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The purpose of this work is to evaluate the effect of total inductance on reactive power compensation limitations in order to determine the ideal capacitance for an LCL-filter. By closely examining critical parameter limits and their ranges for optimal operation in a grid-connected three-phase inverter, significant links between parameters and design criteria are found. Strong LCL-filter design is based on these results. In comparison to percentage-based designs, the suggested design exhibits an astounding 92% reduction in filter capacitance, and it successfully minimizes overall inductance when compared to traditional techniques. The optimal parameters for attaining the necessary performance are determined to be C = 8.7535 uF and Lt = 0.25 mH. Total Harmonic Distortion (THD) of 1.08% for the voltage and 2.3% for the current has been. obtained from the optimized design, which agrees with models and experimental data. The suggested LCLfilter design offers a very practical solution for grid-connected inverters in terms of reactive power compensation limitations, indicating its potential for practical application. [ABSTRACT FROM AUTHOR]

The power delivered by a voltage source inverter needs to be filtered to fulfill grid code requirements. A commonly used filter technology is the LCL-filter. An issue with the LCL-filter is the occurrence of a resonance peak, which can be mitigated with active or passive damping methods. The transfer function of the filter is often used to investigate the frequency response of the system and propose damping methods. The use of an LC-filter combined with a power transformer to form an LCL-filter has not been extensively investigated. Therefore, the study in this article introduces a model for an LC-filter and power transformer for the grid connection and a derived transfer function for the model. The transfer function for the system is validated with simulations and experimental investigations. The results from simulations and the results from a direct solution of the derived analytical function overlap almost perfectly. The magnitudes of the experimental results are approximately 1 dB lower than the simulation and analytical results before the resonance frequency. At the resonance frequency, the experimental results are approximately 13.4 dB lower. The resonance frequency, however, occurs at approximately the same frequency. It is also concluded that the system is significantly damped. [ABSTRACT FROM AUTHOR]

In this paper, an incremental control scheme equipped with harmonic mitigation control terms and a robust full-state observer is designed for a single or multiple parallel-connected LCL-filtered grid-connected inverter (GCI) system under very weak distorted grid (short circuit ratio (SCR) >2). The admittance-based method is used to assess the system stability when single or multiple inverters in parallel are connected to the complex grid (inductive-type ( $L-$ type), and inductive-capacitive-type (LC-type) grid impedances). The analysis shows that the phase angle of inverter output admittance over the entire frequency band (from low frequency up to the Nyquist frequency) is maintained to be lower than 90°. This ensures that the harmonic compensation strategy of the proposed scheme successfully attenuates the low order harmonics of grid voltages without threatening system stability under very weak grid environment. Furthermore, the potential high frequency resonances caused by the interaction between inverter system and LC-type grid impedance are damped well thanks to the passive inverter output admittance in the high frequency range (from the frequency of 13th order harmonic up to the Nyquist frequency). Hardware experiments are conducted for the single LCL-filtered GCI and two parallel-connected LCL-filtered GCIs with identical and non-identical type inverters under weak and complex grid conditions to verify the correctness of the theoretical analyses. Fair comparisons with the conventional schemes are also presented to highlight the superior performance of the proposed scheme.

Nowadays, active power filters represent one of the most efficient means to reduce inactive power components which provides proper quality of electricity at common network connectivity points. Dynamic power losses in the valves that have a significant impact on the efficiency of the converter and, accordingly, determine the feasibility of using these filters in each specific situation, are among their key parameters. Along with the solution of the problem of ensuring the proper quality of electricity at common network connectivity points, the task of reducing dynamic losses in the valves becomes especially relevant. The purpose of the study is to increase the efficiency of active filtration in terms of reducing dynamic losses in the valves while ensuring high-quality voltages at common network connectivity points and currents consumed from the network. To achieve the goal, it is proposed to jointly use a dual-converter active power filter operating in a mode with different conversion frequencies and rated converter capacities, and an interface LCL-filter. Synthesis of converter control is performed. As a control method, the sliding mode control has been used. The efficiency of the proposed system was assessed by modeling in the MATLAB-Simulink application software package. The simulation results confirm the possibility of organizing a mode of operation in which the conversion frequencies and rated capacities of the converters of active power filters are different. In such a case, the currents and voltages consumed from the network at common network connectivity points have an almost perfect harmonic shape; and the phase shift of the network currents relative to the corresponding voltages has a negligible value. It is shown that the organization of the operating mode of converters with different conversion frequencies and rated capacities can significantly reduce the dynamic losses in the switches of active power filters.

