The per-unit impedance of a single-phase electric load is 0.3. The base power is 500 kVA, and the base voltage is 13.8 kV. a. Find the per-unit impedance of the load if 1 MVA and 24 kV are selected as base values. b. Find the ohmic value of the impedance.

114.26

Explanation:

a) Formula for per unit impedance for change of base is

Zpu2 = Zpu1 * (kV1/kV2) ² * (kVA2/kVA1)

Zpu2: New per unit impedance

Zpu1: given per unit impedance

kV1: give base voltage

kV2: New bas votlage

kVA1: given bas power

kVA2: new base power

In the question

Zpu2=?

Zpu1 = 0.3

kV2=24kV

kV1 = 13.8 kV

kVA2 = 1MVA * 1000 = 1000 kVA

kVA1=500kVA

Zpu2 = 0.3 (13.8/24) ² * (1000/500)

Zpu2 = 0.198

b) to find ohmic impedance we will first calculate base value of impedance (Zbase). So,

Zbase = kV²/MVA

Zbase = 13.8² / (500/1000)

Zbase=380.88

Now that we have base value of impedance, Zbase, we can calculate actual ohmic value of impedance (Zactual) by using the following formula:

Zpu=Zactual/Zbase

0.3 = Zactual/380.88

Zactual = 114.26 ohms