Fault Handling at Hybrid High-Voltage AC/DC Transmission Lines With VSC Converters

The generation capacity of renewable energy sources has significantly increased during the last few years. As a consequence, high-power dc transmission systems are key technologies to reinforce the high-voltage transmission system. Regarding the long process of approving for right of way for new lines, a conversion from existing ac into dc lines might be an option. Modular multilevel converters provide the necessary features to avoid technical problems in heavily loaded ac systems; they increase the transmission capacity and system stability very efficiently and assist in preventing cascading disturbances. In this paper, high-voltage direct current transmission systems based on half-bridge topology with and without solid-state breaker and full-bridge (FB) modules are compared. The general behavior during faults on the dc side is described especially with respect to intersystem faults (i.e., faults between the ac and dc systems). Regarding the clearing capability, such disturbances lead to even higher requirements than for pure dc faults, especially the necessary transient interruption voltage (TIV) of the dc breaker design. On the other hand, this adapted design yields some disadvantages with respect to more frequently occurring dc line faults. In contrast, the controllability of the FB solution offers the possibility of an adjustable TIV and does not need to dissipate any energy since the power can be transmitted to the ac side. Therefore, the FB seems suited best for applications in hybrid high-voltage ac/dc systems.