Robust Dichotomy Solution-Based Model Predictive Control for the Grid-Connected Inverters with Disturbance Observer

This paper proposes a robust dichotomy-based model predictive control(DS-MPC)with a fixed switching frequency for the grid-connected inverter(GCI).The proposed fast dichotomy algorithm can select and deduce the optimal voltage vector dynamically through the space vector plane.Therefore,the proposed DS-MPC strategy could ensure dynamic performance and steady-state performance as well.Also,the current control robustness can be improved through DS-MPC with disturbance observer(DO)based on the extended Kalman filter(EKF).The novelty of this control is that the current control with fast dynamic response can be realized in the weak grid,even if the grid voltages are greatly distorted.Simulation and hardware experiments on the weak grid validate the effectiveness of the proposed DS-MPC with the EKF observer approach.

Grid-connected inverter for wind power generation system

In wind power generation system the grid-connected inverter is an important section for energy conversion and transmission, of which the performance has a direct influence on the entire wind power generation system. The mathematical model of the grid-connected inverter is deduced firstly. Then, the space vector pulse width modulation （SVPWM） is analyzed. The power factor can be controlled close to unity, leading or lagging, which is realized based on H-type current controller and grid voltage vector-oriented control. The control strategy is verified by the simulation and experimental results with a good sinusoidal current, a small harmonic component and a fast dynamic response.

Coordination of multiple grid-connected inverters for harmonic compensation

For multiple grid-connected inverters with active filter function,it makes sense to regulate every unit to output maximum active power from photovoltaic arrays,as well as eliminate the harmonic due to the non-linear loads connected to the electric networks.Naturally,a centralized control coordination strategy was proposed for the purpose of high facility utilization,good harmonic compensation ability and unwanted overcompensation condition.Based on a vector decoupling control scheme and generalized instantaneous reactive power theory,the solution was to allocate the harmonic eliminating task for every inverter according to the instantaneous power margin of each.The grid current always keeps sinusoidal in spite of non-linear load change and output active power change for any inverter.The simulation results validate the efficacy of the proposed coordination strategy.

Topology for current-source grid-connected inverter with improved effciency and pulse width modulation control strategy

A novel topology of the current-source grid-connected inverter is proposed based on the immittance converter theory. A control strategy of sine-sine pulse width modulation （PWM） is studied. Compared with the traditional current-source inverter, the power frequency inductors and power frequency transformer are replaced with high frequency inductors and a high frequency transformer. Thus, the proposed inverter has advantages of small volume, low cost, low total harmonic distortion （THD）, low power losses, high power factor （PF） and simple control. Furthermore, grid voltage cannot influence output current of the grid-connected inverter and the current-source inverter with a high PF that approaches one has been realized. Finally, validity of the theory analysis and feasibility of the control scheme are shown by simulation and experimental results.

An Improved Repetitive Control Strategy for LCL Grid-Connected Inverter

This paper proposes a cascade repetitive control strategy based on odd internal mode,and combines it with proportional-integral(PI)control to establish a compound repetitive control system for improving the quality of grid connected current of LCL grid connected inverter.More specifically,the proposed method could effectively improve the control effect of grid-connected current of LCL inverter,restrain current harmonics and reduce the distortion rate of grid-connected current.Simulation experiment is conducted to verify the proposed repetitive control strategy,and the verification results show that,compared with traditional PI control,the proposed improved compound repetitive control strategy has a higher response speed,and the steady-state and dynamic performance have also been significantly improved.

Dual-loop Control Strategy for Grid-connected Inverter with LCL Filter

As to the concrete topology of three-phase LCL type grid-connected inverter with damping resistance, mathematical model was deduced in detail, using method of equivalent transformation to the structure diagram, damping resistance was virtualized, mathematical model under the DQ frame that can realize decoupling control was established, a dual-loop control strategy for grid-connected inverter with LCL filter was proposed, the system stability was analyzed and the design method of controller was given. The proposed method overcame the flaws of loss increase, efficiency reduce and cost increase which were caused by damping resistance in LCL type grid-connected inverter, the system efficiency and power supply quality of the output were improved. Feasibility and effectiveness of the new method were validated by simulation and experimental results.

