A three bus power system with base of 100 MVA and 11 kV whose bus impedances matrix Z-bus for the system given in the following:

$\left[\begin{array}{ccc}j 0.16& j 0.08& j 0.12\\ j 0.08& j 0.24& j 0.16\\ j 0.12& j 0.16& j 0.34\end{array}\right]$

When 5% over voltage occurs in bus-2, fault takes place.

Find the sub-transient current in the fault bus and the bus voltages in the remaining buses during the fault.

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