Grid-connected LCL-filtered Inverter Research Articles

Robust EMPC-Based Frequency-Adaptive Grid Voltage Sensorless Control for an LCL-Filtered Grid-Connected Inverter

A robust explicit model predictive control (EMPC)-based frequency-adaptive grid voltage sensorless control is developed for a grid-connected inverter (GCI) via a linear matrix inequality (LMI) approach under the model parametric uncertainties as well as distorted and imbalanced grid voltages. In order to ensure the quality of grid currents injected into the utility grid even when the system model parameters vary, the proposed control scheme is accomplished by an enhanced prediction model rather than the conventional prediction model obtained by fixed parameters. Furthermore, an LMI-based observer is integrated with the disturbance observer to improve the reference tracking performance and to reject disturbances. The proposed observer is employed for the grid frequency-adaptive control without the need for grid voltage sensors. The proposed current controller and observer employ the LMI scheme to maintain a stable and robust operation of the GCI. The discrete-time frequency response and pole-zero map analyses are utilized to examine the system performance including the stability and robustness against parametric uncertainties. Comprehensive simulation and experimental tests as well as theoretical analyses clearly validate the robustness of the proposed control scheme under various harsh test conditions with non-ideal and unexpected grid and system parametric uncertainties.

Aliasing Suppression Method for a Three-Phase Grid-Connected Photovoltaic Inverter Based on Multi-Sampling and Mean Filtering

In order to reduce the sampling delay and improve bandwidth, sability margin, and the robustness of the active damping in LCL-filtered grid-connected inverters, real-time sampling provides a convenient method. However, aliasing is easily introduced in the control loop because of high-frequency switching harmonics, resulting in a rise in low-order harmonics. To address the challenge of aliasing under real-time sampling, a new method based on multisampling and mean filtering is proposed by combining the proposed harmonic detection and control methods in this paper. This method works by sampling and controlling the fundamental current and selective subharmonic currents separately. The fundamental current control loop without any additional filters maintains real-time sampling, while multisampling and mean filters are applied to the harmonic current control loop. The proposed control method can not only improve the dynamic response performance and control stability of the system but also effectively suppress the selective low-order current harmonics and the aliasing of sampling. Finally, the correctness of the proposed sampling scheme and control strategy is verified by the simulation based on R2018b, MathWorks, Natick, MA, USA and an experimental prototype of a three-phase grid-connected photovoltaic inverter rated at 230 V/50 Hz/40 kW.

Passivity-Based Design for LCL-Filtered Grid-Connected Inverters with Inverter Current Control and Capacitor-Current Active Damping

Passivity-Based Design for LCL-Filtered Grid-Connected Inverters with Inverter Current Control and Capacitor-Current Active Damping

Grid Integration of Livestock Biogas Using Self-Excited Induction Generator and Spark-Ignition Engine

This study developed a grid-connected biogas power generation system for a rural community using a 3-phase 2.2 kW squirrel-cage induction machine as the self-excited induction generator. The generator was driven by a 196 cc single-cylinder spark-ignition engine fueled by biogas. We developed a back-to-back converter that consists of a 3-phase PWM rectifier as the generator-side converter and a single-phase LCL-filtered grid-connected inverter. The generator-side converter transferred the active power to the grid-side converter and supplied the reactive power control back to the generator. The notch filter-based bus voltage control on the generator side mitigated the inter-harmonics in the generator current. The injected grid current complied with the IEEE 1547 standard because of the multi-frequency unbalanced synchronous reference frame control. The proposed system was validated with biogas produced from pig manure at a pig farm in central Thailand, which found a maximum output of 1.2 kW with a thermal system efficiency of 10.7%. The proposed system was scheduled to operate at 1.2 kW for 8 h per day with a levelized cost of 0.07 US$/kWh, 42% cheaper than the retail electricity price, and a payback period of 2.76 years. The proposed system is suitable for a farm with a minimum of 34 pigs.

