A three-phase, Y-connected, 75MVA, 27kV synchronous generator has a synchronous reactance of 9.0/phase. Using rated MVA and voltage as the base values, determine the per-unit reactance. Then refer this per-unit value to a 100-MVA, 30kV base.

Given Information:

ZΩ = 9 per phase

Sbase = 75 MVA

Sbase = 100 MVA

kV = 27 kV

kV = 30 kV

Required Information:

Zpu = ?

Zpu_new = ?

Answer:

Zpu = 0.925 pu

Zpu_new = 1 pu

Explanation:

Zbase = (kVbase) ²/Sbase

Zbase = (27x10³) ²/75x10⁶

Zbase = 9.72

Zpu = Zactual/Zbase

Zpu = 9/9.72

Zpu = 0.925 pu

Now base MVA is changed to 100 MVA and base kV is changed to 30kV so the per unit value becomes

Zbase = (kVbase) ²/Sbase

Zbase = (30x10³) ²/100x10⁶

Zbase = 9

Zpu_new = Zactual/Zbase

Zpu_new = 9/9

Zpu_new = 1 pu