Grid Voltage Sensorless Control Research Articles

Direct-Axis Current Orientation-Based Grid Voltage Sensorless Control Strategy for Vienna Rectifier Under Unbalanced and Distorted Grid Conditions

Direct-Axis Current Orientation-Based Grid Voltage Sensorless Control Strategy for Vienna Rectifier Under Unbalanced and Distorted Grid Conditions

Grid voltage sensorless control of a single‐phase shunt hybrid active power filter

AbstractHere, a grid voltage sensorless control of an LCL‐filtered LC‐tuned single‐phase shunt hybrid active power filter (HAPF) is proposed, which offers high‐quality harmonic current compensation with low hardware requirements. An estimation algorithm replaces the grid voltage sensor and associated circuitry to reduce the size and cost. This paper presents a straightforward and accurate modelling of the suggested structure and its filtering characteristics, followed by a comprehensive yet simple parameter design procedure. An inherent damping technique that uses digital delay rather than virtual or physical damping resistors eliminates the need for additional sensors or extra power losses. As part of the design, a high‐quality reference current is generated for the filter, which is then effectively tracked using a proportional controller with sufficient bandwidth and stability margin. An experimental prototype is implemented to verify the theoretical results, and several steady‐state and transient waveforms are reported to demonstrate the superior performance of the HAPF.

Robust EMPC-Based Frequency-Adaptive Grid Voltage Sensorless Control for an LCL-Filtered Grid-Connected Inverter

A robust explicit model predictive control (EMPC)-based frequency-adaptive grid voltage sensorless control is developed for a grid-connected inverter (GCI) via a linear matrix inequality (LMI) approach under the model parametric uncertainties as well as distorted and imbalanced grid voltages. In order to ensure the quality of grid currents injected into the utility grid even when the system model parameters vary, the proposed control scheme is accomplished by an enhanced prediction model rather than the conventional prediction model obtained by fixed parameters. Furthermore, an LMI-based observer is integrated with the disturbance observer to improve the reference tracking performance and to reject disturbances. The proposed observer is employed for the grid frequency-adaptive control without the need for grid voltage sensors. The proposed current controller and observer employ the LMI scheme to maintain a stable and robust operation of the GCI. The discrete-time frequency response and pole-zero map analyses are utilized to examine the system performance including the stability and robustness against parametric uncertainties. Comprehensive simulation and experimental tests as well as theoretical analyses clearly validate the robustness of the proposed control scheme under various harsh test conditions with non-ideal and unexpected grid and system parametric uncertainties.

Observer based model predictive current control of distorted grid connected single phase rectifier without AC voltage sensor

In this research, a grid voltage sensorless control scheme under the distorted grid voltage condition for a single-phase rectifier was proposed. For eliminating the grid voltage sensor, a time-varying state observer was designed, which provided direct, accurate, and fast estimation of the grid voltage without subsequent complex calculations, even for the presence of harmonics, a DC component, and frequency fluctuations, and it was implemented in the digital processor through Forward Euler discretization. In addition, the aforementioned observer would be partially reused in another observer to modify the input of the PI controller and then reduce the total harmonic distortion of the input current. To warrant robustness for the real world, the proposed observers were investigated in detail for load resistance variations and grid voltage fluctuations. Several simulations and experimentation tests were conducted to verify the effectiveness of proposed scheme. The obtained results indicated that the proposed observers had fast dynamic responses and low estimation errors. The proposed scheme decreased the THD of the input current.

A DC-Offset Reduced Grid-Voltage Sensorless Model Predictive Control Strategy for Grid-Tied Inverters With a Two-Stage Optimal Voltage Vector Searching Method

Grid voltage sensorless model predictive control (MPC) strategies for grid-tied inverters (GTIs) have been studied frequently in recent years. However, the effects of DC-offset are not well addressed, and the current ripples are quite large as only one voltage vector (VV) is applied per control period. To solve these problems, a DC-offset reduced grid voltage sensorless MPC strategy for GTIs with a two-stage optimal VV searching method is proposed. First, a new grid voltage observer is proposed to reduce the effects of DC-offset. Second, by analyzing the proposed observer, it is found that the introduced intermediate variable can be used to estimate the DC-offset contained in the sampled grid current. Thus, the effects of DC-offset on the current control can be compensated accordingly. Third, to reduce the current ripples, a new two-stage optimal VV searching method is proposed on the basis of a detailed theoretical analysis. The novelty of this method is that it searches the phase and amplitude of the optimal VV in two independent stages, which reduces the number of VV that required to be evaluated online obviously. Finally, comparative experimental studies verify the effectiveness of the proposed methods in this paper.