H2/H∞ Robust Observed-State Feedback Control Based on Slack LMI-LQR for LCL-Filtered Inverters

A robust controller for an LCL-filtered grid-connected inverter (GCI) is presented in this paper in the context of the system model uncertainties and non-ideal grid environment. Under the occurrence of exogenous inputs from parameter drifts affecting the controller and observer, the H₂/H∞ approach of the linear quadratic regulator (LQR) control is developed to obtain the state-feedback gain and observer gain with the system stability guarantee by means of Lyapunov stability theory. Furthermore, the use of slack-linear matrix inequalities (LMIs) to handle parameter-dependent Lyapunov functions in synthesizing the robust controller can achieve a less conservative condition than the conventional quadratic stability. System robustness and stability are assessed through the closed-loop pole map and the discrete-time frequency analysis approaches. Hardware experiments are conducted for the LCL-filtered GCI to validate the feasibility and effectiveness of the proposed control method. Fair comparisons with the conventional schemes are also performed to highlight the superior performance of the proposed scheme.

Active damping of LCL-Filtered Grid-Connected inverter based on parallel feedforward compensation strategy

Resonance related to the LCL-filter grid-connected inverter (GCI) is one of the most challenging issues in power electronics. Active damping is a widely used methodology to damp the resonance problem due to its effective control structure. In capacitor current feedback active damping methodologies, a compensator is primarily required in the feedback loop where the damping efficiency depends on the imitated features of the compensator. Such capacitor current feedback damping techniques encounter restrictions in their layout at the critical resonance frequency and may need extra amendment in the delay to assure stability. This paper proposes a different design method called parallel feedforward compensation method, which designs an almost strictly real positive plant and achieved damping with enhanced stability features. In the proposed alternative, a first-order high pass filter is used as a compensator across the LCL filter plant and filter capacitor current feedback loop. The resultant output of this augmented plant is then transmitted back at the reference point of a GCI system to attenuate the undesirable resonance peak. The key benefits of the proposed method are efficient damping without any limitation of critical resonance frequency region, better control characteristics, improved damping area, and better robustness under variations in filter parameters. The simulation and experimental outcomes based on transient response and steady-state analysis are demonstrated to validate the efficacy of the suggested method.

An alternative control synthesis for stability enhancement of digital-controlled LCL-filtered grid-connected inverter

• A control synthesis is proposed to damp capacitor current resonance for LCL-filtered GCI. • The stability and performance characteristics are analyzed and compared with methods from the literature. • Intuitive design, wider stable damping region, better dynamic performance, and higher parametric robustness are the potential benefits of the proposed control synthesis. In distributed power generation systems, the inherent resonance phenomena of LCL -filtered grid-connected inverter is one of the major challenges faced by the integration of renewable energy resources with the electric power grid. In this context, the capacitor current based multi-loop active damping method is employed due to lossless resonance damping and flexible control design. However, the damping loop with the feedback compensation design suffers from relatively higher parameters sensitivity unless employed with the reduced digital delays. The filter parameters fluctuation causes a shift in resonance frequency, which leads to ineffective damping at the critical resonance frequency and may risk the control loop stability. Therefore, an alternative linear control synthesis is proposed to design the capacitor current damping loop in order to reduce the filter parameters sensitivity. The proposed design realizes the equivalent virtual impedance in an alternative manner to suppress the resonance peak and enhanced the stability characteristics of the grid-connected inverter. A detailed stability and performance analysis have been done to show the widened stable damping region and high parametric robustness of the suggested design as compared to the literature method. Simulations and experimental results are presented to confirm the theoretical findings.

Stabilization Design of Three-Phase LCL-Filtered Grid-Connected Inverter Using IDA-PBC Controller

With the aim of improving the stability of renewable energy system with high permeability in the weak grid, a modified passivity-based control based on interconnection and damping assignment (IDA) is presented for LCL-filtered grid-connected inverters to eliminate the interactive resonances. The object is modeled in the form of port-controller Hamiltonian, including the partial differential of stored energy function. On the premise of ensuring Lyapunov stability, a more flexible interconnection matrix design method is applied to simplify the design process. The closed-loop stability is ensured with the selected Hamiltonian energy function at the desired balanced point. Moreover, a step-by-step design process for damping gains is provided to guarantee stable operation and fast dynamic response under variation of complex grid impedance. With the designed injected damping parameters, considering the effect of delay, it is possible for the real part of the inverter output admittance to be positive within the switching frequency. The performance and robustness of the proposed method for LCL-filtered inverter system are validated via simulated and experimental results under both unbalanced and balanced grid conditions.

