LCL Parameters Research Articles

Enhancing performance of shipboard photovoltaic grid-connected inverter through CRNN-LM-BP control optimized by particle swarm optimization of LCL parameters

The global renewable energy portfolio is experiencing a growing proportion of shipboard photovoltaic (PV) energy. Grid-connected system has become the development trend of photovoltaic power generation technology in marine applications because of its high energy utilization efficiency. However, in the grid-connected process, harmonic pollution is easily caused by time-varying marine environment disturbance, which affects the quality of grid-connected power. The current recurrent neural network Levenberg-Marquardt backpropagation (CRNN-LM-BP) method, a control methodology, is introduced in this paper to ensure the robustness, stability, and consistency of the system. In addition, the inductance-capacitance-inductance (LCL) filter is optimized using particle swarm optimization (PSO) to minimize the presence of high frequency harmonics. The system that has been developed possesses the ability to improve the accuracy of steady-state control, operate with high efficiency and effectiveness, and maintain a total harmonic distortion of 1.73 %. Concurrently, streamlining the Jacobian matrix in the LM algorithm expedites convergence. It enables the system to improve its dynamic response, minimize excess, and quickly return to a stable condition when faced with load variations, a wide range of filter parameter adjustments, and voltage fluctuations. The suggested method’s effectiveness and applicability are evaluated by comparing it to the traditional proportional-resonant control, CRNN-BP control, using Matlab/Simulink and the real-time simulator OPAL-RT5700..

Parameter design method of grid-connected inverter microgrid system

LCL filter is widely used in three-phase voltage converters due to its superior high-frequency harmonic suppression ability. In this paper, aiming at the instability problem caused by the resonance peak of the impedance characteristics of the LCL filter third-order system, the relationship between the parameters of the filter inductance, damping resistance, and the harmonic suppression ability is deduced. The impedance analysis method is used to obtain the constraint conditions of the grid stability on the LCL parameters. Based on the stability criterion, a parameter design scheme of passive damping LCL filter is proposed. Finally, experiments are conducted on filters that conform to the proposed design method, and the simulation results verify the rationality and effectiveness of the design method.

Integrated Renewable Smart Grid System Using Fuzzy Based Intelligent Controller

In high power medium voltage applications, the utilization of 5-H Bridge Multi-Level Inverter (MLI) has grown vastly in recent years. The 5-H Bridge MLI can effectively control link voltage as well as power factor. However, the inverter imparts harmonics owing to the high switching frequency. Hence Inductance Capacitance Inductance (LCL) filter is implemented at its output to mitigate harmonics in presence of non-linear load. It’s of highly important to choose LCL parameters wisely in order to attain good filtering effect. This work investigates the application of 5-H Bridge MLI with LCL filter at the output for efficient integration of renewable energy sources on to the grid. A modified fuzzy heuristic approach is used to solve LCL power filter optimization problem at output of 5-H Bridge Multi-Level Inverter. Subject to the constraint’s harmonic distortion and losses of the inverter, multi objective optimization problem is solved to obtain LCL filter optimal values. The optimized LCL filter design was applied to a grid connected cascaded H-bridge MLI of rating 4.5 MW, 11 kV. MATLAB-Simulink is utilized for modelling and simulating the system. The results showed that the system is more efficient and generic. dSPACE Hardware in Loop (HIL) test system was used to verify the efficacy of the prototype system.

Design Method of LCL Filter for Grid-Connected Inverter Based on Particle Swarm Optimization and Screening Method

The trial and error method is the conventional method for LCL parameter design. Its efficiency is low and it is difficult to obtain the optimal parameter. Furthermore, due to the lack of detailed analysis of the high-frequency harmonics in the inverter output voltage, the designed LCL filter cannot effectively filter them, resulting that the quality of the grid-connected current cannot fully meet IEEE 519 standard. Based on the above-mentioned problems, the high-frequency harmonics of inverter output voltage is analyzed in detail in this article, and the multiobjective optimization function, including five optimization objectives, is constructed. Combined with the particle swarm optimization, a screening method is proposed to realize the multiobjective optimization process. Under the proposed method, the LCL parameter, which happens to meet the requirement of IEEE 519 for the high-frequency harmonic distortion rate of the grid-connected current, can be obtained, and the greatest tradeoff of LCL filtering performance and filter cost is achieved. In addition, the proposed method considers the problem of weak grid and provides a complete engineering solution for LCL parameter design in different degrees of weak grid. Finally, the effectiveness and accuracy of the proposed design method are verified by contrast simulation and experiment.

