This paper presents a breaker arrangement concept, the Multi-Line Breaker (MLB), for the protection of multi-terminal high voltage dc (MTdc) networks. Based on the design of a hybrid breaker, the MLB is an economically attractive solution for the protection of multiple dc lines in nodal connection using a single main breaker path. By using commutation units, the MLB directs the fault current through the main breaker in a unidirectional way, irrespective of the fault location. Hence, this study presents the design requirements for the MLB, regarding both hardware and control, and evaluates its operation within a grid. For this reason, a four-terminal half-bridge MMC-based MTdc grid in radial configuration was used and pole-to-ground dc fault conditions were investigated. The dc fault response of the grid with one MLB at the central node is compared to the respective response of the grid when one hybrid breaker is employed at each dc line. The simulations show that the MLB is feasible and that the overall MTdc grid fault response for the two protection systems is very similar. As a result, the design advantages of the MLB make it a promising solution for the dc fault isolation in MTdc grids.

This paper presents a performance evaluation in terms of applicability, response times, energy dissipation, passive components and power semiconductors requirements of the four most promising hybrid DC circuit breaker concepts for HVDC grids. Specific design guidelines for all of the hybrid DC circuit breakers are also shown. The evaluation of the hybrid DC circuit breakers has been performed using PLECS.

In this paper, two new hCB concepts for DC-grids are presented, which use a pulse current to extinguish the arc in the mechanical switch after opening. Both concepts are capable to adjust the pulse current amplitude/waveform to the fault current amplitude and so generate a slow current slope di /dt directly before the zero current crossing without large passive components for a fast and reliable fault clearing. By improving the controllability of the hCB and with a new control concept the capacitor volume of the first concept could be reduced by 60% and the inductor volume by 88% compared to the existing solution. At the cost of more active components, the capacitor volume could be even more reduced by 98.4% compared to the first concept (99.4% compared to the existing solution).

In DC grids, DC circuit breakers (CB) are used to interrupt currents in case of a fault and to dissipate the remaining energy in the line. Independent of the actually DC CB concept, different energy dissipation concepts can be used, which also influence the performance of the DC CB. In this paper, the energy dissipation with different circuit arrangements, where the energy is dissipated in a single loop, in two loops or in two coupled loops, are discussed and compared. Additionally, the effect of different voltage waveforms across the DC CB and the distribution of the current limiting inductance on the input and the output side of the DC CB are investigated.

Circuit breakers with current injection enable fast interruption of fault currents in DC grids. Independent of the topology, a high number of semiconductors and capacitors is required. The minimum required number of semiconductors and capacitors depend mainly on the system voltage and the maximum fault current. However, also the MS characteristics have considerable influence on the required number of semiconductors and capacitors. Therefore, in this paper the influence of the MS characteristics on the arc extinction and the influence of the arc voltage on the DC-CB design is presented. In addition, optimization procedures for current injection circuit breaker are presented, which include the MS characteristics and four different possibilities to include the arc voltage. Finally, three DC-CB topologies are compared in terms of number of semiconductors and capacitors.