This paper presents a novel strategy for online parameter estimation in a hybrid active power filter (HAPF). This HAPF makes use of existing capacitor banks which it combines with an active power filter (APF) in order to dynamically compensate reactive power. The equipment is controlled with finite control set model predictive control (FCS-MPC) due to its already well-known fast dynamic response. The HAPF model is similar to a grid-connected LCL-filtered converter, so the direct control of the HAPF current can cause resonances and instabilities. To solve this, indirect control, using the capacitor voltage and the inverter-side current, is applied in the cost function, which creates high dependency between the system parameters and the equipment capability to compensate the load reactive power. This dependency is evaluated by simulations, in which the capacitor bank reactance is shown to be the most sensitive parameter, and, thus, responsible for inaccuracies in the FCS-MPC references. In order to minimize this problem without increasing the complexity of the FCS-MPC algorithm, an estimation technique, based on adaptive notch filters, is proposed. The proposed algorithm is tested in a laboratory prototype to demonstrate its ability to follow variations in the HAPF capacitor reactance, effectively correcting the reactive power reference and providing dynamic reactive power compensation. During the tests, the proposed algorithm was capable of keeping the supplied reactive power within a 1 % error, even in a situation with 33 % variation in the HAPF capacitor reactance. [ABSTRACT FROM AUTHOR]

The paper deals with the performance checking of an optimally designed LCL-filter connected along the grid-side converter (GSC) in a doubly fed induction generator (DFIG) based wind energy conversion system (WECS) in attenuating harmonics. A repetitive approach is required for designing the parameters of the LCL-filter. The reasons being the explicitness of design parameters and design requirements of the proposed filter like IEEE-519 Std. for harmonic current attenuation, maximum voltage difference allowed in between two sides of the filter to limit switching losses, and limit of reactive power compensation. The proposed filter is compared with the conventional LCL-filter. Based on the proposed method, a minimal inductance and optimal capacitance-based filter is obtained in this paper which is validated using a 2.2 kW DFIG based WECS. Comparative stability analysis of DFIG system with and without proposed filter is performed using small signal stability analysis. The system under study is simulated using MATLAB/Simulink and OPAL-RT 4510, simultaneously. Simulation results validates the effective use of proposed filter design method into DFIG system in terms of harmonic attenuation, total harmonic distortion and reactive power compensation.

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.

This paper presents an improvement of the tuning process of the holistic control published by the authors in previous research to achieve a seamless transition among three operation mode changes and load transients. The proposed tuning approach reveals an improvement in the energy consumption of the Battery Energy Storage System (BESS) during all operation mode transients compared to the holistic control. For this aim, the system addressed is a BESS with the capability to ride through three operation modes of interest: the grid-connected mode as an inverter, the grid-connected mode as a rectifier, and the islanded operation mode. The LCL filter design by using the Butterworth polynomial approach is presented in more detail, and its smooth inherent transient response is preserved when the tuning of the controller gains is carried out by using the same polynomial approach but now including the integral action within the Butterworth polynomial. To reveal the reduction in energy consumption, the closed-loop transfer functions of each mode were used to compare the previous holistic control and this new one named by the authors as the "improved holistic control". The closed-loop system fulfills the frequency and voltage thresholds of the IEEE 1547-2018 standard seamlessly. The simulation runs were conducted on the PSCAD/EMTDC to evaluate the seamless transition among the operation mode changes and load steps. The experimental results in a 617 W, 120 V L − L three-phase converter prototype are included to demonstrate the validity of the improved holistic control. [ABSTRACT FROM AUTHOR]

The power delivered by a voltage source inverter needs to be filtered to fulfill grid code requirements. A commonly used filter technology is the LCL-filter. An issue with the LCL-filter is the occurrence of a resonance peak, which can be mitigated with active or passive damping methods. The transfer function of the filter is often used to investigate the frequency response of the system and propose damping methods. The use of an LC-filter combined with a power transformer to form an LCL-filter has not been extensively investigated. Therefore, the study in this article introduces a model for an LC-filter and power transformer for the grid connection and a derived transfer function for the model. The transfer function for the system is validated with simulations and experimental investigations. The results from simulations and the results from a direct solution of the derived analytical function overlap almost perfectly. The magnitudes of the experimental results are approximately 1 dB lower than the simulation and analytical results before the resonance frequency. At the resonance frequency, the experimental results are approximately 13.4 dB lower. The resonance frequency, however, occurs at approximately the same frequency. It is also concluded that the system is significantly damped.