LMI-Based Model Predictive Current Control for an LCL-Filtered Grid-Connected Inverter under Unexpected Grid and System Uncertainties

To guarantee a system stability and reliable operation of an inductor-capacitor-inductor (LCL)-filtered grid-connected inverter (GCI) under unexpected grid and system uncertainties, a linear matrix inequality (LMI)-based model predictive control (MPC) is presented in this paper. Even though the conventional MPC scheme is constructed by a simple concept, it is difficult to determine an optimized weighting matrix of the MPC cost function against parameter discrepancies. To overcome this problem, the MPC scheme is combined with LMI-based optimization. The system states are estimated by the LMI-based current-type observer in the stationary reference frame to implement the proposed scheme. Additionally, the MPC scheme is combined with the disturbance observer to eliminate offset error, which improves the reference tracking performance. In comparison with the other studies, the proposed control method ensures high robust control performance under grid voltage imbalance, parameter uncertainty, and frequency variation. In addition, the proposed approach achieves a robust active damping even for the grid impedance variation without the need of considering further damping method. The control design step is systematic and straightforward. Furthermore, unlike the conventional schemes, the proposed controller does not require an integral term and the 2nd harmonic compensation term to obtain a good reference tracking performance under grid imbalanced condition, which contributes to the reduction of the controller complexity by decreasing the order of the controller model. To verify the effectiveness of the proposed LMI-based MPC control scheme, the simulation and experiments are carried out by using prototype three-phase GCI. The comprehensive simulation and experimental results clearly demonstrate the robustness of the proposed current controller under various adverse test conditions with unexpected grid and system uncertainties.

A Control Parameters Design Method With Multi-Constrains for an LCL-Filtered Grid-Connected Inverter in a Weak Grid

Under weak grid conditions, the variation of the grid impedance will affect the steady-state and dynamic performance of the LCL-filtered grid-connected inverter and even make the inverter unstable. To ensure the system stability and further improve the dynamic performance in a weak grid, a control parameter design method with multi-constrains considering the system bandwidth for the current controller and active damping is proposed in this paper. First, based on the current controller and active damping with only grid current feedback, the effects of control parameters and grid impedance on the LCL resonant suppression and the performance of the inverter are analyzed. Moreover, the parameter constraints of the controllers are derived considering the grid impedance, including stability, resonance suppression, and margin constraints. Furthermore, as the system bandwidth affects the dynamic performance of the inverter, combined with the obtained multi-constraints, the optimal control parameters are determined by achieving the maximum bandwidth of the system against the impedance variation. Compared with other two methods, when the proposed method is applied, the system can operate with a better dynamic and steady-state performance. Finally, experiments are performed on a 2 kW three-phase grid-connected inverter in the weak grid, which verify the effectiveness of the parameter design method proposed in this paper.

An Improved Three-Level Cascaded Control for LCL-Filtered Grid-Connected Inverter in Complex Grid Impedance Condition

The passivity-based control (PBC) with Euler–Lagrange (EL) model has been used for LCL-filtered grid-connected converter. However, the impacts of the control delay and the interaction resonances under capacitive grid impedance are rarely considered for PBC method. Nonlinear Lyapunov’s stability theory is hard to estimate the stability of the system and accurately design the controller parameters considering the control delay. This paper derived the impedance model of LCL-filtered grid-connected inverter with grid current control based on PBC method. The passive theory and the zero-pole maps are applied to reveal the influence of the digital delay and differential item of inverter current loop. A new three-level cascaded control method is then proposed based on traditional PBC method to achieve stability and high robustness in complex grid. Furthermore, controller parameters are designed accurately to maintain passivity within switching frequency ranges and parameter variations of the LCL filter are investigated. Final validation of the effectiveness and performance of the proposed control structure is performed by simulation and experimentation.

Active damping strategy with differential feedback of grid-side inductor voltage for LCL-filtered grid-connected inverters

Active damping strategy with differential feedback of grid-side inductor voltage for LCL-filtered grid-connected inverters

Observer-based neural network dynamic surface control of single-phase LCL-filtered grid-connected inverters under parametric uncertainties and weak grid condition

This paper focuses on the current control of single-phase LCL-filtered grid-connected inverters in the presence of parametric uncertainties and weak grid condition. Therefore, a novel neural network dynamic surface control (NNDSC) method is proposed by overcoming the problem of ‘explosion of complexity’. In addition, radial basis function neural networks (RBFNNs) are employed to approximate the system parametric uncertainties. Furthermore, by considering practical considerations, a novel state observer (SO) is designed to estimate the inverter-side current and capacitor voltage. As a result, additional current and voltage sensors are not required, which makes the implementation of the proposed approach straightforward and reliable. The origin neighbourhood convergence of estimated and tracking errors is assured through Lyapunov stability theorem and Young’s inequality. The effectiveness and performance of the proposed NNDSC+SO approach is demonstrated through MATLAB/Simpower simulations in view of the reference current changes, LCL filter parametric uncertainties and weak grid condition.

