Power system stability means

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Option C: Both a and b

Explanation & Concepts:
Power system stability refers to the ability of an electric power system to regain a state of operating equilibrium after being subjected to a physical disturbance. It encompasses both the maintenance of steady-state operation and the capability to restore stability following transient or dynamic perturbations.

Working Principle: Stability depends on the balance between electromagnetic torque and mechanical torque in generators. When a disturbance occurs, the rotor speed deviates; the system remains stable if the synchronizing and damping torques are sufficient to return the system to a new or original operating point. The stability is categorized into Steady-State, Transient, and Dynamic stability based on the nature and magnitude of the disturbance.

Key Formulas: $P = \frac{V_s V_r}{X} \sin(\delta)$ — Power-angle relation representing the transfer capability during steady-state; $M \frac{d^2\delta}{dt^2} = P_m - P_e$ — Swing equation governing the rotor dynamics during disturbances

Key Points: Steady-state stability relates to small, slow changes in load.; Transient stability relates to large sudden changes like faults or switching.; The swing equation describes the oscillation of the generator rotor.; Synchronizing torque is essential to pull the rotor back into equilibrium.

Advantages: Ensures continuous and reliable power supply; Prevents cascading outages in a grid

Disadvantages: High computational requirement for simulation; Stability limits restrict maximum power transfer capacity

Applications: Grid protection relay settings; HVDC system control strategies

Additional Info: Stability is generally assessed using the Equal Area Criterion for single machine systems.. Options A and B represent the two fundamental aspects of stability: steady-state maintenance and recovery, making C the comprehensive choice.

Answer: C is correct — Power system stability is defined by both the ability to maintain equilibrium under normal conditions and the capacity to restore it following a disturbance.

Exam Tip: Always remember the Swing Equation $M\ddot{\delta} = P_m - P_e$ as the foundation for all transient stability analysis in competitive exams.

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Option A: Power system remains in a state of equilibrium under normal operating condition

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Option B: To restore an acceptable state of equilibrium after a disturbance

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Option D: None of above

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