This paper suggests an experimental implementation study of the Wind Energy Conversion System (WECS) based on efficient Direct Power Control (DPC). Stand-alone mode for variable wind speed application using Doubly-Fed Induction Generator (DFIG) is studied and developed in this work. Due to the wind power performance limitation of traditional PI controllers, such as overshoot, response time, and static error; IP (integral–Proportional) controllers is replaced instead of the PI to control rotor current d – q components (I rd and I rq) in a Park frame through AC–DC–AC converter. A comparative experimental study was implemented to improve the power quality using L, LC & LCL passive filters between the DFIG's rotor circuit and the inverter. Experimental results prove that the proposed DPC under stand-alone mode with LCL-Type filter could operate in several conditions despite the sudden wind speed variations. It improves the unity power factor grid operation (≈ 0.98), dynamic responses, and the decoupled power control with high wind power performances: good reference tracking, short response time, neglected overshoot, and low power error. The power quality injected into the RL-load satisfied the limit specified by IEEE harmonic standard 519 (less than 5%). • Robust DPC based on the IP controller for WECS-DFIG under stand-alone mode is presented in the paper. • Real-time validation of proposed DPC strategy under wind speed Change and rotor currents references (Ird* and Irq*) variation are tested and proved with high performances in transient and steady states. • Improving power quality injected to the RL-Load is tested and validated (using an LCL-Type filter between the rotor-circuit of DFIG and DC–AC converter). • A detailed experimental study of WECS based on DFIG via dSPACE1103 is validated under severe work conditions; to prove the feasibility of the proposed control. • Implementation simplicity, few switching losses, fast dynamic response, and good power quality injected to the RL-load; are the mains advantages of the proposed control system. [ABSTRACT FROM AUTHOR]

This paper presents a novel strategy for online parameter estimation in a hybrid active power filter (HAPF). This HAPF makes use of existing capacitor banks which it combines with an active power filter (APF) in order to dynamically compensate reactive power. The equipment is controlled with finite control set model predictive control (FCS-MPC) due to its already well-known fast dynamic response. The HAPF model is similar to a grid-connected LCL-filtered converter, so the direct control of the HAPF current can cause resonances and instabilities. To solve this, indirect control, using the capacitor voltage and the inverter-side current, is applied in the cost function, which creates high dependency between the system parameters and the equipment capability to compensate the load reactive power. This dependency is evaluated by simulations, in which the capacitor bank reactance is shown to be the most sensitive parameter, and, thus, responsible for inaccuracies in the FCS-MPC references. In order to minimize this problem without increasing the complexity of the FCS-MPC algorithm, an estimation technique, based on adaptive notch filters, is proposed. The proposed algorithm is tested in a laboratory prototype to demonstrate its ability to follow variations in the HAPF capacitor reactance, effectively correcting the reactive power reference and providing dynamic reactive power compensation. During the tests, the proposed algorithm was capable of keeping the supplied reactive power within a 1% error, even in a situation with 33% variation in the HAPF capacitor reactance.

The paper deals with the performance checking of an optimally designed LCL -filter connected along the grid-side converter (GSC) in a doubly fed induction generator (DFIG) based wind energy conversion system (WECS) in attenuating harmonics. A repetitive approach is required for designing the parameters of the LCL -filter. The reasons being the explicitness of design parameters and design requirements of the proposed filter like IEEE-519 Std. for harmonic current attenuation, maximum voltage difference allowed in between two sides of the filter to limit switching losses, and limit of reactive power compensation. The proposed filter is compared with the conventional LCL -filter. Based on the proposed method, a minimal inductance and optimal capacitance-based filter is obtained in this paper which is validated using a 2.2 kW DFIG based WECS. Comparative stability analysis of DFIG system with and without proposed filter is performed using small signal stability analysis. The system under study is simulated using MATLAB/Simulink and OPAL-RT 4510, simultaneously. Simulation results validates the effective use of proposed filter design method into DFIG system in terms of harmonic attenuation, total harmonic distortion and reactive power compensation. [ABSTRACT FROM AUTHOR]

In this paper, a novel single-loop flatness-based controller (FBC) is proposed to control the grid-side current in a shunt converter connected to a weak grid through an LCL-filter. After its mathematical description, the paper reports controller implementation and some performance comparisons with two distinct implementations of the widely diffused vector current control approach, during balanced and unbalanced grid voltages, and weak grid conditions. Obtained results highlight higher tracking capability and better dynamic response of the proposed FBC. Moreover, because of its reduced negative conductance region, unstable behaviors that can be observed in weak grids appear significantly improved due to a reduced influence of the phase-locked loop system.