Parameter co-design of active damping loop and grid current controller for a 3-phase LCL-filtered grid-connected inverter

Under non-ideal grid conditions, especially the harmonic grid condition, the proportional resonant controller with multiple harmonic resonant controllers (HR-PR controller) is recommended to provide high quality grid currents for the 3-phase LCL-filtered grid-connected inverter. Besides, in order to solve the resonance problem, the active damping loop with the grid-side currents feedback only is preferred. However, parameters of the HR-PR controller and the active damping loop both affect the performance of LCL filter resonance suppression and stability of the inverter, which is analyzed in this paper. Moreover, the stability constraint and the resonance suppression constraint is derived to maintain the good ability of LCL filter resonance suppression and satisfy the stability requirements of the inverter. Considering the wide control bandwidth for the HR-PR controller, a co-design method of the active damping loop and HR-PR controller is proposed in this paper. By introducing the Lagrange multiplier method, the proposed co-design method obtains the optimized parameters for the active damping loop and grid currents controller in z-domain appropriately. A prototype of 3-phase LCL-filtered three-level neutral point clamped grid-connected inverter is established to validate the proposed method.

Optimized Active Disturbance Rejection Control With Resonant Extended State Observer for Grid Voltage Sensorless LCL-Filtered Inverter

A robust optimized active disturbance rejection control (ADRC) based grid voltage sensorless current controller is developed for an LCL-filtered grid-connected inverter (GCI) via a predictive control approach under various sources of disturbance, including the model uncertainties, the LCL inherent resonance phenomenon, and nonideal grid environment. Aiming to improve the sinusoidal reference tracking performance as well as to reject lumped sinusoidal disturbances in the control practice, a resonant extended state observer is integrated into the ADRC structure, which guarantees a rigorous stable operation of inverter for bounded filter parameter uncertainties and adverse grid voltage conditions. The grid frequency adaptability is thoroughly considered in the controller design process and synchronization technique, offering an extra capability for GCI to operate under different grid voltage frequency levels or even the frequency deviation caused by grid fault events. Robustness against parameter uncertainty and system stability is analyzed through the discrete-time frequency analysis and pole-zero map approaches. The simulation and hardware experiments are conducted for GCI with LCL filter parameters designed for two typical regions (i.e., the resonance frequency is less and greater than one-sixth of switching frequency) to validate the theoretical analysis and the effectiveness of the proposed control method.

Aliasing Suppression of Multisampled Current-Controlled LCL-Filtered Inverters

Multisampling control provides an attractive way to reduce the control delays in LCL-filtered grid-connected inverters. Thereby, the bandwidth and stability margin will be improved. However, high-frequency switching harmonics (SHs) are introduced in the control loop when the inverter-side current is sampled. To investigate the effect of multisampled high-frequency SHs, the relationship between the double-update pulsewidth modulation (PWM) and multiupdate PWM is deduced through geometric deduction. It is shown that the multiupdate PWM is equivalent to double-update PWM with sampling instant shift, and the equivalent Nyquist frequency is equal to the switching frequency. Moreover, the nonaveraged value of current is sampled within one switching period and aliased low-order harmonics will appear in the grid-side current. Hence, filtering the multisampled SHs is necessary, and an improved repetitive filter (IRF) is proposed to remove all the sampled SHs and keep the advantage of phase boost using the multisampling control. The method is evaluated with a single-loop inverter-side current control, and its effectiveness is verified through the simulation and experiment.

