Transient stability is very much affected by the type of fault. A three-phase dead short circuit is the most severe fault; the fault severity decreases with two-phase faults and single line-to-ground faults 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

The possible methods that may improve the transient stability are:

(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 a single line or by the 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 the majority of the faults are line-to-ground faults selective single pole opening and reclosing will ensure the transfer of power during the fault and improve stability.

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