Grid Current Research Articles

Performance Analysis of Three-Phase Shunt Active Power Filter for Harmonic Mitigation

Aims: To carried out performance analysis of a shunt active power filter (SAPF) for harmonics mitigation.
 Study Design: Experimental design through simulation studies using P-Q Theory and proportional integral controller.
 Place and Duration of Study: Department of Physics, Nasarawa State University Keffi, main campus, Nigeria, between October 2020 and September 2021.
 Methodology: Primary and secondary data were obtained using AVO Digital Multimeter and manufacturers’ datasheets from Schneider electric website to capture required system parameters, SAPF was designed using a Voltage Source Inverter model to represent the Three-Phase source, and P-Q Theory with PI Control was used for reference current extraction. The SAPF was modeled, designed and simulated using MATLAB-Simulink and analyzed under different nonlinear load conditions and harmonic spectrum to achieve low Total Harmonic Distortion (THD).
 Results: The THD in the unbalanced system voltages before the application of SAPF was found to be 12.6%, 11.4% and 11.2%, while after the application of SAPF was 2.2%, 2.5% and 2.5% for phase voltages a, b and c respectively. The grid currents indicated THD of 27.2%, 30.9% and 31% before application of SAPF and 2.2%, 2.2% and 2.1% after application.
 Conclusion: The use of non-linear loads; has adverse effects on the quality of electric power as well as phase voltage and frequency waveforms. The use of SAPF is of vital importance in improving electric power quality for reliable power supply and quality service delivery.

Seamless Transition for Parallel Inverters With Novel Self-Adaptive Hybrid Controller and Presynchronization Unit

This article proposes a seamless transition strategy between grid-connected (GC) and stand-alone (SA) states for a microgrid (MG) consisting of parallel inverters. Most inverters in MG operate with novel self-adaptive hybrid controllers, and one inverter near the point of common coupling (PCC) acts as presynchronization (PS) unit. When islanding happens, the inverters based on self-adaptive hybrid control can inherently transfer from grid current control in the GC state to droop control in the SA state, critical islanding detection is needless. When the grid restores, the PS unit can regulate its reactive power to influence the output voltage phase of other inverters in MG and realize the PS of MG and grid. The main advantages are concluded into three points. First, seamless transition of parallel inverters is realized without reconfiguration of control structure, and power-sharing among parallel inverters is achieved without communication lines. Second, unlike droop control, the self-adaptive hybrid control can regulate the grid current of the inverter accurately when the grid fluctuates or is distorted. Third, the PS unit can save the complexity and cost of remote communication for PS. The effectiveness of the proposed seamless transition strategy is verified by simulation and experiment results.

A Robust Fractional-Order Control Technique for Stable Performance of Multilevel Converter-Based Grid-Tied DG Units

In this article, we propose a sliding fractional-order (FO) control strategy to provide robustness feature for integrating a multilevel converter into the power grid under the system parameter uncertainties, nonlinear load alterations, and grid voltage sag. All dynamics achieved from the input capacitors and output inductances of the multilevel converter are utilized to design the proposed sliding fractional surfaces (SFSs) such that a robust operation of the grid-tied converter-based system is achieved. Then, an SFS-based Lyapunov function is built to assess the proposed controller stability. A thorough analysis of the Lyapunov function is carried out for distinguishing suitable boundaries of the controller coefficients when the error components of the system state variables are changed, accordingly. Using the derivative of Lyapunov function and SFS, a comprehensive evaluation is executed to detail the regulation procedure for the robustness and fractional PI controller coefficients. Finally, a closed-loop system using the converter current dynamics aims to discern the differences between different values of FO. Experimental tests using dSPACE-1202 along with MATLAB/Simulink simulations are employed to verify the effectiveness of the proposed control technique for providing proper harmonic compensation, enhanced power quality, unity power factor, voltage sag tolerance, and very low total harmonic distortion grid current.

