HVDC transmission practice: Which converter pulse number is generally preferred for line-commutated HVDC links to reduce harmonics and smoothing requirements?

Introduction / Context:
HVDC schemes typically use line-commutated converter bridges. The number of pulses per cycle directly affects characteristic harmonics, filtering needs, and performance. This question targets the standard industry choice for large HVDC links.

Given Data / Assumptions:

• Line-commutated (thyristor) HVDC converters.
• Harmonic reduction and smoother DC current are desirable.
• Transformer phase-shifting allows pulse multiplication.

Concept / Approach:

Two 6-pulse bridges in series/parallel with a 30° phase shift create a 12-pulse converter. This arrangement cancels lower-order characteristic harmonics (e.g., 5th/7th on AC side), lowering filter size and improving DC quality. Hence, 12-pulse is the standard choice for bulk power HVDC.

Step-by-Step Solution:

Verification / Alternative check:

Most classic HVDC installations (e.g., LCC-HVDC) specify 12-pulse at each end as baseline.

Why Other Options Are Wrong:

• 6-pulse alone has higher harmonics and larger filters.
• 3-pulse is obsolete for high-power transmission.
• “Either 12 or 6” is too loose; “generally use” points to 12-pulse.

Common Pitfalls:

Confusing modular multilevel VSC practice (different pulse concept) with line-commutated HVDC.

Final Answer:

12 pulse converters