This paper introduces a holistic control approach applied to a grid-connected converter of a Battery Energy Storage System (BESS). The BESS is mainly used for power peak shaving, frequency supporting, and islanded operation mode which implies multiple control transitions depending on the operation mode. One existing challenge is related to the multiple tuning controllers for the grid-connected converter required for each operation mode, including the rectifier operation. Moreover, when the rectifier operation is running the system is naturally nonlinear which motivates the use of nonlinear controllers. From this standpoint, a strong motivation for this research is the seeking of a holistic control to face all the operation-mode changes seamlessly. Hence, the merit of the proposed holistic control approach consists of using a unique tuning process based on the linear technique of state feedback with integral action for all operation modes, which are: 1) grid-connected mode as an inverter, 2) grid-connected mode as a rectifier, and 3) islanded mode. Of special attention is the grid-connected mode as rectifier since the proposal avoids the introduction of the PWM rectifier model which increases the model order and is inherently nonlinear. A thorough analysis of eigenvalues in open loop and closed loop with the same tuning gains is presented to demonstrate the stability and feasibility of the proposal. An affordable tuning methodology is proposed considering the physical restrictions of the grid-connected converter related to the LCL-filter bandwidth and the switching frequency. To demonstrate the merits of the holistic control approach, several simulations are presented using PSCAD/EMTDC on a 100 kW, 480 V rated BESS using a delta-connected LCL filter for interconnecting with the grid. Simulation results show that the seamless capability with steady-state and transient state operation runs smoothly.

This paper presents an improvement of the tuning process of the holistic control published by the authors in previous research to achieve a seamless transition among three operation mode changes and load transients. The proposed tuning approach reveals an improvement in the energy consumption of the Battery Energy Storage System (BESS) during all operation mode transients compared to the holistic control. For this aim, the system addressed is a BESS with the capability to ride through three operation modes of interest: the grid-connected mode as an inverter, the grid-connected mode as a rectifier, and the islanded operation mode. The LCL filter design by using the Butterworth polynomial approach is presented in more detail, and its smooth inherent transient response is preserved when the tuning of the controller gains is carried out by using the same polynomial approach but now including the integral action within the Butterworth polynomial. To reveal the reduction in energy consumption, the closed-loop transfer functions of each mode were used to compare the previous holistic control and this new one named by the authors as the “improved holistic control”. The closed-loop system fulfills the frequency and voltage thresholds of the IEEE 1547-2018 standard seamlessly. The simulation runs were conducted on the PSCAD/EMTDC to evaluate the seamless transition among the operation mode changes and load steps. The experimental results in a 617 W, 120 VL−L three-phase converter prototype are included to demonstrate the validity of the improved holistic control.

Due to the advantages of superior harmonics attenuation ability and reduced size, the LCL filter has been widely adopted to interface between the inverter and the grid for improving the quality of injected grid currents. However, the high-order characteristics and various constraints of the LCL filter complicate the filter design. Moreover, the stability of the internal current control loop of the individual inverter is susceptible to the inherent LCL -filter resonance peak. Meanwhile, the overall system stability would be aggravated by the external interactions between the inverter and the weak grid as well as among the paralleled inverters. Both the LCL -filter resonance peak and two types of interaction would cause severely distorted grid currents. Motivated by the existing problems, a comprehensive review on the modeling and stability analysis of the LCL -type grid-connected inverters is conducted in this paper. Concretely, the generalized parameter constraints of the LCL filter are outlined to facilitate the passive components selection, and the magnetic integration techniques of filter inductors are also introduced to reduce the weight and size of filter for increasing the power density of the system. Then, the various damping methods for enhancing the individual internal stability and the relevant application issues are also discussed. Furthermore, the impedance-based method for evaluating the system-level interactive stability is subsequently reviewed, with the emphasis on different modeling methods of inverter output impedance and online impedance measurement techniques. Finally, the future research trends on the modeling and stability analysis of LCL -type grid-connected inverters are also presented.

The study suggests a method aimed to improve the performance of practical grid‐connected converters connected to distorted mains, utilising total uncertainty and disturbance estimator (TUDE) based control structures with time‐delay filter. Such a TUDE filter enhances disturbance rejection performance by possessing near‐unity magnitude and near‐zero phase at integer multiples of the base frequency, imposing repetitive‐control‐alike behaviour. However, it is well‐known that in repetitive control, disturbance rejection at mid‐frequencies, were non‐periodic disturbances and noise may exist, is deteriorated. This study presents an enhancement, allowing to retain good disturbance rejection at mid‐frequencies within the low‐frequency portion of control bandwidth (relevant in practical systems) while trading‐off the performance in the high‐frequency control bandwidth part. The proposed methodology is verified by application to output current control of LCL‐filter based inverter feeding distorted AC mains. The validity and performance of the proposed methodology are well‐supported by simulations and experiments. [ABSTRACT FROM AUTHOR]

