In the per-unit system, why use base MVA and how do you convert impedance from one base to another?

In the per-unit system, why use base MVA and how do you convert impedance from one base to another?

• A. Base values eliminate the need to consider impedance; Z_pu,new = Z_pu,old.
• B. Base values are used to simplify time-domain simulation; Z_pu,new = Z_pu,old × (V_base / S_base).
• C. Base values normalize quantities for comparison and calculation; Z_pu,new = Z_pu,old × (S_base,new / S_base,old); actual impedance Z = Z_pu × Z_base where Z_base = V_base^2 / S_base.
• D. Base values are only used for voltage measurements in the field.

Answer: (C)

In the per-unit system, choosing a base MVA (and a base voltage) lets you normalize quantities so impedances from different equipment can be compared and combined easily. The impedance base is Z_base = V_base^2 / S_base, and the actual impedance is Z = Z_pu × Z_base. When you switch to a different apparent-power base while keeping the same voltage base, the per-unit impedance scales with the base change: Z_pu,new = Z_pu,old × (S_base,new / S_base,old). This is because Z_base_new = V_base^2 / S_base,new and Z_pu,new = Z_actual / Z_base_new, while Z_pu,old = Z_actual / Z_base_old. For completeness, you recover real impedance by Z = Z_pu × Z_base, using the base that you’re now adopting. For example, increasing the MVA base (with the same voltage base) will increase the per-unit value proportionally, keeping the actual impedance the same.

In the per-unit system, choosing a base MVA (and a base voltage) lets you normalize quantities so impedances from different equipment can be compared and combined easily. The impedance base is Z_base = V_base^2 / S_base, and the actual impedance is Z = Z_pu × Z_base.

When you switch to a different apparent-power base while keeping the same voltage base, the per-unit impedance scales with the base change: Z_pu,new = Z_pu,old × (S_base,new / S_base,old). This is because Z_base_new = V_base^2 / S_base,new and Z_pu,new = Z_actual / Z_base_new, while Z_pu,old = Z_actual / Z_base_old.

For completeness, you recover real impedance by Z = Z_pu × Z_base, using the base that you’re now adopting. For example, increasing the MVA base (with the same voltage base) will increase the per-unit value proportionally, keeping the actual impedance the same.