Disturbance Observer-Based Robust Model Predictive Control for a Voltage Sensorless Grid-Connected Inverter With an LCL Filter

This paper proposes a disturbance observer-based robust model predictive control (MPC) for a voltage sensorless grid-connected inverter with an inductive-capacitive-inductive (LCL) filter. A full-state estimator and a grid voltage observer are designed to reduce the number of sensors. A lumped disturbance observer, considering the parameter mismatch along with the grid impedance variation, is also designed to eliminate the steady-state error. A cost function, which consists of the error state and control input, is employed in the MPC design. Based on the Lyapunov stability, the full-state observer, voltage estimation, lumped disturbance observer, and the robust controller gains are obtained by solving an optimization problem based on linear matrix inequality (LMI). A frequency response analysis of the entire system is conducted to verify the reference tracking and disturbance rejection outcomes. As a result, the state and grid voltage observer outcomes converge to the actual values as rapidly as possible. The effectiveness of the proposed control method is demonstrated in comparison with the proportional-integral (PI) approach and with a controller recently proposed in the literature. Simulation and experimental results are presented to verify the effectiveness of the proposed method under LCL parameter uncertainties and grid impedance variations.

Grid-connected Inverter Control Strategy Based On Capacitor Reference Voltage Feedforward

In view of the disadvantage that grid-connected power is not high and the phase margin of the grid-connected current is greatly reduced with the increase of impedance of many grid voltage feedforward control strategies. A grid-connected inverter control strategy based on capacitor reference voltage feedforward is proposed. In this control, the error between the filter capacitor voltage and its reference voltage is combined with the proportional controller as the inner loop control, the error between the inverter side current and its reference current is combined with the current controller as the outer loop control to improve the robustness of the system. The phase difference between grid-connected current and grid voltage is reduced by using capacitor reference voltage feedforward. In addition, by using the grid voltage resonance feedforward, the system can not only retains the ability to suppress harmonics, but also prevents the system’s phase margin from decreasing greatly under weak power grid. The proposed strategy has good dynamic performance, it can obtain good grid-connected current quality and high grid-connected power, and it has strong robustness to the change of LCL parameters and the fluctuation of grid voltag. At the same time, it has better adaptability to weak power grid.

Enhanced robustness with damping interval widening strategy of LCL-type converter under weak grid condition

Active damping methods are widely adopted to present a damping effect on the resonance phenomenon without the power loss of LCL-type converters connected to a grid. However, extra sensors are used to implement the damping strategy. In this paper, a sensorless damping strategy where only the grid current is sampled has been proposed. Furthermore, the inner relationship between the tradition capacitor current feedback strategy and the proposed strategy has been clarified. Considering the digital delay and a weak grid, a robust method that uses unit delay feedback to mitigate the phase lag and to widen the effective damping interval has been developed. With this simple measure, stability is enhanced during the Nyquist frequency and robustness is improved when the LCL parameters vary and grid impedance exists. Finally, simulation and experimental results are presented to verify the feasibility and engineering significance of the proposed strategy.

An Integrated Design Approach for LCL-Type Inverter to Improve Its Adaptation in Weak Grid

To improve the robustness of grid-connected inverter against grid impedance in a weak grid an integrated design method of LCL-filter parameters and controller parameters is proposed. In the method the inherent relation of LCL-filter parameters and controller parameters is taken into consideration to realize their optimized match. A parameter normalization scheme is also developed to facilitate the system stability and robustness analysis. Based on the method all normalization parameters can be designed succinctly according to the required stability and robustness. Additionally, the LCL parameter and controller parameter can be achieved immediately by restoring normalization parameters. The proposed design method can guarantee the inverter stability and robustness simultaneously without needing any compensation network, additional hardware, or the complicated iterative computations which cannot be avoided for the conventional inverter design method. Simulation and experiment results have validated the superiority of the proposed inverter design method.

