Virtual Flux Research Articles

Neural Network Quadrature Signal Generator-Based Virtual Flux Estimation for Predictive Control of PWM Converters

In this paper, a frequency-adaptive neural network-based virtual flux (FANN-VF) estimator is developed for sensorless control of a pulse-width modulation converter under unbalanced and distorted grid conditions. This method exploits two parallel FANNs configured as quadrature signal generators. The VF fundamental positive and negative sequence components (PNSCs) are inherently separated in the estimator without employing any cascaded filters. A frequency-locked loop is introduced to accurately track the frequency variations. The estimated VF PNSCs are directly exploited to compute the power references in VF-based predictive direct power control scheme. The developed strategy ensures constant active power and sinusoidal current waveforms under nonideal grid conditions. Simulation and experimental tests are performed to verify the effectiveness of the proposed method.

High-Efficient Direct Power Control Scheme Using Predictive Virtual Flux for Three-Phase Active Rectifiers

In recent years, the pulse-width-modulation (PWM) converter has been found to have extensive applications in renewable energy, industrial fields, and others. The high efficiency requirement is crucial to operating a PWM rectifier in various applications, in addition to the fundamental control objectives of sinusoidal grid currents and the correct DC bus voltage. Additionally, in practical application, another issue arises when the grid voltage frequently experiences distortion, leading to a distorted grid current and a significant rise in total harmonic distortion (THD). To resolve these problems, a model predictive virtual flux-based direct power control (MPVFDPC) with improved power loss performance is proposed based on an integrated switching state predetermination strategy. The proposed MPVFDPC for PWM rectifier inherits the merits of both virtual flux control and direct power control, which have fast dynamic performance and the grid current THD is considerably decreased under distorted grid voltage states. The proposed technique aims to minimize switching loss under ideal and distorted grid voltage states by exploiting the discontinuous modulation concept by using a switching state predetermination strategy. The MPVFDPC with switching state predetermination strategy is proven by employing it in experiments as well as simulations in comparison with previous models: predictive direct power control (Conv. MPDPC) and conventional MPVFDPC (Conv. MPVFDPC). The acquired waveforms and quantitative data are employed to prove the effectiveness of the developed algorithm.

Simulation of cancer progression in bone by a virtual thermal flux with a case study on lumbar vertebrae with multiple myeloma

BackgroundTwo main problems examining the mechanism of cancer progression in the tissues using the computational models are lack of enough knowledge on the effective factors for such events in vivo environments and lack of specific parameters in the available computational models to simulate such complicated reactions. MethodsIn this study, it was tried to simulate the progression of cancerous lesions in the bone tissues by an independent parameter from the anatomical and physiological characteristics of the tissues, so to degrade the orthotropic mechanical properties of the bone tissues, a virtual temperature was determined to be used by a well-known framework for simulation of damages in the composite materials. First, the reliability of the FE model to simulate hyperelastic response in the intervertebral discs (IVDs) and progressive failure in the bony components were verified by simulation of some In-Vitro tests, available in the literature. Then, the progression of the osteolytic damage was simulated in a clinical case with multiple myeloma in the lumbar vertebrae. ResultsThe FE model could simulate stress-shielding and diffusion of lesion in the posterior elements of the damaged vertebra which led to spinal stenosis. The load carrying shares associated with the anterior half and the posterior half of the examined vertebral body and the posterior elements were estimated equal to 41 %, 47 % and 12 %, respectively for the intact condition, that changed to 14 %, 16 % and 70 %, when lesion occupied one third of the vertebral body. ConclusionCorrelation of the FE results with the deformation shapes, observed in the MRIs for the clinical case study, indicated appropriateness of the procedure, proposed for simulation of the progressive osteolytic damage in the vertebral segments. The future studies may follow simulation of tumor growth for various metastatic tissues using the method, established here.

