High-performance digital control of UPS inverters

Ph.DDOCTOR OF PHILOSOPH

Mitigation of Grid-Current Distortion for LCL-Filtered Voltage-Source Inverter with Inverter-Current Feedback Control

LCL filters feature low inductance; thus, the injected grid current from an LCL -filtered voltage-source inverter can be easily distorted by grid-voltage harmonics. This problem is especially tough for the control system with inverter-side current feedback (ICF), since the grid-current harmonics can freely flow into the filter capacitor. In this case, because of the loss of harmonic information, traditional harmonic controllers fail to mitigate the grid-current distortion. Although this problem may be avoided using the grid-voltage feedforward scheme, the required differentiators may cause the noise amplification. In light of the above issue, this paper develops a simple method for the ICF control system to mitigate the grid-current harmonics without extra sensors. In the proposed method, resonant harmonic controllers and an additional compensation loop are adopted at the same time. The potential instability introduced by the compensation loop can be avoided through a special design of the compensation position. Finally, the effectiveness of the proposed method for harmonic rejection is verified by detailed experimental results

Contributions to cascade linear control strategies applied to grid-connected Voltage-Source Converters

El trabajo desarrollado en esta Tesis se centra en optimizar el comportamiento de Voltage-Source Converters (VSCs) cuando son utilizados como interfaz con la red eléctrica, tanto para absorber como para entregar energía de la red con la mejor calidad posible y cumpliendo con los estándares. Para tal fin, esta Tesis se centra en el control de sistemas lineales conectados en cascada aplicados al control de VSCs conectados en paralelo con la red eléctrica a través de un filtro L, especialmente en conexiones con redes débiles y en dos líneas de trabajo: (i) seguimiento de armónicos de las corrientes de red y rechazo de armónicos de las tensiones de red, y (ii) control de la tensión del PCC en caso de desequilibrio. Para ello, esta Tesis realiza contribuciones en el área del control de corriente y control de la tensión del PCC. De entre las técnicas existentes para implementar el control de corriente para compensación armónica, dos de las más utilizadas son el control resonante y el control repetitivo, tanto en ejes de referencia estacionarios como síncronos. Se ha realizado un exhaustivo estudio de diferentes estructuras para implementar tales controles, mostrando su algoritmo adaptativo en frecuencia para cada una de ellas y analizando su carga computacional. Además, se han facilitado directrices básicas para su programación en un DSP. Se ha analizado también el esquema de control de corriente para establecer una comparación entre las diferentes estructuras. Después de estudiar en profundidad el control de corriente de un VSC conectado a la red eléctrica, el segundo control a analizar es el control de tensión del PCC. La presencia de una tensión desequilibrada en el PCC da lugar a la aparición de una componente de corriente de secuencia negativa, que deteriora el comportamiento del sistema de control cuando se emplean las técnicas de control convencionales. Los STATCOMs son bien conocidos por ser una aplicación de potencia capaz de llevar a cabo la regulación de la tensión en el PCC en líneas de distribución que pueden ser susceptibles de sufrir perturbaciones. Esta Tesis propone el uso de un controlador de tensión en ejes de referencia síncronos para compensar una tensión desequilibrada a través de un STATCOM, permitiendo controlar independientemente tanto la secuencia positiva como la secuencia negativa. Además, este controlador incluye aspectos como un mecanismo de antiwindup y droop control para mejorar su comportamiento. Se han realizado varias pruebas experimentales para analizar las características de los controladores de corriente abordados en esta Tesis. Todas ellas han sido realizadas bajo las mismas condiciones de potencia, tensión y corriente, de modo que se pueden extraer resultados comparativos. Estas pruebas pretenden caracterizar la respuesta transitoria, la respuesta en régimen permanente, el comportamiento frente a saltos de frecuencia y la carga computacional de los controladores de corriente estudiados

Measurements, Models, Systems and Design

531 s.

