Solid-state circuit breakers (SSCB) show great promise to become the key element in the protection of low-voltage direct current microgrids. SSCBs operate in the microsecond range and employ semi-conductor devices that have strict safe operation area limits. Therefore, the design of the SSCB needs to consider the effects of fault detection delays and semi-conductor safe operation area limitations. This paper derives SSCB design criteria that consider the effect of different detection methods with different detection delays under varying system constraints. The design space is investigated in a sensitivity analysis, which provides insights into the operation boundaries of SSCB and explains how a combination of fault detection methods can reduce the SSCB size. The insights from the theoretical and sensitivity analysis are used to propose an SSCB design flowchart. SSCB prototype is developed and tested in different scenarios under nominal grid voltage and current. The derived design constraints can be used for efficient SSCB design and also to evaluate the effects of different protection schemes on the required SSCB size.

This paper presents a protection framework for low voltage dc grids, which segments these grids into zones and tiers according to their fault current potential and provided protection. Furthermore, the technology and applications of different protection devices are examined. It is demonstrated that the utilization of fast fault interruption and fault limiting inductors are vital for the protection of low voltage dc grids. Moreover, a design of a solid-state circuit breaker is presented, and this devices is experimentally verified. The experimental results showed that the total time for the detection and interruption of faults can be lower than $1 \mu \mathrm{s}$ with solid-state protection devices.

Since the voltages and currents in dc grids do not have a natural zero-crossing, the protection of these grids is more challenging than the protection of conventional ac grids. Literature presents several unit and non-unit protection schemes that rely on communication, or knowledge about the system's topology and parameters in order to achieve selective protection in these grids. However, communication complicates fast fault detection and interruption, and a system's parameters are subject to uncertainty and change. This paper demonstrates that, in low voltage dc grids, faults propagate fast through the grid and interrupted inductive currents commutate to non-faulted sections of the grid, which both can cause circuit breakers in non-faulted sections to trip. A decentralized plug-and-play protection scheme is proposed that ensures selectivity via an augmented solid-state circuit breaker topology and by utilizing the proposed time-current characteristic. It is experimentally shown that the proposed scheme provides secure and selective fault interruption for radial and meshed low voltage dc grids under various conditions.

This paper proposes a power flows decoupling controller for the triple active bridge converter. The controller is based on a full-order continuous-time model of the TAB converter derived using the generalized average modelling (GAM) technique. GAM uses the Fourier series expansion to decompose the state-space variables into two components, which represent the active power and the reactive power. The controller uses the active power components of the transformer currents to decouple the active power flows between converter ports. Additionally, the implementation of the decoupling controller in the digital domain is detailed in the paper. The decoupling performance of the proposed controller is validated in a hardware experiment.

Triple active bridge (TAB) as an isolated multiport converter is a promising integrated energy system for smart grids or electric vehicles. This article aims to derive and analyze zero voltage switching (ZVS) regions of TAB, in which both switching losses are reduced, and electromagnetic interference issues are mitigated. In the proposed closed-form solution of ZVS criteria, parameters such as the parasitic capacitance of the switches, the leakage inductance of the transformer, the switching frequency, the port voltage, the phase-shift inside and between the full-bridges are all taken into account. The analysis shows how the five degrees of freedom can be used to maintain ZVS operation in various operating points. The analysis and derived closed-form ZVS criteria are experimentally verified using a laboratory prototype. The derived analytical ZVS criteria are a powerful tool to study and optimize the operation of TAB converters.

For a bipolar meshed DC distribution grid, the number of DC transmission lines is greater than converter stations. Therefore, the power flow control (PFC) needs to be introduced to increase the control flexibility of the DC grid. Besides, the unbalanced power between the positive and negative poles in bipolar DC girds may result in a large unbalanced current in the neutral line, causing the pole voltages to deviate from the rated value and increasing the power losses in the bipolar network. This paper aims to realize the PFC of DC distribution grids while suppressing the unbalanced voltage/power of the positive and negative poles. The application of a series parallel-connected PFC in bipolar DC distribution systems is investigated. Subsequently, the relationship between the output voltage of the PFC and the unbalanced power of the bipolar DC distribution grids is derived under the two different control modes of PFC. The simulation model is built in MATLAB/Simulink. The results show that when constant voltage control is applied, the PFC can suppress the unbalanced current in the transmission line, and it can also realize flexible control of the DC power flow.