Virtual Power Grid Flux-Oriented Vector Control for Three-Phase Voltage-Type PWM Rectifier

More considerable power distortions are produced by the non-reversible diode-bridge rectifiers in the power system’s AC-DC conversion technology. In addition, bridge-rectifiers worsen energy quality and decrease power factor. Three-Phase Pulse Width Modulation (3phase-PWM) rectifiers have seen rapid growth in use due to recent breakthroughs in power device technology. Its benefits include a unity power factor, no harmonic distortion, and pulsating direct currents. An actual power regulation technique based on a novel virtual power grid controller is suggested to cheaply regulate a 3phase-PWM converter with good performance. The study introduces a new sensorless control scheme to improve the 3phase-PWM-performance rectifier. The proposed control technique employs the virtual power grid flux-oriented vector control with a Second-Order Generic Integral with Instantaneous Phase Lock Loop (SOGI-IPLL) to overcome the problems inherent in relying only on low-pass filter estimates. The virtual power grid flux value may then determine the phase difference. The IPLL takes the phase angle as an input and is utilized to construct the power-oriented vector control. The proposed power grid flux-oriented control strategy combines the VFidea with SOGI-IPLL to circumvent the restrictions imposed by relying only on an integrator of low-pass filters (LPF) to provide an estimate. A refined Virtual flux (VF) estimator with direct power control can improve management at a cheaper cost and more efficiently. The modeling findings demonstrate that the DC voltage input and power grid throughput of the 3phase-PWM rectifier can be successfully regulated in both the rectification and inversion states, allowing for the effective functioning of the 3phase-PWM rectifier.

A Theory for Self-Sustained Multicentennial Oscillation of the Atlantic Meridional Overturning Circulation. Part II: Role of Temperature

Abstract In the first part of our research on self-sustained multicentennial oscillation of the Atlantic meridional overturning circulation (AMOC), we utilized a hemispheric box model considering only the salinity equations. In this study, we consider both thermal and saline processes in the box model to investigate the AMOC multicentennial oscillation and the role of temperature. The thermal processes have mainly two effects, shortening the oscillation period and stabilizing the system, which are caused by the fast surface temperature restoration and negative feedback between temperature advection and AMOC, respectively. Introducing nonlinearity into the system can lead to self-sustained AMOC oscillation that is controlled by ocean internal dynamics, whose mechanism is generalized as a growing oscillation restrained by nonlinearity. The nonlinearity can arise from subpolar vertical mixing, or a nonlinear relation between the AMOC anomaly and the meridional difference of density anomaly. Linear stability analyses reveal that the eigenmode of the system is sensitive to model parameters, including model geometry, mean strength of the AMOC, and the AMOC’s sensitivity to density perturbation, surface virtual salt flux, and meridional temperature gradient. A larger surface virtual salt flux enhances the positive salinity advection feedback, and a smaller meridional temperature gradient weakens the negative temperature advection feedback. Both processes destabilize the AMOC multicentennial oscillation. Such situations may be expected in the future due to more intense warming and freshwater hosing at the high latitudes of the Northern Hemisphere.

Model Predictive Virtual Flux Control Method for Low Switching Loss Performance in Three-Phase AC/DC Pulse-width-Modulated Converters

Three-phase AC/DC pulse-width-modulated (PWM) converters have been widely employed in various renewable energy systems and industrial applications, which require a high-efficiency power converter operation. This article proposes a technique to reduce switching loss in AC/DC converters by integrating a voltage vector preselection strategy to model predictive virtual flux control. The voltage vector preselection strategy preselects available voltage vectors corresponding to switching states that lead to minimum switching loss in the phase leg, which conducts the highest current. By using preselected voltage vectors, clamping intervals are generated at every fundamental period to maintain the present switching states of the power switches, resulting in the reduction in switching loss. Additionally, by using virtual flux control, the proposed approach can effectively be used under both ideal and distorted source voltage conditions. The proposed method is compared with the conventional model predictive current control and the conventional model predictive virtual flux control. Both a simulation and experiment are performed to validate the correctness and effectiveness of the proposed method, which has been found to decrease the switching loss of an AC/DC converter by up to 15% compared to conventional control schemes at a negligible increase in the input current total harmonic distortion and DC output voltage ripple.