Quasi-Z Source Inverter Control of PV Grid-Connected Based on Fuzzy PCI

The photovoltaic grid-connected inverter is an important interface between the photovoltaic power generation system and power grid.Its high-quality operation is directly related to the output power quality of the power grid.In order to further optimize the control effect of the quasi-Z source grid-connected photovoltaic inverter,a fuzzy proportional complex integral control(PCI)method is proposed for the current internal loop control.This method can eliminate the steady-state error,and has the characteristic of zero steady-state error adjustment for the AC disturbance signal of a specific frequency.The inductance-capacitance-inductance(LCL)filter is adopted in the grid-connected circuit,and the feedback capacitive current is taken as the control variable of the inner loop to form the active damping control method,which can not only effectively suppress the resonance of the LCL circuit,but also significantly inhibit the high-order harmonics in the grid-connected current.Finally,a system simulation model is built in MATLAB/Simulink to verify the superiority and effectiveness of the proposed method.

FCS-MPC Strategy for PV Grid-Connected Inverter Based on MLD Model

In the process of grid-connected photovoltaic power generation,there are high requirements for the quality of the power that the inverter breaks into the grid.In this work,to improve the power quality of the grid-connected inverter into the grid,and the output of the system can meet the grid-connected requirements more quickly and accurately,we exhibit an approach toward establishing a mixed logical dynamical(MLD)model where logic variables were introduced to switch dynamics of the single-phase photovoltaic inverters.Besides,based on the model,our recent efforts in studying the finite control set model predictive control(FCS-MPC)and devising the output current full state observer are exciting for several advantages,including effectively avoiding the problem of the mixed-integer quadratic programming(MIQP),lowering the THD value of the output current of the inverter circuit,improving the quality of the power that the inverter breaks into the grid,and realizing the current output and the grid voltage same frequency and phase to meet grid connection requirements.Finally,the effectiveness of the mentioned methods is verified by MATLAB/Simulink simulation.

The improved PAC method for a three-phase PWM grid-connected inverter

In this paper, a vector regulating principle of the phase and amplitude control PAC method for three-phase grid-connected inverters is presented.To solve the problem of heavy inrush current and slow dynamic response when system starts up, the starting voltage prediction control and the current feed-forward control are proposed and used, which improve the dynamic performance of the system in the PAC.The experimental results carried out on a three-phase grid-connected inverter proved the validity of the proposed method.

A Simulation Research on the Grid-Connected Control Technology of Single-Phase Inverters Based on MATLAB

This paper primarily discusses the main circuit of single-phase inverter circuits.It begins by introducing the research context and the significance of the subject,then discusses the topology of grid-connected single-phase inverter circuits,continues by discussing the control strategy for grid-connected single-phase inverter circuits,realizes a sinusoidal pulse width modulation(SPWM)signal generation circuit and an inverse control algorithm program,and finally ensures good output waveform and fast dynamic response.In view of the hysteresis feature of the grid voltage’s synchronous signal sampling circuit,the acquisition function in digital signal processing(DSP)control chips is applied,and the reasons for the hysteresis phenomenon are thoroughly investigated.The reliability of the SPWM control algorithm is revealed through the results.

Control strategy of LCL-type three-phase photovoltaic grid connected inverter

The grid-connected inverter with LCL filter has the ability of easily attenuating high-frequency current harmonics. However, its suppression effect on the background harmonics in grid voltage is limited. A control strategy is presented, which is composed of an inner loop of capacitor current feedforward, an outer loop of grid-current feedforward and feedforward of grid voltage. The limitations and steps of parameters design for LCL filter are analyzed. Meanwhile, the capacitor current loop is employed to damp the resonant peak caused by the LCL filter and enhance the stability. The properties of different controllers are analyzed and compared, thereinto quasi-proportional-rasonant (PR) controller realizes the control with zero steady-state error of AC variables in static coordinates. In order to suppress the current distortion effected by the background harmonics in grid voltage, the feed-forward function is calculated for the grid-connected inverter with an LCL filter. After simplifying the block diagram, a full-feedforward control strategy for grid voltage is proposed. Theoretical analysis and Matlab/Simulink simulation results show that the proposed method has the advantages of high steady accuracy, fast dynamic response and strong robustness.

