3-Phase Wye to 3-Phase Delta

[stripedbass]

I have a 400A, 120/208 Wye 3-Phase bank of transformers mounted on a utility pole at a site we will be working at. Our equipment requires 200A, 120/240 VAC which would be an open delta, I believe.

Can we use a transformer to accomplish this?

The problem is the power company wants 10K to build us another bank of transformers and the job is only going to last 6 months.

 

[resqcapt19]

You could try a "buck/boost" transformer to boost the 208 to 240 volts. Do you need both 120 and 240 at the machine? The "buck/boost" will also bost the 120 to 138 at the same time it boosts the 208.
Don(resqcapt19)

 

[dpc]

Is your 240V power requirement 3-phase or single-phase? If three phase, it is what is termed a "red-leg" delta. One leg of the 240V delta is center-tapped to derive the required 120V.

This is used quite a bit for irrigation and pump stations where the 120V power requirements are low compared to the three-phase pumping load.

If your load consists primarily of three-phase motors, you may be able to operate with the existing transformers. If they have taps, you adjust for the highest possible voltage. This will put you in the range for running a 230V motor. Many U.S. made motors are "triple-rated" for 460/230/208. They don't run at their specified efficiency, power factor, etc on 208V, but they will run.

Other option is to apply three autotransformers to boost the 208 to 240V.

It depends on your load and how it will be connected.

If it is single-phase, you may be able to buy a single-phase transformer to go from 208V to 120/240 and run on one phase if the existing transformers are large enough. If these are utility-owned transformers, I'd be sceptical about the "400A" rating. You need to know the kVA rating of the tranformers.

dpc

 

[stripedbass]

I need to be more specific. The power company has 3-phase 120/208 Wye transformers. Basically, what I need to do is feed a 200A 120/240 delta 3-phase breaker panel that has 2-pole, single phase breakers and 1-pole single phase breakers. Keeping the single pole breakers off the Hi-Leg.

Our motors are XP rated and up to 15 HP which means they can only be wired for 230/460. 208/230/460 is not an option for some of the motors but is for a couple of others. There is also 120 VAC required for fans, heat, lights, etc.
Also...we have equipment that must run at or near 240 VAC.

I am in the process of working up the load calc sheet and will soon be able to know what the percent single phase load is.

The power company will not re-tap the transformers as they also feed a car wash that is next door to our site.

Don't you love these problems?

 

[dpc]

So your 240V motors are three-phase or single-phase? I'm confused.

If you need three-phase power for motors, I'd go with an autotransformer configuration to boost the 208 to 240V. You can take the 120V from the primary service (208/120V).

So you would have a main service panel rated 208/120V to handle your single-phase 120V loads, then a 208V three-pole feeder to the boost transformer and on to a 240V panel. Or you can put in a small 240 to 120 V transformer for your single-phase loads.

See GE Buy-Log or similar under Specialty Transformers. 200A at 240V, 3p, is about 83 kVA. GE lists a standard model that will give you 78 kVA at 236 volts with a 208V input as an autotransformer.

dpc

 

[busbar]

1.) If the utility has three 50kVA transformers on their pole, then yes, the bank will 'deliver' over 400 amperes.
2.) On paper it is possible to lay out a scheme with a pair of open-delta-connected 240- to 32-volt boost transformers to increase voltage from 208 to 236 volts, but only one phase will remain at 120 volts to neutral. It will not be true 240/120V 3ph 4w delta service. Two phases will be ~150 volts to neutral, and damage may result.
3.) The largest 'stock' buck/boost transformers [5kVA] will only serve a maximum of 156 amperes.
4.) If you could locate three 50kVA 120-to-240V center-tapped transformers, they could be connected to do the job, but your $10k budget would probably be drained.
5.) The utility-owned transformers may have dual 120-volt secondary windings, and could possibly be internally reconfigured for 240V service, but a line crew is not allowed to make such a change in the field, and the units would have to be hauled into the shop for any internal changes.
6.) In many areas any voltage 3ph delta is not very popular. If their engineering/construction standards do not permit it, then it will be difficult to furnish the requested service voltage without paying a price for it. Their metering would have to be reconfigured to accommodate the change. There could be a number of older 240V 3ph installations, but they may not offer that for new service.

 

[busbar]

See:

http://www.electro-mechanical.com/7700-94.pdf

for buck/boost setup. A ‘figure G’ layout will work with the warnings/limitations already posted. Contact the manufacturer to verify, but it may be acceptable to parallel transformers to serve 312 amperes.

 

[stripedbass]

dpc-
Oops...Left out an important fact. Yes, the motors are 3-phase.

Ok, let's see if I'm not too thick here: I can get a 78KVA autotransformer and boost the voltage from 208 to around 240 and feed that to our 120/240 4-Wire existing panel.

Question #1: since I'm feeding from 120/208Y, will the 240 on the secondary side of the autotransformer have a Hi-Leg?

Question #2: Since I'm boosting from 208 to 240 do I carry the neutral through to the 120/240 panel or do I use just 3 wires and make it 3 phase, 240 with a ground.

Next: In our 120/240 panel I install a 2-pole, 240V breaker and feed a 240/120 transformer to supply our single pole, single phase loads (lighting, heater, controls, etc). Basically, I'll need a sub panel.

Question #3: What needs to have ground rods and where does the bonding take place. I guess I'm not that familiar with autotransformers. I'll look that up.

Thanks for all your help so far. It seems that every site we work at has something different and we try to build our equipment as flexible as possible.

stripedbass

 

[dpc]

You won't have a high leg, so your existing 120/240 panel won't have any 120V. You will get three phase conductors and a ground.

But you will need service entrance equipment AHEAD of the autotransformer and probably a meter socket. I think it would be simpler to install a 208/120V panelboard as your service entrance, then sub-feed the autotransformer from it.

The cost of the panelboards is minor compared to the rest of the system.

With this arrangement you can use the main 208/120V service for your 120V power needs.

Refer to Article 250 of the NEC for info on grounding requirements.

 

[stripedbass]

Thanks dpc. Please allow me to be a little more "thick" here: If I have a 3-phase, 3-wire, 120/240 panel with a ground and no high leg wouldn't each phase be 120 VAC to ground? Or...are you saying I have no neutral since I have a 3-wire 120/240 panel and thus do not have 120 VAC from each phase?

 

[dpc]

It doesn't matter what kind of panel you have. The transformer connection determines the voltage. The panel only distributes it.

In a 208/120V three-phase system, you have 208V between each pair of "hot" wires and 120V from **any** hot wire to ground.

If you use a transformer to step up the 208 to 240V, you have 240V between each pair of phase wires, but your voltage to neutral is now 240/1.732 or about 139 volts. You don't have 120V anymore.

The "high leg" you are referring to is based on the use of a 240V delta connection. With a 208/120V system, there is no high leg.

YOu will have to have a service disconnect at the 208/120V voltage. Probably a local electrical contractor could help you out on all this, as well as explaining it the local electrical inspector.

Good luck.

 

[stripedbass]

I got it! Thanks everyone.

stripedbass