Comparison of control techniques for three-phase distributed generation based on VOC and DPC

This paper presents a comparison among control strategies of active rectifiers used for grid connected distributed generation systems from renewable sources. The considered techniques are: Voltage Oriented Control (VOC), Direct Power Control (DPC), two DPC based techniques to minimize the Common Mode (CM) emissions called DPC-EMC1 and DPC-EMC2 set up by the authors and their versions based on virtual flux. All the described techniques have been implemented using Matlab-Simulink® environment and PLECS®. In the results the current injected into the grid, the THD of the injected current vs. generated power and vs. DC link voltage and the CM voltage are shown.

Simple Control Schme of Three-Level PWM Converter Connecting Wind Turbine with Grid.

This paper proposes a simple direct power control using space vector modulation (DPC-SVM) for three-phase three level neutral point clamped (3L-NPC) PWM converter connecting a wind turbine generator with grid. The active and reactive power are used as the control variables instead of the currents applied in conventional control method. Moreover, grid voltage sensors are replaced by a virtual flux (VF) estimator. The theoretical principle of this method as well as the synthesis of the active and reactive power controllers are discussed. It is shown that the proposed method exhibits several features as: simple and robust algorithm as well as simple tuning procedure of PI power controllers. Furthermore the paper presents a LCL-filter design, which fulfils IEEE-SCR 20-50 recommendation. Simulation results have proven an excellent performance and verified the validity of the proposed system.

Mode Predictive Direct Power Control Scheme with Fixed Operation Frequency for a Vienna Rectifier Based on Voltage‐Sensorless

Model predictive direct power control (MPDPC) is a suitable method for the highly nonlinear system of a three‐level Vienna rectifier. However, the traditional MPDPC suffers from irregular switching states, which can decrease current tracking accuracy and generate high current ripple and electromagnetic noise. Additionally, the use of sensors can increase system hardware costs and complexity. To enhance operational reliability and reduce the fault impact of AC voltage sensors, a double closed‐loop control strategy based on voltage‐sensorless MPDPC in the inner loop and sliding mode control (SMC) in the outer loop is proposed. First, the virtual flux model of the three‐phase Vienna rectifier is established in the two‐phase static coordinate system, and the second‐order low‐pass filter is used to improve the voltage observer to estimate the grid‐voltage. Meanwhile, an improved grid voltage observer based on the second‐order low‐pass filter is designed to improve the observation effect. Second, to solve the variable switching operation, current fluctuation and wide harmonic spectrum caused by the traditional MPDPC effectively, a double closed‐loop control strategy with constant switching frequency based on voltage‐sensorless MPDPC in the inner loop and SMC control in the outer loop is proposed. Based on the principle of SVPWM modulation, the modulation voltage of SVPWM is calculated directly, in order to verify the correctness of the theoretical analysis, extensive simulation and matching experimental is presented to verify the correctness of the theoretical analysis. © 2023 The Authors. IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Feedback Linearization Direct Torque Control With Zero-Sequence Current Regulation for DC-Biased VRM Drives

A feedback linearization direct torque control (FL-DTC) scheme with the ability of regulating zero-sequence current is proposed for the DC-biased vernier reluctance machines (DC-VRMs) to improve the steady and dynamic performance of drive system. First, an accurate original nonlinear model of DC-VRM is analyzed in synchronous rotating reference frame with three variables (i.e., the electromagnetic torque, the stator flux, and zero-sequence current). The saturation of self-inductance is considered to ensure the accuracy of model under different load. The virtual 0-axis flux <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ψ₀</i> is firstly defined in this paper. Then, a FL-DTC scheme with zero-sequence current regulation is proposed to improve torque and flux ripple and dynamic response performance. The system stability is verified by the Lyapunov theory. Finally, comparative experiments to conventional PI-based SVM-DTC scheme in steady state and dynamic process are carried out. The advantages of the proposed method are verified, including high control accuracy of torque and flux, fast dynamic response to load change and speed reversal, outstanding low-speed range performance and strong robustness to parameter uncertainties.

