What is the per-unit system and why is it used in power systems analysis?

The per-unit system expresses quantities as fractions of chosen base values, making calculations simpler across different voltage levels. Per-unit value = actual value / base value. Typically, base MVA is chosen (often 100 MVA) and base voltages are transformer rated voltages. Benefits include: transformer per-unit impedances are same on both sides, values fall in predictable ranges (0.05-0.5 for transformers), and analysis becomes voltage-level independent. It simplifies fault calculations and power flow studies.