Converter commutation overlap: identify the dominant cause of overlap between outgoing and incoming devices during line-commutation.

Introduction / Context:
In line-commutated converters, device current transfers from one valve to the next during commutation. The supply inductance resists instantaneous current change, causing an overlap interval when both devices conduct, reducing output voltage momentarily.

Given Data / Assumptions:

• Single-phase or three-phase line-commutated rectifier/inverter.
• Ideal devices, practical ac source with finite inductance.
• Overlap is the finite duration where two valves conduct simultaneously.

Concept / Approach:

By Faraday’s law, di/dt through an inductance L requires applied voltage. The ac source inductance impedes rapid current transfer, creating commutation overlap. Source resistance mainly produces voltage drop but does not prevent instantaneous current transfer as strongly as inductance.

Step-by-Step Reasoning:

Verification / Alternative check:

Classical commutation analysis yields overlap angle μ ≈ (Xs * Id) / V_phase, where Xs is source reactance (ωLs), directly tying μ to inductance.

Why Other Options Are Wrong:

• Resistance: affects losses, not primarily the overlap mechanism.
• “Both”: the principal controlling parameter is inductance; resistance has a secondary effect.
• High firing angle: increases reactive effects but is not the cause.
• Load capacitance: unrelated in line-commutated overlap.

Common Pitfalls:

• Blaming firing angle rather than supply reactance.
• Ignoring that overlap reduces average output voltage.

Final Answer:

source inductance