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Per-Unit Impedance
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Base Impedance
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Base Current
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Base Impedance (1φ)
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The per-unit system is a normalization method that expresses electrical quantities as fractions of chosen base values. It simplifies calculations by eliminating the need to consider voltage levels and makes system analysis more intuitive.
Base impedance is calculated using the formula: Z_base = (kV_base)² / MVA_base. For example, with 138 kV base voltage and 100 MVA base power, the base impedance would be 190.44 ohms.
Per-unit values eliminate the complexity of different voltage levels, provide dimensionless quantities that are easier to compare, and simplify transformer calculations by removing turns ratio considerations.
To change base values, use the formula: Z_pu_new = Z_pu_old × (MVA_new/MVA_old) × (kV_old/kV_new)². This maintains the actual impedance value while expressing it on a new base.
Typical values include: generators (0.1-0.3 p.u.), transformers (0.05-0.15 p.u.), transmission lines (0.01-0.05 p.u. per mile), and motors (0.15-0.25 p.u. subtransient reactance).
Yes, per-unit calculations work for both single-phase and three-phase systems. The key difference is in the base power definition - use single-phase base power for single-phase calculations.
Per-unit calculations are mathematically equivalent to actual value calculations when done correctly. They provide the same accuracy while offering computational advantages in complex power system studies.