What is the result of frequency instability?

Question

Examoogle · Accepted Answer

Option D: Both b and c

Explanation & Concepts:
Frequency instability occurs when there is a significant imbalance between real power generation and consumer demand, leading to deviations from the nominal grid frequency (e.g., 50 Hz in India). This imbalance triggers wide oscillations in frequency and necessitates protective relay operations to prevent system-wide collapse.

Working Principle: In a power system, frequency is a global variable determined by the balance of active power. When $P_{gen}

Key Formulas: $f = \frac{1}{2\pi}\sqrt{\frac{k}{J}}$ — Approximate relationship for governor response; $\Delta f \propto \frac{\Delta P}{K}$ — Frequency deviation relative to power imbalance

Key Points: Frequency instability is primarily a real power (active power) control issue.; Grid frequency reflects the instantaneous balance between production and consumption.; Automatic Generation Control (AGC) works to minimize frequency deviations.; Sudden disconnection of major loads or generation units can cause catastrophic frequency instability.

Advantages: UFLS prevents total system blackout; AGC improves frequency stability in steady-state

Disadvantages: Generator mechanical stress during frequency swings; Potential for cascading outages if protection logic is inadequate

Applications: Load Frequency Control (LFC) in microgrids; Protection schemes for transmission networks

Additional Info: Nominal frequency in India is 50 Hz ± 0.05 Hz.. Option A (Voltage collapse) is primarily associated with Reactive Power ($Q$) and voltage stability issues, not directly frequency.. Option D is correct because frequency swings indicate transient instability, and excessive deviations force generating units to trip for self-protection.

Answer: D is correct — Frequency instability leads to both significant swings in frequency and, if limits are exceeded, the automatic tripping of generating units to protect grid integrity.

Exam Tip: Always remember: Real Power ($P$) controls Frequency ($f$), while Reactive Power ($Q$) controls Voltage ($V$). This is the golden rule for power system stability analysis.

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Option A: Voltage collapse

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Option B: Frequency swings

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Option C: Tripping of generating units

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