As a basic building block in power systems, the three-phase voltage-source inverter (VSI) connects the distributed energy to the grid. For the inductor-capacitor-inductor (LCL)-filter three-phase VSI, according to different current sampling position and different reference frame, there mainly exist four control schemes. Different control schemes present different impedance characteristics in their corresponding determined frequency range. To analyze the existing resonance phenomena due to the variation of grid impedances, the sequence impedance models of LCL-type grid-connected three-phase inverters under different control schemes are presented using the harmonic linearization method. The impedance-based stability analysis approach is then applied to compare the relative stability issues due to the impedance differences at some frequencies and to choose the best control scheme and the better controller parameters regulating method for the LCL-type three-phase VSI. The simulation and experiments both validate the resonance analysis results.

This paper proposes a novel zero-voltage switching (ZVS) DC-DC converter to improve the performances of traditional phase-shifted full-bridge (PSFB) converter. In the primary side, two half-bridge converters (HBCs), leading-HBC and lagging-HBC, are placed in parallel and are driven in a phase-shifted control. The secondary side is a current doubler rectifier (CDR) circuit consisting of four rectified diodes and two output filter inductors. With this structure, the proposed converter can achieve wide ZVS range, minimized circulating current and low current ripple of output filter. Furthermore, the current stress of lagging-HBC is lower than that of leading-HBC, different current-rated switches can be employed for the two HBCs, which is benefit to optimize the ZVS conditions and cost. In this paper, the operational principle and performance of the proposed converter are analyzed in depth. Finally, experimental results from a prototype (320-400V input, 110V/10A output) are presented to validate the theoretical analysis.

The impedance-based method is an effective approach to investigate harmonic interaction problems in weak grids. Modeling and shaping the inverter output impedance are two important aspects of this method. However, for the inverter under finite-control-set model predictive control (FCS-MPC), the output impedance model is difficult to develop due to the nonlinear dynamic characteristics. To explore the FCS-MPC-based solution to harmonic interaction problems in weak grids, this article first derives the mathematical output impedance model of the inverter under FCS-MPC based on the described function method. The developed inverter output impedance model is highly robust to changes in operating conditions and parameters, and the proposed modeling method provides an effective way to describe the impedance characteristics of inverters under FCS-MPC. Then, based on the virtual MPC, a flexible output impedance shaping method for the inverter is proposed. The ability to improve system stability and mitigate harmonics is evaluated through impedance-based analysis, and a design method for the control parameters is presented. Finally, the theoretical model of inverter output impedance and the impedance shaping method developed in this article are verified through simulations and experiments.

Multilevel current reinjection (MLCR) converter has unique advantages over a pulsewidth modulation (PWM) converter in terms of capacity, efficiency, and harmonics, which leads to promising potential in the high voltage direct current transmission application. The imbalance of injection currents is a particularly critical problem that affects the input current harmonics and operation reliability of an MLCR converter. To cope with this problem, the state model of injection circuit is developed, and the decisive factor of its current-sharing performance is analyzed. Based on it, a current-sharing driving method for injection circuit is proposed by applying a phase-shift equal method to the basic switching function of injection switches. Simulation and experimental results show that the current-sharing performance of injection circuit is improved and the input current harmonics are reduced.

For conventional operation, the submodules of a static synchronous compensator based on a T-type converter with modular multilevel dc-link require high capacitance to suppress capacitor voltage ripple. Being different from that, an intriguing discovery reveals the capacitor voltage ripple is in phase with arm voltage in capacitive reactive power mode and opposite in inductive reactive power mode. As a result, a low-capacitance operation is proposed by exploiting this intriguing discovery. To better accommodate the proposed low-capacitance operation, this article presents a hierarchical control strategy that integrates a single PI controller, a model predictive control with a single objective, and an enhanced algorithm for sorting capacitor voltages. Finally, simulation and experimental results validate the effectiveness of the proposed low-capacitance operation and its hierarchical control strategy.

Voltage unbalance is a frequent problem in the power system, and using renewable energy power generation devices to achieve unbalance compensation is a promising solution compared to adding additional equipment. Most of the existing unbalance compensation schemes for inverters are implemented based on cascaded linear controllers, which have obvious disadvantages of complex structure and cumbersome parameter adjustment process. In order to develop a simpler unbalance compensation scheme, this paper first derives the equivalent sequence network (ESN) model of the unbalanced three‐phase power system, and it reveals the important role of transmission line impedance in the unbalance problem. Inspired by the ESN model, an unbalance compensation scheme based on negative sequence virtual impedance (NSVI) is determined. Then the NSVI is embedded in the inverter mathematical model, and the virtual model predictive control (VMPC) of the inverter to achieve unbalance compensation is proposed. Finally, the proposed VMPC method is validated by experiments, and a comparative analysis is carried out with linear control methods. The development process and validation results show that, compared with the unbalance compensation scheme based on cascaded linear controllers, the VMPC method proposed in this paper has a simpler structure and stable performance.