LCL-type Grid-connected Inverter Research Articles

Admittance-Based Stability Analysis of LCL-Type Grid-Connected Inverter Considering AC-Side and AC-DC Frequency Coupling Effects

Admittance-Based Stability Analysis of LCL-Type Grid-Connected Inverter Considering AC-Side and AC-DC Frequency Coupling Effects

Enhancing grid-connected inverter performance under non-ideal grid conditions: a multi-functional multiplexing control strategy

Compared to the traditional thermal power generation, new energy sources, such as wind and photovoltaic systems, are more vulnerable to the effects of non-ideal power grids due to their limited capacity. This susceptibility can jeopardize the safe operation of power equipment, degrade power output quality, and lead to non-compliance with grid-connected specifications. The LCL-type grid-connected inverter is a typical nonlinear system that weakens the controllability of the grid-connected energy. To address these challenges, this study employs feedback linearization theory to transform the inverter into a standard linear system. Subsequently, it utilizes linear system methodologies to develop robust control laws, ultimately introducing a multi-functional multiplexing control strategy for grid-connected inverters based on feedback linearization and Hamilton-Jacobi-Issacs inequality. Simulation results demonstrate that this multi-functional strategy outperforms traditional grid-connected inverter control schemes, effectively mitigating issues related to low short-circuit ratios, voltage fluctuations, imbalances, harmonics, and other non-ideal grid conditions. Furthermore, it significantly expands the system’s adaptability to varying weak grid impedances.

Grid Harmonics Suppression for Three Phase Dual-Frequency Grid-Connected Inverter Based on Feedforward Compensation

Using a low pulse ratio, the electromagnetic interference and switching loss of an inverter can be effectively reduced, particularly in high-power applications. However, due to variations in grid impedance, it is a challenging task to achieve stable operation of an LCL-type grid-connected inverter (GCI) using the active damping method with low pulse ratio. Thus, a novel three-phase dual-frequency GCI is presented to ensure the symmetry of the output power and the stable operation of the system address stability issues. The proposed inverter topology in this article is composed of two inverters in parallel, which are, respectively, a power inverter unit (PIU) and an auxiliary harmonic elimination unit (AHEU). To reduce the switching loss and improve the inverter efficiency, the switching frequency of the PIU is relatively low, injecting current into the grid. Moreover, the feedforward compensation method is used in AHEU. AHEU operates at high switching frequency to generate a current component that is symmetrical with the ripple com-ponent, improving the power quality, without extracting the current harmonic as the current reference. The operating principle of feedforward compensation is explained, and a proper parameter design procedure is presented in this paper. Since L filters are used for the proposed inverter, the system can operate stably where the ratio of switching frequency to fundamental frequency is low. A 10 kW laboratory prototype was built. The experimental results showed that the grid current ripple could be effectively eliminated and the THD of the grid current was 3.01%. The proposed inverter has good stability in a weak grid, and the efficiency of the proposed inverter is 95.98% at rated current, which is 0.81% higher than the traditional GCI, effectively increasing the efficiency of the system.

Robust back-stepping sliding mode control for LCL-type grid-connected inverters in weak grids

Robust back-stepping sliding mode control for LCL-type grid-connected inverters in weak grids

A Control Strategy of LCL-Type Grid-Connected Inverters for Improving the Stability and Harmonic Suppression Capability