This article presents a robust voltage sensorless control scheme to eliminate the need of line-voltage sensors in an LCL-filtered grid-connected inverter system. The proposed control scheme is based on an extended-state estimator structure which consists of a state feedback regulator and an estimator. The main contribution of this article relies on the utilization of the linear matrix inequality approach and the internal model principle to synthesis a robust control scheme that equips a grid-connected inverter with the ability to withstand grid disturbances. As opposed to the existing works, the proposed control scheme can simultaneously deal with all types of grid disturbances, such as grid voltage distortion, grid voltage imbalance, and grid impedance variations. The effectiveness of the proposed controller is highlighted through comparative simulation results and in-depth analyses. Experimental results are also given to validate the practicality of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of a regulator based on a power electronic dc/ac converter that can carry out different functions on the improvement of electric power quality in the three-phase low-voltage grid. The algorithm of the control system is based on the Park–Gorev transform and the use of pulse-width modulation of converter voltage has been suggested. The effect of structure of an output passive converter filter on the operation of the grid regulator has been analyzed. A technique to calculate L- and LCL-filters of a regulator have been considered, as well as mathematical models of a regulator for analysis of its dynamic characteristics and determining the parameters of a control unit in the current feedback circuit, has been presented. Computer simulations that demonstrate the effect of the characteristics of an output filter and parameters of a control system on the dynamic characteristics of a device and level of current modulation distortion of the regulator have been carried out. [ABSTRACT FROM AUTHOR]

Active damping is a popular approach to suppress the resonance in LCL-filtered grid-connected inverter owing to lossless damping and efficient system design. In capacitor voltage (CV) damping methods, a differential compensator is essentially needed in feedback path, where the damping effectiveness depends on emulated derivative characteristics of the compensator. Such CV feedback compensation methods face design limitation at the critical resonance frequency and may require additional delay reduction to ensure stability. In this article, an alternative parallel feedforward compensation method is proposed to synthesize an almost strictly positive real plant and achieved damping with modified stability characteristics. A second-order bandpass section is employed as a compensator in damping loop based on the proposed control scheme. The main features of the proposed method are effective damping without critical resonance frequency damping limit, extended stable damping region, improved control performance, and good robustness under filter parameters and grid inductance variation conditions. The simulation and experimental results are presented to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a new grid current controller for a photovoltaic (PV) module integrated buck inverter using a single pulsewidth modulation switch and an unfolding H-bridge, which significantly reduces switching losses. To achieve this, an internal grid current loop with active damping is presented with an LCL filter to connect it to the utility grid. The problem of the filter natural resonance is solved by means of a virtual impedance (active damping), demanding no additional sensor. Experimental results obtained from a 200-W prototype achieved a European efficiency of 96.27% with a current total harmonic distortion of 3.7%. [ABSTRACT FROM AUTHOR]

Compared with L-and LC-filter, the LCL-filter attracts more and more research interests for its excellent harmonic suppression ability. However, the resonance peak of the LCL-filter may introduce potentially unstable dynamics that need to be carefully damped. Considering the PWM discrete nature and practical implementation in discrete form, to analysis the overall system stability is complex especially when considering the controller and the LCL-filter active damping as a whole system. Moreover different sampling frequency and different delay time that are specific in discrete domain influence the system stability scenario, which increase the analysis complexity. Different from the low order filter, LCL-filter offers two kinds of possible current feedback, which presents totally different impact on overall system stability. In this paper, the stability of the overall system in discrete domain with two kinds of possible current feedback is investigated in detail. Three typical sampling frequency: high sampling frequency, medium sampling frequency and low sampling frequency are considered. And three typical delay time of active damping control: 0, Ts∕2, Ts are introduced into stability analysis process. Experimental results get from a 3 kW laboratory prototype based on the TMS320F2808 are provided to validate the theoretical analysis presented in this paper. [ABSTRACT FROM AUTHOR]

Highlights • An active damping (AD) strategy based on the capacitor voltage derivative is presented. • The approach introduced allows the robust operation of the power converter under different grid conditions. • The stability region of the AD is adapted to stabilize the resonant poles for a given LCL filter. • design instead of imposing constraints to the optimized filter design. • The detailed modeling and analysis allows to accurately tune the AD. Abstract The derivative feedback of the capacitor voltage is one of the most extended active damping strategies, used to eliminate stability problems in grid-connected power converters with an LCL filter. This strategy is equivalent to the implementation of a virtual impedance in parallel with the filter capacitor. This virtual impedance is strongly affected by the control loop delays and frequency, creating changes in the sign of the emulated virtual resistor, and raising instability regions where the active damping is ineffective. As a consequence, the LCL resonance frequency is restricted to vary, as the effective grid inductance changes, within the active damping stability region. This is an additional restriction imposed on the LCL filter design that can compromise the achievement of an optimised design. For this reason, in this work, a different strategy is presented; by adjusting the delay in the active damping feedback path, it becomes stable within the range where the LCL resonance frequency can be located for a given filter design, achieving a robust damping. Analytical expressions are provided to adjust this delay. To widen the stability region of the capacitor voltage derivative active damping, a multisampled derivative is implemented, overcoming its limitations close to the control Nyquist frequency. Experimental and simulation results validate the active damping strategy presented. [ABSTRACT FROM AUTHOR]

