Residential dc microgrids and nanogrids are the emerging technology that is aimed to promote the transition to energy-efficient buildings and provide simple, highly flexible integration of renewables, storages, and loads. At the same time, the mass acceptance of dc buildings is slowed down by the relative immaturity of the dc technology, lack of standardization and general awareness about its potential. Additional efforts from multiple directions are necessary to promote this technology and increase its market attractiveness. In the near-term, it is highly likely that the dc buildings will be connected to the conventional ac distribution grid by a front-end ac-dc converter that provides all the necessary protection and desired functionality. At the same time, the corresponding requirements for this converter have not been yet consolidated. To address this, present paper focuses on various aspects of the integration of dc buildings and includes analysis of related standards, directives, operational and compatibility requirements as well as classification of voltage levels. In addition, power converter configurations and modulation methods are analyzed and compared. A classification of topologies that can provide the required functionality for the application is proposed. Finally, future trends and remaining challenges pointed out to motivate new contributions to this topic.

The dual-active bridge (DAB) topology is commonly preferred in bidirectional applications due to several attractive features, including auto-adjust of power flow, galvanic insulation, wide voltage gain, and zero voltage switching (ZVS) capability over some power ranges. However, the efficiency of the converter drops at light loads because the ZVS range is directly dependent on the circulating current. Assuming that the processed power is variable, the DAB converter's design must find a compromise between extending ZVS ranges and reducing the reactive power processing to ensure higher efficiency. Aiming this compromise, many papers propose hybrid approaches in which the modulation strategy is selected according to the processed power. However, this is not a simple solution because it demands multivariable control and offline optimizations. This paper proposes a modulation strategy to ensure ZVS and reduce the circulating current for the DAB converters. While the usual phase-shift modulation provides ZVS operation in a range of 40% to 100% of rated power, the proposed asymmetrical pulse-width modulation can obtain ZVS operation in a range of 2% to 100% of rated power. Experimental results demonstrated that the proposed strategy improves the converter efficiency for all power ranges, especially at light loads.

The global focus on renewables and energy efficiency makes dc distribution inside buildings an attractive concept, which allows easier integration of generation and loads. When connected to a conventional ac grid, such buildings require a bidirectional ac–dc converter for dc grid forming and energy exchange. Until recently, the works in this direction had a variety of realizations and were mostly not arranged in terms of voltage levels and converter requirements. Newly introduced and actively developing standards open the possibility to make the research of such converters more focused. For example, with a requirement of galvanic isolation, the solution with two stage conversion, where a nonisolated power factor correction rectifier is followed by an isolated dc–dc converter, can be considered as the default approach. In this case, the dual active bridge converter (DAB) for the dc–dc stage is one of the most common industrial solutions. At the same time, this converter has natural limitations in terms of soft-switching range, circulating current, and losses. This article address for the first time the design considerations for DAB topology operating as the dc grid-forming converter with droop control. By including the droop control characteristic in the converter's design, it is possible to simultaneously obtain a wide soft-switching range and low circulating current. The proposed analysis was used to design a 5-kW prototype, which was implemented and tested, showing a flat efficiency curve with a peak value of 97.8%.

Nanogrids for single consumers have gained importance based on the development of standards for prosumers’ electrical installations (PEIs) such as the IEC 60364 series. All distributed energy resources (DERs) are usually integrated into a single place using one or more dc buses to reduce power conversion losses in those systems. Also, an electrical energy management system (EEMS) is required for the nanogrids smart operation, where droop controls and state machines have been considered for power management. The drawback of these strategies is that the power management is poor when droop controls are considered at the second layer because all DERs must contribute to the dc bus voltage regulation. In addition, the state machines are limited due to their reduced flexibility. The main contribution of this article is a distributed price-based power management (d-PBPM) proposed to overcome these challenges. The d-PBPM is based on dc bus signaling and the price theory, and no droop control is required in the second layer, improving the management efficiency. Experimental results show that the d-PBPM makes the nanogrids’ EEMS scalable to new DERs and new dc buses, considering the addition of distributed sources, dc buses, and energy price to an existing nanogrid. The experimental setup includes a dual dc bus nanogrid with photovoltaic (PV) systems, energy storage, loads management, and ac grid interface.

Galvanically isolated ac-dc converters are crucial for integrating distribution generation, energy storage, dc microgrids, and electric vehicles with ac distribution system. The typical industrial approach involves a two-stage setup (ac-dc/dc-dc) with a bulky dc link capacitor, and dual-active bridge converter (dc-dc) for isolation. However, this leads to increased power losses due to multiple power processing stages. To overcome this issue, this paper proposes a three-phase bidirectional isolated ac-dc matrix-converter, as a candidate solution. This topology employs a high-frequency link, eliminating the need for a traditional intermediate dc link found in a standard two-stage solution. As a main contribution, a quasi-resonant modulation strategy is introduced to allow all semiconductors to operate under soft-switching for a wide operating range. A 3.5 kW prototype was built to compare proposed one with the two-stage ac-dc converter. The experimental results show that the proposed converter achieved 96.5% efficiency at nominal power, outperforming the conventional converter, which reached 95.4% efficiency, mainly due to the extra power processing stage in the two-stage solution.

This paper proposes a dispatchable photovoltaic (PV) hybrid inverter for output power tracking without any dependency on the converter’s efficiency and with no power closed loop. The system uses an extra-low-voltage battery energy storage system (BEES) based on a Li-ion battery pack to be applicable for use inside homes and other installations close to the end-user. A bidirectional isolated current-fed dual-active bridge (CF-DAB) converter associated with the batteries provides a wide conversion voltage ratio and ensures safety for the users. The proposed control system shares the DC bus voltage controller between the ac grid interfacing converter (AC-DC) and CF-DAB (DC-DC), eliminating the converter’s efficiency in the reference equations. When dispatchable power is not required, or according to the user’s request, the battery’s charge/discharge current can be specified. A disturbance rejection technique avoids low-frequency current ripple on the battery side. It contributes to the battery’s lifespan. Experimental results presenting the dc bus voltage control, current disturbance rejection, and power dispatching are included to validate the proposal.