The conventional inverter-side current single-loop feedback control scheme is weak in suppressing the grid-side current harmonics, posing a challenge for an inverter to inject high-quality current under distorted grid voltage. With capacitor current compensation added, the control scheme achieves controllability of the grid-side current harmonics so that it can effectively suppress some specific harmonic components. However, due to the stability requirements, only a few low-order harmonic resonance controllers can be applied, which limits the mitigation of high-order harmonics. To tackle this problem, the grid-side current feedback control with inductor–capacitor–inductor (LCL) resonance damping is proposed in this paper. In this case, a higher LCL resonance frequency can be set compared to the inverter-side current single-loop feedback control scheme. Thereby, more resonance controllers can be applied to suppress high-order grid-side current harmonics. The active damping method of capacitor current proportional feedback plus capacitor voltage proportional feedback is adopted because of its high robustness to grid impedance variations. Furthermore, this paper reveals that the applied active damping method has a limitation in that it only considers a single inverter under inductive grid impedance, which cannot eliminate the risk of resonance caused by the interaction of multiple inverters and the grid. To address this issue, a phase lead compensator (PLC) is proposed, eliminating the resonance risk by removing the non-passive region of the inverter output admittance. To retain the advantage of the inverter-side current single-loop feedback control scheme, i.e., only a few measuring devices are required, a digital differentiator is used to calculate the capacitor current from the capacitor voltage. The difference between the measured inverter-side current and the calculated capacitor current is taken to approximate the grid-side current for the feedback control. The control performance is comparable to using the grid-side current for feedback. Simulation and experimental results demonstrate that the proposed control scheme endows the inverter with good stability and current quality without extra measurement devices.

Combined feed-forward control strategy for LCL-type grid-connected inverters

LCL-type grid-connected filter circuits are widely used for line network transmission because of their high ability to suppress current harmonics. However, due to the presence of resonance points in this third-order circuit, the system can generate severe oscillations. In order to improve power quality, a combined feed-forward factor control strategy is proposed in this paper. The control system introduces a combined feed-forward factor and incorporates the idea of dual PI control to ensure that the order of the system is reduced while improving its stability, followed by a detailed derivation of the constraint equations for each link parameter. The simulation verifies that the control scheme can regulate the output voltage of the line network and effectively reduce the harmonic content of the line network, providing a reference for reducing the harmonic content of grid-connected currents in engineering.

An H∞ filter based active damping control strategy for grid-connected inverters with LCL filter applied to wind power system

Since the LCL filter has good performance to attenuate high frequency harmonics, it is widely used in wind power inverters. But it can cause high-frequency oscillations and instability, which limits its application in wind power system. For an LCL-type grid-connected inverter, the conventional capacitor-current-feedback type active damping control strategy can retain the high-frequency characteristics of LCL filter while suppressing the resonant peak effectively, with the drawback of requiring additional current sensors. Therefore, an H∞ filter based active damping strategy of LCL grid-connected inverter is proposed. The state space model and the process noise model of the LCL inverter are built, and then the general model of the standard H∞ control problem is derived, which helps to solve the H∞ filter. The H∞ filter can estimate the filter capacitor current according to sample values of the injected current and the voltage of the point of common coupling, and the estimation accuracy can be guaranteed even if parameter perturbations exist in the LCL filter. The proportional resonant controller is adopted to track the grid current. Therefore, the proposed control strategy can effectively suppress the resonance peak of the LCL filter without additional current sensors and be insensitivity to system parameter perturbations. The simulation and experimental results verify the feasibility and superiority of the proposed method.

Improved Active Disturbance Rejection Control Strategy for LCL-Type Grid-Connected Inverters Based on the Backstepping Method

In the process of grid connection of an LCL inverter, sudden changes in load, high harmonics of the grid voltage, sudden changes in power, and other disturbances often occur. These will cause sharp degradation of the grid-connected power quality, so this paper proposes a new solution to the problem of how to reduce and eliminate disturbances in power quality by using a first-order linear active disturbance rejection control (LADRC) strategy with precise compensation via splitting the total disturbance term. An improved overall direct compensation method for total disturbance was proposed. The use of a subdivision compensation term could avoid the misjudgment arising from the estimation of the overall perturbation by the linear expanded state observer (LESO) within the first few weeks of the feedback when the overall compensation was applied. It aimed to reduce the overshooting caused by the overcompensation of the estimated disturbance term and to shorten the system convergence speed. Backstepping control was introduced to optimize the intermediate quantities of LADRC to estimate the error design outer-loop control law. The controlled quantity tracking the input quantity had excellent characteristics, and could set the desired error range quantity as the purpose of approximation. Therefore, backstepping control was suitable as a feedforward control of the system to preprocess the error in the estimated total disturbance of LESO and feed it into the inner loop improvement LADRC. Secondly, an improved control cascade PWM modulation with a PLL phase-locked loop to regulate the inverter output resulted in the elimination of the effects of internal and external disturbances on the grid-connected current and voltage. Finally, the amplitude–frequency characteristics were analyzed and compared for the trackability and antidisturbance of the improved linear active disturbance rejection controller, showing a good performance of the improved active disturbance rejection. At the same time, comparative simulations were conducted to confirm that the grid-connected current of the LCL inverter could obtain a better stability and grid entry quality in the first-order improved linear active disturbance rejection control.

