Electrical Per-unit Calculation

The per-unit method of calculation in its basic concept is to develop a counterpart impedance or reactance network diagram of the power system involved and resolve these impedances down to a single impedance value through delta, wye, series, parallel conversion equations. While it is possible to do the calculation with the actual system ohmic values, it becomes very complex when several voltages levels are involve since ohmic values at different voltages are not directly compatible. With the aid of the per-unit mathematical technique, it has facilitated this difficulty. While the equivalent diagram and the per-unit mathematics are two separate fields, they usually are used together and referred to as the per-unit method of determining short circuit values.

For Medium voltage power system usually benefit the application of the per-unit
method because the analysis normally undertaken involves a relatively few specific points in an existing system. Also in these system, reactance usually far exceeds resistance high X/R ratio, permitting resistance to be ignored, which greatly simplifies the mathematics.


The primary advantages of the per-unit system are as follow:

1. The per-unit values for transformer impedance, voltage and current are identical when referred to the primary and secondary line ( no need to reflect impedances from one side of the transformer to the other, the transformer is a single impedance).

2. The per-unit values for various components lie within a narrow range ragardless of the equivalent rating.

3. The per-unit values clearly represent the relative values of the circuit quantities. Many of the ubiquitous scaling constants are eliminated.

4. Ideal for computer simulations.