Per Unit Impedance of Transformer

Please study the name plate of the transformer. Per unit impedance or percentage impedance would definitely be mentioned on the transformer name plate. It is a very importnat parameter for any transformer. Please note the same and come back, we will discuss further.

impedance is 6%.

For the transformer, the Per Unit Impedance (%) = Percentage Impedance Voltage = Percentage Impedance (%Z). As already mentioned, it must be mentioned in the transformer name plate data.

See also http://cr4.globalspec.com/thread/41265

- MS

It implies that when you load the transformer to its rated capacity, the Voltage Drop in the transformer secondary windings would be 6% of its no-load secondary voltage, when rated primary voltage is applied to the primary.

It also implies that with the secondary short-ciruited, if you apply 6% of the primary volatge , full-load rated current would flow in the secondary.

Conversely, it gives the short-circuit current supply capability of the transformer for a three phase terminal short circuit on the secondary windings (i.e.) Short circuit current = Transformer Rated Seconadry Full load current / Per unit impedance of the transformer.