• LCL filters are applied to reduce the total harmonic distortion of grid-injected current by inverters. • The stability margins of the LCL-filtered grid-connected inverter will be affected by the resonance frequency of LCL filters. • This paper design optimal active damping of capacitor current feedback and optimal proportional resonant controller. • Metaheuristic algorithms are applied to design optimal parameters of the LCL filter components. The inductor-capacitor-inductor (LCL) filter is used to lower the high-frequency switching noise of a grid-connected inverter (GCI). However, a robust design of the LCL filter is a challenge due to its complex model, variations in the operating conditions of the grid, and its stability gain margin. Therefore, this paper designs an optimal LCL filter for GCI and considers the changing grid conditions. This paper utilized four metaheuristic optimization algorithms, including the Whale Optimization Algorithm, the Circle Search Algorithm, the Particle Swarm Optimization, and the gray Wolf Optimization. These algorithms are applied to minimize the total harmonic distortion (THD) and the error between the reference and real grid current. The robustness of the optimal LCL filter is investigated under transient and steady states of the grid by using MATLAB/SIMULINK and real-time simulations. The obtained results revealed that the optimal LCL filter is capable of reducing the THD under different grid variations. (a) Schematic diagram of LCL filtered grid-connected inverter b) basic block diagram of LCL filtered grid-connected inverter (c) control methodology with Proportional Capacitor current feedback damping loop [Display omitted] [ABSTRACT FROM AUTHOR]

The paper presents a controller design for grid-connected inverters (GCI) with very small dc-link capacitance that are coupled to the grid via an LCL filter. The usual controller designs would fail and result in instability. The proposed controller has a cascaded structure with a current controller as inner control loop and an outer dc-link voltage controller. The controller design is performed in discrete time and it is based on a detailed stability analysis of the dc-link voltage controller to determine the controller parameters which guarantee stability for all operating points. The inner loop is a state-feedback current controller that is designed based on the discrete linear-quadratic regulator (DLQR) theory. An additional integral error feedback assures steady-state accuracy of the current control loop. The simulation and experimental results validate performance and stability of proposed controller design.

This paper comprehensively analyzes the stability of a grid-interfacing inverter with the LCL -filter in the discrete domain, where the LCL-filter, along with the controller, is modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z -domain. Through the poles movement and distribution analysis, the relationship between system stability and the ratio of resonance frequency over sampling frequency is mathematically revealed and calculated as well as the system control gain limit. Moreover, this paper demonstrates that a grid-voltage feedforward regulator would significantly alter the inverter stability in a weak power system. By means of Jury stability criterion, the stability status under different filter resonance frequency is given. The selection of resonance frequency and filter parameters makes a considerable difference on system behavior. Finally, to improve the robustness against grid inductance variation, a conservative design recommendation of filter parameters and control gain is given. Through the tests on a laboratory-scale prototype, the theoretical analysis is validated by experimental results. [ABSTRACT FROM AUTHOR]

The LCL-filter has been recently used with grid-connected converters to mitigate switching ripple harmonics. The LCL-filter presents a better attenuation performance for switching ripple harmonics in comparison with L- and LC-type filters. However, the application of LCL-filter has two basic constraints on the circuit design and the resonance problem. These constraints can be effortlessly overcome for grid-connected converters which operate on the utility fundamental frequency. On the other hand, these constraints become a challenging issue when the LCL-filter is used with shunt active power filter (SAPF) because of the wide operation frequency bandwidth of SAPF. This wide operation bandwidth of SAPF causes a narrow bandwidth for the selection of LCL-filter cutoff frequency and the resonance damping control. In this paper, the application constraints of LCL-filter are discussed and the LCL-filter design is provided for SAPF. In addition, an adaptive notch filter-based single-loop active damping method is developed considering the wide operation bandwidth of SAPF. The proposed method consists of a grid impedance estimation algorithm in order to update notch filter parameters adaptively against grid impedance variations. By the help of the single-loop active damping strategy, the proposed controller does not require any additional sensor for the current measurement of LCL-filter capacitor to damp resonance currents. The stability analysis of proposed controller is performed through pole-zero maps. The proposed method is tested, and its performance is verified with comprehensive case studies of a 400-V 80-kVA SAPF simulation model through MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

