Investigation of a three-phase forced-commutation series capacitor operating with variable-voltage and variable-frequency systems

Abstract

This thesis investigates the use of a three-phase forced-commutation series capacitor (FCSC) for power factor correction in stand-alone variable-voltage variable-frequency (VVVF) systems. Two environmentally-friendly applications are chosen to represent the VVVF systems. The first application is a renewable energy resource as a direct drive wave energy conversion (WEC) buoys. The second application is the more electric aircraft (MEA). In such systems, permanent magnet PM generators are most commonly used. A generator-set generally consists of a three-phase generator connected directly to a conventional three-phase diode bridge rectifier for simplicity and cost reduction. Due to the high inherited inductance of the PM generators used in such applications, this configuration suffers from a poor power factor as a result of commutation overlap. Several controlled series compensator (CSC) topologies have been employed for decades in power systems where the voltage levels and frequency are fixed. However, in applications such as WEC and MEA, the voltage and frequency vary. Therefore, in this work, a variable switched capacitor is used in order to inject a capacitive reactance and therefore compensate the inductive reactance of the generator, which prevents power factor degradation. In a VVVF system, it is important to inject variable capacitive reactance since the inductive reactance changes with frequency variations. Five commonly used CSC circuits are compared and the FCSC is considered as the most suitable circuit topology which is able to cope with a range of frequency variations. This research mainly investigates the performance of the three-phase FCSC circuit when controlled by novel control strategy, in terms of power factor, output voltage, and output power under various load conditions, including constant and variable load. The harmonic content of the three-phase FCSC is also investigated in order to propose this topology for MEA power system. In an aerospace system, the power quality is required to meet high standards and harmonic distortion should not exceed the limited level set by aerospace industry authorities. Therefore, several types of conventional power factor corrector (PFC) are excluded from aerospace systems, due to the associated distortion levels. Preface Abstract iv In this thesis, a novel symmetrical duty cycle control (SDCC) scheme is proposed in order to qualify the three-phase FCSC converter to be employed in different ranges of frequency variation, including 1-3 Hz for wave energy and 50-500 Hz as part of aircraft frequencies. The approach is simple to implement, with no need for a sophisticated controller design. The switch duty cycle is a function of the supply frequency and this allows the FCSC circuit to cope with frequency variation. The modes of operation for both single and three-phase circuit topologies are presented. The three-phase FCSC circuit is designed and tested in the laboratory environment. The performance of the three-phase FCSC circuit when using SDCC is tested experimentally and assessed by comparison of its performance with that of the conventional three-phase diode bridge rectifier. Experimental and simulation results validate the capability of the three-phase FCSC- rectifier to improve the power factor to approximately unity in addition to increasing the output voltage and power at higher voltage and frequency values. However, only limited improvements are achieved at the lower values of the frequency spectrum.