Capacitor Commutation Method for MVDC Hybrid Circuit Breakers

The medium voltage DC (MVDC) type system can connect multiple terminals to a common MVDC bus, so it is possible to connect several renewable DC power sources to the common MVDC bus, but a DC circuit breaker is needed to isolate short circuit accidents that may occur in the MVDC bus. For this purpose, the concept of a hybrid DC circuit breaker that takes advantage of a low conduction loss contact type switch and an arcless-breaking semiconductor switch has been proposed. During break the hybrid switch, a dedicated current commutation device is required to temporarily bypass the load current flowing through the main switch into a semiconductor switch branch. Existing current commutation methods include a proactive method and a reverse current injection method by a LC (Inductor-capacitor) resonant circuit. This paper proposes a power circuit of a new MVDC hybrid circuit breaker using a low withstanding voltage capacitor branch for commutation and a sequence controller according to it, and verifies its operation through an experiment.

A Current-Commutation DC Circuit Breaker with Adaptive Reclosing Capability

DC faults are critical events in a flexible high-voltage dc (HVDC) grid. Thus, ensuring that the power system returns to normal operation rapidly and reliably after fault isolation is very important. This requires a HVDC breaker. In overhead line systems under temporary faults, reclosing is often required. However, once the dc circuit breaker (DCCB) is reclosed directly, the large second overcurrent may occur which could damage the power electronic devices. To avoid this problem, a current-commutation DC circuit breaker with adaptive reclosing capability is proposed. Compared with the traditional auto-reclosing strategy, the second damage under permanent fault condition can be avoided by the proposed DCCB, which can identify the fault property. Compared with the hybrid DCCB, the power electronic breaking branch composed of lots of IGBTs is replaced by the current-commutation branch, which is employed to interrupt bi-directional dc fault current. Moreover, bypassing branch is configured to reduce the energy dissipation of arrester and shorten the fault isolation time effectively. Finally, simulation cases in PSCAD /EMTDC verify the effectiveness and superiorities of the proposed DCCB.