Virtual Flux Estimation 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.

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.

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.

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.

Virtual Flux Voltage-Oriented Vector Control Method of Wide Frequency Active Rectifiers Based on Dual Low-Pass Filter

This article presents a non-AC-side voltage sensor control method applied to More Electric Aircraft rectifiers. The control strategy can operate properly over a wide range of frequencies. This strategy calculates the AC supply frequency through an instantaneous phase-locked loop and feeds it back to a dual low-pass filter. The reconstructed rectifier-side voltage is filtered using two low-pass filters with different scale factors. Then, the values of the two filter outputs are subtracted and the effect of the DC bias due to the initial value of the integration is eliminated. The subtracted value is amplitude-phase compensated to calculate the virtual flux value. The phase angle can then be calculated from the virtual flux value. This phase angle is used for the implementation of the voltage-oriented vector control and as an input to the instantaneous phase-locked loop. Simulation and experimental results show that the use of dual low-pass filters under different frequency conditions improves the speed and accuracy of virtual flux estimation and eliminates DC-side bias errors.

Voltage-Sensorless Natural Frame Control for Single-Phase CHB Converter under Distorted Grid Conditions

In this paper, a control scheme combining grid voltage sensorless control and natural frame control (NFC) has been proposed for single-phase cascaded H-bridge converter (CHB) under distorted grid conditions. Firstly, the virtual flux (VF) signal is estimated by a modified virtual flux estimator (VFE). Then the VF-based NFC is derived, using the estimated sinusoidal VF signal to generate the unit vectors for synchronization, which enables the omitting of the coordinate transformation, current quadrature signal generation, and phase-locked loop (PLL). Aiming at the problem that grid harmonic interference is not well considered in the conventional NFC and conventional voltage sensorless control, a current controller with enhanced harmonic-rejection ability is proposed in this paper. As a result, better steady-state performance and rapid transient response are achieved. The effectiveness of the proposed method has been verified on a prototype of the single-phase 3-cell CHB converter.

Super twisting sliding mode approach applied to voltage orientated control of a stand-alone induction generator

To enhance the robustness and dynamic performance of a self-excited induction generator (SEIG) used in a stand-alone wind energy system (WES), a virtual flux oriented control (VFOC) based on nonlinear super-twisting sliding mode control (STSMC) is adopted. STSMC is used to replace the conventional proportional-integral-Fuzzy Logic Controller (PI-FLC) of the inner current control loops. The combination of the proposed control strategy with space vector modulation (SVM) applied to a PWM rectifier brings many advantages such as reduction in harmonics, and precise and rapid tracking of the references. The performance of the proposed control technique (STSMC-VFOC-SVM) is verified through simulations and compared with the traditional technique (PI-FLC-VFOC-SVM). It shows that the proposed method improves the dynamics of the system with reduced current harmonics. In addition, the use of a virtual flux estimator instead of a phase-locked loop (PLL) eliminates the line voltage sensors and thus increases the reliability of the system.

Voltage Sensorless Direct Power Control for PWM Rectifier Under Distorted Network Using Improved Virtual Flux Estimator

Generally, AC-DC conversion system in the power system use the diode-bridge based rectifiers that cannot be reversible and causes high current distortion. Besides, they reduce the power factor and energy quality. recently, the application of three-phase pulse width modulation (PWM) rectifier has been risen quickly due to breakthrough advancement in the power devices. This converter offers sinusoidal input currents and minimized harmonic distortion and unity power factor. This work presents an improved sensorless control for performance enhancement purpose of the PWM-rectifier. The proposed control strategy uses the virtual flux principle with second-order generalized integrator (SOGI) in order to overcome the accompanying drawbacks of using pure integrator of low-pass filters for estimation. The integration of improved virtual flux estimator with direct power control saves the cost of using many voltage sensors which can increase the reliability and effectivity of the control system. Moreover, it improves control robustness and against highly distorted network. The different proposed approaches have been investigated through simulation using MATLAB/Simulink software.

Sensorless Predictive Direct Power Control PDPC_SVM For PWM Converter Under Different Input Voltage Conditions

In this paper, a new virtual flux (VF) based predictive direct power control (VF_PDPC) applied for three-phase pulse width modulation (PWM) rectifier is proposed. The virtual flux estimation is performed using a pure integrator in series with a new adaptive algorithm in order to cancel dc offset and harmonic distortions in the estimated VF. The introduced structure is able to produce two virtual flux positive sequence components orthogonal output signals under unbalanced and distorted voltage conditions. The main features of the proposed virtual flux estimator are, it's simple structure, accuracy, and fast VF estimation over the excited integrators. Therefore, the estimated VF is then used for robust sensorless VF-PDPC with a constant switching frequency using space vector modulation (SVM) and tested through numerical simulations. The instantaneous active and reactive powers provided by orthogonal (VF) positive sequence components are directly controlled. More importantly, this configuration gives quasi-sinusoidal and balanced current under different input voltage conditions without using the power compensation methods. The results of the simulation confirmed the validity of the proposed virtual flux algorithm and demonstrated excellent performance under different input voltage conditions, complete rejection of disturbances.

