# Transformer Impedance Calculation

#### crtkMan

Joined Dec 13, 2006
1
If have given 3 single phase transformers. Each has an impedance of Z= 2% or 0.02 per unit. If you make a connection of Delta-Delta transformer bank, then what would be your overall impedance (Z%) in Delta?

Asuume that 12kV/480v, 176kVA for single phase. You can make any assumptions you need to calculate your Z.

My calculation: I considered 2% as a single Z value in Y form of connection and then I converted to Delta from the Y.
So, Z(total): (2.2+2.2+2.2)/2 = 6 (using the basic formula for Y to Delta conversion).

#### subtech

Joined Nov 21, 2006
123
It is not necessary to "adjust" or convert impedance values whether the connection is delta or wye. The impedance of each individual transformer or the three units considered as one is the same........2%.
Voltage drop (across the windings) from no load to full rated load will be the same whether the transformer(s) are connected wye or delta. Magnitude of current from each transformer to a bolted fault will be the same as well. (assuming a balanced and infinite source on the primary side)