Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter
TL;DR: In this article, a capacitor-current-feedback active damping with reduced computation delay is proposed, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant.
Abstract: This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.
Citations
More filters
TL;DR: In this article, the authors present an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived.
Abstract: The interconnection stability of a grid-connected voltage-source converter (VSC) can be assessed by the passivity properties of the VSC input admittance. If critical grid resonances fall within regions where the input admittance acts passively, i.e., has nonnegative real part, then their destabilization is generally prevented. This paper presents an overview of passivity-based stability assessment, including techniques for space-vector modeling of VSCs whereby expressions for the input admittance can be derived. Design recommendations for minimizing the negative-real-part region are given as well.
493 citations
TL;DR: In this article, the authors present an overview of the virtual-impedance-based control strategies for voltage-source and current-source converters, and a number of practical examples are demonstrated to illustrate the feasibility of virtual impedances.
Abstract: The virtual impedance concept is increasingly used for the control of power electronic systems. Generally, the virtual impedance loop can either be embedded as an additional degree of freedom for active stabilization and disturbance rejection, or be employed as a command reference generator for the converters to provide ancillary services. This paper presents an overview of the virtual-impedance-based control strategies for voltage-source and current-source converters. The control output impedance shaping attained by the virtual impedances is generalized first using the impedance-based models. Different virtual impedances and their implementation issues are then discussed. A number of practical examples are demonstrated to illustrate the feasibility of virtual impedances. Emerging applications and future trends of virtual impedances in power electronic systems conclude this paper.
468 citations
Cites background from "Capacitor-Current-Feedback Active D..."
...By changing the exponential function of Gd(s) into a trigonometric form, the virtual impedance Zvi,2C i(s) in (22) is derived as [46]...
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TL;DR: In this article, a systematic study on the relationship between the time delay and stability of single-loop controlled grid-connected inverters that employ inverter current feedback or grid current feedback (GCF) was carried out, and the ranges of time delay for system stability were analyzed and deduced in the continuous s-domain and discrete z-domain.
Abstract: 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.
315 citations
TL;DR: In this article, the authors investigated active damping of LCL-filter resonance in a grid-connected voltage-source converter with only grid-current feedback control and showed that the proposed damping technique with a negative high-pass filter along its damping path is equivalent to adding a virtual impedance across the grid-side inductance.
Abstract: This paper investigates active damping of LCL- filter resonance in a grid-connected voltage-source converter with only grid-current feedback control. Basic analysis in the s -domain shows that the proposed damping technique with a negative high-pass filter along its damping path is equivalent to adding a virtual impedance across the grid-side inductance. This added impedance is more precisely represented by a series RL branch in parallel with a negative inductance. The negative inductance helps to mitigate phase lag caused by time delays found in a digitally controlled system. The mitigation of phase-lag, in turn, helps to shrink the region of nonminimum-phase behavior caused by negative virtual resistance inserted unintentionally by most digitally implemented active damping techniques. The presented high-pass-filtered active damping technique with a single grid-current feedback loop is thus a more effective technique, whose systematic design in the z -domain has been developed in this paper. For verification, experimental testing has been performed with results obtained matching the theoretical expectations closely.
301 citations
Cites background from "Capacitor-Current-Feedback Active D..."
...open-loop poles will, in turn, introduce a nonminimum-phase closed-loop behavior to the system [7]....
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TL;DR: A comprehensive overview on the contributions and their classification on the inverter- and grid-side damping measures are presented and some promising damping methods for industrial applications will be discussed.
Abstract: Grid-tied voltage source inverters using LCL filter have been widely adopted in distributed power generation systems (DPGSs). As high-order LCL filters contain multiple resonant frequencies, switching harmonics generated by the inverter and current harmonics generated by the active/passive loads would cause the system resonance, and thus the output current distortion and oscillation. Such phenomenon is particularly critical when the power grid is weak with the unknown grid impedance. In order to stabilize the operation of the DPGS and improve the waveform of the injected currents, many innovative damping methods have been proposed. A comprehensive overview on those contributions and their classification on the inverter- and grid-side damping measures are presented. Based on the concept of the impedance-based stability analysis, all damping methods can ensure the system stability by modifying the effective output impedance of the inverter or the effective grid impedance. Classical damping methods for industrial applications will be analyzed and compared. Finally, the future trends of the impedance-based stability analysis, as well as some promising damping methods, will be discussed.
279 citations
Cites background from "Capacitor-Current-Feedback Active D..."
...[111]–[113] or the other optimization methods [58], [75], [78]....
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...The multiloop ADs include the virtual resistor [73]–[75], the capacitor current feedback [76]–[81], the capacitor voltage feedback [82]–[85], and the LC-trap voltage feedback [86]....
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References
More filters
TL;DR: An overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines is given and the possibility of compensation for low-order harmonics is discussed.
Abstract: Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control
4,655 citations
"Capacitor-Current-Feedback Active D..." refers background in this paper
...Generally, the phase angle of v′ g is obtained through a phase-locked loop (PLL), and the current amplitude reference is generated by the outer voltage loop [1]....
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...As an interface between the DPGSs and power grid, a grid-connected inverter plays an important role in injecting high-quality power into the grid [1]....
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30 Sep 2001
TL;DR: In this article, a step-by-step procedure for designing the LCL filter of a front-end three-phase active rectifier is proposed to reduce the switching frequency ripple at a reasonable cost, while at the same time achieving a high-performance front end rectifier.