Phase Reshaping via All-Pass Filters for Robust LCL-Filter Active Damping

Active damping is a common way to stabilize the current control of LCL -filtered converters. In this paper, the stable region of −180° phase crossing is first identified within a predefined range of grid impedance and LCL parameter variations. Once the phase of the current control loop is in the identified region, a stabilization control can be attained. Subsequently, digital filters can be adopted to achieve active damping by reshaping the open-loop phase. Various digital filters are selected and benchmarked in this paper. It is confirmed that the all-pass filter has a unity gain and adjustable lagging phase before the Nyquist frequency, thereby being a promising solution to the phase reshaping. Therefore, the all-pass filter is employed to move the phase of the open-loop control (i.e., −180° phase crossing) into the targeted region for active damping. Notably, the current controller and the all-pass filter-based active damping can be separately designed, indicating the easy implementation of the active damping. Experimental tests demonstrate that the proposed method can ensure the system stability over a wide range of parameter variations (e.g., grid impedance changes and LCL -filter parameter drifts) while maintaining fast dynamics with the grid-side current control.

An Improved Resonant Frequency Based Systematic LCL Filter Design Method for Grid-Connected Inverter

Resonant frequency of an LCL filter in a grid-connected inverter is an important factor for control stability and grid-injected current quality. However, the constraints on resonant frequency are usually considered as an afterthought in conventional filter design methods. The fact that resonant frequency is affected by both internal and external factors during operation makes it even more critical. Hence, an improved resonant frequency based systematic LCL filter design method is presented in this paper. A resonant frequency band is identified by considering grid-voltage harmonics and control stability boundary. The variation in resonant frequency due to inductive line impedance as well as magnetic permeability of inductor core is taken into account to define the constraints on LCL parameters. The LCL parameters are then systematically derived from the identified constraints with almost no iteration. A division–summation (D-Σ) digital control method is used to validate the design method. Experimental results measured from a 5-kW single-phase grid-connected inverter have verified the effectiveness of the LCL filter design method over conventional methods.

Passivity Enhancement in Renewable Energy Source Based Power Plant With Paralleled Grid-Connected VSIs

Harmonic instability is threatening the operation of renewable energy based power plants where multiple grid connected voltage source inverters (VSIs) are connected in parallel. To analyze and improve the stability of the grid-connected VSIs, the real part of the output admittance of the VSIs is first investigated in this paper. It has been shown that the negative real part of the output admittance will make the system unstable under certain grid condition. Based on the stability analysis, this paper proposes a method to cancel the negative real parts by using VSIs with different LCL filters, sampling frequencies, and control strategies. The proposed method does not require the VSIs to have any active damping function. Three specific cases are studied where the LCL parameters, sampling frequencies, and current control strategies of the VSIs are different. The results show that the combination of different types of grid-connected VSIs can improve the stability of the renewable power plant.

Modeling and Resonance Analysis of Multiparallel Inverters System Under Asynchronous Carriers Conditions

With increasing penetration of renewable energy sources into modern power systems, multiparallel inverters are commonly employed in the interface to the utility grid, giving rise to potential resonance problems. In order to better understand the issue, resonance of multiparallel inverters is analyzed under an asynchronous carrier. First, multiparallel inverter equivalent circuits are modeled within a controlled bandwidth. In order to investigate resonance characteristics under an asynchronous carrier, a crossover filter function is used to separate output currents of different frequencies for a two-parallel inverter to analyze the output current frequency response. With this approach, resonance characteristics are focused under asynchronous conditions, and the impact of shifting LCL parameters on resonance is examined. The influence of paralleled inverter number on output currents is examined, and the analysis is expanded to multiparallel inverter systems to determine the resonance characteristics. Experimental results validate the developed models and the proposed resonance characteristics.

