Hi folks, poor wording on my part. I meant to say that the wild leg was used for three phase only, Some installations will use the wild leg for single phase, line to line loads, but in most installations the wild leg is only used for three phase.
Hi David; it is my understanding that the single phase loading of a four wire delta transformer bank, with three transformers in delta, is limited by the capacity of the smallest transformer. (subject to any adjustments for unequal impedances).
This statement is supported by the load sharing of the double delta connection used to convert a three phase generator to single phase service.
I have found the easiest way to describe this is to first consider an open delta connection.
Assume equal transformers.
If you have an open delta bank supplying "A" to "B", and "B" to "C", they can be considered as a virtual transformer at "C" to "A".
If you draw vector sketches of the voltage, voltage drops, reactive voltage drops and anything else that you can think of, the resultant in every case is equal to the same parameter of a single transformer from "C" to "A".
So, an open delta on "A" to "B" and "B" to "C" can be considered as a single phase transformer from "C" to "A".
The current of a single phase load will split evenly between the in phase transformer and the pair of out of phase transformers.
The single phase capacity of a three phase bank is twice the capacity of a single transformer and 2/3 the three phase capacity.
If any one of the transformers is smaller, that will determine the current limit of the bank.
Open delta does let you mix transformer sizes if need be.
But remember that part of the current in the "open delta" transformers is in phase with the third phase. That means that it is leading by 120 deg in one transformer and lagging by 120 deg in the other transformer. Now try to calculate the currents and transformer loading with a mixed single and three phase load with a bad power factor on the three phase load. The same phase relationships apply to 1/2 the current of a single phase load on a three transformer bank.
I have seen quite a few of these installations. They are usually well oversized. They have to be, because the practice there was to also connect the neutral on a wye primary. (Yes, I know, this is contrary to REA standards.)
As a result, most of these banks spent a good part of the time with one primary fuse blown running open delta with one transformer along for the ride.
Bill
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"Why not the best?"
Jimmy Carter
