delta to wye transformer question

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Old 09-15-08, 06:35 PM
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delta to wye transformer question

I am living at a camp that has a 480VAC (or something like that) distribution system. At each group of buildings there is a delta to wye transformer that steps it down to 208/120v. I would like to have power for my trailer at a location that has an existing transformer with a cut-off switch. Three wires plus a ground come into the switch box and go out to the transformer, however, two of the wires on the load side have been disconnected because the transformer is not currently being used. All the wires have been phase taped so it seems obvious where they need to be reconnected. All three are connected on the transformer primary side, and there are no connections to the secondary. The schematic diagram under the cover seems straightforward and all of the lugs are clearly labelled. What I want to do is to reconnect the two primary wires and then to run power from two phases of the secondary to a 208/120 VAC temporary box within ten feet of the transformer. . We have applied for a permit to build in this location so we will need temporary power there anyway, and apparently the previous owners had run power from the transformer to some unpermitted buildings, and then disconnected it when they vacated. Our maintenance guy is a little concerned about doing this so I just wanted to check if anyone has any suggestions or warnings about potential problems.
 
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Old 09-15-08, 06:51 PM
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This can be simple, but I would strongly recommend that you hire a licensed electrician to take care of this.
If everything is in order, then it should not take long to get you up and running.
If however something is amiss, there is potential for a very dangerous situation.
Voltages of this magnitude and equipment of this type are nothing to cut your teeth on.
 
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Old 09-15-08, 07:16 PM
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I agree with Dezwit. This is not what you want to get your start on. There is other issues to be aware of like wire sizing and grounding. If you wired something up incorrectly and throw the switch you can end up with a large ball of fiery molten steel and copper in a heart beat. Since it sounds like to have most of the equipment you need the cost will only be labor. It would be money well spent.
 
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Old 09-15-08, 07:38 PM
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I am used to working on 480 volts system almost everyday and this is best way to do is have a electrician come out and they can able determed if the transfomer is good and the connections is safe.

The reason why I rather have electrician do this in safe manner due there are some unknowen conditions it may flare up.

I did see few 480 volts shorted out they are not pretty sight when they do short it out.

Merci,Marc
 
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Old 09-19-08, 07:26 PM
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Thanks for the input. I am aware of the dangers in this kind of work. I am not exactly "cutting my teeth" on this project. I have done maintenance in camps for about ten years now, oftentimes sorting out and fixing nightmare problems created by unconscientious electricians (Yes, they are out there, but I am quite sure they are not the type of people who would post on, or even bother to read this bulletin board) However, I have to admit this is the first opportunity I have had to actually work with a "high voltage" distribution transformer--something I have studied, but haven't had any practical experience with. The camp director is a friend of mine and he would have to "sign off" on anything I do before it gets done. He may well decide to call in an electrician, but his budget is stretched right now so it will take quite a while. It would be a little more helpful if anyone could be specific about the potential problems. For instance, some questions I have are:

1. I know it is important to balance the load on three phase systems, but the amount of power in this case is small enough that this is probably not a concern. However, is there a problem having one or two phases not connected to any wires or loads at all (i.e. induced voltages within the transformer), or should the transformer be robust enough to handle this?

2. I would assume since the feed wires have all been phase taped, that the correct way to reconnect the wires would be red to red, yellow to yellow, oarnge to oarnge, but if the person who put the tape on screwed up will it make any difference to the transformer? (Seems to me that since the phases are 120 degrees apart, no matter how the connections are permuted the transformer should work--the only thing that should change is the "rotation" of the electric field within the apparatus.

3. What are some of the other potential problems with transformers? Could I check for them with a high quality insulation tester?

Again, I may end up not doing this project myself--it definitely wouldn't be worth losing my friend's trust and confidence--but I would like know a little more in making the assessment. At the very least it seems like a good opportunity to learn a little more about this type of system. I would truly appreciate any more advice.
 
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Old 09-20-08, 02:43 AM
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Originally Posted by Gregor View Post
1. I know it is important to balance the load on three phase systems, but the amount of power in this case is small enough that this is probably not a concern. However, is there a problem having one or two phases not connected to any wires or loads at all (i.e. induced voltages within the transformer), or should the transformer be robust enough to handle this?
Balancing a three-phase load is only important when an engineer is designing a system, in order to maximize the amount of power available without overloading any one wire. If you went through your campsite and wired a bunch of single phase transformers between A - B, and B - C, but no A - C, you'd end up with the B wire maxing out long before A and C. That unused three-phase transformer you have is three single-phase transformers standing side by side, if you use one phase only you simply have a bunch of unused copper in there.

