Combined control and protection system for improved performance of HVDC links in the presence of AC faults

One of the problems of DC links is that, in the case of severe AC faults at the inverter side due to successive commutation failures, their operation is interrupted and they are unable to assist the integrated AC-DC system to recover. This is particularly true in the case of weak systems. The paper develops the theory and design of a control system which, in the case of AC faults at the inverter side, overrides temporarily the normal firing control and tries to maintain the operation of the DC link uninterrupted. The proposed control system was tested on an AC-DC system simulated on the digital computer. The model system used, the results obtained and their evaluation are presented in the paper. The simulation results show that the proposed control system is particularly effective in the case of line-to-ground faults. In the case of multiple phase faults, the control system is less effective and temporary blocking of the bridge remains the only alternative control action.