Current Control For Grid-connected Inverter Research Articles

Model Predictive Current Control for Grid-connected Inverter Considering the PLL Dynamics

Model Predictive Current Control for Grid-connected Inverter Considering the PLL Dynamics

CONSTRAINT CURRENT CONTROL FOR GRID-CONNECTED POWER INVERTER

Recent study has paid much attention in the areas of renewable energy (solar and wind) as an alternative method to derive electrical power rather than going by fossil fuels (coal, natural gases etc.) which constantly emits carbon dioxide and other harmful substances into the atmosphere. The emission of these undesirable harmful substances into the atmosphere have caused climatic changes for example global warming, acid rain, low precipitations and unwanted desert encroachment. These badly affect the quality life of humans and animals. In response to these problems, methods of reducing carbon content emissions become necessary through the use of renewable energy sources (Photovoltaic system, Wind power, Fuel cell etc.). As a result, research on grid-connected inverter have recently become a very hot topic as a means of interfacing renewable energy sources to utility grid. With good interfacing, the renewable energy sources can be able to solve not only the problem of carbon emissions into the atmosphere but also to efficiently support the grid from increased demand of electrical power. Thus, this research has focused on designing a constraint current controller for grid-connected inverter using linear quadratic regulations (LQR) method. The idea of using LQR control design as opposed to classical PI controller is that The LQR provides optimal current control by careful tuning of the input and state weighting matrices and therefore systematic control design can be achieved. Another advantage of LQR method is that constraint handling can be address through an offline optimization technique. This is necessary in order to protect the inverter system components (semiconductor switches) and improve its reliability.

Fixed Switching Frequency Model Predictive Current Control for Grid-Connected Inverter With Improved Dynamic and Steady State Performance

Fixed Switching Frequency Model Predictive Current Control for Grid-Connected Inverter With Improved Dynamic and Steady State Performance

A harmonic suppression strategy for grid-connected inverters based on quadrature sinewave extractor

Grid-connected inverters need to reduce current harmonics as much as possible. After introducing the input signal’s fundamental and main harmonic quadrature components, a discrete state model is created, and the discrete observer design method is used to propose a harmonic extraction algorithm called quadrature sinewave extractor (QSE). The QSE is a stable recursive operator with the advantages of no phase displacement and the ability to eliminate mutual influence between harmonic components. Compared to the widely used proportional multi-resonant controller, QSE can reduce current harmonics and improve system stable performance by using it in the current control of grid-connected inverters. Finally, comparative experiments on a three-phase grid-connected inverter are used to verify the proposed method’s effectiveness.

Adaptive Current Control for Grid-Connected Inverter with Dynamic Recurrent Fuzzy-Neural-Network

The grid-connected inverter is a vital power electronic equipment connecting distributed generation (DG) systems to the utility grid. The quality of the grid-connected current is directly related to the safe and stable operation of the grid-connected system. This study successfully constructed a robust control system for a grid-connected inverter through a dynamic recurrent fuzzy-neural-network imitating sliding-mode control (DRFNNISMC) framework. Firstly, the dynamic model considering system uncertainties of the grid-connected inverter is described for the global integral sliding-mode control (GISMC) design. In order to overcome the chattering phenomena and the dependence of the dynamic information in the GISMC, a model-free dynamic recurrent fuzzy-neural-network (DRFNN) is proposed as a major controller to approximate the GISMC law without the extra compensator. In the DRFNN, a Petri net with varied threshold is incorporated to fire the rules, and only the parameters of the fired rules are adapted to alleviate the computational workload. Moreover, the network is designed with internal recurrent loops to improve the dynamic mapping capability considering the uncertainties in the control system. In addition, to assure the parameter convergence in the adaptation and the stability of the designed control system, the adaptation laws for the parameters of the DRFNN are deduced by the projection theorem and Lyapunov stability theory. Finally, the experimental comparisons with the GISMC scheme are performed in an inverter prototype to verify the superior performance of the proposed DRFNNISMC framework for the grid-connected current control.

Repetitive Predictive Control for Current Control of Grid-Connected Inverter Under Distorted Voltage Conditions

A repetitive predictive control for grid-connected inverter current control scheme is presented in this paper under voltage harmonic distortion in the stationary reference frame. Predictive control is an approach that uses a receding horizon to achieve the optimal track for the reference. In this approach, the repetitive controller behavior is added to the predictive control to increase its performance under distorted voltage conditions. In addition, to apply the controller in the generation system, controller is designed from the perspectives of the power system. The controller is designed considering the state-space model in the stationary reference frame. The controller performance was verified using a laboratory experimental setup under distorted grid voltage condition. Experimental results confirm that the proposed controller can effectively suppress harmonics under both normal operation and distorted grid voltage conditions, and also satisfy the requirements stipulated in IEEE Std. 1547.2-2008.