Model Predictive Control for Grid-Connected Current-Source Converter With Enhanced Robustness and Grid-Current Feedback Only

The current-source converter (CSC) features the second-order characteristic, and multivariable feedback is necessary to suppress the filter resonance. Model predictive control (MPC) has shown advantages in terms of the multiple-input–multiple-output (MIMO) system. This article contributes to the MPC scheme applied to CSC from two aspects. First, based on the typical one-step prediction, existing MPC with both grid-current and capacitor voltage feedback has been applied to mitigate the resonance issue but lacks the investigation and improvement of system robustness to parameter mismatch, especially in such a second-order system. Thus, the dual-disturbance observer is proposed, where both the additional capacitor voltage reference and the lumped input disturbance caused by parameter mismatch are estimated online. As a result, the grid-current reference tracking performance can be improved under mismatched parameters. Second, to reveal the resonance principle with different state feedback types, the tracking process of the grid current is intuitively analyzed in the time domain. The MPC scheme is further assessed by increasing the prediction horizon length, and the results show that the CSC can work well with only gird-current feedback. In this case, both the weighting factor and corresponding reference estimation for the capacitor voltage can be avoided, thus significantly simplifying the design stage and reducing the effect caused by the parameter mismatch. Experimental results verify the effectiveness of the proposed scheme and perspectives.

A Novel Framework of Extended Active Power and Its Application Into Low Complexity MPC for Enhanced Steady-State Performance

This article proposes a novel low complexity model predictive control (LC-MPC) for the performance improvement of the ac/dc converter under unbalanced grid voltages. Although the finite control set model predictive control (FCS-MPC) is a very effective control method, whose cost function is evaluated for all discrete switching states, presenting a high calculation burden for the efficient control. To obtain a reduced harmonic of the grid current and a low calculation burden under unbalanced grid conditions, a new framework of the extended active power is proposed and a negative conjugate of new complex power is chosen as a control variable. In the proposed LC-MPC method, the best voltage vector is obtained with only one prediction whether it is the proposed single-vector-based LC-MPC (SVLC-MPC) or double-vector-based LC-MPC (DVLC-MPC) method, and the optimal duration time using the framework of the extended active power is derived in detail based on the principle of minimizing the error of the new complex power. The proposed LC-MPC has a similar dynamic performance with the prior model predictive control (MPC) methods and obtains much better steady-state performance, i.e., the reduced harmonics of grid currents, under severely unbalanced grid voltages. The comparative experiments confirm the effectiveness of the proposed LC-MPC method.

Wind turbine doubly-fed induction generator defects diagnosis under voltage dips

Voltage dips are brief but frequent disturbances that can be defined as the most important problem affecting all electrical grids, it cannot be completely avoided and represent the main priority for the industry. This study remains part of monitoring the quality of electrical energy’s field. Its aim is to apply two approaches signals for improving the failure recognition in wind turbine conversion chain. One is based on the combination of spectral analysis of stator current by fast Fourier transform and Lissajous curves technique of grid currents. The second approach is based on the stator current analysis by discrete wavelet transform which is considered an ideal tool for this purpose due to its time-frequency localization. These defects diagnosis methods have shown their efficiency in detection, visualization and analysis of voltage dips in order to develop a control strategy for a wind turbine requiring continuity of its service despite the presence of voltage dips.

Three-Phase Grid-Connected Inverter Power Control under Unbalanced Grid Conditions Using a Proportional-Resonant Control Method

Proposed in this article is bidirectional real and reactive power control of a three-phase grid-connected inverter under unbalanced grid conditions using a proportional-resonance controller. Different unbalanced grid conditions have been studied, such as unbalanced three-phase load and unbalanced grid impedance. These unbalanced scenarios generate unbalanced grid currents and unbalanced point-of-common-coupling (PCC) voltages, causing large oscillations in both real and reactive power transferred to the grid. The purposes of the suggested technique are to balance the grid currents and point-of-common-coupling voltages as well as control the power injected into the grid. As a result, balanced PCC voltages are guaranteed, the oscillation in real and reactive power is reduced, and power control is achieved. The proposed method’s performance has been verified in MATLAB/SIMULINK simulation software, and different experimental results have been obtained using a real-time interface platform, dSPACE DS1202.

