What is the primary purpose of the per-unit system in power system analysis?

What is the primary purpose of the per-unit system in power system analysis?

• A. To convert all quantities to real numbers
• B. To normalize system quantities to a common base, simplifying comparisons and calculations across components with different voltage/current levels
• C. To simplify by ignoring resistive losses
• D. To only analyze DC systems

Answer: (B)

Think of the per-unit system as putting everything on the same scale. By choosing base values for power and voltage, you convert voltages, currents, impedances, and power into per-unit quantities. This makes components operating at different voltage levels directly comparable and lets you scale results consistently when you move between bases. The math aligns with Ohm’s law in per-unit form: V_pu = V / V_base, I_pu = I / I_base, Z_pu = Z / Z_base, with Z_base = V_base^2 / S_base. As a result, quantities stay in a similar range (often around 0.8–1.2 in normal operation), which greatly simplifies analyses like power flow and fault studies. It does not ignore losses, nor is it limited to DC systems, and it isn’t simply about converting to real numbers.

Think of the per-unit system as putting everything on the same scale. By choosing base values for power and voltage, you convert voltages, currents, impedances, and power into per-unit quantities. This makes components operating at different voltage levels directly comparable and lets you scale results consistently when you move between bases. The math aligns with Ohm’s law in per-unit form: V_pu = V / V_base, I_pu = I / I_base, Z_pu = Z / Z_base, with Z_base = V_base^2 / S_base. As a result, quantities stay in a similar range (often around 0.8–1.2 in normal operation), which greatly simplifies analyses like power flow and fault studies. It does not ignore losses, nor is it limited to DC systems, and it isn’t simply about converting to real numbers.