Question

A three-phase, 600 MVA, 13.8KV AC generator has a synchronous reactance of 2.0 per unit. The generator is connected to a system for which the specified bases are 100MVA and 345KV. a) Find the per-unit value of the generator synchronous reactance on the specified bases. b) Find the ohmic value of the synchronous reactance. Problem 4 A single-phase source is connected to an electrical load. The load

Answer 1

Answer:

(a) 0.00053

(b) 0.1 mΩ

Explanation:

• New per-unit reactance is given as:

           New Per-unit reactance = $2* \frac{100}{600} * (\frac{13.8}{345}) ^{2} = 0.00053$

• Ohmic reactance : $\frac{13.8^2}{600} = 0.31 ohm$
• Ohmic per unit : 0.31 * 0.00053 = 0.1 mΩ

Answer 2

Given Information:

Zpu_old = 2 pu

Sbase_new = 100 MVA

Sbase_old = 600 MVA

kV_old = 13.8 kV

kV_new = 345 kV

Required Information:

Zpu_new = ?

ZΩ = ?

Answer:

Zpu_new = 0.000533 pu

ZΩ = 0.634 Ω

Explanation:

a) Find the per-unit value of the generator synchronous reactance on the specified bases.

When the base kVA and base kV are changed then we use following relation to update the per unit values.

Zpu_new = Zpu_old*(Sbase_new/Sbase_old)*(kV_old/kV_new)²

Zpu_new = 2*(100x10⁶/600x10⁶)*(13.8x10³/345x10³)²

Zpu_new = 0.000533 pu

b) Find the ohmic value of the synchronous reactance.

ZΩ = Zbase*Zpu_new

Where Zbase is calculated as

Zbase = (kVbase)²/Sbase

Zbase = (345x10³)²/100x10⁶

Zbase = 1190.25 Ω

ZΩ = Zbase*Zpu_new = 1190.25*0.000533 = 0.634 Ω