Inverter Current Feedback Research Articles

Unified Active Damping Strategy Based on Generalized Virtual Impedance in LCL-Type Grid-Connected Inverter

Extensive solutions 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> filter resonance. However, the time delay caused by computation and modulation process aggravates the complexity of current loop analysis. In this paper, a generalized virtual impedance model for six kinds of commonly used active damping methods is proposed, and the physical meaning is established by introducing the metric of “damping factor”. To ensure system robustness in a wide range of grid conditions, a hybrid active damping strategy that combines inverter current feedback and capacitor voltage feedforward is proposed. Importantly, this method only requires inverter current and capacitor voltage sensors, which are the basic variables for over-current protection, power control and synchronization. The superposition theorem is also utilized to analyze the damping capability provided by parasitic resistances, current feedback loop and voltage feedforward loop. Finally, experimental results verify the feasibility and robustness of the proposed damping method.

A General Single-Sensor Damping Framework for LCL-Equipped High-Speed PMSM Drives

This letter proposes a general single-sensor active damping framework for LCL-equipped high-speed permanent magnet synchronous machines. By the proposed method, arbitrary damping assignment and stability for the LCL resonance within the Nyquist frequency are achieved with only the inverter-current feedback or motor-current feedback. Moreover, the simplified analytical expression between the physical parameters and the damping performance enables automatic tuning for different drive systems.

An Improved Active Damping Method Based on Single-Loop Inverter Current Control for LCL Resonance in Grid-Connected Inverters

This paper investigates active damping of LCL filter resonance in grid-connected inverters with only inverter current feedback control, since it only needs to sample one current to realize both current control and inverter protection. The traditional single-loop inverter current control (SLICC) can damp the LCL filter resonance actively. However, if the control delay is considered in digital control, the system stability will depend on the ratio of the LCL resonance frequency f res to the sampling frequency f s , and the valid damping region is only up to f s / 6 . Considering that the design region of the LCL resonance frequency f res is up to f s / 2 , the system can easily become unstable due to the LCL resonance frequency shifting. Thus, this paper proposes an improved active damping method based on SLICC, including the asymmetric regular sampling method and delay compensation method. The improved sampling method minimizes the control delay without introducing a switching ripple, and the delay compensation method further compensates for the delay effect. With a proper parameter design, the upper limit of the valid damping region is extended up to f s / 2 , which can cover all the possible resonance frequencies, and it has inherent robustness against grid-impedance variation. Finally, a few simulations in MATLAB/SIMULINK and experiments based on a 6 kW prototype are performed to verify the theoretical analysis.

H ∞ robust control with improved harmonics suppression for inverter‐based distributed generation systems

Voltage source inverter is a key element in an inverter-based distributed generation (DG) system for the integration of renewable energy resources with an existing power grid. Interconnection standards for the requirements of inverter output current quality are one of the foremost demands imposed by distribution system operators. The operation of a stabilised DG system with LCL -filter meeting current quality requirements is challenging due to variation in system parameters and non-ideal grid voltages. In this context, this study proposes a robust controller design and modified control structure to improve current quality in the presence of distorted grid voltages. The controller is designed based on general mixed sensitivity problem formulation to achieve robustness against system parameters variation and ensure high-quality injected current under normal grid voltage. The control performance of the designed robust controller with a feed-forward loop is investigated under distorted grid voltage. Furthermore, an inverter current compensation is proposed, employed by an additional inverter current feedback loop through the high-pass compensator. The current quality investigation with variation in different parameters and low power operation shows enhanced control performance of the proposed solution under distorted as well as normal grid voltages. The main features of the proposed scheme are the simplicity in design, easy implementation, and better harmonics rejection capability without limiting the control loop stability. The simulation and experimental results validate the effectiveness of the proposed method .

Design and control of an LCL-type single-phase grid-connected inverter withinverter current feedback using the phase-delay method

In this study, a novel single-phase grid-connected microinverter system and its control applications are introduced for solar energy systems. The proposed system consists of two stages to transfer solar power to the grid. In the first stage, an isolated high-gain DC/DC converter is used to increase low solar panel output voltage. In the second stage, an inverter is used to supply a sinusoidal current to the grid. Moreover, a proportional resonant controller is adopted to reduce grid current total harmonic distortions (THDs) and an LCL filter is used to provide better harmonic attenuation. However, the ratio between the sampling frequency $f_{s}$ and the resonance frequency $f_{res\, }$should be greater than 6 in the LCL filter with the inverter current feedback for a stable system. In order to obtain higher phase margins, the sampling frequency should be increased, which increases the inverter switching frequency. The present study shows that $f_{s}/f_{res} $can be lower than 6 by placing a phase delay on the inverter current feedback path to guarantee adequate stability margins. The effectiveness and feasibility of the proposed method are confirmed by the experimental test results based on a 300-W laboratory prototype.

A novel dual closed-loop control scheme based on repetitive control for grid-connected inverters with an LCL filter

Grid-connected inverters with LCL filters need high steady-state control accuracy, fast dynamic response performance, and strong robustness to guarantee the power quality. However, there are many problems in traditional control strategies that restrict improvements to control system performance, such as poor dynamic performance of traditional single-repetitive control, large ripples, low steady-state accuracy of inverter current feedback based repetitive dual-loop control or grid-current feedback based single-loop proportional-integral control. In this paper, a novel dual closed-loop repetitive control strategy based on grid current feedback is proposed for single-phase grid-connected inverters with LCL filters. The proportional-integral inner loop is stabilized by using an inherent one-beat delay achieved by digital controller. Based on the inner loop system, a detailed design scheme of a repetitive controller is presented, through which direct control of the grid current is realized, the reference is tracked perfectly to a zero phase shift, and high-attenuation gain is achieved in the high frequency range. In particular, the gird-voltage feed forward control and current reference feedforward control are adopted to suppress grid-voltage disturbance and increase dynamic tracking performance. Finally, the simulation and experimental results show that the proposed method has the advantages of high steady-state accuracy, fast dynamic response, and anti-disturbance ability.

