Supervisors:

1. Dr. Hamid A. Toliyat,  Dept. of Electrical Engineering, Texas A&M University , Texas, USA.
2. Dr. Reza Ghazi, Dept. of Electrical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran
 

Abstract :

Nonlinear Loads produce non-sinusoidal currents in the power system. This harmonics cause many difficulties such as reducing the power factor and interference on communication and control systems. Many efforts have been performed to reject this unfavorable current components, and modern compensation strategies are based on using the active filters . The two main parts of an active filter are :

- Harmonic Detector System.

- Inverter Based Compensator System.

This thesis is a comprehensive research on the “ Active Filtering” . Enough attention has been paid on investigating the Basic concepts, reviewing the previously published methods , and investigating the advantages and weaknesses of each.

In the detection field , after evaluating the present methods including the well known p-q theory which is the state of the art , a new method based on the p-q theory and by using the IIR digital filters named “ Indirect Method” has been proposed and its superiority over the classical methods has been shown by digital computer simulations.

In the compensator field , presenting Fuzzy Logic Based PWM schemes are the main goal of the thesis. The proposed schemes optimize the THD or DF and switching loss of inverter . Before it , the performance of all important types of PWM Modulation schemes, Programmed & Carrier Based PWMs to be used in the active filters has been evaluated. Then several new Fuzzy Logic Based PWM systems such as Centroid Based and Equal Area Fuzzy PWM have been proposed and their superiority over conventional PWM systems has been shown . Also the Genetic Algorithms the well known numerical optimization methods have been used for optimizing the Fuzzy Modulators. Also the system speed and its performance in over modulation conditions have been analyzed .

Before using the proposed Fuzzy PWMs in the closed loop Current Regulated PWM (CRPWM) system, because of digital nature of fuzzy modulators it was felt that it is necessary to perform a separate research work on digital control of CRPWM system. This research led to some new important results. The results were very interesting and they were presented as a new theorem on sampling rate of PWM signals in digital control of CRPWM systems where the feedback signal contains a large amounts of high frequency ripples due to switching behavior of the inverter. In contrast with classic sampling theorem that says the sampling frequency should be higher than twice the signal bandwidth, further studies about the PWM signals showed that in the case where the PWM modulation is double edge and symmetric the sampling frequency can be reduced to be equal to the switching frequency of the inverter. The proposed theorem was analytically proved in the frequency domain and successfully verified by both the digital computer simulation and experiments on a practical system in laboratory . Also for adapting the under experiment system to the theoretical conditions some modifications were performed in the practical system. for example, because of opto -coupler and power devices delay we made to perform the delayed sampling techniques.

Experimental results successfully certified the presented theories and the performance of the several PWM techniques such as natural sampling and regular sampling ( both single edge and double edge) were practically examined and compared . This theoretical & practical results clearly showed the importance and advantages of symmetric PWM techniques. Thus , according to it the previously proposed Equal Area Fuzzy PWM which is a symmetric PWM strategy was selected to be used in the closed loop CRPWM system.

In the final stage, optimal control of the closed loop system has been attended . A hybrid objective function containing a term concerns the THD of inverter output and a term which affected by the transient and steady state response of the closed loop system has been defined . Then optimizing the Fuzzy PWM was performed busing the Genetic Algorithms and the resulting systemhas both the optimal THD and optimal time domain characteristics.