How it works
An HVDC link uses a rectifier at the sending end (AC to DC) and an inverter at the receiving end (DC to AC), both built from 12-pulse thyristor valve groups to reduce harmonic content. The 12-pulse configuration uses two 6-pulse converters fed from star and delta secondaries of the converter transformer, phase-shifted by 30°, cancelling the 5th and 7th harmonics. DC voltage is controlled by varying the firing angle α of the thyristors: Vd = Vd0·cos α, where Vd0 = (3√6/π)·Vm for a 3-phase bridge. Monopolar HVDC uses one high-voltage conductor with ground or sea return; bipolar HVDC uses two conductors at +V and −V, providing redundancy if one pole fails.
Key points to remember
HVDC advantages over AC include: no reactive power on the DC link (no charging current), no skin effect losses, no stability limit on line length, and asynchronous interconnection of grids at different frequencies (e.g., 50 Hz India to 60 Hz grid). Disadvantages are high converter station cost and the inability to use transformers to step DC voltage up or down. Harmonics generated by 6-pulse converters are of order 6k±1 (5th, 7th, 11th, 13th…); 12-pulse eliminates 5th and 7th, leaving 11th, 13th as dominant. Reactive power consumption of the converter is approximately 50–60% of the rated DC power — large capacitor banks or SVCs at the converter bus compensate for this. VSC-HVDC using IGBTs allows black-start capability and independent P and Q control, a key advantage over line-commutated HVDC.
Exam tip
Every Anna University power systems paper asks you to list four advantages of HVDC over HVAC and explain the principle of the 12-pulse converter — answer the advantages first with one line each, then sketch the 12-pulse configuration with star and delta transformers clearly labelled.