Consider the following statements regarding Voltage Source Converters (VSC):1.They use self-commutating devices like IGBT.2.They can independently control active and reactive power.3.They require strong AC system for operation.Which options stated above is/are correct?

Correct Answer: B - 1 and 2 only

Quick Concept Explanation

The key distinction is commutation. LCC relies on the AC system for commutation and consumes reactive power; VSC uses self-commutating devices and synthesized AC voltage, giving flexible P/Q control and better weak-grid capability. Key relation: Conceptually, active power is strongly related to converter voltage-angle difference, while reactive power is strongly related to voltage-magnitude difference (subject to network/converter limits).

Exam focus: VSC keyword pair: self-commutated + independent/flexible P-Q control. Common trap: Transferring the “strong AC system required” limitation from LCC to VSC.

Statement-wise Verification

• Statement 1 - Correct.
  They use self-commutating devices like IGBT.
  Modern VSC-HVDC uses turn-on/turn-off controllable semiconductor valves (classically IGBT-based; modern MMC implementations use IGBT submodules or equivalent self-commutating devices).
• Statement 2 - Correct.
  They can independently control active and reactive power.
  By controlling converter voltage magnitude and phase, VSC can regulate active and reactive power substantially independently within converter limits.
• Statement 3 - Incorrect.
  They require strong AC system for operation.
  Requiring a strong AC system is a major limitation of LCC-HVDC, not VSC; VSC is well suited to weak grids, offshore networks and black-start/passive-network support.

Core Concept

The key distinction is commutation. LCC relies on the AC system for commutation and consumes reactive power; VSC uses self-commutating devices and synthesized AC voltage, giving flexible P/Q control and better weak-grid capability.

Formula / Key Relationship

Conceptually, active power is strongly related to converter voltage-angle difference, while reactive power is strongly related to voltage-magnitude difference (subject to network/converter limits).

Step-by-Step Check

Device technology, control capability and weak-grid behavior independently give truth set {1,2}. Option B.

Why the Other Options Are Wrong

Option A is incorrect because it includes incorrect statement(s) 3 and omits correct statement(s) 1. Option C is incorrect because it omits correct statement(s) 2. Option D is incorrect because it includes incorrect statement(s) 3.

Answer-Key Verification Note

The uploaded provisional key marks D . After independent technical verification, the defensible answer is B . This discrepancy is stated explicitly rather than silently copying the provisional key.

Exam Shortcut / Approach

VSC keyword pair: self-commutated + independent/flexible P-Q control.

Common Mistake

Transferring the “strong AC system required” limitation from LCC to VSC.

Quick Revision

Self-Commutation and P-Q Control is linked with VSC-HVDC, HVDC Transmission and Power Systems. VSC keyword pair: self-commutated + independent/flexible P-Q control.