DC-Link voltage control of a multifunctional PV-GCVSI to reduce VSI switching losses

ABSTRACT The reference input dc voltage of grid connected voltage source inverter (GCVSI) is altered in this article based on power (both active and reactive) injection from GCVSI for minimising inverter switching losses, and total harmonic distortion (THD) of grid current. If the GCVSI is exclusively used for actual power injection from the photo-voltaic (PV) system to the utility grid, the needed input dc voltage of GCVSI is twice the utility grid phase peak voltage. According to the literature, GCVSI is also used to compensate for power quality issues, and in these circumstances, the dc-link voltage value is determined by the maximum power injection of GCVSI. Under reduced load, and reduced power generation from the PV system, keeping maximum GCVSI input dc voltage reference causes extra voltage stress at GCVSI switches, and the grid current THD is worse due to high ripple current in filter inductance. Therefore, to reduce the inverter switching losses and to improve the grid current THD, the relationship between the GCVSI input dc voltage reference and the power flow from GCVSI has been formulated. The proposed work is verified using simulation studies, as well as hardware studies, and the results are compared with existing methods for validation.

Three-Phase Grid-Connected Inverter Power Control under Unbalanced Grid Conditions Using a Time-Domain Symmetrical Components Extraction Method

Presented in this paper is a method of bidirectional real and reactive power control of a three-phase grid-connected inverter under unbalanced grid situations. Unbalanced three-phase load and unbalanced grid impedance are illustrations of unbalanced grid issues that have been investigated. As a result, both grid currents and point-of-common-coupling (PCC) voltages will be unbalanced. The real and reactive power that is delivered to the grid oscillates by a significant amount in these unbalanced conditions. A time-domain symmetrical components extraction approach is used to calculate the inverter’s reference currents from the negative- and zero-sequence components of the measured currents. The suggested approach corrects unbalanced grid currents and unbalanced PCC voltages, and provides the desired real and reactive power to the grid when unbalanced situations exist. As a consequence, power oscillations will be eliminated, and power control will be possible. The suggested method’s performance is supported by simulation, and various experimental results are obtained utilizing the dSPACE DS1202 real-time interface platform.

Neural Network Controlled Solar PV Battery Powered Unified Power Quality Conditioner for Grid Connected Operation

The Unified Power Quality Conditioner (UPQC) is a technology that has successfully addressed power quality issues. In this paper, a photovoltaic system with battery storage powered Unified Power Quality Conditioner is presented. Total harmonic distortion of the grid current during extreme voltage sag and swell conditions is more than 5% when UPQC is controlled with synchronous reference frame theory (SRF) and instantaneous reactive power theory (PQ) control. The shunt active filter of the UPQC is controlled by the artificial neural network to overcome the above problem. The proposed artificial neural network controller helps to simplify the control complexity and mitigate power quality issues effectively. This study aims to use a neural network to control a shunt active filter of the UPQC to maximise the supply of active power loads and grid and also used to mitigate the harmonic problem due to non-linear loads in the grid. The performance of the model is tested under various case scenarios, including non-linear load conditions, unbalanced load conditions, and voltage sag and voltage swell conditions. The simulations were performed in MATLAB/Simulink software. The results showed excellent performance of the proposed approach and were compared with PQ and SRF control. The percent total harmonic distortion (%THD) of the grid current was measured and discussed for all cases. The results show that the %THD is within the acceptable limits of IEEE-519 (less than 5%) in all test case scenarios by the proposed controller.