Renewable Energy Source (RES)-based power plants need to control the active and reactive power at the Point of Common Connection (PCC) with the grid, in order to comply with the requirements of the Transmission System Operators (TSOs). This point is normally far away from the power converter station, and the cables and step-up transformers have a non-neglectable influence on the delivered power. In order to overcome this drawback, this paper presents a control algorithm that permits one to control remotely the power injected at the PCC, by adjusting the local controller of the Voltage Source Converters (VSCs). In this work, the synchronization with the grid is done based on the Virtual Flux (VF) concept. The results reveals that the VF estimation is able to produce a reliable estimation of the grid voltage in any point of the network, and makes it possible to calculate the necessary current reference for injecting a desired active and reactive power at a point that can be some kilometres away. In this paper the main principle for this remote power control is presented. Likewise, the simulation and experimental results will be shown in order to analyse the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

This study presents an intelligent metaheuristics-based design procedure for the Proportional-Integral (PI) controllers tuning in the direct power control scheme for 1.5 MW Doubly Fed Induction Generator (DFIG) based Wind Turbine (WT) systems. The PI controllers’ gains tuning is formulated as a constrained optimization problem under nonlinear and non-smooth operational constraints. Such a formulated tuning problem is efficiently solved by means of the proposed Thermal Exchange Optimization (TEO) algorithm. To evaluate the effectiveness of the introduced TEO metaheuristic, an empirical comparison study with the homologous particle swarm optimization, genetic algorithm, harmony search algorithm, water cycle algorithm, and grasshopper optimization algorithm is achieved. The proposed TEO algorithm is ensured to perform several desired operational characteristics of DFIG for the active/reactive power and DC-link voltage simultaneously. This is performed by solving a multi-objective function optimization problem through a weighted-sum approach. The proposed control strategy is investigated in MATLAB/environment and the results proved the capabilities of the proposed control system in tracking and control under different scenarios. Moreover, a statistical analysis using non-parametric Friedman and Bonferroni−Dunn’s tests demonstrates that the TEO algorithm gives very competitive results in solving global optimization problems in comparison to the other reported metaheuristic algorithms.

The development of photovoltaic systems has increased due to the high demand for clean and green energy. Used worldwide for distributed power generation, the photovoltaic power must not only be maximized, but also adapted to the global standards on harmonic emissions limits. Therefore, tracking the Maximum Power Point and filtering harmonics became necessary. A three-phase grid-connected Photovoltaic system consisting of a photovoltaic (PV) array, a boost converter and a three-phase PWM inverter coupled through a low-pass filter to the electrical grid is studied in this paper. Two new control designs are developed to maximize power in the side of PV panels and to impose a unity power factor in the side of the grid, the first for the three-phase grid-connected systems with an L-filter and the second for the same systems with an LCL-filter. Both controls are analyzed and compared in details and their stability is demonstrated using Lyapunov analysis. The theoretical analysis and simulation results clearly illustrate that the proposed controllers have achieved the objectives. In order to demonstrate the good performance of the LCL-filter in minimizing the harmonics produced by the inverter, a comparison between the frequency responses of the injected currents using the LCL-filter and the L-filter is carried out. [ABSTRACT FROM AUTHOR]

This paper addresses the design and implementation of a droop controlled three phase bidirectional AC-DC converter for more electric aircraft (MEA) applications. A unified control strategy using a droop characteristic is presented to achieve the bidirectional power flow between the AC and DC source while maintaining a stable DC output voltage. Based on the rigorous control design, the converter is capable of operating over a wide frequency range with a unity power factor. The key findings obtained from the theoretical analysis are confirmed by simulation studies and further validated by a scaled down laboratory prototype. The practical results show good performance of the bidirectional converter and demonstrate the effectiveness of the proposed control strategies. [ABSTRACT FROM AUTHOR]

This paper presents an implementation methodology for a current harmonic compensation technique of three-phase grid-connected voltage-source converter (VSC), which was applied under the grid voltage distortion and frequency variation. The proportional–integral plus multi-resonant controllers (PIMR) were used to mitigate the high-order harmonic current. The harmonic compensators were used in the synchronous reference frame at harmonic orders 6th and 12th for suppression of the grid voltage harmonic orders 5th, 7th, 11th, and 13th with a total harmonic distortion of 4.69%. The PIMR controller was transformed to the discrete-time domain and implemented on a TMS320F28379D digital signal processor. The PIMR controller was compared with the conventional multiple synchronous reference frame (PIMSR) controller. The two control schemes were validated with a hardware-in-the-loop (HiL) and with a 5 kW three-phase LCL -filtered grid-connected VSC. The PIMR and PIMSR controllers exhibited excellent harmonic rejection on the grid current with a total harmonic distortion close to 1%, which complied with the IEEE 1547 standard. The grid current can track the reference command with a fast dynamic response. Moreover, the PIMR controller consumed a computational time of 1. 625 μ s , 35% of the PIMSR controller. [ABSTRACT FROM AUTHOR]

