Base Power / Base Voltage

Base power and base voltage are the two independently chosen reference values that anchor the per-unit system used in power system analysis. All other base quantities are mathematically derived from these two.

Key Aspects of Base Power and Base Voltage:

• Base Power (S_base): Measured in MVA, it is usually a convenient round number, commonly 100 MVA for transmission system studies or the rated power of a specific piece of equipment, and remains constant throughout the entire system regardless of voltage level.
• Base Voltage (V_base): Measured in kV, it is typically chosen as the nominal voltage at each bus and changes across transformer windings according to the turns ratio, forming a consistent chain of base voltages throughout the network.
• Derived Quantities: Base current = S_base ÷ (√3 × V_base) for three-phase systems; base impedance = V_base² ÷ S_base. These relationships ensure dimensional consistency across the per-unit framework.
• Base Conversion: When equipment impedance is given on a different base than the system base, it must be converted: Z_new = Z_old × (S_base_new ÷ S_base_old) × (V_base_old ÷ V_base_new)². This is one of the most frequent calculations in power system studies.
• Practical Impact: The choice of base values affects only the per-unit representation, not the physical behavior of the system. A well-chosen base simplifies analysis and makes results more intuitive for engineers reviewing network models.