2. I would assume since the feed wires have all been phase taped, that the correct way to reconnect the wires would be red to red, yellow to yellow, oarnge to oarnge, but if the person who put the tape on screwed up will it make any difference to the transformer? (Seems to me that since the phases are 120 degrees apart, no matter how the connections are permuted the transformer should work--the only thing that should change is the "rotation" of the electric field within the apparatus.
This is true. Three-phase rotation only matters if you are hooking up objects like three-phase motors whose actual rotation depends on the electrical order. Feeding a transformer for receptacle-type circuits, it doesn't matter. Just make sure that the ground is clearly separate.

3. What are some of the other potential problems with transformers? Could I check for them with a high quality insulation tester?
Two issues I can think of involve the insulation on the xfmr coils. First is that there may be physical damage. Testing for this is done with a megger, which is basically a high voltage static generator. With *everything* disconnected, you hook it up to one coil lead at a time and charge it. An inability to hold the high voltage means there's a leak. Actual operation of one is a bit more complicated than that... for one thing, after you're done testing each component, you need to safely ground it out. This isn't practical to do with cheap leather gloves and a length of 12ga wire. Using such a tool should be doable for you, but read the instructions and tread carefully...
Second insulation issue is that any xfmr which has been sitting unused for more than a day or two has probably gotten moisture into the interior wrappings and parts. To get rid of this, hook the xfmr up with no loads attached and let it sit for a couple of days. It will get warm from its internal resistance, and dry out. Then shut it off and hook up your loads.

One other issue: Make sure that the xfmr is properly grounded, and the ground tied to the neutral bar. The xfmr is the starting point of a new electrical system, if it's not properly grounded you can end up with hot-to-ground voltages much larger than the 120 listed on the panel.
 
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Old 09-20-08, 09:15 PM
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Thank You mukansamonkey.

I have a Fluke insulation tester that I believe automatically de-energizes the circuit when the test button is released. It's a digital tester which hopefully will work as well as the older style meggers.

On your last point, I understand the reason to tie the neutral and ground together at the panel. Does this require installing a grounding electrode, or is its proximity to the transformer (no more than ten feet away) adequate if the transformer is properly grounded? I seem to recall that utility transformers are supposed to be tied to grounding electrodes, and I guess I can do some resistance testing to see if this one has been properly grounded--possibly some sort of ufer, as it is sitting on a cement pad. I will read up on all this, but any info you can give me will be much appreciated. Thanks again.
 
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Old 09-21-08, 01:43 AM
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The neutral bar inside the xfrm must be grounded, either to an ufer or to a ground rod driven next to it. Closer the better, with grounds. Code says that your xfmr and distribution panel get treated as a service, since your primary power is over 240V. So from the xfmr to the distro panel, you don't need to run a separate ground. However, the panel should have a main disconnect in it, other wise you have no service disconnect. The one thing I'm not entirely clear on is whether the disconnect upstream from the xfmr qualifies as a service disconnect because it's so close... I don't think it does because it's part of the higher voltage system, but I can't dig up a rule that makes it clear.
 
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Old 09-23-08, 12:30 PM
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I was doing some reading and was a little surprised to see that for a derived system the neutral is bonded to ground either at the transformer or the main service panel, but not both. This is different than what I've seen for utility pole transformers where I believe the neutral is bonded at the transformer and the main service panel. I am not sure the exact reason for the difference, but several possibilities come to mind. As for the disconnect, it is easy enough to use a service panel with a main breaker. Thinking about the disconnect upstream of the primary, can that be a problem if, say, someone swiches it off while there is a large load on the system? Kind of seems similar to opening the points on an old distributor coil.
 
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Old 09-23-08, 03:21 PM
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Gregor.,

Normally with the SDS { seperated derived system } the netural pretty much SOP to bond at the transformer OX lug and very rare to do the otherway and make sure you have bonding strap on the transformer on it as well.

Speaking of primary { 480 v } disconnect it don't affect not too much when they go off mode but energized { start up } that is most critcal part once you pass that stage you will be fine after that.

The reason why I always say start up is pretty circital due the inrush current to engerized the transfomer and some case it can kick out the primary breaker if the secondary load is pretty well loaded up.

The best safe way is make sure you have secondary breaker is off before you turn on the primary breaker so that way you can reduce the inrush current a bit and also it will stableized a little then turn on the secondary breaker.

Merci,Marc
 
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