Virtual Flux Estimation for Sensorless Predictive Control of PWM Rectifiers Under Unbalanced and Distorted Grid Conditions

In this article, a new neural network-based virtual flux (NN-VF) estimator is proposed for sensorless control of a pulsewidth modulation rectifier under unbalanced and distorted grid conditions. In this estimator, the VF vector is reconstructed through an emulated ideal integrator. Thereafter, positive and negative sequence (PNS) VF components are extracted using an NN-based PNS components separator. A Lyapunov's theory-based convergence analysis is performed for optimal tuning of the NN-VF estimator. Accordingly, accurate and fast estimation is achieved. A VF-based predictive direct power control (VF-PDPC) model, including the estimated VF positive sequence components, is formulated. A startup procedure is considered for smooth starting under unbalanced grid conditions. Feasibility and robustness of the developed VF-PDPC are verified by experiments. Mainly, obtained results demonstrate that the startup procedure reduces settling time and avoids over currents; the VF-PDPC achieves better current waveforms than those obtained by the conventional PDPC under unbalanced grid conditions; the NN-VF estimator presents similar dynamic performances as the second-order generalized integrator that uses grid voltages measurement.

Frequency-Adaptive Virtual Flux Estimator-Based Predictive Power Control With Suppression of Dc Voltage Ripples Under Unbalanced Network

This article presents a closed-loop virtual-flux (VF) estimator for grid voltage sensorless control of pulsewidth modulation (PWM) rectifier under unbalanced network. Compared with the prior open-loop VF estimator, the proposed method shows better filtering capability and improved robustness against load variation. Considering the grid frequency may deviate from the nominal value in practical application, an adaptive method is further developed for online frequency estimation. Additionally, the finite control set-model predictive power control with power compensation is implemented based on the proposed VF estimator to attenuate dc voltage ripples under unbalanced network. Theoretical analysis show that the twice-grid-frequency ripple component in the dc voltage can be effectively suppressed even with inductance mismatches. Experimental tests on a two-level PWM rectifier confirm the effectiveness of the proposed method.

Virtual Flux Direct Power Control for PWM Rectifiers Based on an Adaptive Sliding Mode Observer

In the traditional virtual flux estimation for a three-phase pulsewidth modulation rectifier, the integration element causes problems for the initial value and dc bias, and the unstable grid voltage induces a nonconstant flux amplitude. To address these issues, an improved direct power control scheme based on an adaptive sliding mode observer is proposed in this work. The observer employs a sigmoid function as switch function to estimate the grid-side source voltage. Meanwhile, an adaptive compensator instead of pure integral element is also designed to dynamically adjust compensation. The stability of this observer is proved by the Lyapunov function; moreover, simulations and experimental results indicate that this new virtual flux observer substantially improves the observation accuracy based on voltage sensorless control. The application of this strategy successfully suppresses the fluctuation of the dynamic voltage response in the dc bus, eliminating high-frequency noise from the grid side, while simultaneously boosting the power quality.

Remote Power Control Injection of Grid-Connected Power Converters Based on Virtual Flux

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

A power quality enhanced grid voltage sensorless predictive direct power control for active front end rectifiers

In this research work, a power quality enhanced grid voltage sensorless deadbeat predictive direct power control (DB-DPC) approach for AC/DC power converters based on latest virtual flux (VF) estimation, DB-DPC and space vector pulse width modulation (SVPWM) is proposed. At first, the grid voltage sensorless technique with switching table based DPC, which is a conventional approach, is discussed and its performance is analyzed under both increase of load as well as decrease of load conditions. Further, the proposed technique performance is also analyzed under the same circumstance and its performance is compared with the traditional method. The proposed technique has numerous advantages over the conventional method such as constant switching frequency, better active and reactive power control, good regulation of dc bus voltage, and enhanced power quality performance. As the proposed approach utilizes VF, DB-PC, and SVPWM techniques, this ensure line voltage sensorless approach, excellent control dynamics and constant switching frequency, respectively. At the end, experimental validation is done to confirm the supremacy of the proposed technique under load varying conditions and has a phenomenal performance under these circumstances.

A Comparative Study of Methods for Estimating Virtual Flux at the Point of Common Coupling in Grid-Connected Voltage Source Converters With LCL Filter

Grid synchronization based on virtual flux (VF) estimation allows for control of grid-connected power converter without depending on ac-voltage measurements. This is useful in voltage-sensor-less applications for reducing cost and complexity of the control hardware, and can be utilized in case of limited reliability or availability of voltage measurements at the intended point of synchronization to the grid. However, for voltage source converters (VSC) with LCL filters, the influence of the capacitor current must be taken into account to ensure accurate VF estimation at the point of common coupling (PCC) with the grid. This paper presents a comparative evaluation of three VF-based methods for grid synchronization of VSCs with LCL filters, with three different ways of obtaining the capacitor current. The VF estimation in the first method is based only on the measured converter currents. The second method includes capacitor voltage measurements used for estimating the capacitor currents, while the capacitor currents are measured in the third approach. Comparative results from time-domain simulations are presented, demonstrating good performance of the estimation and accurate control of the active and reactive power at the PCC with all three methods, as long as sufficiently accurate filter parameters and current measurements are available. However, the approach based on capacitor current measurements is sensitive to noise due to the high ripple current compared with the fundamental frequency current in the capacitors. The operation of a converter with VF-based grid synchronization including estimation of the capacitor current is demonstrated by experimental results, verifying the voltage sensor-less operation with LCL filter.