Hybrid Model Predictive Current and Voltage Control for LCL-Filtered Grid-Connected Inverter

The inductive-capacitive-inductive (LCL)-filtered grid-connected inverter features a high-order plant, complex parameter design, and vulnerability under disturbance factors. The transfer-function-based active damping (TFAD) scheme is commonly adopted to mitigate the resonance problem but with cascaded control loops and high design complexity. On the contrary, multiobjective optimization can be easily realized in model predictive control (MPC) via a predefined cost function. As the coupling dynamic among state variables, a hybrid cost function, which contains the grid current, inverter-side current, and capacitor voltage tracking, is designed to guarantee system stability. The optimized control law is generated according to the minimization of the designed cost function, which is further transformed into the driving pulse by the modulator stage. In this process, the weighting factors are exactly arranged to place the desired closed-loop poles. The control law can ensure zero error tracking of grid current reference, which is better than the state feedback way. The hybrid cost function with modified reference is also proposed to reject the harmonics caused by the unhealthy grid condition. The control delay is compensated by using the forecast reference and the predictive state variable estimated from the observer. The proposed scheme performs the intuitive concept, reduced tuning complexity, and fast dynamic response. Simulated results and experiments are presented to verify the proposed control strategy.

Stability Assessment of Current Controller with Harmonic Compensator for LCL-Filtered Grid-Connected Inverter under Distorted Weak Grid

An assessment of the stability and performance of current controllers with harmonic compensators is presented for an inductive-capacitive-inductive (LCL)-filtered grid-connected inverter under distorted weak grid conditions. By using two typical current control schemes which are the direct current controller with the capacitor current-based active damping and integral-resonant state feedback current controller, the closed-loop system stability and current control performance are investigated in the presence of both uncertain grid impedance and distorted grid. Even though the controller stability has been investigated under weak grid in several studies, the stability assessment of the entire current control scheme, including the harmonic resonant controllers, still needs a further comprehensive investigation. The system stability is analyzed by obtaining the movement of the closed-loop poles in the discrete-time domain when the grid impedance varies. To fully study the impact of distorted weak grid condition on the LCL filters, three LCL filter parameter sets giving the resonance frequency in different frequency bands are chosen for the purpose of evaluating the system robustness and grid-injected current quality. In order to support the presented theoretical analyses, comprehensive simulation and experimental results based on 32-bit DSP TMS320F28335 to control 2 kVA grid-connected inverter are presented in terms of grid current quality and control stability in the environment of both uncertain grid impedance and distorted grid.

Highly Accurate Digital Current Controllers for Single-Phase LCL-Filtered Grid-Connected Inverters

Photovoltaic (PV) systems are the most promising technology for residential installation as an alternative source of energy. To interface the primary source of PV to the electrical grid, an LCL-filtered inverter is being broadly adopted due to its low volume compared to the L-filtered one and the superior ability to filter high-frequency harmonics. In this context, this paper proposes highly accurate digital current controllers for single-phase LCL-filtered grid-connected inverters. The proposed controllers are: Integral-single-lead, integral double-lead, integral double-lead taking into account the effect of pulse width modulation (PWM) delay and the proportional-resonant (PR). These controllers are different from the traditional Proportional-Integral (PI), Proportional-Derivative (PD), and Proportional-Integral-Derivative (PID). One of the novelties of this paper is the detailed, step-by-step procedure for tuning each parameter of the proposed digital controllers considering the dynamic behavior of the LCL filter. The proposed PR has a different and more straightforward tuning methodology than those procedures commonly found in the literature. Therefore, this paper is an attractive tool for a fast, accurate, and reliable way to tune digital current controllers for a single-phase LCL-filtered grid-connected inverter. The controllers were verified in the digital signal controller (DSC) TMS320F28335 while the power structure runs in a hardware-in-loop (HIL device). Results show the efficacy of the proposed controllers.

Mitigating Disturbance in Harmonic Voltage Using Grid-side Current Feedback for Grid-connected LCL-filtered Inverter

As the interface between new energy and power grid, the grid-connected LCL-filtered inverter plays a key role in energy conversion. However, it performs poor at rejecting grid background harmonics that distort the grid-side current and affect the quality of the output power of the inverter. Thus, a feedback strategy for the grid-side current employing the proportional integral and resonant controller (PI + HC) is used to mitigate the harmonics of the grid-side current and improves the quality of the output power of the grid-connected LCL-filtered inverter. Owing to the grid-side current feedback control system is unstable, improved High Pass Filter (HPF) active damping based on unit delay feedback is used to guarantee stability and effectiveness of PI + HC. The proposed method for mitigating grid-side current harmonics is validated by a detailed simulation and the results of experiments.