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

 

[wareagle]

"I frequently need 120 volt delta power."
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.

 

[davidbeach]

I'm not sure why you would need 120V delta, but it does exist. At least one class of diesel submarines used a 120V delta power system. If you are taking a a 480V delta primary winding apart and reconnecting it in wye, you will have a 480Y/277V - 120Y/69V transformer; a bit of a strange beast, but not impossible. What you need to do though is bring a neutral to the primary of the transformer, connect it to the H0 terminal you created, then jumper it to the X0 terminal. There is not a need to carry it further, out to the load if it is not necessary at the load. You will also need to carry the primary equipment grounding conductor with your 120V circuits. This will not be a separately derived system, unlike the secondary of a delta-wye transformer.

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.

 

[steve96]

Thanks, but nope I mean 120 volt 3 phase delta (I acturally get Y because the transformer secondary is hard wired this way). The navy uses a lot of it. I do get this as I described and the secondary phase-to-phase is 120 volts. Phase to neutral is 120/1.73 = 69 volts. But now I wonder about any issues I don't understand concerning the Y - Y configuration as well as the ungrounded primary neutral and grounded secondary neutral.
Steve

 

[steve96]

Thanks davidbeach, you posted while I was composing. Rephrasing your advice, I should connect the new primary neutral to the existing secondary neutral.

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

 

[itsmoked]

There are some 120V 3phase motors in 'my' rail cars.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

If you leave the H0 "terminal" floating, you will wind up with voltage imbalances due to load imbalances. So unless your 120V system is entirely balanced you need a connection to the neutral on the primary. Once that connection is made, you have two choices regarding X0.

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.

 

[waross]

A pair of 480:120 V transformers connected in open delta will give you 120 volt delta power.
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

 

[steve96]

davidbeach,
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

 

[davidbeach]

H0 is the primary neutral point of the transformer. I've used H0 to refer to the terminal and primary neutral to refer to the conductor connected to that terminal; likewise for the secondary. So, connect H0 of the transformer to the neutral of the primary system with a primary neutral conductor and you should be good to go.

In your original post you said:

I end up with 69 volts to ground on my grounded Y neutral conductor in the secondary...

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.

 

[rhpope]

I have come across some things I have read that suggest the Navy 120V Delta power runs at 400 Hz. Can anyone comfirm this? I have an old generator that came off of a Naval ship. It was built by Hobart and appears to date from somewhere around the 30's or 40's. It has the same 120V 3 phase output, but I have not noticed anything on it stating that it is 400 Hz. Since it may be 400 Hz, can it be used for anything?

 

[waross]

Look at the physical size. A 400 Hz. machine needs only about 1/7th of the iron in the core that a 60 Hz. machine needs. It will be a lot smaller physically. You can correlate the rated speed with the number of poles to determine the frequency.
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

 

[rhpope]

Waross,

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

 

[steve96]

The navy uses 400 Hz and 60 Hz power. Both at nominal voltages of 440 and 115. 115 volt 3 phase delta 60 Hz is common, as is 115 volt 3 phase delta 400 Hz. Most of the 400 Hz I've seen came from electric motor generators or solid state frequency changers (vs 400 Hz engine generator or turbine generator). I'm not sure if 400 Hz was used before radar (pre World war 2).

Steve

 

[waross]

400 Hz was also used in aviation because of the reduced weight for a given size.
respectfully

 

[DanDel]

steve96, when you say 120v delta power, I assume you mean 3-phase 120V ungrounded, which was a relatively common system years ago, but is not allowed now per the NEC in the USA anyway.
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?

 

[steve96]

I reconnected the transformer primary from delta to wye. This reduced the phase to phase voltage on the secondary from 208 to 120 volts. I just finished running some 120 volt 3 phase delta (Navy) equipment off of this and always had within 3% phase to phase voltage balance. I did not have the center of the wye primary or the wye secondary connected to anything or to each other. From above, is seems I would be better off connecting the center of the primary wye to the neutral for my site power (the center of my main site tranformer's 480 volt 3 phase wye secondary - which is grounded).

 

[waross]

I think that the code requires you to ground the secondary wye point regardless of whether you use a secondary neutral or not. I would connect the transformer primary wye point to your 480 volt neutral but not to ground. It will act as a neutral and conduct any neutral currents caused by unbalanced loads and any harmonic currents reflected from the secondary. A single phase ground fault in the secondary will not cause a ground current in the primary, but it will cause neutral currents.
Other than that, your connection sounds good.
respectfully

 

[steve96]

waross,
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

 

[davidbeach]

steve96, I doubt that you would meet the exceptions that would allow your system to be ungrounded per the NEC. In my opinion, your best bet would be to connect H0 to the neutral of your site power and then connect H0 to X0. At that point you do not have a separately derived system and your 120V system shares a neutral with the 480V system. Not exactly conventional, but neither is a wye-wye transformer, or a 120V phase-to-to phase voltage.

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.