Inverter-side Current Feedback Research Articles

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.

Analysis and comparison of two methods for reducing the impact of time delay on inverter stability

To more effectively select the method that reduces the impact of time delay in digital control on inverter stability and improve the robustness of an inverter, a discrete model of the inverter with inverter-side current feedback control is first established in the two-phase stationary frame in this paper. Then, based on this model, systematic study is carried out on the inverter stability under half sampling time delay method and linear predictor method respectively. It is found that, compared with one sampling time delay method, half sampling time delay method and linear predictor method are both able to reduce the influence of time delay on inverter stability. Moreover, analysis shows that half sampling time delay method have a better performance than linear predictor method. Finally, experiment results are presented to prove the correctness of theoretical analysis.

Active Damping Control for LCL Filters With Inverter-Side Current Feedback Only

In the past decade, extensive active damping solutions based on state variable feedback have been proposed to damp <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> resonance. In this letter, an innovative control strategy, which can achieve overcurrent protection, injected power control, and active damping at the same time, is proposed with the inverter-side current feedback only. Moreover, to avoid the mathematical limitations when solving characteristic roots of a third-order system, the damping-factor-based design method is proposed for the first time, by analyzing its equivalent circuit. The optimal damping factor, which is recommended to be positive and as large as possible, is searched to achieve superior resonance suppression effect. Finally, the proposed control and design method is verified by experimental results and shows good robustness both in stiff and ultraweak grid conditions.

Robust Active Damping Control for LCL-Type Shunt Active Power Filters

A dual-loop current control is discussed for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -type shunt active power filters (APFs), where inverter-side current feedback and point of common coupling (PCC) voltage feedforward both provide active damping (AD) effects for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> resonance. However, two ADs are equivalently applied to different elements of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filter with narrow damping boundaries, which causes less robustness against grid impedance variation. Hence, this paper has presented a comprehensive approach to strengthen the system robustness against grid impedance variation. On the basis of phase compensated resonant unit, a delay-compensation scheme is firstly proposed to extend the damping boundary of inverter-side current feedback up to almost Nyquist frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{s}/2$ </tex-math></inline-formula> ). Theory analysis considering PCC voltage feedforward indicates that the control system can keep robust to grid impedance when the initial <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> resonance frequency falls between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{s}/3$ </tex-math></inline-formula> and the extended damping boundary. Moreover, a typical weighted proportional-resonant feedforward scheme is suggested to prevent high-frequency resonant units from inducing system instability under a large grid impedance. We finally validate the effectiveness of the proposed approach through experiments.

Feedforward phase compensation method of LCL grid‐connected inverter based on all‐pass filter in weak grid

Inverter-side current feedback control (ICFC) has been extensively adopted in distributed generation systems because of its simple implementation and better consistency with actual operating conditions. At the same time, when there are a large number of background harmonics in the grid, capacitor voltage feedforward (CVF) is usually added to the system for its suppression. This method could make the system keep the first-order characteristics under the analogue control. However, under digital control, due to the digital control delay, a reverse resonance peak will be generated in the loop gain, which makes the system unstable in the weak grid. In order to solve the aforementioned problems, this study proposes a feedforward phase compensation method of LCL grid-connected inverter based on the all-pass filter (AF). By introducing AF into the CVF channel, the phase lag in the range of reverse resonance peak frequency is compensated, so as to enhance the robustness of the system in the weak grid. At the same time, this study gives the detailed design process of the proposed method. Experimental results on a 3-kW prototype are provided, and the effectiveness of the proposed control method is verified..

Current Control of LCL-Type Shunt APFs: Damping Characteristics, Stability Analysis, and Robust Design Against Grid Impedance Variation

This article is focused on a dual-loop current control (grid current loop and inverter-side current loop) with the point of common coupling (PCC) voltage feedforward control for digitally controlled LCL-type shunt active power filters (APFs). However, both inverter-side current feedback and PCC voltage feedforward introduce active damping (AD) effects on different LCL elements, leading to uncertain system damping characteristics and robustness against grid impedance variation. To address this issue, the accurate system damping characteristics are evaluated through open-loop pole analysis, and the benchmarking of system stability and robustness is revealed with respect to various control and system parameters. Moreover, the system instability may occur under a large grid impedance due to high-frequency resonant units of current controllers, and an improved PCC voltage feedforward control is presented to avoid this problem. Based on these findings, a robust design procedure is proposed to obtain proper control parameters and enhance system stability and robustness. Experimental results on a 30-kVA APF prototype have finally validated the correctness and effectiveness of the proposed analysis and method.