Passivity-Oriented Design of LCL-Type Grid-Connected Inverters With Luenberger Observer-Based Active Damping

The frequency-domain passivity theory offers an effective way to assess the stability of inverters in a complex grid. In this article, a unified impedance model, suitable for either inverter current control (ICC) or grid current control (GCC) of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -type grid-connected inverters with observer-based capacitor current feedback active damping, is built to facilitate the passivity-based stability assessment and controller parameter design. With the passivity analysis, it is found that when the antiresonant frequency of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter is in certain ranges, i.e., (0.056 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\omega _s$</tex-math></inline-formula> , 0.20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\omega _s$</tex-math></inline-formula> ) for ICC and (0.046 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\omega _s$</tex-math></inline-formula> , 0.23 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\omega _s$</tex-math></inline-formula> ) for GCC, all frequencies’ passive output admittance of the inverter can be achieved via proposed parameter design guidelines. Due to the utilization of the observer and all frequencies’ passive output admittance property, not only extra current sensors for active damping can be saved, but also the inverter can be connected and stably operated in a grid regardless of the grid impedance. The validity of the theoretical analysis and effectiveness of the proposed method are verified by using experimental results on a laboratory prototype.

Indirect Current Control Method Based on Reference Current Compensation of an LCL-Type Grid-Connected Inverter

NPC (neutral point clamped converter) is widely used in medium- and high-voltage grid connection because of its small power devices, simple overall structure, and easy control. This paper presents an improved indirect current control strategy for an LCL-type NPC. Firstly, the reference current is compensated to suppress the resonant peak of the LCL filter and improve the tracking accuracy of the output current. Then, with the relationship between state variables, the control structure is optimized by introducing the non-PLL control method, so that the sensors are reduced to only two sets. Finally, the traditional differential operator is improved to compensate the calculation delay in digital control. The experimental results show that the amplitude of thee grid-connected current is consistent with the reference value, the phase deviation is reduced from 7° to about 1°, and the transient response time is shortened to 1/3 of that of the traditional method.

Robust Delay Compensation Strategy for LCL-Type Grid-Connected Inverter in Weak Grid

Capacitor-current-feedback active damping has been widely adopted in digitally controlled LCL-type grid-connected inverters (GCIs). However, their digital control delay causes the system stability to deteriorate within a negative damping range. Particularly, in a weak grid, such GCIs are prone to be unstable against the grid impedance variation. To address the above problem, a delay compensation strategy is proposed for an LCL-type GCI, in which a second-order low-pass filter (LPF) is inserted into the capacitor-current-feedback path. This increases the virtual-positive-resistance-region from one-sixth of the sampling frequency to almost within the Nyquist frequency. Therefore, the robustness of the grid-connected inverter can be significantly enhanced against the grid impedance variation. Furthermore, to ensure the system stability and improve the injecting current tracking performance, a proportional complex integral (PCI) strategy is adopted. More specifically, a robustness analysis is conducted under discrete domain. Finally, a 2-kW prototype is constructed, and the experimental results confirm the effectiveness of the theoretical expectations.