Neural Filter Based Integrator for Virtual Flux Estimation in Direct Power Control of Three-Phase PWM Rectifiers

In this paper, new neural filter (NF) based integrator (NF-I) is developed for virtual flux (VF) estimation in direct power control (DPC) of a three-phase pulse width modulation (PWM) rectifier. The main advantages of the proposed NF-I are its simple structure, accuracy and provides fast VF estimation compared to the traditional first order low-pass (FOLP) filter. The NF capability to online filtering distorted signals is exploited for extracting fundamental components of VF obtained from a pure integration. Numerical simulations of the proposed NF-I inserted in the VF based DPC (VF-DPC) are carried out under different grid voltage conditions. Steady state and dynamic performances of the NF-I are compared with those obtained with the traditional FOLP filter. Simulation results illustrate good performances of the NF-I inserted in the VF-DPC strategy.

Direct power control in pulse-width modulation rectifier based on virtual flux estimation

Pulse-width modulation rectifier plays an important role in the field of electric energy conversion. It has many advantages such as the grid side current sinusoidal, bidirectional energy flow, and unit power factor. In order to improve the power factor, this article proposes a new control method for the three-phase pulse-width modulation rectifier, that is, direct power control strategy based on virtual flux orientation. The original concept of AC motor stator flux is introduced into the proposed scheme for the three-phase pulse-width modulation rectifier so as to remove the grid voltage sampling circuit. The instantaneous power is estimated by the virtual flux vector orientation to realize the pulse-width modulation rectifier control in unbalanced condition. The two-order filter replaces the pure integrator in this process of implementation to solve the initial value problem of this pure integrator and achieve the better filtering property. With the estimation of virtual flux, the control strategy with s...

Sensors fault diagnosis and fault tolerant control for grid connected PV system

This paper discusses a Sensors Fault Detection and Isolation (FDI) and Fault Tolerant Control (FTC) of three phase inverter for PV system application. All sensors used in the inverter control strategy are addressed in this work. The developed FDI algorithm focuses on residuals generation; it is defined by the different between measurement and reconstituted signal. So, estimators block are investigated, such as, two Models References Adaptive Systems (MRAS) are developed to reconstitute the information of three phase line currents and dc link voltage sensors. The reconstituted information is injected to the corresponding FDI algorithm. Grid voltage sensors FDI is presented in this contribution trough the use of scaled virtual flux estimator, based on Second Order Generalized Integrator and Frequency Looked Loop. When a fault detected, FTC is ensured by replacing the erroneous measurement by its corresponding estimator. Theoretical study and simulations results are presented to validate the effectiveness of the proposed method.

Accurate Voltage Parameter Estimation for Grid Synchronization in Single-Phase Power Systems

This paper presents an adaptive observer-based approach to estimate voltage parameters, including frequency, amplitude, and phase angle, for single-phase power systems. In contrast to most existing estimation methods of grid voltage parameters, in this study, grid voltage is treated as a dynamic system related to an unknown grid frequency. Based on adaptive observer theory, a full-order adaptive observer is proposed to estimate voltage parameters. A Lyapunov function-based argument is employed to ensure that the proposed estimation method of voltage parameters has zero steady-state error, even when frequency varies or phase angle jumps significantly. Meanwhile, a reduced-order adaptive observer is designed as the simplified version of the proposed full-order observer. Compared with the frequency-adaptive virtual flux estimation, the proposed adaptive observers exhibit better dynamic response to track the actual grid voltage frequency, amplitude, and phase angle. Simulations and experiments have been conducted to validate the effectiveness of the proposed observers.

Voltage sensorless predictive direct power control of three‐phase PWM converters

This study develops a voltage sensorless predictive direct power control (PDPC) strategy for three-phase pulsewidth modulation (PWM) converters using accurate virtual flux (VF) estimation. This proposed PDPC can realise simultaneous control of the instantaneous active and reactive power, and achieve a constant switching frequency operation by adopting an on-line corrective space vector modulation modulator. AC voltage sensors are removed by using the following two techniques. One is that an accurate VF observer is employed to compensate the magnitude and phase errors introduced by the first-order low pass filter. The other is that grid voltages are estimated using the VF based estimation of instantaneous active and reactive power as intermediate variables. In addition, the time delay existing in a real system is taken into account in the proposed control model. Comparative results demonstrate the excellent performance of the proposed approach compared with the conventional look-up-table DPC and the conventional PDPC.