Virtual Flux Direct Power Control for Vienna Rectifier Under Unbalanced Grid

The Vienna rectifier is being broadly utilized in many industrial applications, which makes it essential to address its challenges. This article conducts the virtual flux-based direct power control and proposes an improved switching table for the Vienna rectifier under a balanced or unbalanced grid's voltage. Accordingly, the proposed method improves the THD of the three-phase input current when the grid voltage is unbalanced. Moreover, employing the virtual flux concept eliminates using three-phase input voltage. This results in reducing the cost of the converter. One of the challenges in the Vienna rectifier is to consider the current direction in the control design. Accordingly, the proposed switching table is designed based on the input current's sector. Furthermore, the positive and negative sequences of the input currents and virtual flux are utilized to correctly estimate the instantaneous input active and reactive power under the unbalanced grid voltage condition. Moreover, the proposed method can balance the output capacitors in balanced and unbalanced grid voltage conditions. This method possesses a high dynamic response due to hysteresis bands. Finally, a 430-watt laboratory prototype of the Vienna rectifier is implemented, and the accuracy of the proposed VFDPC is verified through the experimental results.

Resiliency Enhancement Control of Converter Fed Smart Renewable Micro Grids Against Multiple Risks

Increasing the penetration level of converter-based renewable power plants, along with the advanced grid code requirements, has made the role of the control of grid-connected voltage source converters more important. In this paper, an improved combined control method for the grid-connected converters is proposed to improve the resiliency of the microgrid against multiple risks. Two main objectives of the proposed method are to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i)</i> improve the steady state and dynamic performance of grid-connected converter, and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii)</i> enhance the resiliency of the system. It copes with high power fluctuations, severe load changes, symmetrical and asymmetrical short circuit faults, and unbalanced grid conditions. As a result, no extra means are needed to tackle each undesirable conditions. The advantages of a current control scheme are integrated with a logical switching table in the proposed control method to improve its steady state and dynamic performance. The virtual flux estimation is eliminated from the control system to reduce the dependency of the system on the parameters. It provides stable operation under normal and abnormal conditions such as unbalanced voltages, voltage sag, faults, etc. The performance of the proposed control method is compared with a virtual-based direct power control and a conventional combined control to confirm its effectiveness superiority through simulation and experimental results.

Understanding grain virtual water flux dynamics and drivers from a socio-ecohydrological perspective: A case study of landlocked developing countries of Africa

The virtual water (VW) approach offers a crucial heuristic tool to analyze water and food security by considering the water embedded in grain during the whole production process. African landlocked developing countries (LLDCs) continually suffer from an escalating food crisis and water resources and socioeconomic policy and water conservation strategy derived from the water-food nexus of virtual water trade (VWT) systems and resources may be the solution. Using a trade gravity model and multilateral data, this study evaluated grain VW flux patterns and 11 main drivers of VWT between 16 African LLDCs and their partners. Besides, the feedback path of VW flows corresponding to the socio-ecohydrological variability was studied using the impulse response function. The findings revealed that net virtual water import (VWI) varied across all 16 African LLDCs, ranging from 1.67 Bm3 to 10.28 Bm3 during 2000 and 2020, with an estimated yearly grain VWI of 105.61 Bm3. Green, blue, and grey water accounted for about 79.3%, 14.7%, and 5.98% of the total grain VWI, respectively. Ethiopia had the highest grain VWI among the African LLDCs. Grain VWI fluxes were significantly and positively driven by population growth and per capita GDP, which were expected to continue in the future. It was concluded that the quantitative analysis of grain VWT patterns and driving forces using the VW theory for LLDCs can be instrumental for guiding the socioeconomic policy and water conservation strategy. To effectively achieve sustainable water security and grain production, earmarked future VW strategy is required. Decision-makers should incorporate rational VW strategies and socio-ecohydrological factors into the monitoring system from a multidisciplinary perspective.