Presynchronization Control for Grid-Connected Inverters Without Grid Voltage Sensors

The grid voltage sensorless control for grid-connected inverters samples the line current to estimate the voltage at the point-of-common-coupling and achieve grid synchronization. The sensorless control tends to enlarge in-rush currents and fails to connect to the grid. For addressing this issue, this letter proposes a presynchronization control strategy to achieve a fast and reliable connection to the grid. By applying periodic pulses with a designed duty cycle, the converter operates in the rectifier mode with unit power factor before being connected to the ac grid as an inverter. The initial phase of the ac grid can thereby be estimated by the sampled current, where a synchronous reference frame phase-locked loop is applied to obtain the current phase. The proposed strategy is validated by experimental tests.

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.

Grid Voltage Sensorless Model Predictive Control for a Single-Phase T-Type Rectifier With an Active Power Decoupling Circuit

This paper proposes grid voltage sensorless model predictive control for a single-phase T-type rectifier with an active power decoupling circuit. The proposed sensorless technique is based on a model reference adaptive system (MRAS) and tested under distorted grid conditions. This study also examines the relationship among the ripple energy, the dc-link capacitor, and the active power decoupling circuit capacitor. The developed control technique is proposed to ensure the following objectives; (1) sensorless grid voltage estimation; (2) the second-order ripple power elimination; (3) reference current generation based on power equilibrium; (4) ensuring unity power factor under all operating conditions; and (5) capacitor voltage balance. The developed control structure offers simplicity and it is cost-effective due to the absence of a grid voltage sensor. An experimental prototype is established, and the main results, including the steady-state and dynamic performances, are presented to validate the effectiveness of the proposed control.

Grid Voltage Estimation Based on Integral Resonant Current Controller for LCL-Filtered Grid-Connected Inverter without AC Voltage Sensors

A high reliability of a grid-connected inverter (GCI) system at reasonable cost is a critical requirement for maximizing renewable energy potential in the electrical energy market. Several grid voltage sensorless control approaches have been investigated not only to eliminate the vulnerability of faulty sensors but also to further reduce the GCI commercial price. In this paper, a frequency adaptive integral-resonant full-state feedback current control scheme with the facilitation of a full-state observer is adopted for a grid-connected inductive–capacitive–inductive (LCL) filtered inverter without sensing the grid voltages. The proposed scheme actively damps the filter resonance and ensures the robustness of the inverter system against unexpected severe grid conditions with low cost and simplified hardware construction. The synchronization of the inverter with the main grid is accomplished by the proposed current controller-based grid voltage estimator, in which the grid frequency and phase angle can be detected effectively. In addition, the actual grid voltages are precisely regenerated to ensure the stable performance of the full-state observer. A safe start-up procedure is also presented for the grid voltage sensorless control of the LCL-filtered inverter to avoid a critical overcurrent and long settling time during the start-up instant, offering a stable and reliable inverter system operation with low computational burden. The effectiveness and feasibility of the proposed voltage sensorless current control scheme are validated by the simulation and experimental results under non-ideal grid conditions such as the harmonic distortion, grid frequency variation, and sudden grid phase angle jump.