High Efficiency PCS with Scan-Type MPPT Control for a Grid-Connected PV Power Generation System

For a standalone PV （photovoltaic） power generation system, the author previously proposed a new MPPT （maximum power point tracking） control method in which the I-V characteristics are scanned with a detection interval control that operates at specified intervals and monitors the maximum power point. The author has obtained satisfactory results using this new MPPT control method. This paper investigates the application of the new MPPT control method for a PCS （power conditioning system） in a grid-connected type PV power generation system. The experimental results clearly demonstrate that the developed PCS offers outstanding effectiveness in tracking the maximum power point in partially shaded environments.

Norton equivalent circuit for current control loop of grid-connected system

Modeling method for the current control loop of a grid-connected PWM inverter with the LCL output filter was discussed.Firstly,the current control loop with the LCL inverter-side current as feedback was established.Then,parameters of PI controller were calculated on the basis of an equivalent controlled object.Finally,Norton equivalent circuit for the current control loop of grid-connected system was derived by integrating one control equation,which connected the PWM inverter output voltage and the LCL inverter-side current,with two circuit equations,separately using the LCL inverter-side current and the injected current as loop currents.With the induced Norton equivalent circuit,system-level resonant and unstable issues on real grid-connected system applied in weak distributed power systems can be easily analyzed.The validity of substituting Norton equivalent circuit for grid-connected system is verified by simulation and experiment.

Testing PV Central Inverters＇ Compliance with International Grid Codes

In many electrical grids worldwide, the rising amount of installed PV （photovoltaic） power entails a considerable influence of PV systems on grid quality and stability. Consequently, in the wake of the revised German medium voltage directives issued in 2009, new requirements for PV inverters have been established internationally. At Fraunhofer ISE＇s Inverter Laboratory, approximately 25 large PV inverters with a nominal power of up to 880 kVA have been characterized in the past three years. In this period, the focus of many inverter manufacturers has begun to shift from traditional European markets towards an international perspective. Therefore, experiences with numerous different grid codes have been gained by our team. This work summarizes the similarities and differences between these grid codes. Additionally, several requirements that have proved to be critical will be examined. Finally, the adequacy of these grid codes to guarantee the safe and reliable operation of electrical grids is discussed.

Generalized coupling resonance modeling, analysis, and active damping of multi-parallel inverters in microgrid operating in grid-connected mode

This paper firstly presents an equivalent coupling circuit modeling of multi-parallel inverters in microgrid operating in grid-connected mode. By using the model, the coupling resonance phenomena are explicitly investigated through the mathematical approach, and the intrinsic and extrinsic resonances exist widely in microgrid.Considering the inverter own reference current, other inverters reference current, and grid harmonic voltage, the distributions of resonance peaks with the growth in the number of inverters are obtained. Then, an active damping control parameter design method is proposed to attenuate coupling resonance, and the most salient feature is that the optimal range of the damping parameter can be easily located through an initiatively graphic method. Finally,simulations and experiments verify the validity of the proposed modeling and method.

Impedance Modeling and Stability Analysis of PV Grid-connected Inverter Systems Considering Frequency Coupling

Impedance analysis is an effective method to analyze the oscillation issue associated with grid-connected photovoltaic systems.However,the existing impedance modeling of a gridconnected photovoltaic inverter usually only considers the effect of a single perturbation frequency,ignoring the coupling frequency response between the internal control loops of a grid-connected inverter,which severely affects the accuracy of the stability analysis.Hence,a method of impedance modeling and stability analysis for grid-connected photovoltaic inverters considering cross-coupling frequency is proposed in this paper.First,the generation mechanism of frequency coupling in gridconnected photovoltaic inverters,and the relationship between the coupling frequency and perturbation frequency are analyzed.Secondly,a sequence impedance model of grid-connected photovoltaic systems considering the coupling frequency is established by using the harmonic linearization method.The impact of DC bus voltage control strategy on frequency coupling characteristics of a grid-connected photovoltaic system is evaluated,and the impact of a coupling frequency term on system stability is quantitatively analyzed.Finally,the advantages of the proposed method are verified by several simulations.The results show that the proposed impedance model can accurately predict the potential resonance points of the system,and the coupling frequency characteristics will become much stronger with smaller DC bus capacitance or larger bandwidth of the DC bus controller.