A Variable Switching Frequency Space Vector Pulsewidth Modulation Technique Using Virtual Flux Ripple

The variable switching frequency space vector pulse width modulation technique based on current ripple prediction (CRP-VSFSVPWM) can effectively suppress electromagnetic noise, but its effectiveness depends on the accuracy of load parameters. In this paper, a virtual flux ripple prediction model is developed. Similar to the current ripple prediction model, it is also a time-domain ripple prediction model, but it does not require load parameters and has low complexity. Furthermore, a variable switching frequency space vector modulation strategy based on three-phase virtual flux ripple is designed in this paper. The proposed method is fully validated by comparison with the fixed switching frequency space vector modulation and CRP-VSFSVPWM on a three-phase inverter. Experimental results confirm that the proposed technique can significantly suppress electromagnetic noise while maintaining current harmonic performance, and its performance is not affected by load parameters.

Virtual Three-Level Model Predictive Flux Control With Reduced Computational Burden and Switching Frequency for Induction Motors

Model predictive flux control (MPFC) is an alternative control solution to the classic model predictive torque control (MPTC) for driving induction motors (IMs) without the complexity of tuning the weighting factors. However, when applied in motor drives fed by a two-level converter, it results in higher flux and torque ripples. To improve its performance, this article proposes a virtual three-level MPFC (V3-MPFC) method. The virtual vector method increases the total number of voltage vectors crucial for improving the closed-loop performances of the model predictive control. To reduce the computational burden, direct torque control and novel deadbeat (DB) control strategies are proposed. Compared with existing methods, the proposed DB-based method reduces V3-MPFC to a suboptimization problem and still ensures that the suboptimal solution is equivalent to the global optimal solution provided by V3-MPFC. In addition, the redundant states are exploited to lower the switching frequency (SF) without extending the cost function. The simulation and experiment results show that, compared with existing methods, the proposed strategy effectively reduces the flux and torque ripples, and requires a lower SF and execution time.

Grid-Forming Control of Voltage Source Converters Based on the Virtual-Flux Orientation

The increasing penetration of renewable generation in power systems is causing growing concern about system stability. Grid-forming (GFM) control has been appointed as the technology required for achieving a high penetration of renewables in the grid, as it successfully contributes to the power system stability. This paper proposes a novel grid-forming control scheme for voltage source converters (VSC) achieving the operational characteristics of a virtual synchronous machine (VSM). The proposed scheme is based on the orientation of a defined virtual flux to a reference axis obtained from the emulation of the synchronous generator swing equation. The virtual flux is obtained by the integration of the VSC internal voltage and it is oriented to the reference axis by means of a flux controller that also controls the flux magnitude. In this way, the VSC synchronism is maintained, operating as a voltage source connected to the grid. Moreover, the flux orientation in turn allows to control the VSC active power, while the flux magnitude control allows to regulate the VSC reactive power or terminal voltage in isolated mode. The paper also demonstrates that the stability of GFM converters is negatively affected when emulating high inertia values. So, a stabilizer is also proposed for damping these modes. Finally, due to fact that GFM converters behave as voltage sources, their low voltage ride through capability becomes an issue, because they naturally respond to faults with high short-circuit currents, that cannot be withstand by power electronics converters. Therefore, a method for limiting fault currents is proposed as well in the paper. The proposed grid-forming control scheme has been implemented in a real-time control platform and validated using comprehensive simulation models in a real-time simulator for assessing its grid-forming capability.