Abstract: This paper proposes a step-by-step procedure for designing the LCL filter of a front-end three-phase active rectifier. The primary goal is to reduce the switching frequency ripple at a reasonable cost, while at the same time achieving a high-performance front-end rectifier (as characterized by a rapid dynamic response and good stability margin). An example LCL filter design is reported and a filter has been built and tested using the values obtained from this design. The experimental results demonstrate the performance of the design procedure both for the LCL filter and for the rectifier controller. The system is stable and the grid current harmonic content is low both in the lowand high-frequency ranges. Moreover, the good agreement that was obtained between simulation and experimental results validates the proposed approach. Hence, the design procedure and the simulation model provide a powerful tool to design an LCL-filter-based active rectifier while avoiding trial-and-error procedures that can result in having to build several filter prototypes.
2,147 citations
"Capacitor-Current-Feedback Active D..." refers background in this paper
...This passive damping solution is very simple and highly reliable, but it results in power loss [3], [4]....
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...Specifically, fr is typically required to be lower than one half of the switching frequency fsw to ensure the effective attenuation of the switching harmonics [3], [29], and fs is generally set to be 2fsw [10], [11]....
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TL;DR: In this paper, the authors analyze the stability problems of grid connected inverters used in distributed generation and demonstrate that the use of active damping helps to stabilise the system in respect to many different kinds of resonances.
Abstract: The aim of this paper is to analyze the stability problems of grid connected inverters used in distributed generation. Complex controllers (e.g., multiple rotating dq-frames or resonant-based) are often required to compensate low frequency grid voltage background distortion and an LCL-filter is usually adopted for the high frequency one. The possible wide range of grid impedance values (distributed generation is suited for remote areas with radial distribution plants) challenge the stability and the effectiveness of the LCL-filter-based current controlled system. It has been found out and it will be demonstrated in this paper that the use of active damping helps to stabilise the system in respect to many different kinds of resonances. The use of active damping results in an easy plug-in feature of the generation system in a vast range of grid conditions and in a more flexible operation of the overall system able to manage sudden grid changes. In the paper, a vast measurement campaign made on a single-phase system and on a three-phase system used as scale prototypes for photovoltaic and wind turbines, respectively, validate the analysis.
997 citations
"Capacitor-Current-Feedback Active D..." refers background in this paper
...As reported in [16], a 40% drop of the resonance frequency can be yielded as the grid impedance varies up to a typical 10% per-unit (PU)....
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...Therefore, it is necessary to evaluate the system robustness against the variation of fr , which is commonly caused by the variation of Lg [16]....
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...Generally, the grid impedance at the point of common coupling (PCC) mainly consists of inductance and resistance [16]....
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...All the controller parameters are properly designed with assumption of Lg = 0, and the system stability is examined with Lg varies up to 10% PU, which corresponds to a short-circuit ratio of 10 [16]....
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...To deal with a typical ±1% variation of the grid fundamental frequency [16], ωi = 1%·2πfo = π rad/s is set....
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TL;DR: In this article, the authors present an analytical method to determine the best possible gains that can be achieved for any class of practical linear AC current controller, including stationary frame PI regulators, stationary frame P+ resonant (PR) controllers, and synchronous d- q frame controllers.
Abstract: Current regulation plays an important role in modern power electronic AC conversion systems The most direct strategy to regulate such currents is to use a simple closed loop proportional-integral (PI) regulator, which has no theoretical stability limits as the proportional and integral gains are increased, since it is only a second order system However, pulsewidth modulation (PWM) transport and controller sampling delays limit the gain values that can be achieved in practical systems Taking these limitations into account, this paper presents an analytical method to determine the best possible gains that can be achieved for any class of practical linear AC current controller The analysis shows that the maximum possible proportional gain is determined by the plant series inductance, the DC bus voltage and the transport and sampling delays, while the maximum possible integral gain is determined primarily by the transport and sampling delays The work is applicable to stationary frame PI regulators, stationary frame controllers with back electromotive force compensation, stationary frame P+ resonant (PR) controllers, and synchronous d- q frame controllers, since they all have identical proportional and integral gains that must be optimized for any particular application
655 citations
"Capacitor-Current-Feedback Active D..." refers background in this paper
...has been updated, and it is definitely half sampling period [11]....
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...Specifically, fr is typically required to be lower than one half of the switching frequency fsw to ensure the effective attenuation of the switching harmonics [3], [29], and fs is generally set to be 2fsw [10], [11]....
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Book•
01 Jan 2006TL;DR: This book presents the reader, whether an electrical engineering student in power electronics or a design engineer, a selection of power converter control problems and their basic digital solutions, based on the most widespread digital control techniques, to give a basic, introductory knowledge of thedigital control techniques applied to power converters.
Abstract: This book presents the reader, whether an electrical engineering student in power electronics or a design engineer, a selection of power converter control problems and their basic digital solutions, based on the most widespread digital control techniques. The presentation is primarily focused on different applications of the same power converter topology, the half-bridge voltage source inverter, considered both in its single- and three-phase implementation. This is chosen as the test case because, besides being simple and well known, it allows the discussion of a significant spectrum of the most frequently encountered digital control applications in power electronics, from digital pulse width modulation (DPWM) and space vector modulation (SVM), to inverter output current and voltage control, ending with the relatively more complex VSI applications related to the so called smart-grid scenario. This book aims to serve two purposes: (1) to give a basic, introductory knowledge of the digital control techniques applied to power converters; and (2) to raise the interest for discrete time control theory, stimulating new developments in its application to switching power converters.
537 citations
"Capacitor-Current-Feedback Active D..." refers background in this paper
...mental component) without requiring low-pass filtering [10]....
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...place at the beginning and in the middle of a switching period) [10]....
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...Specifically, fr is typically required to be lower than one half of the switching frequency fsw to ensure the effective attenuation of the switching harmonics [3], [29], and fs is generally set to be 2fsw [10], [11]....
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