HVDC converter using self-commutated devices

A three-phase bridge for a high-voltage direct-current (HVDC) converter which uses auxiliary (bypass) thyristor valves connected to tapping points on the transformer secondary windings has been previously described. The performance is enhanced by using self- or force-commutated devices for these valves, and three different schemes are examined. Experimental tests on a laboratory model validate the theoretical results. Fast and continuous control of the DC voltage is possible with a typical reduction of 20%-30% of the reactive volt-ampere (VAr) absorption compared with a conventional scheme. An additional feature of the scheme is that the transformer tap changer could have a reduced range with fewer taps or even be eliminated.