Renewable Energy Source (RES)-based power plants need to control the active and reactive power at the Point of Common Connection (PCC) with the grid, in order to comply with the requirements of the Transmission System Operators (TSOs). This point is normally far away from the power converter station, and the cables and step-up transformers have a non-neglectable influence on the delivered power. In order to overcome this drawback, this paper presents a control algorithm that permits one to control remotely the power injected at the PCC, by adjusting the local controller of the Voltage Source Converters (VSCs). In this work, the synchronization with the grid is done based on the Virtual Flux (VF) concept. The results reveals that the VF estimation is able to produce a reliable estimation of the grid voltage in any point of the network, and makes it possible to calculate the necessary current reference for injecting a desired active and reactive power at a point that can be some kilometres away. In this paper the main principle for this remote power control is presented. Likewise, the simulation and experimental results will be shown in order to analyse the effectiveness of the proposed system.

• 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. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper assesses the feasibility of three-phase pulse-width modulated (PWM) three-level voltage-source converters (VSC), namely neutral-point-clamped (3L-NPC) and T-type (3L-T) as alternatives to two-level converter (2L-VSC) for low-voltage multimegawatt renewable energy grid-connected converter applications. For this purpose, a novel design algorithm that takes switching frequency, capacity factor, modulation index, PWM scheme(s), and converter topology(ies) as input is unveiled to provide the design steps and performance benchmark for the specified topologies and modulation methods. Design of the LCL filter is also addressed, and equivalent output ripple performance criteria is used to benchmark the converter performances for a set of selected filter parameters. Main contribution of this algorithm is that it covers cost and operating factors of the design during the hardware design phase by introducing/modifying the terms of total cost of ownership (TCO) and return on investment (ROI) time considering the major energy markets. The design is detailed for 1-MVA system, and the results are summarized for 0.5 and 2-MVA systems. The study shows that 3L topologies perform more efficiently than 2L-VSC (T-type being the most prominent—with the shortest ROI), whereas the difference becomes much more prominent at the multimegawatt range, provided that all three yield equivalent outputs fulfilling the stringent grid codes. [ABSTRACT FROM PUBLISHER]

The middle capacitor voltage of an LCL-filter, if fed back for synchronization, can be used for active damping. An extra sensor for measuring the capacitor current is then avoided. Relating the capacitor voltage to existing popular damping techniques designed with capacitor current feedback would however demand a noise-sensitive derivative term. Digital implementation of this derivative term is generally a challenge with many methods presently developed for resolving it. These methods are however still facing drawbacks, which have comprehensively been explained in this paper. Two derivatives are then proposed, based on either second-order or nonideal generalized integrator. Performances of these derivatives have been found to match the ideal “s” function closely. Active damping based on capacitor voltage feedback can therefore be realized accurately. Experimental results presented have verified the effectiveness of the proposed derivative, which can similarly be used with other applications, where differentiation is needed. [ABSTRACT FROM AUTHOR]

In this study, the transient process when the single‐phase open‐line fault (SPOLF) occurs in the three‐phase AC–DC pulse width modulation rectifier with LCL filter (LCL‐PWMR) is theoretically analysed for the first time and the corresponding diagnosis scheme is proposed. First, the equivalent circuit of the LCL‐PWMR is built and the detected phase voltage (DPV) detected by the three‐phase grid voltage sensors under fault conditions are derived in s‐domain. The result shows that the DPV in the open‐line phase is not equal to zero, while consists of a fundamental component plus a resonant component with specific frequency. Furthermore, a diagnosis scheme for the SPOLF based on 2‐order band‐pass filter (BPF) connected in series with enhanced phase‐locked loop (EPLL) is proposed. BPF is used to eliminate the fundamental component of the sum of three‐phase DPVs and EPLL to estimate the amplitude of resonant component. Then the SPOLF can be detected when the amplitude of resonant component is beyond the allowed threshold. Furthermore, the adaptability of the proposed diagnosis scheme for the system with different rated power is verified theoretically. Detailed simulations tests based on Matlab/Simulink software and experimental tests are carried out to demonstrate the validity of the theoretical analysis and feasibility of the proposed diagnosis scheme. [ABSTRACT FROM AUTHOR]