Per-Unit System

The per-unit (p.u.) system is a normalization method widely used in power engineering that expresses electrical quantities, voltage, current, power, and impedance, as dimensionless ratios relative to chosen base values rather than in their actual SI units. The fundamental calculation is: per-unit value = actual value ÷ base value.

Key Aspects of the Per-Unit System:

• Base Selection: Two base quantities are independently chosen (typically base power in MVA and base voltage in kV); all others, base current and base impedance, are automatically derived through Ohm's law relationships.
• Transformer Simplification: Impedances referred from one side of a transformer to another remain unchanged in per-unit, eliminating turns ratios from the analysis and dramatically simplifying multi-voltage network calculations.
• Error Detection: Since similar apparatus (generators, transformers, motors) of different physical sizes have per-unit impedances within narrow, well-known ranges, values far from the expected range immediately flag data errors.
• Numerical Conditioning: All quantities are of similar order of magnitude (near 1.0), reducing rounding errors and improving convergence in computer simulations.
• Industry Standard: The per-unit system is the standard framework for power flow studies, short-circuit analysis, stability assessment, and protection coordination. Equipment manufacturers specify impedance values in per-unit or percent on nameplates.