THE PER UNIT SYSTEM - PART - 01 - INTRODUCTION

Power transmission lines are operated at voltage levels where kV is the most convenient unit to express voltage.  Because of a large amount of power transmitted, kilowatts or megawatts and kilovolt-amperes or megavolt-amperes are the common terms.

In a large interconnected power system operating at various voltage levels and various capacity equipment. It is very easy for analyzing the network with per unit system of quantities rather than in absolute values of quantities.

DEFINITION OF PER UNIT

The numerical per unit value of any quantity is defined as the ratio of the quantity to its base expressed as a decimal. Per unit values are dimensionless.

Per unit value = 

Ratio in percent is 100 time the value of per unit.

Percent value =

Both the percent and per unit method of calculation are simplified often more informative than the use of actual amperes, ohms, and volts.

The per unit method has an advantage over the percent method because the product or two quantities expressed in per unit itself, but the product of two quantities expressed in percent must be divided by 100 to obtain the result in percent.

For example, if the base voltage of 66kV is selected, a voltage of 11kV, 33kV and 66kV would be specified as 0.17, 0.33 and 1.00 per unit respectively or 17, 33 and 100 percent.

SELECTION OF BASE VALUE

In electrical engineering there are three basic quantities are voltage, current, and impedance. Any two of them as the base quantity, the third one automatically will have a base value depending upon the other two.

In power system calculations, usually volt-amperes (S) voltage (V) are specified. Hence S_b and V_b are chosen as the base value. The base current I_b and base impedance Z_b are expressed in terms of Sb and Vb.

Actual value	Base value	Per unit value
Ia = actual current	Ib=base current	Per unit current = Ia / Ib
Va = actual voltage	Vb=base voltage	per unit voltage = Va / Vb
Za = actual impedance	Zb=base impedance	Per unit impedance = Za/Zb
Ya = actual admittance	Ya=actual admittance	Per unit admittance = Ya/Yb
SVA = actual voltamperes	Sb = base voltamperes	Per unit voltamperes = SVA / Sb