*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.

>

> 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.

>

> Are there any applications or systems engineers out there who can give

> me their thoughts on this? Thanks.

> 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.

>

> 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.

>

> Are there any applications or systems engineers out there who can give

> me their thoughts on this? Thanks.

(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