With the rising popularity of the power electronic based systems with integrated energy storage, the multi-port isolated converter topologies are gaining popularity. In contemporary literature, the triple active bridge (TAB) converter is the most popular among these topologies. However, the TAB was not yet described with the continuous-time full-order model. In this paper, the continuous-time full-order model of the TAB converter is derived. The derived model is validated with the measurement of the control-to-output transfer functions. The derived model can provide useful insights into the operation of the converter and can be used for controller design.

Scalable and robust low-voltage direct current (LVdc) distribution networks require solutions, allowing flexible power flow control and reliable short-circuit protection. In this paper, the continuous full-order large-and small-signal models of a partially rated power flow control converter (PFCC) are derived utilizing the generalized averaging method. The large-signal model of the PFCC is coupled with a model of the LVdc grid. Due to the state-space representation, the combined model of the PFCC and the LVdc grid is suitable for easy algorithmization, and efficient simulation. These advantages make them essential tools for studying and optimizing of scalable LVdc systems with decentralized power flow control based on the PFCC. The PFCC models provide insights into controller design and stability analysis. The models are experimentally validated, and the functionality of the PFCC is demonstrated in a laboratory-scale microgrid.

Flexible power flow control is one of the main challenges in the development of the meshed low voltage direct current distribution system. The most widely adopted approach to achieve flexible power control in the network is to use various solid-state based solutions which are rated for the peak power of the grid. In this paper, we propose a solution based on a converter which has several times smaller power rating than the grid power rating. Due to the multi-port nature of the proposed solution, it is well suited for the bipolar networks.

The two main challenges of meshed low voltage DC grids today are the flexible control of power flow and the short-circuit protection. The conventional approach to deal with both problems is to incorporate galvanically isolated DC-DC converters with integrated short-circuit protection, which are rated for the full power rating of the grid. In this paper, we describe an approach based on the combination of a converter which has a partial power rating with respect to the grids power rating and a circuit breaker with full rating with respect to the grid. Based on the review of abnormal operating conditions of the power flow control converter and its requirements on protection. We propose a new protection strategy for the power flow control converter and evaluate the applicable circuit breaker technologies based on the review of the protection requirements.

Inside the meshed LVDC distribution grids the power flow is predominantly limited by the line impedances. In order to achieve economical and flexible operation of the meshed LVDC distribution grids, it is required to control the power flow. For that end, the line impedances need to be adjustable. The line impedance can be controlled by a DC-DC transformer, however, it needs to be rated for the full grid power. In order to reduce the installation costs, a partially rated device is desirable. Therefore, a partially rated power flow control converter (PFCC) is proposed. This paper presents the PFCC performance estimate and demonstrates the PFCC functionality. The PFCC is an economical solution for increasing the controllability of the power flow in the meshed LVDC distribution grids. However, due to high step down ratio inside the PFCC achieving efficient performance is challenging. Furthermore, due to unavoidable MOSFET paralleling the part count rises. The proposed PFCC consists of two cascaded converters, therefore the control range and stability depends, besides else, on the impedance interaction between the two stages.

Increased proliferation of the renewable energy sources (RES) brings more power electronic devices to the power distribution. Modularity on the converter level is one of the key concepts enabling flexibility, scalability and high availability of the new solutions for the distribution networks(e.g. for low voltage DC). To understand the trends and performance trade-offs it is important to describe and classify different aspects of the modularity concept. This paper presents a comprehensive literature review, classification of modular power electronics and outlook on future research. Different aspects of modularity are identified and described from the perspective of a converter designer, manufacturer and a user. It is illustrated that depending on a combination of different aspects, different modules are selected resulting in different technical challenges. These aspect can be considered as a mapping tool for the functional description and physical realization of the power converter.