Transient stability is very much affected by the type of the fault. A three phase dead short circuit is the most severe fault; the fault severity decreasing with two phase fault and single line-to ground fault in that order.
If the fault is farther from the generator the severity will be less than in the case of a fault occurring at the terminals of the generator.
Power transferred during fault also plays a major role. When, part of the power generated is transferred to the load, the accelerating power is reduced to that extent.
Theoretically an increase in the value of inertia constant M reduces the angle through which the rotor swings farther during a fault. However, this is not a practical proposition since, increasing M means, increasing the dimensions of the machine, which is uneconomical.
The dimensions of the machine are determined by the output desired from the machine and stability cannot be the criterion. Also, increasing M may interfere with speed governing system.
Thus looking at the swing equations
(i) Increase of system voltages, and use of automatic voltage regulators.
(ii) Use of quick response excitation systems
(iii) Compensation for transfer reactance XI2 so that Pe increases and Pm - Pe = Pa reduces.
(iv) Use of high speed circuit breakers which reduce the fault duration time and hence the accelerating power.
When faults occur, the system voltage drops. Support to the system voltages by automatic voltage controllers and fast acting excitation systems will improve the power transfer during the fault and reduce the rotor swing.
Reduction in transfer reactance is possible only when parallel lines are used in place of single line or by use of bundle conductors. Other theoretical methods such as reducing the spacing between the conductors-and increasing the size of the conductors are not practicable and are uneconomical.
Quick opening of circuit breakers and single pole reclosing is helpful. Since majority of the faults are line to ground faults selective single pole opening and reclosing will ensure transfer of power during the fault and improve stability.