Innovative Grid-Connected Photovoltaic Systems Control Based on Complex-Vector-Filter

The research presented in this paper explains how the complex-vector-filter (CVF) method can help in minimizing the current harmonic of a grid-tied photovoltaic system. In fact, the harmonic-free positive sequence (HFPS) load current is used to produce referential sinusoidal currents. This control stabilizes the grid’s currents under unbalanced load currents, as well as mitigates undesirable harmonic load currents, while feeding clean active power to the grid. Thanks to the proposed controller, the performance, such as robustness, as well as the stability and dynamics of the CVF are more effective compared to the proportional-integral (PI) with phase-locked-loop (PLL) controller. Moreover, the CVF ensures robustness and stability during the synchronization between the photovoltaic (PV) generator and the utility grid system. The PI&PLL control presents higher active and reactive power fluctuations during synchronization. On the other hand, the CVF ensures the elimination of the reactive power fluctuations during synchronization. The performance of the proposed CVF is validated by simulation through MATLAB software. Under all conditions, the grid current, considering harmonics, is within the limits set by the IEEE-519 power quality standard, where a total harmonic distortion (THD) of 1.56% was achieved in the case of feeding a non-linear load.

Design and control of PV integrated qZS-CMI system with meta heuristic assisted artificial neural network

“Quasi-Z source cascaded multilevel inverter (qZS-CMI) is a rising topology in photovoltaic (PV) power system. This resolves the drawbacks of traditional CMI-oriented PV systems as it has better balanced dc-link voltage”. However, limited works are there for revealing controller design and modeling of qZS-CMI-oriented systems. This work introduces a controller model for a 3-phase qZS-CMI design. Two different phases are exploited in this work. In Phase I, an optimized “Second order Sliding Mode Controller (SoSMC)” is introduced to determine the PV voltage. For current regulation, Phase II uses an "Optimized Proportional Resonant (PR) controller with Artificial Neural Network (ANN)." A new "Fitness Enabled-Rider Optimization Algorithm (FE-ROA)" is used to optimize control gains in both phases. By using the FE-ROA convergence is improved. In Phase II, the suggested FE-ROA is used to train the ANN. The primary goal of phase I is to minimize the discrepancy between the reference and PV voltage, whereas phase II aims to minimize the error between computed and link grid current. As a result, the introduced model achieves "grid-connected current injection," and system-level management improves "Maximum Power Point Tracking (MPPT)." Finally, the effectiveness of chosen methodology is demonstrated using a variety of measures.

An H∞ filter based active damping control strategy for grid-connected inverters with LCL filter applied to wind power system

Since the LCL filter has good performance to attenuate high frequency harmonics, it is widely used in wind power inverters. But it can cause high-frequency oscillations and instability, which limits its application in wind power system. For an LCL-type grid-connected inverter, the conventional capacitor-current-feedback type active damping control strategy can retain the high-frequency characteristics of LCL filter while suppressing the resonant peak effectively, with the drawback of requiring additional current sensors. Therefore, an H∞ filter based active damping strategy of LCL grid-connected inverter is proposed. The state space model and the process noise model of the LCL inverter are built, and then the general model of the standard H∞ control problem is derived, which helps to solve the H∞ filter. The H∞ filter can estimate the filter capacitor current according to sample values of the injected current and the voltage of the point of common coupling, and the estimation accuracy can be guaranteed even if parameter perturbations exist in the LCL filter. The proportional resonant controller is adopted to track the grid current. Therefore, the proposed control strategy can effectively suppress the resonance peak of the LCL filter without additional current sensors and be insensitivity to system parameter perturbations. The simulation and experimental results verify the feasibility and superiority of the proposed method.

