Load Current Observer Research Articles

Investigations on the Modulation Strategies for Performance Improvement of a Controlled Wind Energy System

The challenges faced in an isolated wind energy conversion system (WECS) are larger transient times, high steady-state error, and larger harmonic content. To overcome these issues, an adaptive voltage controller (AVC) along with the load current observer (LCO) could be the better proposition. However, the AVC and LCO, in conjunction with the conventional space vector pulse width modulation (SVPWM) technique to operate the three-phase inverter of WECS, would not be able to further improve these parameters. This paper proposes the use of the unified voltage SVPWM (UVSVPWM) technique along with the AVC and LCO, which could improve the transient behavior by about 30% as well as reduce the harmonic content of the load voltage and current by about 70% and 2%, respectively. This paper considers an isolated WECS connected to the linear load, which is operated under balanced as well as unbalanced load conditions. The proposed control technique is verified for both the balanced and unbalanced cases using MATLAB/Simulink.

Load Voltage and Current Observers for Series–Series Wireless Power Transfer System

Because a transmitter-side controller of an inductive power transfer (IPT) system cannot directly measure its load conditions, i.e., receiver-side output voltages and currents, wireless communications are commonly used in many IPT systems. However, the wireless communication can cause a devastating failure of the IPT system because of its tens of milliseconds delays and frequent data losses. The mismatch between the transmitted and the received power can destroy the receiver-side power converters. In this article, receiver-side load voltage and current observers for a series–series tuned IPT system are proposed using the well-known disturbance observer theories. A load current observer that has a 10 kHz estimation bandwidth and a load voltage observer that has a 200 Hz bandwidth are proposed. The estimated load voltage and current have much faster dynamics than the wireless feedback signals without any data loss. Therefore, the proposed observers can bring significant advances in reliability and performance of the IPT system. Design guidelines for the observers that have low steady-state errors and fast dynamics are suggested. The theoretical performances of the proposed observers are evaluated by simulations and experiments. The simulated frequency- and time-domain responses of the proposed observers are consistent with the theoretical results. An experimental testbed was built to evaluate the proposed observers. The estimated current follows its actual value in 0.1 ms and the estimated voltage follows its measured value in 4 ms as expected from the simulations.

Research on feedforward control of four-quadrant converter based on load current observer

In the field of heavy-duty transportation of electric locomotives, the power of traction converters can reach MW, but the switching frequency needs to be kept at several hundred Hz. In addition, the converter needs to maintain a high transient response to load changes and keep grid-side harmonics at a low level, which is difficult to meet simultaneously. More critically, load disturbances often occur in electric locomotives during operation, which may lead to DC voltage pulsations and more serious overvoltage faults, which in turn lead to an increase in grid harmonics and a reduction in the stability of the traction system. To address the above challenges, this paper proposes a state observer-based power feedforward control method for high-power locomotives, which helps to improve the dynamic performance of the converter while minimizing harmonics to the grid, thus achieving more stable operation of the traction system. Finally, the feasibility and effectiveness of the proposed method are verified by simulations and multi-condition experiments on a 1.2MW electric traction platform.

Multilayer Model Predictive Control for a Voltage Mode Digital Power Amplifier

The application of the finite control set model predictive control to cascaded inverters is severely limited by its computational complexity. In this paper, a load observer based multilayer model predictive control is proposed for the voltage mode digital power amplifier employing cascaded full-bridge neutral point clamped inverter, which can avoid the use of load current sensor and greatly reduce the controller computation without affecting its dynamic performance. The discrete mathematical model of the voltage mode digital power amplifier employing cascaded full-bridge neutral point clamped inverter is established with filter inductor current and filter capacitor voltage as state variables. A load current observer is designed based on this to avoid the use of load current observer. Based on the discrete model and the observed load current, the upper layer of the multilayer model predictive control determines the optimal level that minimizes the cost function. The middle layer allocates the optimal level to each submodule in order to achieve capacitor voltage balancing. The lower layer determines the switching state of each submodule in order to reduce switching actions. Finally, the experimental results based on the designed nine-level prototype show that the develop multilayer model predictive control lead to acceptable steady state, dynamic and robust performance, with only 1.37% of the run time of the traditional model predictive control.

