A Modification in Thyristor-Based Hybrid DC Circuit Breaker for DC-Side Fault Identification in Auto-Reclosing of MMC-HVDC Systems

Safe reclosing of high-voltage direct current (HVdc) systems after dc-side fault current interruption is essential. Otherwise, a second strike can endanger the components of dc circuit breakers (DCCBs) and modular multilevel converters (MMCs). Thus, it is necessary to identify the fault type before auto-reclosing of MMC-HVdc systems. This article proposes a method that enables an MMC-HVdc system equipped with the thyristor-based hybrid DCCB introduced by Alstom to identify and localize the dc-side fault. In the proposed method, adding some capacitors with low capacitance parallel to the thyristors in one part of an auxiliary branch enables the hybrid DCCB to inject active pulses. To clarify, by turning a specified number of the thyristors on, a desirable number of capacitors parallel with the off thyristors are applied, and a second-order circuit is formed, resulting in the injection of active pulses with controllable amplitude and width. According to the traveling wave theory, the difference in the reflection of the injected pulse determines whether the fault is permanent or temporary. The efficacy of the proposed method is validated by an MMC-HVdc test system built into a PSCAD/EMTDC platform. Furthermore, generating active pulses with the modified hybrid DCCB is verified by a scaled-down prototype.