Recently, Finite Control Set Model Predictive Control (FCS-MPC) has been successfully applied in the grid-tied inverter with LCL filter. However, to achieve active damping and grid synchronization, many sensors are required, increasing cost and complexity. In addition, a considerable computational delay should be addressed when it is experimentally implemented, which may degrade the performance of overall system. In order to reduce the number of sensors, eliminate the computational delay, and enhance the control reliability of system, a novel FCS-MPC strategy with merely grid-injected current sensors is proposed, which contains four compositions: virtual flux observer, state observer, delay compensation and FCS-MPC algorithm based on estimations. A 3-kW/3-phase/110V experimental platform is established to validate that utilizing the proposed observations-based control method with only grid-injected current sensors is capable to obtain satisfactory performance of grid synchronization and high quality grid-injected current both under balanced and unbalanced grid voltage condition.

Nowadays, various Sliding Mode Control (SMC) methods have been successfully applied to digitally controlled Grid-Connected Inverter (GCI) with LCL filter. However, how to design of the Pulse Width Modulation based SMC (PWM-SMC) controller need be further explored, especially upon the large variation of parameters drift and the delay issue. In this paper, the essence of two classic SMC methods used in power converter area is first analyzed in detail. Thus, a novel design of PWM-SMC controller using linearized model for three-phase GCI with LCL filter is proposed. Based on this, a three-loop step-by-step design of the PWM-SMC controller is developed, by using the closed-loop pole locations. Robust analysis against parameters drift is also studied. In addition, a discrete state observer is adopted to reduce the number of sensors. Further, a discussion between the proposed control strategy with the existing SMC methods and the full-state feedback controller is carried out. Finally, a 3 kW lab device designed on the dSPACE controller is constructed to verify the feasibility of the proposed strategy and the correctness of theoretical analysis.

A single-phase transformer-less unified power quality conditioner (TL-UPQC) is presented. Apart from having no isolation transformer, the proposed structure utilizes four switching devices only, forming two half-bridge voltage-source inverters-one connected in parallel with the load and another one connected in series with the ac mains. The two inverters share the same dc link. The parallel inverter, which is controlled by a hysteresis current controller, is used to shape the current drawn from the ac mains and regulate the dc-link voltage. The series inverter, which is controlled by a boundary controller with second-order switching surface, is used to regulate the steady-state load voltage and provide voltage sag/swell ride-through. A dc-link capacitor voltage balancing control that coordinates the operations of the hysteresis and boundary controllers is designed. Modeling, design, and analysis of the whole system will be given. A 1 kVA, 110 V, 60 Hz prototype has been built and evaluated on a setup with a nonlinear load. The steady-state and transient responses under a voltage sag will be given. Experimental results are favorably compared with the theoretical predictions and the performance of other UPQCs.

An active grid impedance cancellator using the concept of series active filter to suppress the effect of the grid disturbance and stabilize the single-phase grid-connected inverters with an LCL filter operating under variable grid conditions is presented. Harmonic interaction between the inverter and the grid is thereby avoided owing to the cancellation of equivalent grid impedance. More importantly, the impedance cancellator offers an active damping function to ease the heavy burden of the inverter control, such as power control, phase locked loop, current regulation, impedance shaping, etc. The impedance cancellator is a full-bridge dc-ac converter having no passive inductive-capacitive (LC) filter. It is connected in series with the inverter output and is operated as a negative virtual grid impedance. As the VA rating of the impedance cancellator is low, the efficiency of the entire system is not sacrificed. Starting with the impedance-based analytical modeling method, the basic principle of equivalent grid impedance cancellator is derived and studied. Then, the digital control strategy and the modeling of equivalent grid impedance cancellator are examined. The experimental results of a prototype cancellator for a single-phase inverter is favorably compared with theoretical predictions.

Owing to the strong robustness against system parameter changes and external perturbations, the passivity-based control (PBC) has been widely adopted in grid-tied inverter (GTI). However, for a PBC-based GTI with LCL-filter, there are three damping gains and two interactively coupled-feedforward terms in the control loop, resulting in the controller design challenge for engineers. In order to help electrical engineers to well design the PBC controller for LCL-filtered GTI, a new design of the damping gains is proposed, by limiting the inherent steady-state error of grid-injected current. Furthermore, the state observer is also adopted to reduce the number of sensors. The robustness against the parameters shift and wide grid impedance variation is also addressed. The effectiveness of the proposed control design strategy will be verified through experimental results on a 3 kW/3-phase/110 V experimental lab setup.

The instability issues of the three-phase grid-tied inverters are major concerns in the distributed generated systems. However due to the coupling effects in dq frame, the instability mechanisms of the three-phase inverters have not yet been clearly revealed. This paper presents a divided DQ small-signal model of the grid-tied inverter in dq frame. The specific expressions of small-signal output admittance of the inverter are investigated based on the proposed model, which can reveal the influencing mechanism of coupling effects and PLL on the stability of the inverter. Then a stability analysis method based the divided DQ small-signal model is proposed, by which the procedure of checking the stability of the grid-tied inverters with strong/weak grid is executed step by step, and it could be helpful for the system parameters design. Furthermore, the stability the multi-paralleled inverters system is also studied, and it indicates that as the number of the paralleled inverters increases, the parameters of PLL could be adjusted for stable operation. The theoretical stability analysis results are verified by simulations and experiments.