WITHDRAWN: Performance analysis of isolated wind energy conversion system connected to unbalanced loads

The load tension and frequency are impaired when the unbalanced load is unexpectedly attached to the insulated wind energy transformation grid (IWECS). By using the appropriate control and pulse width modulation technologies for the IWECS, the results may be reduced. The controller consists of an Adaptive Tense Controller (AVC) to regulate the overall harmonic distortion (USVPW) and a Unified Space Vector Pulsenwidth (USVPW) modulator for controlling the Inverter output tension and frequency, the Load Current Observer (LCO). This article simulates the IWECS with MATLAB/Simulink and discusses the results of the study with LCO, AVC and combined space vector PWM modulation technology.

Improved Continuous Control Set Model Predictive Control for Three-Phase CVCF Inverters: Fuzzy Logic Approach

This paper presents an improved continuous control set model predictive control (CCS-MPC) scheme using a fuzzy logic approach (FLA) for three-phase constant voltage constant frequency (CVCF) inverters with an output LC filter. The proposed method can enhance the output voltage quality by using an optimal predictive control law (OPCL). Additionally, the proposed method corrects the input voltage commands through the FLA-based online gain tuning aiming to satisfy the hardware constraint. Thus, the proposed FLA-based CCS-MPC can guarantee an acceptable output voltage with a faster transient response, smaller steady-state error (SSE), and lower total harmonic distortion (THD). Moreover, a Kalman filter-based load current observer is used to significantly improve the reliability and robustness. Comparative experimental studies with another CCS-MPC method are conducted on a prototype three-phase CVCF inverter using TI TMS320F28335 DSP to validate the improved performance under the step-changes of linear/unbalanced/nonlinear loads with uncertain parameters.

Performance improvement of voltage controlled isolated wind power generation system consisting of linear and non-linear loads

In an isolated mode operation of wind power generating system (WPGS), operating voltage and frequency are affected under sudden change of load because of more transient time, higher total harmonic distortion. To minimize the effects, the WEGS should be operated with suitable controller and the inverter should be operated with required pulse width modulation (PWM) technique. In this paper, the proposed WPGS is simulated using MATLAB/Simulink with adaptive voltage controller (AVC) and load current observer (LCO) and the inverter is operating with Unified voltage Space Vector PWM (USVPWM) technique to improve the transient behaviour of the system. The same proposed system is also simulated with conventional space vector pulse width modulation (SVPWM) techniques. The simulation results will be analyzed in various load conditions. The conventional SVPWM results compared with the USVPWM results like transient time, load voltage and load current total harmonic distortion under balanced three-phase load.

Design of the Current and the Voltage Observers for Active-Load-Balancer (ALB) in Model Predictive Control System

In this paper, two observers for the model predictive control system of the active load balancer (ALB), are proposed. An unbalanced single-phase load in the distributed power system causes significant difficulties. An active load balancer is an efficient approach to tackle these issues. Model predictive control is one of the centralized proper control system for a power electronic converter. The main drawback of this control system is the use of numerous voltage and current sensors. Therefore, using of observers instead of sensors makes the implementation of the model predictive control system more cost effective. These two observers, estimate the load current and PCC voltage, fast and accurate as possible. This paper presents load current observer based on furrier series and voltage observer based on nonlinear newton-raphson method. The experimental results obtained from a 4 kW prototype ALB validate the theoretical analysis. The experimental results reveal that the proposed observers can achieve high accuracy and low costs as well as a high-quality grid side current.

Load Current Observer and Adaptive Voltage Controller for Standalone Wind Energy System with Linear and Non-linear Loads

The Wind Energy Conversion System (WECS) is one of the most important renewable energy systems in the existing power network, which can be operated in either grid connected or standalone mode. In the standalone WECS, voltage and frequency variations are more due to the wind speed variations, load changes and switching surges. In this paper, space vector pulse width modulation based adaptive voltage controller with load current observer is used to control the voltage and frequency, for the standalone or isolated WECS. The isolated system is simulated using MATLAB/Simulink and the results are analysed when three-phase non-linear and linear (Resistive and Inductive) balanced and unbalanced loads are connected.

