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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.

Multiactive bridge converters (MAB) have become a widely-researched candidate for the integration of multiple renewable sources, storage, and loads for a variety of applications, from robust smart grids to more-electric aircraft. Connecting multiple dc ports reduces power conversion stress, improves efficiency, reduces material billing, and increases power density. However, the power flows between the ports of an MAB converter are magnetically coupled via the high-frequency (HF) transformer, making it difficult to control. This article presents an MAB converter configuration with a rigid voltage source on the magnetizing inductance of the transformer resulting in inherently decoupled power flows. As a result, the configuration allows independent power flow control tuning of the rest of the ports. The theory behind the power flow decoupling of the proposed MAB configuration is analyzed in detail using a reduced-order model. A 2-kW, 100-kHz Si-C-based four-port MAB converter laboratory prototype is built and tested, showing completely decoupled control loops with fast transient response regardless of their control bandwidths. The proposed configuration therefore makes the operation and design of the MAB family of converters much more feasible for any number of ports and precludes the need for a high-performance dynamic decoupling controller.

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.

This paper analyses the fault current characteristics of a flexible DC grid with a power flow controller (PFC) and proposes a coordinated control method for suppression of fault currents between the PFC and the DC circuit breaker (DCCB). The equivalent circuits of the PFC during different stages of the DC fault are analyzed. Moreover, the influence of the output voltage of the PFC on the fault current is studied. After that, the current suppression characteristics of the current limiting reactor (CLR) and PFC are investigated. Due to the output voltage of the PFC can be flexibly adjusted during the DC fault, the fault current can be suppressed significantly by adjusting the output voltage of the PFC when the DC fault is detected. This paper adopts a bang-bang control for PFC during the fault. Finally, a simulation model was built in Matlab/Simulink to verify the fault current characteristics of the DC grid containing the PFC, and the coordinated control of the PFC was verified.

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.

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.

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.

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.

In meshed low voltage DC distribution grids, one of the main challenges is achieving controllability of the power flow. The most common approach to achieve the full control over the power flow in the meshed DC grid is to deploy DC-DC converters rated for the full power rating of the grid. The drawbacks of this solution are costs and losses of such DC-DC converters. To reduce the costs and the losses from the system standpoint, we introduce a partially rated power flow control converter. This paper presents the control modes of the partially rated power flow control converter and experimentally demonstrates its functionality. The proposed power flow control converter consists of two cascaded converters. On one side the converter is rated for the maximum grid voltage but only a fraction of the grid current, and on the other side, it is rated for the maximum grid current but only a fraction of the grid voltage. The proposed converter can operate in three modes and demonstrates the power flow control capability.

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.

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.