**7. Conclusions**

The lack of a zero-crossing makes interrupting inductive currents more challenging in dc grids than in conventional ac grids. Furthermore, fast fault interruption is often required for low voltage dc grids in order to reduce the current stress on the components in the grid and prevent blackouts. Moreover, meshed topologies and bi-directional power flow complicate fault detection and selectivity. To ensure selectivity, literature presents several schemes that rely on communication, or knowledge about the system's topology and parameters. However, fast fault interruption is difficult when communication is used, and systems are subject to uncertainty and change.

Because of the fast fault interruption and meshed system structures, it is difficult to ensure protection selectivity in low voltage dc grids with non-unit protection schemes. It was experimentally shown that the current and current rate of change of circuit breakers in non-faulted regions of the grid can exceed their thresholds before the faulted circuit breakers can clear the fault. Furthermore, interrupted inductive currents temporarily commutate to healthy parts of the system, causing overcurrents. As a consequence of these two phenomena, undesired tripping can occur of circuit breakers in non-faulted parts of the system.

This paper proposes a decentralized plug-and-play protection scheme that ensures security and selectivity, without utilizing communication and with minimal knowledge of the system. The protection scheme delays fault propagation by introducing RC dampers at both ends of the solid-state circuit breaker, which then forms a second order filter for the fault. Furthermore, commutated inductive overcurrents are ignored by incorporating the lines' worst time constant into a time-current characteristic. Additionally, design guidelines were provided for the proposed solid-state circuit breaker topology. Several experiments were carried out that showed that the proposed protection scheme provides secure and selective fault interruption for radial and meshed low voltage dc grids.

Although the proposed protection scheme provides an effective and robust solution, further research is still required. The effectiveness of the protection scheme for medium and high voltage grids must be investigated. Moreover, more research is required on solid-state protection devices, since they are still inefficient and expensive compared to their mechanical counterparts. Additionally, SSCBs have potential for providing additional functionality to the grid, with or without utilizing a communication infrastructure.

**Author Contributions:** Conceptualization, N.H.v.d.B. and P.P.; methodology, N.H.v.d.B. and P.P.; validation, N.H.v.d.B. and T.B.S.; formal analysis, N.H.v.d.B.; investigation, N.H.v.d.B. and T.B.S.; resources, T.B.S.; writing–original draft preparation, N.H.v.d.B.; writing–review and editing, L.M.R.-E., M.T.J.S. and T.B.S.; visualization, N.H.v.d.B.; supervision, L.M.R.-E., M.T.J.S. and T.B.S.; project administration, P.B.; funding acquisition, P.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project has received funding in the framework of the joint programming initiative ERA-Net Smart Grids Plus, with support from the European Union's Horizon 2020 research and innovation programme.

**Conflicts of Interest:** The authors declare no conflict of interest. Furthermore, the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