Current Control of Grid-Connected Inverter With LCL Filter Based on Extended-State Observer Estimations Using Single Sensor and Achieving Improved Robust Observation Dynamics

In the context of distributed generation and renewable energy penetration toward smart grid, grid-connected inverter with LCL filter has drawn many attentions, whose current control conventionally requires several sensors to realize active damping and grid synchronization. In this paper, a novel full status observation strategy based on extended state observer (ESO) is proposed using inverter current feedback only and without grid voltage sensor. The proposed observation strategy contains observation and transformation process. Unlike conventional Luenberger observer, by using ESO, the system parameters do not appear in observation process, thus the observer dynamics and parameter mismatch error can be separately handled, providing more robust observation dynamics during parameter variation. Parameter mismatch study was carried out, and it is found that purposely choosing the observer parameters smaller than the real value can achieve relatively low estimation error and a large adaption range for parameter variation. The proposed observation strategy is simple to implement, without the need of expert knowledge-based parameter tuning such as pole placement. Experimental tests validated that the proposed observation-based control is able to give satisfactory performance in both dynamic and steady states, as well as adaption for system parameter variation.

Design and Analysis of Robust Active Damping for LCL Filters Using Digital Notch Filters

Resonant poles of LCL filters may challenge the entire system stability especially in digital-controlled pulse width modulation (PWM) inverters. In order to tackle the resonance issues, many active damping solutions have been reported. For instance, a notch filter can be employed to damp the resonance, where the notch frequency should be aligned exactly to the resonant frequency of the LCL filter. However, parameter variations of the LCL filter as well as the time delay appearing in digital control systems will induce resonance drifting, and thus break this alignment, possibly deteriorating the original damping. In this paper, the effectiveness of the notch filter-based active damping is first explored, considering the drifts of the resonant frequency. It is revealed that when the resonant frequency drifts away from its nominal value, the phase lead or lag introduced by the notch filter may make itself fail to damp the resonance. Specifically, the phase lag can make the current control stable despite of the resonant frequency drifting, when the grid current is fed back. In contrast, in the case of an inverter current feedback control, the influence of the phase lead or lag on the active damping is dependent on the actual resonant frequency. Accordingly, in this paper, the notch frequency is designed away from the nominal resonant frequency to tolerate the resonance drifting, being the proposed robust active damping. Simulations and experiments performed on a 2.2-kW three-phase grid-connected PWM inverter verify the effectiveness of the proposed design for robust active damping using digital notch filters.

Pseudo-Derivative-Feedback Current Control for Three-Phase Grid-Connected Inverters With &lt;italic&gt;LCL &lt;/italic&gt; Filters

Pseudo-derivative-feedback (PDF) control is, for the first time, applied to the current control of three-phase grid-connected inverters with LCL filters, which significantly improves the transient response of the system to a step change in the reference input through the elimination of overshoot and oscillation. Two PDF controllers with different terms in the inner feedback path are developed for an inverter current feedback system and a grid current feedback system, respectively. For the inverter current feedback system, a simple PDF controller with a proportional term is used. The influence of the controller parameters on the transient performance is discussed in detail. Compared with a PI controller, the PDF controller is able to eliminate the transient overshoot and oscillation easily, while maintaining a fast response. For the gird current feedback system, a PDF controller with a proportional term and a second-order derivative is developed. The implementation and design of the PDF controller are presented. Active damping is achieved with only the feedback from the grid current, and at the same time, the system transient response is improved. Both theoretical analysis and experimental results verify the advantages of the PDF control over PI control methods.

임의 파형 발생기를 위한 단일 루프 전압 제어기 설계

This study presents a design method for a single-loop voltage controller that is suitable for an arbitrary waveform generator (AWG). The voltage control algorithm of AWG should ensure high dynamic performance and should attain sufficient robustness to disturbances such as inverter nonlinearity, sensor noise, and load current. By analyzing the power circuit of AWG, control limitation and control target are presented to improve the dynamic performance of AWG. The proposed voltage control algorithm is composed of a single-loop output voltage control, an inverter current feedback term to improve transient response, and a load current feedforward term to prevent voltage distortion. The guideline for setting control gain is presented based on output filter parameters and digital time delay. The performance of the proposed algorithm is proven by experimental results through comparison with the conventional algorithm.

Delay-Dependent Stability of Single-Loop Controlled Grid-Connected Inverters with &lt;italic&gt;LCL&lt;/italic&gt; Filters

LCL filters have been widely used for grid-connected inverters. However, the problem that how time delay affects the stability of digitally controlled grid-connected inverters with LCL filters has not been fully studied. In this paper, a systematic study is carried out on the relationship between the time delay and stability of single-loop controlled grid-connected inverters that employ inverter current feedback (ICF) or grid current feedback (GCF). The ranges of time delay for system stability are analyzed and deduced in the continuous s -domain and discrete z -domain. It is shown that in the optimal range, the existence of time delay weakens the stability of the ICF loop, whereas a proper time delay is required for the GCF loop. The present work explains, for the first time, why different conclusions on the stability of ICF loop and GCF loop have been drawn in previous studies. To improve system stability, a linear predictor-based time delay reduction method is proposed for ICF, while a time delay addition method is used for GCF. A controller design method is then presented that guarantees adequate stability margins. The delay-dependent stability study is verified by simulation and experiment.