In series line compensation, the voltage stability is

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Improves

Explanation: Series line compensation involves adding a capacitor in series with the transmission line to reduce the overall inductive reactance of the system. By reducing the line impedance ($X_{eq} = X_L - X_C$), the power transfer capability is enhanced, and the voltage regulation is improved.

Working Principle: The transmission line voltage drop is proportional to the reactive component of the impedance. By inserting a series capacitor, the total line reactance is neutralized, which decreases the voltage drop across the line for a given power flow. This leads to better voltage regulation and increased stability margins by pushing the power-angle curve higher, allowing more stable power transfer for the same angular displacement.

Key Formulas: $P = \frac{V_s V_r}{X_L - X_C} \sin \delta$ — Power transfer capability increases as $X_C$ reduces net reactance; $V_{drop} \approx I(R \cos \phi + (X_L - X_C) \sin \phi)$ — Voltage drop expression showing reduction with series compensation

Key Points: Series compensation is used primarily in long-distance EHV transmission lines.; It directly reduces the inductive voltage drop by compensating for line inductance.; It increases the steady-state power transfer limit of the transmission system.; Potential risk includes sub-synchronous resonance (SSR) if not properly controlled.

Advantages: Increased power transmission capacity; Improved transient and steady-state stability; Better voltage profile regulation; Effective control of load sharing between parallel lines

Disadvantages: Risk of sub-synchronous resonance (SSR); Requires high-voltage insulation and protection for capacitor banks; Need for complex fast-acting protective bypass switches

Applications: Long EHV and UHV transmission lines; Interconnection between large grids; Lines where power flow control is required dynamically

Additional Info: Series compensation is generally preferred for long lines ($>300$ km) to improve voltage regulation without the need for additional generating stations.. Option D is incorrect because compensation is a stability enhancement measure, not a cause of instability.

Answer: B is correct — Series line compensation reduces the total line reactance, which minimizes voltage drops and increases the maximum stable power transfer capacity of the system.

Exam Tip: Always remember that while series compensation improves stability, shunt compensation is primarily used for voltage regulation at load ends. Distinguishing between these two is critical for exam questions.

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