Current harmonic reduction for grid-connected photovoltaic system (PV) based on improved control of three-phase seven-level PUC inverter

In this article, we have improved the control for three-phase of packed U-Cell seven-level inverter, the latter considered as a new multilevel converter topology and has the advantage of very high energy efficiency, and the number of components reduced but has disadvantages related to the harmonics injected into the grid causing switching states if the number of levels increased. In this paper, an improved control based on the sinusoidal control (SPWM), and space vector control (SVPWM) has been proposed for the seven-level packed PUC-Cell inverter. The Proportional resonant (PR) controllers are implemented to adjust the grid currents in the synchronous DQ frame and to maintain synchronism between the inverter and the grid, a Phase-locked loop technique (PLL) can be used. This control is composed of two regulation loops the first allows for regulation of the active and reactive currents delivered to the grid and the second regulates the DC voltage. The evaluation of our results obtained is based on the value of the total harmonic distortion obtained (THD ≪ 5 % of the international standard). This control strategy eliminates current harmonics, improves the quality of power delivered into the grid from the photovoltaic system. An analysis and comparison between this seven-level PUC-cell inverter structure, and the conventional seven-level cascade inverter structure is done, and the simulation results in Matlab/Simulink demonstrate the effectiveness and feasibility of the proposed control method.

Proportional‐resonant based control strategy for grid‐connected packed‐E cell inverters with Lyapunov filter‐based PLL

A proportional-resonant (PR)-based current control strategy for grid-connected packed-E cell (PEC) inverter is presented. Unlike the existing control strategies, which are based on proportional-integral controller, developed for packed-U cell and PEC inverters, the proposed PR-based control strategy achieves zero steady-state error in the grid current. Furthermore, it eliminates the necessity of employing the control loop for regulating the dc capacitor voltages. Also, the grid current synchronization is achieved by using a Lyapunov filter-based phase locked loop (PLL). The consequence of using Lyapunov filter-based PLL is that sinusoidal synchronization signal can be extracted from the distorted grid voltage. The performances of the proposed control strategy under steady-state, load variation and non-ideal grid voltage are investigated on a laboratory-scale experimental prototype. It is reported that the control of both dc- and ac-side variables of the system are accomplished.

Hybrid MLI Topology Using Open-End Windings for Active Power Filter Applications

Different multilevel converter topologies have been presented for achieving more output voltage steps, hence improving system performance and lowering costs. In this paper, a hybrid multilevel inverter (MLI) topology is proposed for active-power-filter applications. The proposed MLI is a combination of two standard topologies: the cascaded H-bridge and the three-phase cascaded voltage source inverter. This configuration enhances the voltage levels of the proposed MLI while using fewer switches than typical MLI topologies. The proposed MLI was developed in the MATLAB/Simulink environment, and a closed-loop control technique was used to achieve a unity power factor connection of the PV modules to the grid, as well as to compensate for harmonics caused by nonlinear loads. To demonstrate that the configuration was working correctly and that the control was precise, the proposed MLI was constructed in a laboratory. A MicroLabBox real-time controller handled data acquisition and switch gating. The proposed topology was experimentally connected to the grid and the MLI was experimentally used as an active power filter to compensate for the harmonics generated due to nonlinear loads. This control technique was able to generating a sinusoidal grid current that was in phase with the grid voltage, and the grid current’s total harmonic distortion was within acceptable limits. To validate the practicability of the proposed MLI, both simulation and experimental results are presented.

Sequential Model Predictive Fault-Tolerance Control for T-Type Three-Level Grid-Connected Converters With LCL Filters

Recently, fault-tolerance control (FTC) methods have been proposed to further improve the reliability of T-type three-level converters. However, the application of model predictive control in FTCs is rare, and the neutral point (NP) voltage imbalance before an open-switch fault occurs has not been solved. This paper focuses on these two problems and proposes two FTC control methods: sequential model predictive control (SMPC) and sequential model predictive tolerance control (SMPTC). Both methods are implemented by designing a sequential predictive control that first considers the NP voltage balance and then grid current tracking. Through the staggered operation of the two methods, a T-type grid-connected converter with an LCL filter can operate normally after an open-switch fault occurs. Moreover, both methods can achieve the NP voltage balance and excellent grid current outputs, eliminate the time-consuming selection of weighting factors, and reduce the number of calculation loops compared with the traditional model predictive tolerance control, thereby improving the control flexibility. The performance and comparison of the open-switch FTC with respect to the NP voltage oscillations, total harmonic distortion of the grid current, and pole voltage were demonstrated using numerous simulation and experimental results.