A Systematic Approach via IIR Filters for Enhancing the Robustness of LCL-Type Shunt Active Power Filters to Grid Impedance

A dual-loop current control method (grid current loop and inverter-side current loop) is promising for digitally controlled LCL -type shunt active power filters (APFs), due to its inherent active damping effect for LCL resonance. However, the damping region is only up to one-sixth of the sampling frequency $(f_{s}/ 6)$ , which leads to limited system robustness against grid impedance variation at the point of common coupling (PCC). Hence, in this study, the robustness of such control is discussed through mathematic derivations and diagram analysis, which reveals that both the system delays and PCC voltage feedforward affect the system robustness. On the basis of theoretical analysis, a systematic approach via infinite impulse response (IIR) filters has been proposed for enhancing the robustness of grid impedance. Thereof, a second-order IIR filter-based inverter-side current feedback is proposed to widen the damping region up to almost the Nyquist frequency $(f_{s}/ 2)$ , while a band-stop IIR filter-based PCC voltage feedforward is proposed to achieve the superior stability margins under a large grid impedance in comparison with unit feedforward. Finally, experimental results on a 30-kVA APF prototype have verified the effectiveness of the proposed approach.

A Novel Inverter-Side Current Control Method of $LCL$ -Filtered Inverters Based on High-Pass- Filtered Capacitor Voltage Feedforward

LCL filters have been widely used as the interface between inverters and power grids, but damping methods must be taken to solve the resonance of LCL filters. Considering overcurrent protection and inherent active damping, this paper focuses on the inverter-side current feedback (ICF) control. Some literatures have proposed that the capacitor voltage feedforward (CVF) is beneficial to the stability of the ICF control. However, in practical applications, it is found that the ICF control with CVF mainly faces two problems. Firstly, the CVF can be seen as a positive feedback loop, and it tends to cause low-frequency oscillation, especially in weak grid. Secondly, low-frequency harmonics in the grid voltage can easily distort the grid-side current of the inverter, even though the inverter-side current has been regulated well. In order to solve these two problems simultaneously, this paper proposes a novel inverter-side current control method, which adds a high-pass filter into the CVF path. In this paper, the design process of the high-pass filter parameters is analyzed, and the specific voltage feedforward structure of the proposed method to avoid the inverter startup inrush current is also discussed in detail. Simulations in MATLAB/SIMULINK and experiments based on a 6.6 kW prototype have been utilized to verify the effectiveness of the proposed method.

Analysis and Validation for an Inverter-side Current Controller in LCL Grid-connected Power Systems

Transformerless grid-connected inverters offer greater efficiencies when transferring power from renewable energy sources to the electrical grid. If the grid-inverter connection is done with an LCL filter, high attenuation of switching harmonics is achieved while preserving a small-size output filter. However, damping must be included in the controller to assure closed-loop stability. This paper proposes a reference computation methodology for the inverter-side current feedback in a photovoltaic (PV) generation system connected to the grid through an LCL filter. Theoretical analysis of the closed-loop system stability and of the steady-state performance are presented as well as experimental validation of the closed-loop performance. The feedback controller includes active damping and relies on a resonant control structure which improves the ability of dealing with grid harmonic distortion. The controller uses a reduced set of measurements, which requires the inverter-side current and grid voltage only, and assures a power factor close to unity.

The Dual-Current Control Strategy of Grid-Connected Inverter With LCL Filter

In the renewable energy generation systems, the grid-connected inverter with LCL filter (GCI- LCL ) is an important device to realize dc–ac power conversion and connect the distributed power generation system and power grid. However, the resonance problems of the LCL filter may bring instability to GCI- LCL . Besides, the grid impedance and the time delay in digital controllers also have a harmful influence on the stability of GCI- LCL . To solve these problems, a dual-current active damping control strategy based on the inverter-side current and grid-connected current feedback for GCI- LCL is proposed in this paper, and the systematic parameters design process is investigated. Compared with the conventional damping strategies, a resonance suppression for the LCL filter, a larger stability margin, and a better anti-interference capability in a weak grid can be achieved under the proposed strategy. In addition, by analyzing the active damping effect of the proposed strategy, a simplified model in a low-frequency band for GCI- LCL is proposed to enhance the anti-interference capability in the weak grid and simplify the parameters design process. Finally, a GCI- LCL prototype is constructed and the experiment results verify the correctness and feasibility of the analysis and proposed strategy.