Design and Development of FPGA based Controllers for Photovoltaic Power System

In the recent years owing to increased energy consumption and consequent rise in crude oil price and global climatic change have motivated researchers to focus towards harnessing power from renewable energy resources such as photovoltaic (PV), fuel cell, biomass and wind energy systems. Among the different renewable resources, PV technology is one of the fastest growing technologies, because of abundance availability of solar irradiance and it has no adverse environmental impacts. But, the cost of PV energy is higher than the other conventional sources owing to its low PV conversion efficiency. Therefore, research opportunities lie in applying power electronics and control techniques for harvesting PV power at higher efficiencies for appropriate utilization. For simulation, analysis and control design of a PV power system, an accurate model of the PV cell is essential because PV cell is the basic bulding block of a PV power system. To maximise the power generation of a PV system it is necessary that the PV array should be operated at the maximum power point. A maximum power point tracker (MPPT) is required in the PV system to enable it to operate at the MPP. The output current-voltage (I-V) and power-voltage (P-V) characteristics of a PV vell are non-linear and hence its power fluctuates in accordance with the variation in solar irradiance and temperature. During the last decade, a lot of research has been directed to develop efficient MPPT schemes. But, research opportunities are still promising for designing new MPPT algorithms and to address their digital implementation issues. Further, there lies challenge to design MPPTs that can handle partial shading conditions. The thesis first proposes development of new MPPT algorithms and different pulse width modulated-voltage source inverter control strategies for a PV system. Firstly an integral sliding mode MPPT controller (ISMC) has been proposed for achieving an effective MPPT scheme, and then a modified P&O MPPT controller is developed which is implemented using a real-time digital simulator called Opal-RT. The performance of the modified ISMC is compared with that of the conventional proportional integral (PI) MPPT controller using both MATLAB simulation and real-time experimentation. The performance of the modified P&O MPPT controller with fixed step size is compared with that of the conventional incremental conductance (Inc Cond) and P&O MPPT controllers, and these are validated by using Opal-RT and subsequently through FPGA implementation. A modified incremental conductance MPPT controller with variable step size is then proposed for handling partial shading conditions. The tracking performance of the proposed modified Inc Cond MPPT controller is also compared with that of the conventional Inc Cond MPPT controller, from the obtained results by using Opal-RT. Further, an experimental prototype PV set-up is developed in the laboratory to implement the proposed MPPT algorithms on the physical hardware. After having developed efficient parameter extraction algorithms for a PV panel, the thesis subsequently proposes five new MPPT algorithms such as Integral sliding mode MPPT, modified P&O MPPT, modified Inc Cond MPPT, Model predictive MPPT, and modified Inc Cond variable step size MPPT controllers. All these developed MPPT algorithms have been implemented on a Solar array simulator (SAS) PV system, in MATLAB/SIMULINK, OPAL-RT and on a prototype hardware PV set-up. From the obtained results, it is found that these MPPTs adjust the power of a PV system effectively to its maximum power value smoothly with fast response and accuracy whilst reducing the fluctuations in its power. Tracking performance of all these proposed MPPT algorithms are found to be superior to some of the existing MPPTs such as perturb and observe (P&O), incremental conductance (INC), HCC and adaptive HCC. Further more, a PV system is observed to be stable with all these proposed MPPTs. From the results obtained it is also confirmed that the proposed modified P&O MPPT exhibits better MPP tracking performance in terms of quick settling time and least steady state error. Further, less voltage fluctuation and less maximum overshoot are observed in the case of the proposed modified Inc Cond MPPT among all the proposed MPPT algorithms. The proposed controllers are also well suited to all weather conditions. A grid connected PV system involves a power conversion from DC power into AC power. Due to high switching frequencies of this conversion by inverter, there is a power loss. An efficient control scheme needs to be developed for integrating the PV system to the grid. The thesis then proposes a Model Predictive Control (MPC) for integrating a PV system to the grid. The performance of the MPC is compared with conventional hysteresis current controller (HCC) and also with that of an adaptive HCC (AHCC) through a real-time simulatin using the Opal-RT then through FPGA implementations. FPGA implementation of the controllers such as HCC, AHCC and MPC were also performed by using LABVIEW configured with NI-cRIO-9014 platform. For elimination of current harmonic and reactive power of the grid connected PV system, there is a need of designing a filter. The PV system based shunt active power filter (SAPF) with modified incremental conductance MPPT controller with variable step size is then designed. From the MATLAB simulation and real-time digital simulation studies it is envisaged that the proposed PV based SAPF exhibits good harmonics compensation

Design and Implementation of Internal Model Based Controllers for DC/ AC Power Converters

The aim of this thesis is to design and implement an advanced control system for a working three-phase DC to AC power converter. Compared to' the traditional PI controller used widely in industry, the new voltage controller can track the reference voltage with improved accuracy and efficiency in the presence of different kind of local loads, and also works well in the single phase voltage control. This voltage controller is combined with a power controller to yield a complete controller. An important aspect of this work is the hardware implementation of the whole system. Main parts ofthis thesis are: ???????? 1. Review ofH-infinity and repetitive control techniques and their applications in power converters. 2. Design of a new voltage controller to eliminate the DC component in the output voltages, and taking into account the practical issues such as the processing delay due to the digital signal processor (DSP) implementation. 3. Modelling and simulation of the converter system incorporating different control techniques and with different kinds of loads. 4. Hardware implementation and the two-processor controller. The parallel communication between the DSPs. 5. The main problems encountered in???????????????????? hardware implementation and programming. The software used to initialize DSPs, implement the discretetime voltage controller and other functions such ~ generations of space vector pulse width modulation (SVPWM) signals, circuit protections, analog to digital (AD) cOl)versions, data transmission, etc. 6. Experimental results the under circumstances of no load connected to the converter, pure three-phase resistive loads, three-phase unbalanced resistive' loads and the series resistor-inductor loads. /Imperial Users onl

Model Based Design Environment for the Embedded System Case Study: Inverter Power Supply

学位記番号：工博甲43

Enhanced controller for grid-connected modular multilevel converters in distorted utility grids

This paper is about the control of Modular multilevel converters, an innovative technology in the design of converters, which is beginning to be included in real installations. Papers about this topic include simulation models, circulating current reduction, voltage modulators, capacitor voltage balancing and control issues. The scheme for current source regulation used in this article includes all control loops, which are, from the outermost to innermost, DC bus voltage regulator, current regulator, voltage modulator, capacitor voltage balancing, and a PLL for the synchronization to the grid. Disposition-sinusoidal pulse width modulation is used as the voltage modulator, and an enhanced control strategy in the stationary reference frame for 3-phase MMCs is used for the inner current control loops. Very detailed simulations of the complete control system have been performed for both the enhanced control strategy in the stationary reference frame, and the well-known control in the synchronous reference frame, as well as some experiments using the hardware-in-the-loop simulation technique. The validation of these control strategies is made by a comparison of the capability of each one to compensate the harmonic distortions of the utility grid according to the grid code. The correct operation has been tested in the case of a strong/weak grid, unbalances and grid failures.This work has been partially supported by a grant from the Spanish Government as a part of 673 Project Ref. TEC2016-80136-P, entitled “Nuevas topologías para convertidores en MT para grandes 674 Instalaciones Fotovoltaicas” (A. B. Rey-Boué