Research on the Control Strategy of PWM Rectifier under Unbalanced Grid

Background: Aiming at the three-phase voltage source PWM rectifier (VSR), there have been some researches on the neural network control strategies, but the unbalanced power grid condition is always neglected. Meanwhile, about the self-detection technology of the unbalanced power grid voltage, there are still few researches. Methods: Under the unbalanced power grid conditions, this work presents a power grid voltage sensorless control strategy of the three-phase VSR. Based on the radial basis function neural network (RBF) theory, a newly PI controller with parameter self-tuning function is studied, and the RBF-PI controller is used for the closed-loop controls of the voltage and current variables. By means of T/4 delay method, the positive- and negative-sequence components of the power grid side current, and those of the equivalent virtual flux-linkage of unbalanced power grid are extracted. From the equivalent virtual flux-linkage, the unbalanced power grid voltage is reconstructed online. Results: Under the load mutation condition, the power grid voltage sensorless control of threephase VSR is achieved, the system response characteristics are analyzed. From the simulation experimental results, it can be seen that the unbalanced power grid voltage can be quickly tracked, and the sinusoidal control of the power grid side current can be achieved. In addition, the control system has a series of advantages, such as a faster dynamic response speed, a stronger robustness and a smaller DC side voltage ripple. Conclusion: The proposed unbalanced power grid voltage sensorless control strategy of threephase VSR is effective.

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.

Frequency Adaptive Current Control Scheme for Grid-connected Inverter without Grid Voltage Sensors Based on Gradient Steepest Descent Method

This paper presents a frequency adaptive grid voltage sensorless control scheme of a grid-connected inductive–capacitive–inductive (LCL)-filtered inverter, which is based on an adaptive current controller and a grid voltage observer. The frequency adaptive current controller is constructed by a full-state feedback regulator with the augmentation of multiple control terms to restrain not only the inherent resonance phenomenon that is caused by LCL filter, but also current harmonic distortions from an adverse grid environment. The number of required sensing devices is minimized in the proposed scheme by means of a discrete-time current-type observer, which estimates the system state variables, and gradient-method-based observers, which estimate the grid voltages and frequency simultaneously at different grid conditions. The estimated grid frequency is utilized in the current control loop to provide high-quality grid-injected currents, even under harmonic distortions and the frequency variation of grid voltages. As a result, the grid frequency adaptive control performance as well as the robustness against distorted grid voltages can be realized. Finally, an inverter synchronization task without using grid voltage sensors is accomplished by a fundamental grid voltage filter and a phase-locked loop to detect the actual grid phase angle. The stability and convergence performance of the proposed observers have been studied by means of the Lyapunov theory to ensure a high accuracy tracking performance of estimated variables. Simulation and experimental results are presented to validate the feasibility and the effectiveness of the proposed control approach.

Grid Voltage Sensorless Model-Based Predictive Power Control of PWM Rectifiers Based on Sliding Mode Virtual Flux Observer

In this paper, a grid voltage sensorless model predictive control is proposed based on a sliding mode virtual flux observer (SMVFO). The proposed SMVFO shows good inherent filtering capacity, and thus there is no high-frequency chattering problem. In addition, the proposed SMVFO is designed based on the closed-loop current estimation. Not only is DC-drift issue solved but also dynamic response is enhanced when compared with the prior open-loop virtual flux observer. To verify the effectiveness of the presented SMVFO, it is further integrated into finite control set-model predictive control (FCS-MPC) for pulse width modulator (PWM) rectifiers. The whole control algorithm features simplicity and improved cost-effectiveness due to the absence of modulator and grid voltage sensors. As the SMVFO can predict current at the next sampling instant while estimating virtual flux accurately, the proposed SMVFO assisted FCS-MPC is comparable to its counterpart using measured grid voltage. The simulation and experimental tests were carried out on a two-level voltage source PWM rectifier to validate the effectiveness of the proposed method.

Grid Voltage Sensorless Control of Single Phase Grid Tied Inverter for Renewable Energy Systems Applications

This paper introduces a grid voltage sensorless control scheme using damped resonant compensator for single phase grid interactive inverter interfacing renewable energy-based distributed generation plants with the grid. Elimination of grid voltage sensor reduces the cost and enhances the system reliability. A delay compensated model predictive current controller is also implemented with the estimation scheme to achieve the grid side control task, which includes reduction in harmonic contamination in the injected current and regulating the power factor during dynamic conditions like change in power demands and grid voltage distortions. The grid synchronization process is accomplished without a phase locked loop using the estimated grid voltage signal. A 1 kW grid tied single phase inverter is modeled and simulated for performance validation of the control scheme. The proposed control strategy is also corroborated by hardware implementation using ds1104 real time controller.

