LCL-type Inverter Research Articles

LADRC-based grid-connected control strategy for single-phase LCL-type inverters.

To ensure that grid-connected currents are of high quality, it is crucial to optimize the dynamic performance of grid-connected inverters and their control. This study suggests using a combination of reduced-order linear active disturbance rejection control (LADRC) and a Proportional-Integral (PI) controller. By applying this control strategy to a single-phase photovoltaic grid-connected system, the system's ability to suppress grid harmonics is significantly improved. The validity and effectiveness of this control approach have been confirmed through simulations and experiments. The results show that the LADRC-based control system is robust and capable of rejecting disturbances, resulting in a significant reduction in the Total Harmonic Distortion (THD) of grid-connected currents. Comparative analysis with traditional control methods demonstrates the superior performance of the proposed approach.

Digital Control Design Scheme for an LCL-Type Inverter Based on QPR Controller With GVWF Under Weak Grid Conditions

Digital Control Design Scheme for an LCL-Type Inverter Based on QPR Controller With GVWF Under Weak Grid Conditions

Design and control of LCL-type grid-tied PV power conditioning system based on inverter and grid side currents double feedback

ABSTRACT Solar energy as a limitless, clean, and cost-effective energy source could be converted to electrical power by photovoltaic cells. In this paper, a photovoltaic power conditioner, which is connected to the low voltage grid via an LCL filter, is presented. This power conditioning system supplies high-quality current and has high stability against disturbances. Since the generated PV voltage is lower than the grid voltage, a DC-DC converter is utilized. Also, DC voltage is also converted to AC by an LCL-type inverter. To ensure the resonance elimination yielded from the LCL filter, inverter and grid side currents, a double feedback strategy is employed. By considering resonance damping, stability margin, and the grid-connected requirements, a systematic design procedure for the suggested strategy is investigated to ensure the system stability in the low-voltage network. In comparison with the conventional strategies, the suggested strategy gives suitable resonance damping capability, higher marginal stability, and better grid-connected properties. Simulation results of the PV power conditioning system illustrate the accurate performance of the control system, suitable tracking of the PV array maximum power point, and high-quality current injection into the grid.

Robustness Investigation of Multi-Inverter Paralleled Grid-Connected System With LCL-Filter Based on the Grid-Impedance Allocation Mechanism

Due to the existence of grid impedance, there is coupling resonance problem which may cause the multi-inverter grid-connected system to become unstable. This article focuses on universal mechanism for multimodule grid-connected system composed of LCL-type inverters. By establishing the multi-input multi-output matrix of the multi-inverter system, the system stability is divided into two parts, i.e., the self-stability and the interactive-stability. Then, based on elaborate deduction for the interactive-stability, the equivalent allocation mechanism of grid impedance is raised and a stability criterion for multimodule inverters system is proposed. In addition, on the basis of exploring the effect of the digital-control delay on the stability of the multi-inverter system in the weak grid, the phase-lead compensator combining with the optimal capacitor-current-feedback coefficient is adopted to improve the system robustness against grid impedance variation. Finally, the effectiveness of the theoretical analysis is verified by experimental results.

A Frequency Adaptive Scheme Based on Newton Structure of PRRC for LCL-Type Inverter Connected with Weak Grid

Repetitive control (RC) is gradually used in inverters tied with weak grid. To achieve the zero steady-state error tracking of inverter current and compensate the harmonic distortion caused by frequency fluctuation, a frequency adaptive (FA) control scheme for LCL-type inverter connected with weak grid is proposed. This scheme adopts a proportional resonance (PR) controller in parallel with RC (PRRC) to overcome the disadvantages caused by RC inherent one-cycle time delay. A fractional delay (FD) filter based on the Newton structure is proposed to approximate the fraction item of fs/f, where fs and f are sample frequency and grid frequency, respectively. The structure of the proposed FD filter is relatively simple; moreover, coefficients of the filter maintain constant so as not to need online tuning even when grid frequency fluctuates, which decreases the computational burden considerably. The feasibility and effectiveness of the proposed FA control scheme, named as Newton-FAPRRC, are all verified by the simulation and experimental results.

Grid-Voltage-Feedback Active Damping With Lead Compensation for LCL-Type Inverter Connected to Weak Grid

To improve the robustness and stability of the LCL inverter connected to weak grid, this paper proposes a novel active damping based on voltage of point common coupling ( v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pcc</sub> ) feedback. A phase lead compensator is proposed in the v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pcc</sub> feedback loop to compensate system phase margin (PM). Theoretical analysis of the effectiveness, detailed parameter setting, stability analysis for proposed active damping adopting lead compensator, named as LC AD, are all described. Simulation and experiment results show that comparing to conventional high pass filter (HPF) AD, the proposed LC AD provides more effective damping effect for the LCL inverter adopting quasi-proportional resonant (QPR) controller under weak grid.

