*Discussion*

The experimental results show that the proposed decentralized plug-and-play protection scheme ensures selectivity for different dc grids under various conditions. Firstly, the systems were tested with varying impedance between the sources and the faults. In the experiments, short-circuits with low fault impedance generally trip the di/dt detection, while short-circuits with high fault impedance generally trip the overcurrent detection. Secondly, the experiments showed that the SSCBs are capable of interrupting non-inductive and inductive fault currents, dissipating the inductive energy in the MOV's when there is no alternative path for inductive currents. Thirdly, it is shown that the protection scheme achieves this for both radial and meshed systems.

The main advantages of the proposed protection scheme are that no communication infrastructure is required and that only minimal knowledge about the system is needed. Furthermore, because the SSCBs are designed for the worst-case scenario, the approach is not sensitive to uncertainties and disturbances in the system. Moreover, the protection scheme is resilient to failures, since upstream circuit breakers trip if downstream circuit breakers fail. Additionally, it is important to note that the RC dampers do not increase the steady state losses of the SSCBs, although they slightly increase losses during transient situations.

The main disadvantage of the proposed protection scheme is that the components in the system need to sustain twice the nominal current for up to *t*max. Furthermore, the worst-case time constant of the lines in the system need to be known, although this is not influenced by the length of the lines or their configuration. Moreover, care must be taken that the normal behavior of sources and loads, such as inrush currents, does not cause unnecessary tripping. However, the time-current characteristic can be adapted to avoid tripping on these events. Additionally, although solid-state circuit breakers solve a lot of challenges with regards to the protection of dc grids, they are still relatively inefficient and expensive compared to mechanical circuit breakers.

For the industrial application of the proposed plug-and-play SSCBs, different design considerations can be made. For example, the SSCB's current can be measured utilizing the voltage over the switches instead of using a high-bandwidth hall-sensor, or the di/dt measurement can be omitted in order to reduce complexity. Furthermore, semiconductor switches with lower cost or lower on-state resistance can be used, or multiple switches can be put in parallel or series. Overall, trade-offs can be made for the electrical performance of the SSCB versus its cost. However, detailed cost analysis and optimization of SSCBs were outside of the scope of this paper.

The proposed plug-and-play protection scheme provides a solid foundation for the protection of low voltage dc systems. In future research, the scheme's applicability to systems with longer distribution lines, such as medium and high voltage transmission systems, can be investigated. In those studies the impact of the propagation delay on the effectiveness of the approach must be examined. Furthermore, more cost and energy efficient solid-state circuit breaker topologies must be explored. Moreover, it can be researched how the SSCBs can provide additional functionality, such as controlling the power flow and black-starting dc grids. Additionally, a communication infrastructure can be used to improve performance or add functionality.