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.

State Observer for Grid-Voltage Sensorless Control of a Converter Under Unbalanced Conditions

This paper deals with grid-voltage sensorless synchronization and control under unbalanced grid conditions. A three-phase grid-connected converter equipped with an LCL filter is considered, and no signals other than the converter currents and the dc-link voltage are measured for control. An augmented adaptive state observer is proposed for estimation of the positive- and negative-sequence components of the grid voltage. Dynamic performance limitations of the proposed method and effects of parameter errors are analyzed. The proposed observer is tested as part of a sensorless control system. Experimental results show that the proposed method works well even under highly unbalanced grid conditions.

Direct Power Control Strategy of PWM Rectifier Based on Improved Virtual Flux-Linkage Observer

In order to achieve the low cost and high performance control of three-phase PWM rectifier, a direct power control (DPC) strategy based on a new-style virtual flux-linkage observer is proposed. The model of three-phase PWM rectifier and the principle of virtual flux-linkage vector control are introduced firstly. Then, in order to avoid the effect of integral initial value and cumulative deviation, three first-order low-pass filters are cascaded to replace the pure integral link; an improved virtual flux-linkage observer of three-phase power grid is presented. From the observed virtual flux-linkage, the voltage and instantaneous power of three-phase power grid are online estimated. On this basis, the power grid voltage sensorless direct power control system of three-phase PWM rectifier is designed. Simulation results have shown that, both in the rectifying state and in the inverting state, the power grid side current and the DC side voltage of three-phase PWM rectifier all can be effectively controlled; the high power factor operation of three-phase PWM rectifier is realized.

State Observer for Grid-Voltage Sensorless Control of a Converter Equipped With an LCL Filter: Direct Discrete-Time Design

Synchronization with the power system is an essential part of control of grid-connected converters. This paper proposes a grid-voltage sensorless synchronization and control scheme for a converter equipped with an LCL filter, measuring only the converter currents and the dc voltage. A discrete-time pole-placement design method is used to formulate an adaptive full-order observer for the estimation of the frequency, angle, and magnitude of the grid voltage. The proposed discrete-time design method enables straightforward implementation and suits low sampling rates better than its continuous-time counterpart. The analytically derived design is experimentally validated, and the results demonstrate rapid convergence of the estimated quantities. Moreover, the experimental tests show that grid-voltage sensorless operation is possible under balanced and unbalanced grid disturbances and distorted grid conditions.

Improved grid voltage sensorless control strategy for railway power conditioners

Rapid development of high-speed trains confronts the power grid with serious power quality problems. In this study, to compensate power quality, an improved grid voltage sensorless control method for the railway power conditioner (RPC) is proposed. The proposed control strategy utilises a moving average filter to better detect the compensating currents and a proportional-resonant controller to control the compensating currents. Moreover, a sensorless virtual flux method based on second order low-pass filters is presented to replace the AC voltage sensors. Using the proposed strategy, the dynamic and steady-state characteristic of the RPC is significantly enhanced. Through simulation tests, the effectiveness of the proposed methods is confirmed.

Improved Virtual-Flux-Linkage Observation Method of PWM Rectifier

In order to improve the virtual flux oriented vector control performance of PWM rectifier, an improved observation method of virtual flux-linkage is proposed. To avoid the relevant problems of pure integrator, and to achieve the accurate observation of the grid voltage’s phase, three first order low-pass filters are cascaded to displace the pure integrator. Based on the SVPWM modulation module and the presented algorithm, grid voltage sensorless control system of three-phase PWM rectifier is given. Simulation results have shown that: either in rectifying or inverting status, the amplitude and phase of virtual flux-linkage can be observed accurately, i.e. the control system of PWM rectifier can operate reliably without the grid voltage sensors; the presented estimation method of virtual flux-linkage is effective.