Smart Battery Chargers (SBCs) implementing grid-forming (GFM) control strategies are a promising solution to provide voltage and frequency support, increasing the grid reliability. Typically, GFM studies consider inverters with high DC-link capacitance. Therefore, there is a research gap in two-stage DC-AC converters with small DC-link capacitance implementing GFM strategies. This article proposes a novel approach to implement a GFM control strategy along with active power decoupling (APD) in an isolated, single-phase, electrolytic-capacitor-free two-stage DC-AC structure. The structure is composed of a voltage source inverter (VSI), DC-linked by film capacitors, with a dual-active-bridge series-resonant (DABSR) DC-DC converter. High DC-link ripple is allowed and managed by the APD. Hence, electrolytic capacitors are avoided, increasing the converter lifetime. In the proposed approach, the VSI implements the GFM strategy, operating in the four quadrants of the active and reactive power plane. However, the DABSR allows galvanic isolation, average DC-link voltage control, and suppression of the low-frequency ripple on the battery current, minimizing the impact in the battery lifetime. Design criteria are given for the DC-link voltage controller, active-reactive power controllers, inner inverter controllers, and APD technique. The control strategy is validated for vehicle-to-grid and stand-alone vehicle-to-home applications, using hardware-in-the-loop for a 2kW test setup.

An isolated single stage bidirectional AC-DC converter with a series-connected buffer to power decoupling is proposed to interface battery with the single phase grid. None electrolytic capacitor is used in the proposed structure. The converter is a modified AC-DC DAB series-resonant single phase converter where a third active bridge is used to power decoupling and it is series-connected with the rectified grid voltage active bridge. A very practical bidirectional power decoupling technique which controls the voltage amplitude in the series-connected buffer (SCB) is proposed. Unlike existing structures, two independent time-variant duty ratios is applied on both AC active bridges (rectified grid and the series-connected buffer) and fixed duty ratio on DC side (battery). The proposed modulation allows the power decoupling of the low frequency AC grid voltage variations in the series-resonant circuit. As a result, the steady-state response of the proposed AC-DC converter with SCB is equivalent of a DC-DC DAB with a series-resonant circuit. Decoupled close loop control for the grid current and for the SCB voltage are implemented. Furthermore, reactive power in the grid is compensated in the tank circuit with fixed duty ratio on DC side. Bidirectional power flow is controlled with phase shift modulation. High efficiency is obtained due to Zero Voltage Switching (ZVS) in all MOSFETs. Finally, the 1kW prototype is validated experimentally.

The massive integration of renewable energies into the grid using fast-response converters without inertia generates issues such as inertia reduction, temporary voltage violations, and power quality reduction. The system inertia reduction is a critical problem that could lead to grid frequency exceeding the acceptable range, resulting in undesirable load-shedding or even large-scale blackouts. To overcome these issues, the use of electric vehicle bidirectional chargers (EVBCs) implementing functionalities such as distributed virtual inertia (VI), long-term frequency support, voltage support by reactive power, and harmonics compensation, has been proposed as a possible solution. This article proposes a novel control strategy to manage a hybrid energy storage system (HESS) composed of dc-link capacitors and battery, through an isolated two-stage ac–dc converter (composed of a dual active bridge resonant type dc–dc converter cascaded to a voltage source inverter), intended for off-board EVBCs. The HESS management allows decoupling of the active power dynamic response since dc-link capacitors supply the fast dynamic response for VI support whereas the battery supplies the slower dynamic response for long-term frequency support, respectively. Hence, the VI support does not affect the battery lifetime. Simulations and experimental results are presented for a 2.5 kW prototype to validate VI, frequency-voltage support along with harmonics compensation.

A novel three-port, isolated, and single-stage DC/AC three-phase converter is proposed. This converter aims at interfacing a renewable energy source, a battery pack, and the three-phase electric grid. The battery and grid ports have bidirectional power flow capability. It is comprised of three active bridges, a HF AC-link three-winding transformer, and a series-resonant LC circuit in the AC-port. In contrast to the conventional three-port series-resonant converter, this structure only includes one LC tank, which makes it more reliable and less complex. The proposed modulation consists in phase shift control under a high and fixed switching frequency. Duty ratio depending of the grid phase is introduced on the AC-side. An extra phase angle is added to control real and reactive power transfer. This topology can be considered as the superposition of a DAB DC/DC and a series-resonant DC/AC three-phase converter. As a result, the currents flowing through the transformer's windings are quasi-sinusoidal and trapezoidal waves, which permits low rms and peak current magnitudes. Steady-state analysis, design procedure, and simulation results are presented for a 1kW converter.

A novel modulation strategy and close loop control is proposed for an isolated, highly efficient bidirectional single stage DC/AC converter dedicated to connection of batteries to single phase grid. Modulation, design and close loop control are discussed to 500W converter prototype. The converter is composed of two active bridges, a series resonant circuit, an HF transformer and a synchronous rectifier. The novel modulation strategy keep phase shift modulation which allows a bidirectional power flow between the battery and the grid but, an additional duty ratio control in both actives bridges is proposed. With this modulation the converter can works with any AC grid amplitude and frequency without any change in the converter structure. Controller type PR+PD and DQ are analyzed. Bidirectional power flow has been validated with power factor closest to unity. High efficiency of 96% has been obtained owing to Zero Voltage Switching (ZVS) in all MOSFETs. Finally, the modulation and the controller have been validated experimentally for different cases.

Single-stage isolated and bidirectional (SSIB) AC–DC converters have a high potential for future solid-state transformers and smart battery chargers due to their reduced volume and high efficiency. However, there is a research gap for SSIB reactive power injection. This article introduces an SSIB three-phase AC–DC converter composed of three low frequency rectifiers linked by tiny film capacitors with a quad-active-bridge series-resonant (QABSR) DC–DC. A novel QAB modulation is proposed to solve three issues: (1) Three DC inputs with high ripple compensation, (2) active–reactive power injection, and (3) minimization of high-frequency (HF) transformers currents. The rectified grid voltages were modulated by time-variant duty ratio (DR) angles. In contrast, the DC source was modulated by a fixed DR (FDR) angle along with a phase-shift angle which changes according to the grid current amplitude. A constant HF current amplitude with minimum value was obtained. It is shown that the HF current amplitude is increased for reactive power injection. Hence, the FDR angle was used to compensate for this increase. Active and reactive power control were validated in a 2 kW prototype. Compared with other structures, tiny DC-link capacitors and smaller L filters were used. Moreover, higher efficiency (96%) and smaller grid currents THDi (3%) were obtained.