Capacitor-Current-Feedback With Improved Delay Compensation for LCL-Type Grid-Connected Inverter to Achieve High Robustness in Weak Grid

To attenuate the resonance of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter, capacitor-current-feedback (CCF) active damping has been extensively adopted in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -Type grid-connected inverters. Owing to the appearance of a negative damping region caused by the digital control delay, however, the damping performance has significantly deteriorated, thus the system is susceptible to being unstable under weak grid operation. To address this issue, this paper proposes an improved delay compensation method, which can effectively extend the boundary frequency of the positive equivalent resistance region from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_s</i> /6 to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_s</i> /4. Furthermore, the resonance frequency forbidden region of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter can be eliminated. In doing so, strong robustness against the grid impedance variation and high noise immunity can be achieved. To guarantee the system stability and obtain good control performance after compensation, a detailed parameters design procedure for the current regulator and the CCF coefficient is further presented. At last, experimental results are provided to confirm the theoretical analysis and verify the effectiveness of the proposed delay compensation method.

A resonant damping control and analysis for LCL-type grid-connected inverter

The inverter becomes an essential part in the distributed energy units, where an inductor–capacitor–inductor (LCL) filter is an up-to-date adoption for grid interfacing. However, the challenging issue of resonance related to the LCL-filter worsens the dynamic control characteristics and produces stability threat for the voltage source inverter system. The proper design of inverter control plays a substantial part in ensuring a steady state operation and a high quality of grid injected current according to grid connection codes. This paper offers a different design method of inverter control which alters the inner damping loop structure to enhance the damping and stability features of the inverter system. In the proposed solution, a modified compensator is employed through the LCL filter network and filter capacitor current feedback loop, the resultant augmented plant’s output is then sent back at the reference point of a grid-connected inverter system to damp the unwanted resonance spike. This particular arrangement to form a damping loop is termed as parallel feedforward compensation. The damping loop is implemented by calculating the filter capacitor current, and a first order high-pass filter is employed as a damper in the suggested arrangement. Moreover, for comparative studies and analysis, the conventional filter capacitor current feedback active damping method is selected. In addition, the stability margins and control performance of the open current loop are examined under different filter parameters values. The substantial results of the suggested structure are quicker dynamic response, comparatively improved resonance damping, better tracking performance and higher delay compensation capability. A laboratory prototype is established to confirm the effectiveness of the suggested scheme on the bases of control performance and stability margins.

An Ignored Culprit of Harmonic Oscillation in LCL-Type Grid-Connected Inverter: Resonant Pole Cancelation

Resonant pole cancelation is usually adopted in the current control or active damping to tackle the filter resonance in the LCL-type grid-connected inverter. However, its potential impact on the system stability has not been adequately assessed. This article provides a comprehensive investigation on three typical control schemes that utilize resonant pole cancelation. An important finding is drawn that the resonant pole cancelation violates the so-called internal stability, which can give rise to the undesired harmonic oscillation in grid current. From the physical insights, it is revealed that the resonant pole cancelation does not essentially eliminate the LCL resonance but merely “hide” it in the system, which can still be triggered and, thereby, turns into the culprit of the harmonic oscillation. The theoretical expectation is validated by the simulation and experimental results.

An improved capacitor voltage full feedforward control strategy for LCL‐type grid‐connected inverter based on control delay compensation

For the LCL-type grid-connected inverter, when the capacitor voltage feedforward is applied, the delay in the digital control system could change the phase characteristics of capacitor voltage feedback and affect the stability of the system. To solve the problem, a delay compensation method based on lead compensator is proposed in this paper, which can reduce the impacts of control delay on the capacitor voltage full feedforward control system. In this study, a lead compensator is introduced into the feedforward channel to compensate the phase lag, and the compensation function is designed to compensate for the phase margin but not change the system amplitude gain. The proposed strategy can keep the phase margin of the system above 25° when the grid impedance changes from 0 to 2.6 mH (0.1 p.u.), which enhances the robustness of the grid-connected inverter under weak condition. The grid current distortion caused by grid background harmonics is greatly reduced and is kept within 4%. Moreover, the proposed strategy has fast dynamics while ensuring the system stability. Finally, the simulation and experimental results are presented to verify the effectiveness and robustness of the proposed control strategy.

