A Hybrid DC circuit breaker (DC CB) topology is one of the topologies tested and analysed in the PROMOTioN project. It is characterized by low losses and fast opening time, with a 320kV, 16kA prototype being tested in WP10 of the project. This DC CB will be available in both a unidirectional and a bidirectional version. The bidirectional version will have two main semiconductor valves (which are similar to the valves in a VSC converter) – one for each direction of current. A bidirectional version may be required in some DC grid topologies involving lower numbers of DC CBs, or in other DC grid applications dependent on the requirements for protection against DC bus faults or DC CB failure (backup protection).

The work in WP6 concerns DC CB characterisation primarily employing DC CB models and scaled low-power DC CB demonstrators. In task 6.7, one focus of study has been on possible cost reduction of bidirectional DC CBs. Two novel bidirectional topologies have been developed, where only a single unidirectional main valve is employed. Since one of the two high-voltage valves is eliminated, cost savings could be significant. The first topology operates two novel double-throw mechanical switches in tandem in order to change the direction of the main valve current. The second topology uses low-voltage diodes to route the current through the main valve and four novel fast mechanical switches to isolate the main valve in the direction required. Performance of the new designs has been extensively analysed on the hybrid DC CB simulation models developed earlier in WP6. It was confirmed that performance (opening speed, current/voltage ratings, …) is similar to the original hybrid DC CB. In order to further evaluate the new designs, low power prototypes for both new topologies have been built and tested at the University of Aberdeen laboratory. The prototypes are integrated into the low-power hardware DC CB demonstrator and DC CB test circuit built in WP6. The image to the left shows two of these new double-throw mechanical switches. The researchers at Aberdeen HVDC Research Centre have performed numerous tests with up to 500A interrupting current and around 1.5kV voltage which have confirmed successful operation.   
More details on this study can be found in the journal article on Bidirectional Hybrid HVDC CB with a single HV Valve.