Alternative convertor for h.v. d.c. transmission. Three single-phase bridges connected in series

The Graetz (3-phase) bridge circuit is the arrangement usually chosen for a 3-phase d.c. convertor, because it has the best valve and transformer utilisation, and hence lowest valve costs. This bridge has the disadvantage that the alternating current is fixed relative to the direct current, which means that the reactive-power requirement is dependent upon the active power. If three single-phase bridge circuits are used, however, it is possible to vary the relationship between the alternating and the direct currents, and hence it is possible to change reactive power without varying active power. This facility is particularly useful when a number of convertors are connected in series on the d.c. side, when the direct current is common to all convertors and voltage control is used to set the active power level. The operation of a convertor using three single-phase bridge circuits is analysed, and it is shown that, although the bridge is more expensive on valves, it has operational advantages over the conventional Graetz bridge. The theoretical and practical operating limitations are discussed, and it is shown that the convertor can be operated continuously over its complete power range with a reactive-power requirement of only 70% of the rated convertor apparent power, compared with 100% for a Graetz bridge. The cost of terminal plant other than valves is shown to be the same for both convertor circuits. In a brief economic assessment, it is shown that the overall cost of the new convertor may be only 14% higher than a similarly rated Graetz bridge