Current control of grid-connected inverter using integral sliding mode control and resonant compensation

<p>To eliminate the adverse effect from parameter variations as well as distorted grid conditions, a current control scheme of an LCL-filtered grid-connected inverter using a discrete integral sliding mode control (ISMC) and resonant compensation is presented. The proposed scheme is constructed based on the cascaded multiloop structure, in which three control loops are composed of grid-side current control, capacitor voltage control, and inverter-side current control. An active damping to suppress the resonance caused by LCL filter can be effectively realized by means of the inverter-side feedback control loop. Furthermore, the seamless transfer operation between the grid-connected mode and islanded mode is achieved by the capacitor voltage control loop. To retain a high tracking performance and robustness of the ISMC as well as an excellent harmonic compensation capability of the resonant control (RC) scheme at the same time, two control methods are combined in the proposed current controller. As a result, the proposed scheme yields a high quality of the injected grid currents and fast dynamic response even under distorted grid conditions. Furthermore, to reduce the number of sensors, a discrete-time reduced-order state observer is introduced. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed scheme.</p>

Design of a novel robust current controller for grid-connected inverter against grid impedance variations

In this paper, a simple low order robust current controller with capacitor-current-feedback active damping is developed to reject grid impedance variations. Two control parameters among five are identified as the parameters that affect the system’s stability significantly. Thus, the goal of the robust controller design is to fix the three control parameters, varying only the other two, and to determine the robust stability region in the two-parameter plane by using D-decomposition method under the specified uncertainty region. The D-decomposition method maps the roots of closed-loop characteristic polynomials onto the graphic control parameter plane since certain relationship exists between the value of the control parameters and the value of the characteristic roots. By constraining all the closed-loop poles inside the unity circle in z-plane, the two control parameters are then the solutions to the characteristic polynomials which mathematically determines the robust stability region. Any parameters selected from this region can reject the specified grid impedance uncertainty. A robust performance region (settling time and damping ratio) is further determined inside the robust stability region, thus achieving robust stability, fast response and high quality current injection. All the analytical studies and simulation results verified the control design. The developed simple low order robust controller has been proven to be able to guarantee robust stability and achieve desired robust performance with the specified grid impedance uncertainty.

A Voltage-sensorless Current Control of Grid-connected Inverter Using Frequency-adaptive Observer

Abstract This paper presents a grid voltage-sensorless current control design based on the linear quadratic regulator (LQR) approach for an LCL-filtered grid-connected inverter. The proposed scheme relies only on the information from the grid-side current sensors to implement the control algorithm as well as to synchronize the inverter system with the utility grid. Basically, the construction of current controller consists of a full-state feedback regulator augmented with an integral control term for achieving control objectives, and a frequency-adaptive observer to estimate the system state variables and grid voltage parameters even under a non-ideal grid environment with frequency variation. A systematic design method based on the LQR approach is introduced to obtain optimal gains for the controller as well as the adaptive observer. The effectiveness of the proposed scheme is validated through the simulation results.

Optimal and Systematic Design of Current Controller for Grid-Connected Inverters

A design approach for grid-connected inverter controllers for distributed and renewable energy system applications is proposed, where a high number of controller gains can be designed optimally in a fairly systematic way. The linear quadratic regulator problem is first modified to accommodate sinusoidal signal tracking through a meaningful state-space transformation. The cost function is modified to explicitly include the tracking error, so that its weights are designed in a transparent and systematic way. The proposed technique is applied to the well-known control structure comprising the fundamental and harmonic resonant controllers. Second, the control structure is rearranged to reject the distortions from both the grid voltage and the reference signal, and the proposed design technique is applied to this new structure. It is shown that the desired features of active damping for $LCL$ filters and robust performance against system uncertainties, harmonics, and disturbances are achieved. Third, the controller and the systematic design procedure are extended for inverters with $LLCL$ filters. Proposed control structures are designed and simulated and then implemented on a digital signal processor. Results confirm features of the method and its ability to address control system challenges in high power density and efficient inverter applications.

A Reference Compensation Current Control of Grid-Connected Inverter for Three-Phase Distributed Generators

Renewable energy resources (RES) are being increasingly connected in distribution systems by utilizing power electronic converters. However, the extensive use of power electronics has resulted in a rise in power quality (PQ) concerns faced by the utility. A novel control strategy implementing reference compensation current was proposed in this paper. So that these grid-connected inverters can achieve maximum benefits when they installed in 3-phase 4-wire distribution systems. The inverter is controlled to perform as a multi-function device by incorporating active power filter functionality. The inverter can thus be utilized as: 1) power converter to transfer active power from RES to the grid, and 2) load reactive power demand support; 3) current harmonics compensation at PCC; and 4) current unbalance and neutral current compensation in case of 3-phase 4-wire system. Moreover, with adequate control of grid-interfacing inverter, all the four objectives can be accomplished either individually or simultaneously. Simulation results show the validity and capability of the novel proposed control strategy.