This paper proposes a new single-stage high-frequency link based single phase Solid State Transformer (SST) topology. Switching tube multiplexing is carried out to reduce the number of switching devices and the corresponding modulation strategy is given. An optimized hybrid control analysis for reducing the average conduction current of IGBT and inductor was performed. By switching different control modes, the average current value of the SST inductor is kept being the minimum, so as to reduce the conduction losses of IGBTs in the SST. As the core unit of the SST is the dual active bridge (DAB) converter, the proposed optimal control is based on extended phase shift control (EPS), instead of triple phase shift control (TPS), that makes the implementation method very simple. And the feasibility of topology and optimized hybrid control scheme is verified through experiments.

Three-phase four-leg voltage-source inverter has been extensively investigated in recent years for its compactness, small size, and high efficiency, and it has been proved to be the best solution for providing transformer-less neutral connection to three-phase asymmetric loads. The main purpose of this article is to apply 3-D unbalanced coordinate transformation to the four-leg inverter, which supplies stand-alone asymmetric loads, to maintain the balance of the output capacitor voltages, and obtain simpler control structure and better control performance. First, a double closed-loop PI control structure can be designed based on the Clarke and Park transformations. Second, the phases and amplitudes of the capacitor voltages are locked based on the 3-D unbalanced coordinate transformation. Furthermore, the Clarke transformation oriented on the inductor currents is replaced by a 3-D unbalanced coordinate transformation, and the corresponding unbalance indices are continuously adjusted according to the unbalance indices of the capacitor voltages, so as to recover the capacitor voltages to the balanced condition. The simulation and experimental results of the whole control system for the four-leg inverter are provided in the end, so as to verify the feasibility and effectiveness of the 3-D unbalanced coordinate transformation for application to the power electronic systems.

Cascaded H-bridge (CHB) converter has become an attractive topology for future large-scale photovoltaic (PV) plants in medium-voltage microgrids. However, the unequal irradiation and aging degree of PV arrays will lead to imbalanced and inconsistent output power between the three phases. This article presents a novel approach to integrating PV and energy storage (ES) systems inherent in microgrids, utilizing a hybrid CHB-based energy router (HCHB-ER), which is configured with a limited number of submodules to connect with the centralized ES. The proposed topology can handle serious interphase power imbalance and respond to grid-side power scheduling, without the appearance of overmodulation and common-mode voltage rise caused by traditional zero-sequence voltage injection methods. Under the three major constraints from the modulation level, the centralized energy storage maximum intrapower regulation boundary and submodule configuration ratio are analyzed in detail. Additionally, the power imbalance regulation capability of the proposed HCHB-ER across various configuration ratios is thoroughly evaluated and compared with the zero-sequence injection methods. Finally, the correctness and effectiveness of the proposed strategy are verified by simulations and experiments.

This article proposes a common-mode voltage (CMV) suppression strategy for the cascaded H-bridge (CHB) based motor drive. CMV can be categorized into two components: the low-frequency component associated with the motor frequency, and the high-frequency component related to the switching frequency. Low-frequency CMV primarily originates from the voltage fluctuations of submodules (SMs), and a topology method of CHB cascading four-port dual-active bridges (CHB-FDAB) is proposed. The CHB-FDAB, in conjunction with synchronous modulation of FDAB three full-bridges cascading with three-phase SMs, can effectively mitigate SM voltage fluctuations and prevent the occurrence of low-frequency CMV, which can also create a reduction for cost and volume of system. High-frequency CMV is generated by the modulation of CHB-stage, and a novel carrier phase-shift pulse-optimization PWM (CPSPO-PWM) strategy is proposed. The CPSPO-PWM can ensure that the difference between the output voltage sums of the left and right arms of the all three-phase SMs is zero, effectively suppressing the generation of high-frequency CMV. Additionally, CPSPO-PWM exhibits minimal increases in current ripple, torque ripple, and system power loss. As a result, the complete suppression of CMV throughout the entire speed range of motor can be achieved. Finally, simulation and experimental results validate the effectiveness of the proposed strategy.

This article proposes a control strategy for three-phase four-leg PWM rectifier under unbalanced grid voltage conditions, which aims at achieving not only no second harmonic in dc output voltage, but also unity power factor operation for each phase. First, the amplitude-phase-locked loop is designed based on three-dimensional unbalanced coordinate transformation to detect the phases of three-phase unbalanced grid voltages, which can be selected as the reference phases of three-phase currents to achieve unity power factor operation for each phase. The second harmonic in dc output voltage can be detected based on Fourier transform principle, and the linear control laws of current amplitude ratios can be designed to suppress second-harmonic ripple completely. The above control objectives for four-leg PWM rectifier can both be achieved with a small dc-link capacitor and without the introduction of an active power compensator, which helps simplify circuit structure and improve power density. The simulation and experimental results are provided in the end to verify the feasibility and effectiveness of the proposed control strategy.

Due to differences of solar irradiance, ambient temperatures, or inconsistent degradation of photovoltaic (PV) modules, the unbalanced output power between cascaded H-bridge (CHB) legs will lead to the unbalanced or even distorted grid currents between three phases. This article proposes a novel CHB-based PV grid-tied system integrating centralized energy storage (CHB-PV/ES), which can realize power balanced operation by utilizing the centralized energy storage (ES) interconnecting the CHB three-phase sub-modules (SMs) by isolated DC/DC converter. In addition, the centralized ES group can facilitate the regulation between grid-tied power references and PV power generation, such as stabilizing the power injected into the grid by smoothing the PV power, as well as responding to changes in grid-tied power references. The configuration and security domain of PV and centralized ES SMs are critical factors for CHB-PV/ES, which are thoroughly analyzed in this article to ensure safe system operation. The simulation and experimental results validate the effectiveness of the proposed CHB-PV/ES scheme.