HVDC transmission converters Which pulse-number converter is predominantly used in classic line-commutated high-voltage direct current (HVDC) transmission schemes?

Introduction / Context:
In line-commutated HVDC, the converter topology and pulse number determine characteristic harmonics, filtering requirements, and performance. Utilities standardised on particular pulse numbers to reduce harmonics on both AC and DC sides while keeping station complexity manageable.

Given Data / Assumptions:

• Traditional LCC (thyristor-based) HVDC systems.
• 12-pulse arrangements formed by two 6-pulse bridges with a 30° phase shift.
• Focus is on common practice, not rare specialised schemes.

Concept / Approach:

A 12-pulse converter cancels the lowest characteristic harmonics (notably the 5th and 7th on the AC side and the 6th on the DC side), substantially cutting filter size and losses compared with a single 6-pulse bridge. It is realised using a converter transformer with star/delta secondaries to achieve the requisite 30° phase displacement between the two 6-pulse bridges.

Step-by-Step Solution:

Verification / Alternative check:

Utility specifications and classic HVDC references cite the 12-pulse arrangement as the norm for thyristor-based terminals.

Why Other Options Are Wrong:

3-pulse and 6-pulse have poor harmonic performance; 24-pulse exists but is complex and uncommon for full stations; 18-pulse is rare in HVDC LCC practice.

Common Pitfalls:

Assuming “more pulses is always better” without considering transformer design, cost, and practicality; in practice 12-pulse strikes the balance.

Final Answer:

12-pulse converter