Transformer Impedance Question

There is no substitute for understanding the problem yourself. You should understand it well enough to solve it yourself. If you do not know enough to tell which engineer (if any) is giving good advice, you are in over your head and should not be practicing electrical engineering at a level that requires a PE license.

Bill

-- Fermez le Bush--about two years to go.

| We have a situation where a customer needs to put three single-phase, | 50 KVA pole mounted transformers into a three-phase bank. The voltage | of each transformer is 13800-277/480Y. Two of the units have an | impedance of 2.9%. The other unit has a 1.6% impedance.

If each transformer is single phase to begin with then it is either a

277 volt secondary or a 480 volt secondary. If 277 then wired WYE they can be 480Y/277. If 480 then the best they can do is 480 delta (unless you want to go with 831Y/480).

| Here's the problem. One transformer design engineer is telling me that | the Wye connection will prevent any significant load and/or voltage | imbalances. Another transformer design engineer is telling me that the | Wye connection will not correct for the impedance difference.

What is the primary configuration? Wye or delta?

What kind of imbalance is your concern? I'd probably worry more about the imbalance between transformers being paralleled. To know what kind of imbalance you would have with 3 277 volt pigs in a WYE configuration it might be nice to know what kind of load you are expecting. The more load, the more the 1.6% one will be overpowering the others. But still,

1.6% would be just a few volts more than 2.9% even at full rated load.

Fault conditions might get weirder.

| Are there any applications or systems engineers out there who can give | me their thoughts on this? Thanks.

Not me. Power is an "interesting hobby" for me. When I have built data centers I have specified necessary loads and voltages, and that it for my professional "power experience".

| Steve Lockridge | Alfa Transformer |

So are you just doing sales consulting? You guys should at least have a professional engineer educated or certified in power design on contract to call upon.

The Y connection will not correct unbalances. However, depending on the load, the voltage unbalances would likely be small. Single phase loads will create larger unbalances.

We have several engineers that we consult with. That is where the problem began. The answers were split about 50/50 between "it is a problem" and "it isn't a problem." Those were transformer design engineers. The reason I posted here was to see if there were some applications or systems engineers who could give me a definitive answer.

Steve Lockridge snipped-for-privacy@alfatransformer.com

No connection will compensate for the difference. Voltage imbalance (voltage at each phase divided by the average) will be minimal under normal balanced load, but as was pointed out it could be worse with single-phase loads.

There is a bigger problem, however. Maximum fault current on the low impedance phase could be up to 80% higher than on the other phases. There is a danger that someone will analyze this using the higher impedance, not realizing that they differ, which will give too low a value and result in inadequately rated load equipment.

Recommended practice is to use three identical transformers. Why ask for problems?

Ben Miller