Which of the following is the long term stability in a generator driven system?

Question

Examoogle · Accepted Answer

Frequency Stability

Explanation: Frequency stability refers to the ability of a power system to maintain steady frequency following a severe system upset resulting in a significant imbalance between generation and load. It is classified as a long-term stability phenomenon because the restoration of balance involves slow-acting processes like governor response, AGC, and boiler controls.

Working Principle: System frequency is governed by the rotational inertia of generators and the power balance equation: $P_m - P_e = M \cdot \frac{d\omega}{dt}$. When a disturbance occurs, the mismatch between mechanical input ($P_m$) and electrical demand ($P_e$) causes the rotor speed to deviate. Frequency stability analysis involves evaluating the effectiveness of Primary Frequency Control (governor action) and Secondary Frequency Control (AGC) over timescales ranging from seconds to several minutes.

Key Formulas: $P_m - P_e = M \frac{d\omega}{dt}$ — Swing equation relating power imbalance to speed change; $\Delta f = -R \cdot \Delta P$ — Relation between frequency deviation and governor regulation

Key Points: Frequency stability is concerned with the system's ability to maintain frequency within permissible limits.; The time frame for frequency stability spans from a few seconds to minutes.; Generator governor systems provide the primary defense against frequency instability.; Long-term voltage stability is separate, involving tap changers and thermal load characteristics.

Advantages: Ensures synchronization of interconnected grids; Prevents damage to turbine blades due to resonance at off-nominal frequencies

Disadvantages: Slow response compared to transient stability; Dependency on complex multi-loop control systems

Applications: Load frequency control (LFC); Grid code compliance and power interchange regulation

Additional Info: Transient stability is a short-term phenomenon (typically. Option A (Rotor-angle stability) is primarily a short-term/transient concern. Option D (Long-term voltage stability) involves slow dynamics like transformer tap changing and thermostats, but the question specifically refers to the generator-driven response for frequency.

Answer: C is correct — Frequency stability is a long-term stability phenomenon involving the balance between system generation and load regulated by governors and AGC.

Exam Tip: Always distinguish stability by timescale: Rotor-angle is micro-seconds to seconds, while frequency and voltage stability span from seconds to minutes (long-term).

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