Finite control set model predictive control scheme of four-switch three-phase rectifier with load current observer

Three-phase rectifier is typically realized by six power switches. However, this rectifier is fault sensitive in power switches. To enable continued controllable operation, the grid phase with fault rectifier leg can be connected to center tap of the dc-link capacitors, known as the four-switch three-phase rectifier (FSTPR), using hardware reconfiguration. However, the symmetry of three-phase currents and reliable operation of the FSTPR cannot be retained due to the offset of the two-capacitor voltages. This paper proposes a finite control set model predictive control (FCS-MPC) to obtain the balanced three-phase current with the offset of two-capacitor voltages suppressed. The PI-Controller-free FCS-MPC with a second-order Luenberger observer is adopted to improve the dynamic performance of FSTPR. The performance of the proposed control scheme is illustrated by extensive simulation and experimental results. The comparison with the conventional voltage-oriented-control, which is based on PI controller and pulse width modulation (PWM), is also presented to show the superiority of the proposed FCS-MPC.

An Optimal PR Control Strategy with Load Current Observer for a Three-Phase Voltage Source Inverter

Inverter voltage control is an important task in the operation of a DC/AC microgrid system. To improve the inverter voltage control dynamics, traditional approaches attempt to measure and feedforward the load current, which, however, needs remote measurement with communications in a microgrid system with distributed loads. In this paper, a load current observer (LCO) based control strategy, which does not need remote measurement, is proposed for sinusoidal signals tracking control of a three-phase inverter of the microgrid. With LCO, the load current is estimated precisely, acting as the feedforward of the dual-loop control, which can effectively enlarge the stability margin of the control system and improve the dynamic response to load disturbance. Furthermore, multiple PR regulators are applied in this strategy conducted in a stationary frame to suppress the transient fluctuations and the total harmonic distortion (THD) of the output voltage and achieve faster transient performance compared with traditional dual-loop control in a rotating dq0 frame under instantaneous change of various types of load (i.e., balanced load, unbalanced load, and nonlinear load). The parameters of multiple PR regulators are analyzed and selected through the root locus method and the stability of the whole control system is evaluated and analyzed. Finally, the validity of the proposed approach is verified through simulations and a three-phase prototype test system with a TMS320F28335 DSP.

An Observer-Based Optimal Voltage Control Scheme for Three-Phase UPS Systems

This paper proposes a simple optimal voltage control method for three-phase uninterruptible-power-supply systems. The proposed voltage controller is composed of a feedback control term and a compensating control term. The former term is designed to make the system errors converge to zero, whereas the latter term is applied to compensate for the system uncertainties. Moreover, the optimal load current observer is used to optimize system cost and reliability. Particularly, the closed-loop stability of an observer-based optimal voltage control law is mathematically proven by showing that the whole states of the augmented observer-based control system errors exponentially converge to zero. Unlike previous algorithms, the proposed method can make a tradeoff between control input magnitude and tracking error by simply choosing proper performance indexes. The effectiveness of the proposed controller is validated through simulations on MATLAB/Simulink and experiments on a prototype 600-VA testbed with a TMS320LF28335 DSP. Finally, the comparative results for the proposed scheme and the conventional feedback linearization control scheme are presented to demonstrate that the proposed algorithm achieves an excellent performance such as fast transient response, small steady-state error, and low total harmonic distortion under load step change, unbalanced load, and nonlinear load with the parameter variations.

The FCS-MPC research for a new inverter

Founding an accurate mathematical model for power electronic circuit is the primary work in order to design a superior controller. Considering that conventional switching function model only describes controlled transition, while ignored the conditional transition of the circuit, this paper founds a more accurate mixed logical dynamic (MLD) model for a new inverter and takes the MLD model as prediction model to research the finite control set model predictive control (FCS-MPC) for new inverter. FCS-MPC takes full account of distribution properties of inverter, the switch state that minimises objective function in FCS is selected as the control of inverter, without any control loop and modulator, which solves the optimisation problem of MPC. In addition, this paper designs load current observer for inverter based on least square estimation, which enhanced the robust of controller; its validity is verified by the simulation and experiment.

