Correct answer: C — actual value in any units / base value in same units
Definition of a per-unit quantity
The per-unit system expresses a quantity relative to a selected reference or base value of the same physical quantity. It is defined as:
Voltage is divided by base voltage, current by base current, and impedance by base impedance. Both values must be expressed in the same units before division. Their units then cancel, leaving a dimensionless ratio.
Example with unit conversion
Step 1: Choose the actual and base values. Suppose the actual voltage is 5,500 V and the selected base voltage is 11 kV.
Step 2: Express both in the same unit. Convert the base voltage: 11 kV = 11,000 V.
Step 3: Divide actual value by base value. The normalized voltage is:
Vpu = 5,500/11,000 = 0.5 pu
Percentage value = 100 × 0.5 = 50%
A value of 1 pu means actual equals base, 0.5 pu means half the base, and 1.1 pu means 10% above the base. The same actual quantity can have different per-unit values on different bases, so the chosen base must be stated when interpreting or comparing results. This definition directly matches option C.