A Step-by-Step Design Procedure of a Robust Control Design for Grid-Connected Inverter by LCL Filter in a Weak and Harmonically Distorted Grid

The current-controlled grid-connected inverter with LCL filter is widely utilized in the distributed power generation systems at remote places with weak grids. Oscillations in weak grids have raised the stability issue in grid-tied inverter systems, and the quality of the injected power into the grid is highly affected by the variations of grid-side inductance. In this study, a multi-loop control has been applied in the LCL-type grid-connected inverter based on capacitor current feedback control method. Computational delays in the control unit can remarkably influence the damping capability of this method. The accuracy of the modeling has been increased by considering the computational delays of the capacitor current active damping inner loop as well as the grid current outer loop. The impact of the computational delay is eliminated by the proposed approach, and the performance of the control system has been remarkably enhanced. The harmonic attenuation of the grid voltage has been improved by utilizing virtual series and parallel impedances, which leads to strong stability-robustness and high harmonic-rejection-ability even in severe variations of the grid-side inductance. Simulation results verify the credibility of the proposed approach, and the quality of the injected power is proved to be improved in a weak and harmonically distorted grid.

Notch correction strategy of LCL-type grid-connected inverters based on GCFAD

Considering the control delay, grid-current-feedback-active-damping(GCFAD) method can ensure the stability of the system through the reasonable design of parameters, but when the grid impedance changes, the system may be unstable because the LCL resonance peak cannot be effectively suppressed. In this paper, a notch correction strategy is proposed. The amplitude drop characteristic of the notch filter is used to effectively suppress the LCL resonance peak in this paper, so as to ensure that the system has sufficient amplitude margin when the grid impedance changes. The robustness of the system is greatly improved to the grid impedance when the GCFAD method is adopted. Finally, the simulation model verifies the effectiveness of the proposed strategy.

Parallel Resonance Mechanism Analysis and Suppression Method for LCL Type Grid-Connected Inverter

Parallel Resonance Mechanism Analysis and Suppression Method for LCL Type Grid-Connected Inverter

Passivity-Based Design of Repetitive Controller for $LCL$-Type Grid-Connected Inverters Suitable for Microgrid Applications

A repetitive controller (RC) is one of the most promising candidates for harmonic compensation when designing the current controller of grid-connected inverters. The discrete-time closed-loop transfer function based frequency response method can be used to shape RC, which is applicable for a single inverter connecting to well-known inductive-impedance grids, but cannot guarantee the stability in the case of capacitive-impedance grids or complex systems, such as microgrids. To address this issue, this article presents a new passivity-based design method for an RC for LCL-type grid-connected inverters with either inverter-side or grid-side current control. With the proposed design guidelines, the output admittance of the RC-controlled inverter is tuned to be passive in all frequencies, so that it can be plug and play connected to a grid regardless of grid impedance. Meanwhile, thanks to infinite gains of an RC at the fundamental frequency and its multiples as the high-quality grid injected current in accordance with IEC 61000-3-4 standard is ensured even in the presence of distorted grid conditions. Finally, experimental results with an established lab prototype are provided to verify the effectiveness of the proposed approach.

A novel harmonic impedance compensator based on POHMR-type RC under gird voltage distortion

The LCL-type grid-connected inverter can suppress high-frequency current harmonics but the inject grid current of the inverter will be worse under grid voltage distortion, therefore harmonic impedance compensation technology is required for this condition. The proportional Multi Resonant (PMR) controller is a feasible method for this purpose, but this scheme needs a large calculation burden and complex design. To overcome these drawbacks, a novel Proportional Odd harmonic Multi Resonant-type Repetitive Controller (POHMR-type RC) is proposed to compensate for the harmonic impedance, which means the grid voltage distortion hardly influence injected grid current. Compared to the PMR controller, the proposed scheme omits the even harmonic resonant controllers and has much more odd harmonic resonant controllers, which also reduces the memory space consumption, calculation burden and controller complexity. The proposed scheme also has better dynamic performance than conventional RC. When the proposed scheme is proposed, the inject grid current is still sinusoidal under strong grid voltage distortion and the transient response is desired. Experimental results verify the feasibility of the POHMR-type RC.