Fractional order harmonic disturbance observer control for three-phase [formula omitted]-type inverter

The quality of output current of LCL-type voltage source inverter (VSI) is degraded by the grid voltage distortion, the dead time effect, and the parameter mismatches. This paper proposes a novel fractional order harmonic disturbance observer-based control (FOHDO) for the three-phase LCL-type inverter to suppress all the dc- and ac-component periodic disturbances. Firstly, a harmonic disturbance observer based control for LCL-type inverter is proposed with the internal model knowledge of the periodic disturbances signals. Subsequently, the FOHDO based on Lagrange interpolating polynomial fractional order delay approximation is put forward to enhance the adaptability of the grid frequency. Then, the stability and robustness performance of the proposed FOHDO for LCL-type inverter are evaluated, which also involves a detailed parameter design process. Finally, the effectiveness and performance of the proposed FOHDO method are verified by both experimental and simulation results.

Analysis and design of L + LCL‐filtered dual‐frequency single‐phase grid‐connected inverter

To increase the efficiency of the grid-connected inverter, this study proposes an L + LCL-filtered dual-frequency single-phase grid-connected inverter. The proposed inverter consists of the low-frequency unit and high-frequency unit. The low-frequency unit transmits power to the grid at the low switching frequency. The high-frequency unit uses the feed-forward method to eliminate grid switching current ripples without current harmonics detection. The passive damping LCL filter is used to attenuate high-frequency unit switching harmonics. The operation principle of the proposed inverter is analysed, and the parameter design method of the inverter is introduced. An experimental prototype is built to test the performances of the inverter. Simulation and experimental results show that the grid current harmonics are effectively suppressed, and the efficiency of the proposed inverter can be higher than that of the L-type inverter and LCL-type inverter.

Research on Adaptive Suppression of LCL Converter Resonance Grid-Connected System

LCL-type converters are widely used in grid-connected systems due to their small size and good filtering performance. However, the resonance suppression problem brought by the LCL filter cannot be ignored. The capacitive current feedback is a commonly used resonance suppression method. In applications, the grid impedance can cause LCL filter resonance. Thus, this paper presents an adaptive resonance suppression method based on the RBF network optimized by particle swarm optimization. This method optimizes the initial parameters of the RBF network through particle swarm optimization, identifies the parameters of the PI controller by RBF neural network’s own identification capability, and updates the active damping coefficient based on constraints such as stability margin, thereby realizing the LCL-type inverter to maintain the system stability when the grid impedance changes. The effectiveness of the method is verified by experiments.

Design of Discrete-Time Backstepping Sliding- Mode Control for LCL-Type Grid-Connected Inverter

This study designs a discrete-time backstepping sliding-mode control (DTBSMC) method for an LCL-type grid-connected inverter. Firstly, the dynamic model of a discrete-time three-order system is derived, and a discrete-time backstepping control method cascading with the sliding-mode control theory is designed via Lyapunov stability verification. Moreover, the system state equation is transformed into a special form by using a time-varying mapping for overcoming the difficulty of a non-causal problem. Besides, through the recursively subsystem design for the high-order LCL-type inverter, the asymptotic system stability can be ensured by step-by-step virtual control designs. In addition, the proposed method can combine both the advantages of the backstepping control method and sliding-mode control theory. Therefore, the inverter system can have strong robustness under the condition of a power grid with varied grid impedances. Finally, the effectiveness of the proposed DTBSMC method is verified by numerical simulations and experimental results in comparison with a traditional proportional-resonant (PR) method and a backstepping control method.

Passivity-Based Stabilization of LCL-Type Grid-Connected Inverters via a General Admittance Model

For LCL-type grid-connected inverters, inverter-side current, grid-side current, capacitor current and voltage are four commonly used variables in the control serving as different roles, such as control objective, active damping, synchronization, etc. This article builds a general admittance model, in which the total output admittance is decomposed into four paralleled subadmittance corresponding to the above four variables in the control law, to facilitate the passivity-based stability assessment and controller parameter optimization for different control objectives and schemes. Based on the passivity analysis, an appropriate combination scheme of subadmittance and a simple unified analytical controller parameter design method are proposed to obtain a passive output admittance from 0 Hz up to Nyquist frequency for either inverter-side or grid-side current control. Therefore, it can not only ensure the stable operation of LCL-type inverters regardless of grid impedance (either inductive or capacitive) but also avoid the conventional design constraints of an LCL filter, i.e., f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> <; or > f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> /6 is required for the single-loop inverter-side or the grid-side current control. Finally, experiments verify the theoretical results.