Speed Tracking for IFOC Induction Motor Speed Control Using Hybrid Sensorless Speed Estimator Based on Flux Error for Electric Vehicles Application

This paper presents hybrid sensorless speed tracking by an indirect field-oriented control (IFOC) for an induction motor (IM). The sensorless model is based on an improved virtual estimation topology model to predict the virtual speed and flux of the IM using stator current components. The hybrid sensorless model, defined as a modification of voltage with a rotor flux-oriented current model, was also implemented with proportional-integral (PI) control for comparison with the conventional voltage model (CVM). The suggested adaptive mechanism for PI control in the hybrid estimator was able to compensate for the back-EMF error from the rotor flux-oriented current model into the voltage model and change the air gap flux of the IM. An accurate rotor flux position was estimated and used to estimate the speed with low speed error. This IFOC model, with various speed change references, was tested in a simulation environment by using the MATLAB/Simulink program. The proposed hybrid estimator was tested in two different EV operations, which were reverse and forward operations. The effectiveness of the proposed estimator was analyzed for its transient and steady-state performances based on settling time, recovery time and the overshoot and speed error percentages. All the results were in good agreement in terms of the stability of the speed and current controller with minimum speed error obtained, where the average errors were 0.08% and 0.16% for high speed and lower speed, respectively.

Size determination of interior defects by reconstruction of subsurface virtual heat flux for step heating thermography

Step heating thermography is a non-destructive testing (NDT) technique that inspects interior defects by observing surface temperature rises during a long pulse heat stimulation. Due to the heat diffusion induced blurring of defect shapes, it is a challenging task to accurately determine the sizes of deep and small defects. This work focuses on estimating the size of a subsurface defect by an inversion of temperature images for the step heating thermography, to overcome the information loss during the propagation of thermal waves. We assume a space- and time-dependent virtual heat flux on the defect surface, which is reconstructed from surface temperature measurements by solving a two-dimensional inverse heat conduction problem. The local future time concept is combined with the Tikhonov's regularization to stabilize the inverse solution. A prudent choice of the regularization parameter is carried out through the L-curve. The feasibility of the proposed approach is assessed by numerical simulations and experiments. It is shown that the inversion brings an improvement in defect sizing accuracy for defects with low width-to-depth ratios and an efficient suppression of measurement noises.

Comparative study of the virtual flux method and immersed boundary method coupled with regularized lattice Boltzmann method for suspension flow simulations

Suspension flows are ubiquitous in many fields, and reducing computational costs for numerical simulations of suspension flows remains a great challenge. Numerical simulations of particle-laden flows were performed here using the regularized lattice Boltzmann method (RLBM). RLBM, coupled with the virtual flux method (VFM) or the immersed boundary method (IBM) with the multi-direct forcing approach, conducted a series of three simulations for stationary and moving boundaries. The VFM and IBM results were compared for validation, and the computational efficiency was evaluated. For the two simulations using stationary and moving boundaries (i.e., a steady flow past a stationary cylinder and a plane Poiseuille flow including a single moving circular particle), the results based on the VFM and IBM were qualitatively comparable. The computational time for IBM was longer than that for VFM due to the iterations in its scheme. The pressure-driven suspension flow simulations showed that a dilute suspension's (<5%) relative viscosity was almost equivalent. The difference between the results obtained by the VFM and IBM in the semidilute regime (>10%) was discussed in terms of particle rotational motion. Even though the scalability for parallel computing was comparable between the VFM and IBM, the computational time could be effectively reduced by using the VFM instead of the IBM because of the increased suspension concentration.

Robust Predictive Control of Grid-Tied Modular Multilevel Converters for HVDC Systems With Virtual-Flux Based Online Inductance Estimation

Direct model predictive control (DMPC) has become an effective control alternative for the grid-tied modular multilevel converter. However, classical DMPC of the grid-tied MMC relies on accurate system model. System parameter variations will deteriorate the control performances of power, grid current, and circulating current. Besides, the inherent fewer switching actions of DMPC will also lead to poor circulating current performance. To solve these issues, this work proposes a robust DMPC solution for the grid-tied MMC. Novelties lie in two aspects. First, a virtual-flux (VF) based online inductance estimator is proposed, which utilizes the VF estimation error caused by inductance deviation. It achieves fast and accurate estimation of AC equivalent inductance, and reduces the dependency on system parameters. Thereafter, we propose a hybrid predictive control frame with a simple proportional-resonant (PR) controller, which considerably suppresses the circulating current. Finally, experimental results verify the effectiveness of the proposed solution.