will I generate any unexpected neutral currents this way?
I also use the 3 single phase transformer delta-delta approach. This has the advantage of providing the full power of the three individual trandformers, while the Y-Y method is I think limited by the primary current to 1/1.73 of the nameplate power.
Steve
RE: Y to Y transformer connection
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Y to Y transformer connection
You could ground X0 and NOT connect it to H0 or you could connect it to H0 and NOT ground it. Under normal conditions you will find minimal difference between the two. There becomes a difference in how those two respond when there is a ground fault on the secondary as the wye-wye transformer is not a ground source the way a delta wye transformer is.
If the H0 and XO points are connected to the primary neutral and not grounded, a ground fault on the secondary will show up as a ground fault on the primary and can be detected and cleared by a ground fault device on the primary. If the H0 is connected to the primary neutral, but not to the X0 while X0 is grounded a ground fault on the secondary will drive a zero sequence current on the secondary, fault current up the X0 and out on the phase windings. That current on the phase windings will also be on the primary phase windings, and flow down into the primary neutral. This fault current circulating in the neutral of the primary system will not be seen as a ground fault and you will have to depend on the primary phase device to clear the fault. So, either connection could be used depending on how you want the primary system to see a ground fault on the secondary system.
Yes, the reconnected transformer will have to be derated to 1/1.73 of its original rating as the primary windings can not accept additional current to offset the reduced voltage.
RE: Y to Y transformer connection
A 15 KVA transformer will give you 125 amps at 120 volts. That's 25.9 KVA. on open delta three phase. If you need the full 30 KVA, use 25 KVA transformers.
Two 15 KVA single phase transformers will probably be the ceapest solution and there are no circulating current issues with open delta.
This may be a separately derived system or you can call it a buck connection and connect the corners of the 480 volt system and the 120 volt system, whichever is to your advantage.
respectfully
RE: Y to Y transformer connection
My ignorance is really starting to show. I thought "H0" and "primary neutral" were the same thing and were both referring to the center point of my primary Y setup. Same on the secondary for "X0" and "secondary neutral". If this is the case and X0 is grounded, what should I do with H0? I'd like to get away from voltage imbalance due to load imbalance if possible. No ground fault devices are involved other than fuses or circuit breakers on the primary and secondary power cables.
waross,
I've used open delta a few times. I think it generates noise - either on the power lines or EMI. Either way it seems to be problematic in our environment with lots of instrumentation and sensitive equipment.
Thanks
Steve
RE: Y to Y transformer connection
In your original post you said:I assume what you really meant was that you are getting 69V from each phase conductor to the grounded neutral. It's too easy to read your statement as a measurement of 69V from the neutral to ground, and that would be a definite problem if it is supposed to be grounded.
RE: Y to Y transformer connection
RE: Y to Y transformer connection
You can use it for any load that is not frequency sensitive. That is mainly incandescent light bulbs and resistance heaters. You can use thransformers at about 6 or 7 times their original voltage rating if the insulation will stand it.
respectfully
RE: Y to Y transformer connection
I will have to go back and take a second look at the nameplate on the generator head to see if it lists anything about the frequency or not. How do I physically determine the number of poles in the generator and from there correlate that to its frequency?
If it is 400 Hz, then is there some type of devcie that is practical to use the will change the frequency to 60 Hz?
Just for reference, the generator is about 24" in diameter and about 32" long powered by a rather large 6 cyl. gasoline enigne that runs at 1200rpm with the generator rated at 35KW.
Non-frequency sensitive loads means that it cannot be used for powering things such as electric motors, correct?
I assume when you speak of using the transformers at higher voltages that you mean I could use transformers to step up the voltage from 120V Delta to say 240 Delta or 480 Delta....That is step up to a more usable voltage.
Thanks,
Roger
RE: Y to Y transformer connection
Steve
RE: Y to Y transformer connection
respectfully
RE: Y to Y transformer connection
If you connect your 480V delta - 208/120V wye transformer to get 120/69V wye, you either are using 277V phase-to-phase as a source on the delta side, or you have changed the turns ratio of the transformer.
This transformer has only one neutral, which is on the wye side, so where does the discussion about two neutrals come from?
What are you using for a source voltage and where is it connected?
RE: Y to Y transformer connection
RE: Y to Y transformer connection
Other than that, your connection sounds good.
respectfully
RE: Y to Y transformer connection
Thanks, the neutral of my 480 incoming power is grounded. Should I still follow your recommendations?
I don't think NEC applies strictly to our laboratory setting. Any engineering principles behind NEC would, of course, apply.
Steve
RE: Y to Y transformer connection
If you encounter an AHJ who just can't wrap his mind around your system, you could connect H0 to the site power neutral and ground X0, but that would be a less desirable connection. With no connections to X0, it won't really matter, but if there were ever any 69V loads (line-to-neutral), a grounded X0 would put load current on the building ground system to get back to the neutral of the 480V system while in the case where H0 and X0 are jumpered, the neutral current stays on a neutral.
RE: Y to Y transformer connection
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
RE: Y to Y transformer connection
RE: Y to Y transformer connection
respectfully
RE: Y to Y transformer connection
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.
RE: Y to Y transformer connection