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ADVANTAGES OF PER UNIT SYSTEM

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

  1. 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.
  2. 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.
  3. Per unit impedances of electrical equipment of similar type usually lie within a narrow range, when the equipment ratings are used as base values.
  4. Transformer connections do not affect the per unit values.
  5. Manufacturers usually specify the impedances of machines and transformers in per unit or percent of name plate ratings.
  6. Transformers can be replaced by their equivalent series impedances.
  7. Equipment impedances can be easily estimated since their per unit impedances lie within a relatively narrow range.
  8. Reduced calculations in three-phase systems.
  9. By the choice of voltage bases, the solution of networks containing several transformers is easy.
  10. More usefully for digital computation.
  11. For apparatus of the same general type the p.u. and volt drops or losses are in the same order, regardless of size.
  12. For transformers the p.u. of impedances are same for primary and secondary sides.

PER UNIT CONVERSION PROCEDURE OF SINGLE PHASE

  1. Pick a VA base for the entire system, Sbase
  2. Pick a voltage base for each different voltage level, Vbase.
  3. Voltage bases are related by transformer turns ratios.
  4. Voltages are line to neutral.
  5. Calculate the impedance base, Zbase = (Vbase)2/Sbase
  6. Calculate the current base, Ibase =Vbase/Zbase
  7. Convert actual values to per unit
  8. Convert to per unit (p.u.) (many problems are already in per unit)
  9. Solve
  10. Convert back to actual as necessary

Comments

Unknown said…
Superb and simple explanation 👌
Wednesday, 13 February 2019 at 09:38:00 GMT+5
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