Transformer/Panel install & grounding

[tpmc]

I have been ask to install a 240v 3 phase panel. I have a 3 wire 208v 3 phase buss, no neutral or ground other than the frame. I installed a buss plug have the conduit ran to the transformer, also from the transformer to the new panel. Ready to wire. I plan on connecting the 208v feed to the secondary side of the transformer in order to get the 240v. I know I need to connect the 208v wires to X1,X2 & X3. 480v wires to H1,H2 & H3. Here's my questions: I don't belive I should connect anything to X0. Should I connect a wire between the buss plug and the frame of the transformer? Also from the transformer to the 240v panel, where do I get my ground? ( From the frame of the transformer?) Also shoud I corner ground the 240v? If so how?

Any help appreciated!!

 

[davidbeach]

Where does the 480V enter the picture? Sounds like you need to discuss this with a local engineer who can look at the installation and advise you from there.

 

[tpmc]

David,
My bad there is no 480v I mistakenly entered it It should be 240v.

 

[waross]

You should run a conductor sized per code from the 120/208 volt source to supply the ground for the transformer case. This may also serve as the equipment ground for the 240 volt system. The 240 volt system does not need a system ground, but it does need an equipment ground as mentioned above.
I'm wondering, with a wye/delta connection will you connect the neutral of the 208 volt supply and deal with overheating of the transformer and possible burnout in the event of unequal line to neutral voltages, or phase loss situations,
or will you leave the neutral floating and accept the overvoltage switching transients?
I would have used a pair of autotransformers in open delta boost. The transformers are a lot smaller so it's a lot cheaper. It avoids the wye/delta issues and you will have a grounded system.
respectfully.

 

[EEJaime]

What kind of loads are you anticipating? As Waross said there seems to be a simpler and less costly way of doing this. A Buck-Boost connection would allow you to feed a sizeable load with a minimally sized transformer. Just a thought.

 

[tpmc]

As mentioed above everything is already ran. Have talk to 2 local large electrical contractor companies. I will call them company "A" & company "B". Company "A" had quoted doing it this way. Both Company "A" & "B" told me to run a ground from the buss plug to the frame of the transformer. Then to the groung on the 3 phase panel. Company "B" also stated I should ground 1 corner of the 240 delta to the transformer chassis ground. Then also from the transformers chassis ground connect a ground wire to the building ground. Not sure why I need to corner ground the Delta 240, since I'm only using the phase voltage(no 120V).
Sounds like the auto transformers may have been a better choice, had I known sooner. Have not used them.

 

[waross]

The Canadian and US electrical codes recognized two classes of grounding. Equipment grounding and system grounding.
Grounding the corner of the 240 volts is a system ground and is subject to different requirements.
There is a different table for conductor sizes and some more stringent requirements for installation.
The reason that the contractor proposes to connect the phase to the building ground is that it would not be allowed ground the system by connecting the phase to the grounded transformer case.
Grounding the system may not be required but it is good practice. There are some issues with drive ratings (voltage) on a corner grounded system but the same issues apply to an ungrounded system because you must consider the possibility of the effect of a ground on the system.
It sounds as if your contractor is on the right page. I would go for the corner ground.
You may still be able to justify a change to autotransformers.
for a 100 KVA 3 phase load, The phase current at 240 volts would be be about 240 amps.
A single phase transformer rated for 240:24 volts and boost service (often no extra cost) will boost 208 volts up to 229 volts, adequate for 230 volt motors.
Only the 24 volt winding will carry the 240 amps so the transformer must be rated for 5.76 KVA.
You are looking at two, 7.5 KVA transformers instead of one 100 KVA transformer.
If you need a little more voltage, look for some 240:32 volt transformers. They will get you up to 236 volts. 7.68 KVA load, so use 2 x 10 KVA, transformers, still a substantial saving, and you are over 98% of desired voltage. You will have uneven phase to neutral voltages on two phases but you are not using the neutral so no problem. This is no longer a separately derived system so your existing grounding system will be usable.
I once worked for a company who had two power systems in their plant. 600 volts and 480 volts. They had equipment rated at 575 volts and 460 volts. It seemed that every piece of equipment that came in was the wrong voltage for the feed at the location. We had pairs of small dry type transformers all over the plant, adjusting voltages for individual machines. We never had a problem.
respectfully

 

[tpmc]

Guys , I really do appreciate the help! I'm trying to understand this. Both Company "A" & "B" told me to run a ground from the buss plug to the frame of the transformer. Then to the ground on the 3 phase panel. Company "B" also stated I should ground 1 corner of the 240 delta to the transformer chassis ground. Then also from the transformers chassis ground connect a ground wire to the building ground.
Waross you state "The reason that the contractor proposes to connect the phase to the building ground is that it would not be allowed ground the system by connecting the phase to the grounded transformer case."
Help me here, if I'm reading this correct Contractor B is saying to connect the corner ground to the Chassis ground and your saying it's not allowed.
Do you really need to corner ground a Delta if your only going to use the phase voltages?
Also does corner grounding a Delta give you 120v. Or sholud it be a stinger?
The info on the auto transformers is interesting. I would like to know more. I would use this to my advantage in the future.
I was looking into auto transformers. Since this is a project for a different dept. They had an AR approved for a remodel project. They were adding this electrical modification on money left over. I was told the chances of any extra wouuldn't happen. So looks like I need to make this work. Knowing its inexpensive I will pursue this further.

Thanks,
TPMC