A Novel Control Method For Grid Side Inverters Under Generalized Unbalanced Operating Conditions

This thesis provides a summary on renewable energy sources integration into the grid, using an inverter, along with a comprehensive literature research on variety of available control methods. A new generalized method for grid side inverter control under unbalanced operating conditions is also proposed. The presented control method provides complete harmonic elimination in line currents and DC link voltage with adjustable power factor. The method is general, and can be used for all levels of imbalance in grid voltages and line impedances. The control algorithm proposed in this work has been implemented by using MATLAB Simulink and dSPACE RT1104 control system. Simulation and experimental results presented in this thesis are in excellent agreement

A Fast Diagnosis Method for Both IGBT Faults and Current Sensor Faults in Grid-Tied Three-Phase Inverters With Two Current Sensors

© 1986-2012 IEEE. This article considers fault detection in the case of a three-phase three-wire (3P3W) inverter, when only two current sensors are used to save cost or due to a faulty current sensor. With two current sensors, there is no current method addressing the diagnosis of both IGBT open-circuit (OC) faults and current sensor faults. In order to solve this problem, this article proposes a method which innovatively combines two kinds of diagnosis variables, line voltage deviations and phase voltage deviations. The unique faulty characteristics of diagnosis variables for each fault are extracted and utilized to distinguish the fault. Using an average model, the method only needs the signals already available in the controller. Both IGBT OC faults and current sensor faults can be detected quickly in inverter mode and rectifier mode, so that the converter can be protected in a timely way to avoid further damages. In addition, error-adaptive thresholds are adopted to make the method robust. Effects such as system unbalance are analyzed to ensure that the method is robust and feasible. Simulation and experimental results are used to verify and validate the effectiveness of the method

Predictive Voltage Control of Direct Matrix Converters with Improved Output Voltage for Renewable Distributed Generation

© 2013 IEEE. This paper proposes a predictive voltage control strategy for a direct matrix converter used in a renewable energy distributed generation (DG) system. A direct matrix converter with LC filters is controlled in order to work as a stable voltage supply for loads. This is especially relevant for the stand-alone operation of a renewable DG where a stable sinusoidal voltage, with desired amplitude and frequency under various load conditions, is the main control objective. The model predictive control is employed to regulate the matrix converter so that it produces stable sinusoidal voltages for different loads. With predictive control, many other control objectives, e.g., input power factor, common-mode voltage, and switching frequency, can be achieved depending on the application. To reduce the number of required measurements and sensors, this paper utilizes observers and makes the use of the switch matrices. In addition, the voltage transfer ratio can be improved with the proposed strategy. The controller is tested under various conditions including intermittent disturbance, nonlinear loads, and unbalanced loads. The proposed controller is effective, simple, and easy to implement. The simulation and experimental results verify the effectiveness of the proposed scheme and control strategy. This proposed scheme can be potentially used in microgrid applications

Harmonic current extraction of shunt active power filter based on prediction current technique - Hysteresis PWM

Due to the wide spread of power electronics equipment in modern electrical systems, the increase of the harmonics disturbance in the ac mains currents has became a major concern due to the adverse effects on all analysis and simulation using MATLAB-SIMULINK of a three-phase shunt active equipment. This paper presents the Shunt Active Power Filter (SAPF) to compensate the generated harmonics by 3-phase Rectifier Bridge fed R-L load. The harmonic current extraction is based on prediction current extraction technique -hysteresis PWM generation pattern

Grid Voltages Estimation for Three-Phase PWM Rectifiers Control Without AC Voltage Sensors

This paper proposes a new ac voltage sensorless control scheme for the three-phase pulse-width modulation rectifier. A new startup process to ensure a smooth starting of the system is also proposed. The sensorless control scheme uses an adaptive neural (AN) estimator inserted in voltage-oriented control to eliminate the grid voltage sensors. The developed AN estimator combines an AN network in series with an AN filter. The AN estimator structure leads to simple, accurate, and fast grid voltages estimation, and makes it ideal for low-cost digital signal processor implementation. Lyapunov-based stability and parameters tuning of the AN estimator are performed. Simulation and experimental tests are carried out to verify the feasibility and effectiveness of the AN estimator. Obtained results show that the proposed AN estimator presented faster convergence and better accuracy than the second-order generalized integrator-based estimator; the new startup procedure avoided the overcurrent and reduced the settling time; and the AN estimator presented high performances even under distorted and unbalanced grid voltages

Operation of Grid-Connected Inverter under Unbalanced Grid Conditions Using Indirect Voltage Sensoring

Abstract The grid connected voltage source inverter is now the most widely used interface for connecting renewable power generation to the grid. Control of this device is a key aspect to ensure the performance, reliability and life span of the renewable power generation system. Conventionally, the current control of the grid connected inverter is based on the measured grid side voltage. The power and the power factor at the receiving end, which is usually defined as the point of common coupling, can be controlled accurately. This controller topology has been widely used and many control methods have been developed aiming at objectives such as increasing system stability, decreasing harmonic injection, and improving transient response of the system. However, in case of the voltage measurement is not available, i.e. a faulty voltage sensor, the conventional current control topology will be disabled for lack of information of the grid voltage. This would decrease the reliability and efficiency of the system thus should be improved. voltage-sensor-less In this research, a current control system for the grid connected inverter system not relying on the information provided by the a.c. side voltage sensors will be developed with compliance to the recommendations issued to the performances of the distribution generations such as the harmonic limitations and the fault-ride-through capabilities. Three problem will be addressed and solved. Firstly, the a.c. side voltage should be acquired without the use of a.c. side voltage sensors. This is achieved by adopting an a.c. side voltage estimation algorithm. Secondly, the grid connected inverter should be able to start-up without synchronising to the grid while keep the current injected in a safe range. This is achieved by the newly designed start-up process. Thirdly, the grid connected inverter should be able to ride-through grid faults and providing support to the grid. The transient response of the grid connected inverter is the key measure to define the performance. In this study, a faster symmetrical component decomposition method is proposed to improve the transient response of the current control, without relying on grid voltage sensors. The proposed system is verified by both simulation and experimental tests, with analyses and insight aiming at general applications of the proposed method and algorithms

Sensorless Unbalance Modelling and Estimation as an Ancillary Service for LV 4-Wire/3-Phase Power Converters

This paper describes a method to provide LV four-wire three-phase power converters with the capability of unbalance estimation as an ancillary service to the main role that they play in the distribution system (distributed generator, energy storage system, drive, etc). Typically, dedicated grid/load current sensors are needed to effectively comply with unbalance compensation tasks, increasing system cost and reducing reliability. This is due to the difficulties that arises in the extraction of the zero and negative voltage components from the voltages at the point of common coupling, such as the inadequate resolution of full-scaled voltage sensors and limited spectral separation. In this paper, the proposed method does not rely on additional sensors to those typically used in VSCs, and in any case, those sensors are limited to the point of connection of the power converter. Impedance estimation only using converter-side current sensors is implemented by adding a high frequency voltage excitation over the fundamental command. A new model approach is proposed for the real-time extraction of system impedance using a complex-valued compact form. Considering the voltage source at that frequency to be unique in the grid, it will be proved the impedance can be estimated and, thus, the resulting negative sequence current which is used for unbalance compensation. For the zero sequence, an especial arrangement of the converter voltage sensors together with a repetitive controller is used