Conventional dual-active-bridge (DAB) dc-dc converters have a limited structure to balance the bipolar voltage level of the bipolar dc distribution system. Therefore, to regulate the bipolar voltage level, additional voltage balancers are needed for the DAB converters, which eventually requires more devices resulting in low power conversion efficiency and low power density. This paper proposes an enhanced DAB dc-dc converter which can balance the bipolar voltage level without the additional voltage balancers. The bipolar voltage level can be tightly regulated under entire unbalanced load condition and even when there is no-load at one side of the dc poles. Besides, its zero voltage switching (ZVS) capability can be extended to cover the entire load range, including the no-load condition. Furthermore, the proposed converter can be operated by using a simple single phase-shift modulation. Therefore, its control algorithm can be easily implemented in digital controllers. Finally, the theoretical analysis and the effectiveness of the proposed DAB converter are validated by a 3-kW prototype converter.

Low-voltage direct current (LVDC) distribution has attracted attention due to increased DC loads, the popularization of electric vehicles, energy storage systems (ESS), and renewable energy sources such as photovoltaic (PV). This paper studies a ±750 V bipolar DC distribution system and applies a 3-level neutral-point clamped (NPC) AC/DC converter for LVDC distribution. However, the 3-level NPC converter is fundamental in the neutral-point (NP) imbalance problem. This paper discusses the NP balance control method using zero-sequence voltage among various solutions to solve NP imbalance. However, since the zero-sequence voltage for NP balance control is limited, the NP voltage cannot be controlled to be balanced when extreme load differences occur. To maintain microgrid stability with bipolar LVDC distribution, it is necessary to control the NP voltage balance, even in an imbalance of extreme load. In addition, due to the bipolar LVDC distribution, the pole where a short-circuit condition occurs limits the short current until the circuit breaker operates, and a pole without a short-circuit condition must supply a stable voltage. Since the conventional 3-level NPC AC/DC converter alone cannot satisfy both functions, an additional DC/DC converter is proposed, analyzed, and verified. This paper is about a 3-level NPC AC/DC converter system for LVDC distribution. It can be used for the imbalance and short-circuit condition in bipolar LVDC distribution through the prototype of the 300 kW 3-level NPC AC/DC converter system and experimented and verified in various conditions.

This paper presents the implementation of a single-phase solid-state transformer (SST) for the interface between a 13.2 kV medium voltage alternative current (MVAC) network and a 750 V bipolar DC distribution. The SST has ten cascaded subunits in consideration of the device rating and modulation index (MI). Each subunit consists of an AC/DC stage and a DC/DC stage with a high frequency isolated transformer (HFIT). The AC/DC stage consists of cascaded H-bridges (CHBs) to cope with the MVAC. The DC/DC stage employs a triple active bridge (TAB) converter for bipolar DC distribution. Topology analysis and controller design for this specific structure are discussed. In addition, the insulation of HFIT used in DC/DC converters is also discussed. A simple balancing controller at the AC/DC stage and a current sharing controller at the DC/DC stage are used to prevent DC-link voltage unbalance caused by the cascaded structure. The discussions are validated using a 150 kW single-phase 21-level SST prototype at the laboratory level.

This paper proposes a DC-bus signaling (DBS) method for autonomous power management in a DC microgrid, used to improve its reliability. Centralized power management systems require communication between the power sources and loads. However, the DBS method operates based on the common DC-bus voltage and does not require communication. Based on the DC-bus voltage band, the DC-bus voltage can be used to inform the status of the DC-bus in various scenarios. The DC microgrid operates independently to maintain the system stably in the DC-bus voltage band. The DC microgrid can be divided into a grid-connected mode and an islanding mode. This paper proposes a control strategy based on power management of various independent components in islanding mode. In addition, the autonomous control method for switching the converter’s operation between grid-connected mode and islanding mode is proposed. A DC microgrid test bed consisting of a grid-connected AC/DC converter, a bidirectional DC/DC converter, a renewable energy simulator, DC home appliances and a DC-bus protector is used to test the proposed control strategy. The proposed autonomous control strategy is experimentally verified using the DC microgrid test bed.

This paper presents a 13.2 kV class 3-phase solid-state transformer (SST) based on EtherCAT communication. In general, when the structure of the unit module is determined, the number of high-frequency isolated transformers (HFIT) is also proportional to the number of modules. The structure most considered in SST is a 1:1 combination of AC/DC converter and DC/DC converter. To optimally implement a 3-phase SST, a topology for reducing passive elements such as switching elements and HFIT is proposed. It also describes the design of HFIT used in DC/DC converter. EtherCAT communication with high transmission speed and expandability is applied to control the SST composed of unit modules stably, and a multi-core microcontroller unit (MCU) is applied to achieve both a high-speed communication cycle and complicated control algorithm execution. The discussions are validated using a 300 kW 13.2 kV class 3-phase SST prototype in various conditions.

One of the main disadvantages of the multi-level solid-state transformer (SST) system is the voltage imbalance on the output of the rectifier modules. This voltage imbalance can be caused by parameter mismatch of the active and passive components, different loads, and the floating structure of the high voltage DC-links. Some studies have been done to solve this voltage imbalance problem. A common way to avoid this imbalance is to balance the voltage of DC-links at the AC/DC conversion stage and balance the power between the modules at the DC/DC conversion stage. Most of these methods require a complex balancing controller or additional circuits. This paper proposes a novel dual active bridge (DAB) converter specialized in power balancing in a single-phase 5-level SST system. The proposed DAB converter does not require any additional balancing controllers or techniques for power balancing. The performance of the proposed DAB converter was verified by simulation and experiments using a 3 kW 5-level SST prototype system.