Behaviour of h.v. d.c. links under unbalanced-a.c.-fault conditions

A theoretical analysis is made of the production of triplen-harmonic currents by a convertor bridge using symmetrical firing control when supplied with unbalanced 3-phase voltages. It is shown that these harmonic currents can pass into the a.c. system regardless of the transformer connections. Results are obtained from a digital-computer program for the dynamic behaviour of an a.c.-d.c. interconnected system during single- and 2-phase faults, and particular importance is given to their effect on a weak a.c. system.