Control Strategy Based on $\text{H}\infty$ Repetitive Controller With Active Damping for Islanded Microgrid

To overcome the drawbacks of the proportional plus integral (PI) control based strategy, i.e., limitation for harmonic suppression and performance deterioration under load variation, a voltage control strategy based on the $\text{H}\infty $ repetitive controller with inverter-side current feedback damping is proposed for the inverter in islanded microgrid. The $\text{H}\infty $ repetitive controller provides good robustness against parameter perturbation and ensures the output voltage of low total harmonic distortion. Compared with other damping methods, a good resonance suppression and a better attenuation of the high-order harmonics can be achieved under the proposed strategy, which further improves the control performance of the voltage repetitive control system. A state space model including the damping coefficient is built to implement order reduction of the high-order controller, thereby simplifying the digital implementation while nearly without performance degradation. Moreover, the frequency response analysis shows that the order reduction can improve the adaptability of the controller to parameter variation. Finally, experiments and simulations verify the correctness of the analysis.

Mitigation of Grid-Current Distortion for LCL-Filtered Voltage-Source Inverter With Inverter-Current Feedback Control

LCL filters feature low inductance; thus, the injected grid current from an LCL -filtered voltage-source inverter can be easily distorted by grid-voltage harmonics. This problem is especially tough for the control system with inverter-side current feedback (ICF), since the grid-current harmonics can freely flow into the filter capacitor. In this case, because of the loss of harmonic information, traditional harmonic controllers fail to mitigate the grid-current distortion. Although this problem may be avoided using the grid-voltage feedforward scheme, the required differentiators may cause the noise amplification. In light of the above issue, this paper develops a simple method for the ICF control system to mitigate the grid-current harmonics without extra sensors. In the proposed method, resonant harmonic controllers and an additional compensation loop are adopted at the same time. The potential instability introduced by the compensation loop can be avoided through a special design of the compensation position. Finally, the effectiveness of the proposed method for harmonic rejection is verified by detailed experimental results.

Study of a Current Control Strategy Based on Multisampling for High-Power Grid-Connected Inverters With an LCL filter

The high-power grid-connected inverters with LCL filters have been widely used. Current control plays a key role in the grid-connected inverter control system. To cope with the inherent resonance of the LCL filter, active damping (AD) methods are usually employed. However, the AD performance is impaired by control delays which are introduced in the digital control implementation process. Besides, the lag phase due to control delays limits the system bandwidth and stability margin. The effects of control delays are more noteworthy in the high-power grid-connected inverter due to its low switching frequency. This paper investigates the current control strategy based on multisampling for the high-power grid-connected inverter with the LCL filter. First, the multisampled AD scheme is studied, which can reduce control delays effectively and improve AD performance. Besides, the multisampled control without additional damping whether passive or active is researched. Through the inverter-side current feedback, the system can realize single-current-loop control based on multisampling. Thus, the control system is simplified and it can be stable, and achieve better dynamic performance. Finally, experimental results show that the proposed control schemes are effective.

A Grid Voltage Feed-forward Control Strategy Based on The Virtual Impedance

This paper initially investigates the conventional LCL filter grid-connected inverts system, which introduces the control strategy with inverter-side current feedback, and analyzes the deficiencies within the strategy. The stability of the controller can be improved by adopting the active damping scheme. However, the output current of the inverter is severely influenced by the external grid voltage distortion and the grid low-order harmonic voltage. In order to reduce this impact and gain high-quality power, a novel control strategy is proposed in this paper, in which the virtual complex impedance is introduced to feed grid voltage forward. By using the proposed control strategy, the total harmonic distortion can be greatly decreased, allowing the grid current to synchronize the reference current well. In addition, the modulation of the virtual complex impedance by controller can also quantificationally regulate the equivalent output impedance, which brings great convenience for inverters system designers to calculating and devising. Simulation results are presented to prove the validity and improvements achieved by the proposed controller.