Abstract
Two-level voltage-source converters and half-bridge modular multilevel converters are among the most popular types of HVDC converters. One of their serious drawbacks is their vulnerable nature to dc-side faults, since the freewheeling diodes act as a rectifier bridge and feed the dc faults. The severity of dc-side faults can be limited by connecting double thyristor switches across the semiconductor devices. By turning them on, the ac current contribution into the dc side is eliminated and the dc-link current will freely decay to zero. The main disadvantages of this method are: high dv/dt stresses across thyristors during normal conditions, and the absence of bypassing for the freewheeling diodes during dc faults since they are sharing the fault current with thyristors. This paper proposes combining and connecting the double thyristor switches across the ac output terminals of the HVDC converter. The proposed protection scheme provides advantages, such as lower dv/dt stresses and lower voltage rating of thyristor switches in addition to providing full segregation between the converter semiconductor devices and ac grid during dc-side faults. A simulation case study has been carried out to demonstrate the effectiveness of the proposed scheme.