Harmonic Voltage Distortion Damping Method for Parallel-Connected LCL-Type Inverters in Islanded Operation

For parallel-connected LCL -type inverters in islanded operation, it is difficult to reshape the large harmonic impedance by the conventional capacitor voltage control due to the presence of grid-side inductor, which seriously deteriorates the voltage quality at the point of common coupling (PCC) under nonlinear loads. To address this issue, a harmonic voltage distortion damping method is proposed in this paper, which mainly includes a direct output voltage control and the parallel virtual-admittance control. First, the direct output voltage control converts the grid-side inductor into the equivalent output impedance of the inverter itself by changing the voltage feedback variable from capacitor voltage to output voltage, and then the harmonic impedance is reshaped from a large value to a small value with the P + Repetitive controller. However, it is found that the direct output voltage control fails to damp PCC voltage distortion with droop-frequency offset. That is because its voltage controller cannot guarantee the resonance bandwidth of main harmonic frequency, and then a slight droop-frequency offset can cause a large increase in the harmonic impedance. For this, the parallel virtual-admittance control is further proposed to limit the size of the output impedance to no more than that of the impedance in parallel, which effectively reduces the adverse effect of droop-frequency offset on the quality of the PCC voltage. Further, the parallel virtual-admittance control is simplified with the same harmonic voltage distortion damping consequent. The system stability conditions are analyzed in detail, and its proper control parameters are selected without complicated trial. Finally, the experimental results validate the proposed control method.

An Integrated Design Approach for LCL-Type Inverter to Improve Its Adaptation in Weak Grid

To improve the robustness of grid-connected inverter against grid impedance in a weak grid an integrated design method of LCL-filter parameters and controller parameters is proposed. In the method the inherent relation of LCL-filter parameters and controller parameters is taken into consideration to realize their optimized match. A parameter normalization scheme is also developed to facilitate the system stability and robustness analysis. Based on the method all normalization parameters can be designed succinctly according to the required stability and robustness. Additionally, the LCL parameter and controller parameter can be achieved immediately by restoring normalization parameters. The proposed design method can guarantee the inverter stability and robustness simultaneously without needing any compensation network, additional hardware, or the complicated iterative computations which cannot be avoided for the conventional inverter design method. Simulation and experiment results have validated the superiority of the proposed inverter design method.

Effect of phase‐locked loop on small‐signal perturbation modelling and stability analysis for three‐phase LCL‐type inverter connected to weak grid

For three-phase LCL-type inverter connected to weak grid, the bandwidth and dynamics of phase-locked loop (PLL) directly affect small-signal perturbation modelling, which causes the control dq frame of PLL being no longer aligned with the system dq frame of weak grid. Thus, considering the effect of PLL, a small-signal perturbation admittance model is firstly built. Secondly, by the rotating matrix, variables in the system dq frame are transformed into the control dq frame. Then, the interaction between two dq frames is established based on an admittance model in consideration of the PLL, and the inverter output admittance is obtained. Finally, the effect of PLL, grid impedance and LCL-type filter parameters on system stability are analysed. As a conclusion, with an increase in PLL bandwidth and grid impedance, the system stability gradually is decreased, even resulting in the system instability. In addition, the larger the active damping coefficient is, the better the system stability is. While, the increase of filter capacitance and grid-side inductance leads to the decrease of system stability. The conclusions have certain guiding significance and reference value for the design and safe operation of three-phase LCL-type grid-connected inverter. The simulation and experiment verify the validity of theoretical analysis.

ディジタル制御による系統連系LCL形インバータの安定性解析

A grid-connected inverter is indispensable for photovoltaic power generation and smart grid systems, and it must be designed for stable operation. The impedance method based on the Nyquist criterion is often utilized to analyze the stability of grid-connected inverter systems. The impedance method is based on the eigen values of the product of the inverter output admittance and the line impedance matrices in the frequency domain. However, the frequency characteristics have so far been derived only for inverters with analog control systems. A new frequency analysis method for inverters with digital control systems is proposed in this paper. First, a stability analysis example for a three-phase LCL-type inverter controlled digitally is shown and the results are compared and validated with those by simulation using a Saber simulator. Finally, they are also compared and validated with experimental results digitally controlled by a DSP-based system.