Stability Analysis and Improvement of the Capacitor Current Active Damping of the LCL Filters in Grid-Connected Applications

For grid-connected LCL-filtered inverters, dual-loop current control with an inner-loop active damping (AD) based on capacitor current feedback is generally used for the sake of current quality. However, existing studies on capacitor current feedback AD with a control delay do not reveal the mathematical relation among the dual-loop stability, capacitor current feedback factor, delay time and LCL parameters. The robustness was not investigated through mathematical derivations. Thus, this paper aims to provide a systematic study of dual-loop current control in a digitally-controlled inverter. At first, the stable region of the inner-loop AD is derived. Then, the dual-loop stability and robustness are analyzed by mathematical derivations when the inner-loop AD is stable and unstable. Robust design principles for the inner-loop AD feedback factor and the outer-loop current controller are derived. Most importantly, ensuring the stability of the inner-loop AD is critical for achieving high robustness against a large grid impedance. Then, several improved approaches are proposed and synthesized. The limitations and benefits of all of the approaches are identified to help engineers apply capacitor current feedback AD in practice.

Stability Analysis of Grid-Connected Inverters with an LCL Filter Considering Grid Impedance

Under high grid impedance conditions, it is difficult to guarantee the stability of grid-connected inverters with an LCL filter designed based on ideal grid conditions. In this paper, the theoretical basis for output impedance calculation is introduced. Based on the small-signal model, the d-d channel closed-loop output impedance models adopting the converter-side current control method and the grid-side current control method are derived, respectively. Specifically, this paper shows how to simplify the stability analysis which is usually complemented based on the generalized Nyquist stability criterion (GNC). The stability of each current-controlled grid-connected system is analyzed via the proposed simplified method. Moreover, the influence of the LCL parameters on the stability margin of grid-connected inverter controlled with converter-side current is studied. It is shown that the stability of grid-connected systems is fully determined by the d-d channel output admittance of the grid-connected inverter and the inductive component of the grid impedance. Experimental results validate the proposed theoretical stability analysis.

Generalized Design of High Performance Shunt Active Power Filter With Output LCL Filter

This paper concentrates on the design, control, and implementation of an LCL-filter-based shunt active power filter (SAPF), which can effectively compensate for harmonic currents produced by nonlinear loads in a three-phase three-wire power system. With an LCL filter added at its output, the proposed SAPF offers superior switching harmonic suppression using much reduced passive filtering elements. Its output currents thus have high slew rate for tracking the targeted reference closely. Smaller inductance of the LCL filter also means smaller harmonic voltage drop across the passive output filter, which in turn minimizes the possibility of overmodulation, particularly for cases where high modulation index is desired. These advantages, together with overall system stability, are guaranteed only through proper consideration of critical design and control issues, like the selection of LCL parameters, interactions between resonance damping and harmonic compensation, bandwidth design of the closed-loop system, and active damping implementation with fewer current sensors. These described design concerns, together with their generalized design procedure, are applied to an analytical example, and eventually verified by both simulation and experimental results.

Robust proportional resonant regulator for grid-connected voltage source inverter (VSI) using direct pole placement design method

Based on the introduction of a dual-loop current control strategy for a grid-connected inverter, an averaged switching model of a grid-connected inverter with an LCL-filter in discrete domain is built under a stationary frame and a proportional resonant (PR) regulator is adopted in the current loop to track the given fundamental sinusoidal current. The impacts of PR parameters, LCL parameters and digital delay on the root locus are studied, respectively. The parameters of the LCL-filter and the PR current regulator loop are designed to assure system stability and dynamic response during a wide power range by using the pole placements method. Finally, a 10 kW prototype of a grid-connected inverter with an LCL-filter is set up to verify the effectiveness, the practicality and robustness of the proposed PR current design method.

Filter Optimization for Grid Interactive Voltage Source Inverters

Higher order LCL filters are essential in meeting the interconnection standard requirement for grid-connected voltage source converters. LCL filters offer better harmonic attenuation and better efficiency at a smaller size when compared to the traditional L filters. The focus of this paper is to analyze the LCL filter design procedure from the point of view of power loss and efficiency. The IEEE 1547-2008 specifications for high-frequency current ripple are used as a major constraint early in the design to ensure that all subsequent optimizations are still compliant with the standards. Power loss in each individual filter component is calculated on a per-phase basis. The total inductance per unit of the LCL filter is varied, and LCL parameter values which give the highest efficiency while simultaneously meeting the stringent standard requirements are identified. The power loss and harmonic output spectrum of the grid-connected LCL filter is experimentally verified, and measurements confirm the predicted trends.