Percentage impedance of a transformer is the percentage of rated voltage required to circulate full-load current when the secondary is short-circuited. %Z $=\frac{V_{sc}}{V_{rated}}\times 100$

It rep represents the transformer's equivalent impedance in % on its own rating.

Effects of Higher % Impedance

• ✓ Lower short-circuit current
• ✓Better fault current limitation
• ⨯ Higher voltage drop
• ⨯ Poorer voltage regulation

Effects of Lower % Impedance

• ✓ Better voltage regulation
• ✓ Smaller voltage drop
• ⨯ Very high short-circuit current
• ⨯ Higher mechanical stress during fault

Given,

$Z_{22} = j0.24$

$Prefault voltage at Bus-2 = 1.05 ри$

Sub-transient Fault Current,

$I_f=\frac{1.05}{j 0.24}=-j4.375 ри$

$| If |= 4.375 pu$

Base current,

$100\times {10}^6$

$I_{base} =\frac{100 \times {10}^6}{\sqrt{3} \times 11\times {10}^3}$

$I_f= 22.96 kA$

Bus Voltages During Fault,

$V_i= V_i^0 - Z_{i2}{l}_f$

Bus-1:

$V_1 =1-(j0.08) (- j 4.375) =1-0.35$

$= 0.65 ри$

Bus-2 (fault bus), $V_2=0$

Bus-3:

$V_3 =1-(j0.16) (- j 4.375) = 1-0.70$

$= 0.30 pu$