PER UNIT SYSTEM
The per-unit system expressed the voltages, currents,
powers, impedances, and other electrical quantities basis by the equation:
Quantity per unit (pu) = Actual value/ Base value of
quantity
ADVANTAGES OF PER UNIT SYSTEM
- While performing calculations, referring quantities from one side of the transformer to the other side serious errors may be committed. This can be avoided by using per unit system.
- Voltages, currents and impedances expressed in per unit do not change when they are referred from one side of transformer to the other side. This is a great advantage.
- Per unit impedances of electrical equipment of similar type usually lie within a narrow range, when the equipment ratings are used as base values.
- Transformer connections do not affect the per unit values.
- Manufacturers usually specify the impedances of machines and transformers in per unit or percent of name plate ratings.
- Transformers can be replaced by their equivalent series impedances.
- Equipment impedances can be easily estimated since their per unit impedances lie within a relatively narrow range.
- Reduced calculations in three-phase systems.
- By the choice of voltage bases, the solution of networks containing several transformers is easy.
- More usefully for digital computation.
- For apparatus of the same general type the p.u. and volt drops or losses are in the same order, regardless of size.
- For transformers the p.u. of impedances are same for primary and secondary sides.
PER UNIT CONVERSION PROCEDURE OF SINGLE PHASE
- Pick a VA base for the entire system, Sbase
- Pick a voltage base for each different voltage level, Vbase.
- Voltage bases are related by transformer turns ratios.
- Voltages are line to neutral.
- Calculate the impedance base, Zbase = (Vbase)2/Sbase
- Calculate the current base, Ibase =Vbase/Zbase
- Convert actual values to per unit
- Convert to per unit (p.u.) (many problems are already in per unit)
- Solve
- Convert back to actual as necessary