Intelligent voltage control strategy for three-phase UPS inverters with output LC filter

This paper presents a supervisory fuzzy neural network control (SFNNC) method for a three-phase inverter of uninterruptible power supplies (UPSs). The proposed voltage controller is comprised of a fuzzy neural network control (FNNC) term and a supervisory control term. The FNNC term is deliberately employed to estimate the uncertain terms, and the supervisory control term is designed based on the sliding mode technique to stabilise the system dynamic errors. To improve the learning capability, the FNNC term incorporates an online parameter training methodology, using the gradient descent method and Lyapunov stability theory. Besides, a linear load current observer that estimates the load currents is used to exclude the load current sensors. The proposed SFNN controller and the observer are robust to the filter inductance variations, and their stability analyses are described in detail. The experimental results obtained on a prototype UPS test bed with a TMS320F28335 DSP are presented to validate the feasibility of the proposed scheme. Verification results demonstrate that the proposed control strategy can achieve smaller steady-state error and lower total harmonic distortion when subjected to nonlinear or unbalanced loads compared to the conventional sliding mode control method.

Design and Stability Analysis of a Fuzzy Adaptive SMC System for Three-Phase UPS Inverter

This paper proposes a combined fuzzy adaptive sliding-mode voltage controller (FASVC) for a three-phase UPS inverter. The proposed FASVC encapsulates two control terms: a fuzzy adaptive compensation control term, which solves the problem of parameter uncertainties, and a sliding-mode feedback control term, which stabilizes the error dynamics of the system. To extract precise load current information, the proposed method uses a conventional load current observer instead of current sensors. In addition, the stability of the proposed control scheme is fully guaranteed by using the Lyapunov stability theory. It is shown that the proposed FASVC can attain excellent voltage regulation features such as a fast dynamic response, low total harmonic distortion (THD), and a small steady-state error under sudden load disturbances, nonlinear loads, and unbalanced loads in the existence of the parameter uncertainties. Finally, experimental results are obtained from a prototype 1 kVA three-phase UPS inverter system via a TMS320F28335 DSP. A comparison of these results with those obtained from a conventional sliding-mode controller (SMC) confirms the superior transient and steady-state performances of the proposed control technique.

An Adaptive Voltage Control Strategy of Three-Phase Inverter for Stand-Alone Distributed Generation Systems

This paper proposes an adaptive control method of three-phase inverters for stand-alone distributed generation systems (DGSs). The proposed voltage controller includes two control terms: an adaptive compensating term and a stabilizing term. The adaptive compensating control term is constructed to avoid directly calculating the time derivatives of state variables. Meanwhile, the stabilizing control term is designed to asymptotically stabilize the error dynamics of the system. Also, a fourth-order optimal load current observer is proposed to reduce the number of current sensors and enhance the system reliability and cost effectiveness. The stability of the proposed voltage controller and the proposed load current observer is fully proven by using Lyapunov theory. The proposed control system can establish good voltage regulation such as fast dynamic response, small steady-state error, and low total harmonic distortion under sudden load change, unbalanced load, and nonlinear load. Finally, the validity of the proposed control strategy is verified through simulations and experiments on a prototype DGS test bed with a TMS320F28335 DSP. For a comparative study, the control scheme of feedback linearization for multi-input and multioutput is implemented, and its results are presented in this paper.

Load current observer based feed-forward DC bus voltage control for active rectifiers

In this paper, load current observer based feedforward control of DC bus voltage for a three-phase active rectifier is presented. The paper focuses on: (1) design of conventional proportional-integral (PI) current controller with fastest possible response, (2) the digital current control loop time-delay compensation, (3) design of dc-link-load current observer for identifying the dc load current without using actual current sensors and (4) the feedforward control of dc bus voltage based on the designed current controller and dc link load current observer. A prototype is developed based on the proposed control scheme and expected results have been achieved.

Design and analysis of a feedforward control scheme for a three-phase voltage source pulse width modulation rectifier using sensorless load current signal

A load current observer based feedforward control scheme for three-phase voltage source pulse width modulation (PWM) active rectifiers is presented. The observer is designed to identify the load current disturbance, which is then forwarded to an inner current controller. Details of the design of the inner current controller and outer dc voltage regulator are also presented. To cope with the delay time of the digital control loop, Smith-predictor is employed in the current control loop to stop overshoots and oscillations during transient current regulation. Simulation model has been set up using commercially available VisSim software and then implemented in a digitally controlled prototype of a three-phase PWM active rectifier. The simulation and experimental results show that without using an actual load current sensor, the proposed observer based feedforward control scheme provides almost the same performance as the traditional feedforward control based on actual load current measurement.