Generalized Model Predictive Pulse Pattern Control Based on Small-Signal Modeling—Part 1: Algorithm

A model predictive controller based on optimized pulse patterns is proposed that is suitable for higher-order linear systems, such as converters with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$LC$</tex-math></inline-formula> filters. The controller manipulates the switching times of an optimized pulse pattern. The switching time modifications are approximated by the strengths of impulses, which are based on a small-signal linearization around the nominal switching instants. With these, the evolution of the state variables over the prediction horizon can be described by a set of linear differential equations. An objective function penalizes the predicted tracking error of the controlled variables, such as the converter currents, filter capacitor voltages, and grid currents, over a prediction horizon. Thanks to the use of impulse strengths, the underlying optimization problem is a convex quadratic program, which can be solved in real time to determine the switching time modifications of the pulse pattern to be applied by the controller.

Flexible Ripple Minimization Technique for Wind-Solar Renewable Energy System Under Unbalanced and Distorted Grid Conditions

This article focusses on power quality issues related with the operation of a wind energy conversion system employing a doubly fed induction generator (DFIG), integrated with a solar photovoltaic array, under unbalanced, and distorted grid conditions. The multiple-order DFIG electromagnetic torque ripples and grid active power ripples, resulting from the unbalanced and distorted grid, can be mitigated, however, at the cost of highly unbalanced and distorted currents. This article provides a possible solution towards a tradeoff by developing a flexible ripple minimization technique based on a cross-feedback extended second order generalized integrator for harmonics sequence separation (CextSOGI-HSS). The CextSOGI-HSS based current compensation enables a controlled minimization of ripple as per the customizable flexibility quotient. Furthermore, the current compensation technique employs an improved recursive least squares (IRLS) adaptive filtering algorithm for processing DFIG stator currents and local load currents. The IRLS algorithm improves grid currents harmonics spectra by filtering-out higher order harmonics from DFIG stator currents and nonlinear load currents, meanwhile, the effects of any load or stator currents unbalance on grid currents are also mitigated. Test results from a developed laboratory prototype confirm the efficacy of the presented flexible ripple minimization control technique.

Multi-Objective Control Strategy for Power Quality Improvement in Wind-Solar Distributed Generation System Under Harmonically Distorted Grid

Grid-interactive wind-solar distributed generation system (DGS) experiences substantial power quality (PQ) challenges pertaining to incessant voltage distortions in the utility grid and nonlinear loads at the point of common intersection. Such system non-idealities cause distortions in grid currents and infuse ripples in grid active and reactive powers. The effect of harmonically distorted voltages on DGS is further intensified in case of wind generation by means of a doubly-fed induction generator (DFIG), as its stator is directly coupled to the grid. Significant performance degradation with regard to distortions in DFIG currents, and ripples in stator power as well as DFIG torque, are observed. In this context, a control strategy, based on multiresonant higher order generalized integrator for harmonic estimation (MRHGI-HE) and a ϵ-normalized sign regressor least mean fourth (ϵ-NSRLMF) algorithm, for DGS employing solar photovoltaic array and DFIG-based wind generator, is presented in this article. Based on selected objective, the control approach puts-forth a feasible solution for alleviating the aforementioned PQ challenges. The MRHGI-HE provides improved harmonics estimation and superior disturbance rejection, than its conventional counterpart. Besides, ϵ-NSRLMF algorithm exhibits enhanced system dynamics, as compared with least mean fourth algorithm. The multiobjective control strategy is corroborated through extensive simulation and experimental studies, and results demonstrate the PQ improvements in the DGS.