Stability of LCL-filtered grid-connected inverters with capacitor current feedback active damping considering controller time delays

This paper investigates the stability of LCL-filtered grid-connected inverters with capacitor current feedback(CCF) active damping. The impact of time delays in the digital controller on active damping and its equivalent virtual impedance is analyzed. The inherent relationship between these time delays and stability is illustrated.Specially, a critical value of the CCF active damping coefficient kdamp_cis proposed to define three distinct regions of stability evaluation. If kdamp_c[ 0, a sufficient but smaller damping coefficient(kdamp\ kdamp_c) is recommended as optimum damping solution;if kdamp_c= 0,system will be unstable irrespective of active damping;and if kdamp_c\ 0, active damping is not necessary to design a stable system. Necessary conditions to ensure stability are identified;guidelines for controller design are then presented to optimize the performance of active damping and dynamic response. Simulation and experimental results confirm the presented analysis.

Impact of phase-locked loop on stability of active damped LCL-filter-based grid-connected inverters with capacitor voltage feedback

Active damped LCL-filter-based inverters have been widely used for grid-connected distributed generation(DG) systems. In weak grids, however, the phase-locked loop(PLL) dynamics may detrimentally affect the stability of grid-connected inverters due to interaction between the PLL and the controller. In order to solve the problem, the impact of PLL dynamics on small-signal stability is investigated for the active damped LCL-filtered grid-connected inverters with capacitor voltage feedback. The system closed-loop transfer function is established based on the Norton equivalent model by taking the PLL dynamics into account. Using an established model, the system stability boundary is identified from the viewpoint of PLL bandwidth and current regulator gain. The accuracy of the ranges of stability for the PLL bandwidth and current regulator gain is verified by both simulation and experimental results.

LCL-resonance damping strategies for grid-connected inverters with LCL filters: a comprehensive review

Grid-connected LCL-filtered inverters are commonly used for distributed power generators. The LCL resonance should be treated properly. Recently, many strategies have been used to damp the resonance, but the relationships between different damping strategies have not been thoroughly investigated. Thus, this study analyses the essential mechanisms of LCL-resonance damping and reviews state-of-the-art resonance damping strategies.Existing resonance damping strategies are classified into those with single-state and multi-state feedback. Singlestate feedback strategies damp the LCL resonance using feedback of a voltage or current state at the resonance frequency. Multi-state feedback strategies are summarized as zero-placement and pole-placement strategies, where the zero-placement strategy configures the zeros of a novel state combined by multi-state feedback, while the poleplacement strategy aims to assign the closed-loop poles freely. Based on these mechanisms, an investigation of single-state and multi-state feedback is presented, including detailed comparisons of the existing strategies. Finally,some future research directions that can improve LCL-filtered inverter performance and minimize their implementation costs are summarized.

The Resonance Suppression for Parallel Photovoltaic Grid-connected Inverters in Weak Grid

Obvious resonance peak will be generated when parallel photovoltaic grid-connected inverters are connected to the weak grid with high grid impedance, which seriously affects the stability of grid-connected operation of the photovoltaic system. To overcome the problems mentioned above, the mathematical model of the parallel photovoltaic inverters is established. Several factors including the impact of the reference current of the grid-connected inverter, the grid voltage interference and the current disturbance between the photovoltaic inverters in parallel with the grid-connected inverters are analyzed. The grid impedance and the LCL filter of the photovoltaic inverter system are found to be the key elements which lead to existence of resonance peak. This paper presents the branch voltage and current double feedback suppression method under the premise of not changing the topological structure of the photovoltaic inverter, which effectively handles the resonance peak, weakens the harmonic content of the grid current of the photovoltaic grid-connected inverter and the voltage at the point of common coupling, and improves the stability of the parallel operation of the photovoltaic grid-connected inverters in weak grid. At last, the simulation model is established to verify the reliability of this suppression method.