published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 1995. / Includes bibliographical references.

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No description available.

This thesis presents the development of a generalised computer program to calculate harmonic currents and voltages in six and twelve-pulse thyristor controlled reactors under non-ideal conditions. Thyristor controlled reactors are a relatively new source of harmonic distortion in power systems. The steady state characteristic harmonics are well known. Other non-characteristic harmonics can, however, be generated. A detailed representation is therefore necessary. Apart from the most common non-ideal conditions, such as voltage, impedance and firing pulse unbalances, this work allows voltage harmonic distortions, two firing pulse control methods, the effect of the feedback control in the equally spaced firing pulse control and the effect of the step-down transformer saturation. The effect of the a.c. system impedance, filters and capacitor banks is also included. Four different models of filters were implemented. With non-infinite a.c. systems, the harmonic currents generated are not totally absorbed by the filters. The remaining distortion may affect the main busbar voltages. Therefore, an iterative method was adopted in which the distorted voltages calculated at the end of one iteration are used to calculate the new currents and voltages. The process is repeated until convergence is reached. Several cases were then studied using this program which was then joined to an existing steady-state converter harmonic program. For instance, the need for a more complete representation of controlled reactors, converters and a.c. system network is illustrated. This study begins considering an hvdc station under ideal conditions which are then gradually moved towards more real conditions. The influence of the a.c. system representation in harmonic studies is also discussed. This analysis also compares the performance of two filter designs, namely the tuned and the damped filters. A study of harmonic magnification in the presence of a.c. and d.c. resonances is also included. The harmonic calculations program presented in this thesis is able to study so many conditions of operation of converters and/or thyristor controlled reactors that it is impractical to show all the possible cases. For instance, filters and capacitor banks can be installed at the converter busbar or at any controlled reactor busbar. Furthermore, the three-phase calculation approach allows studies in which some abnormal operation, such as the absence of a filter branch or capacitor bank at one phase, can be observed.

Th. doct-ing.--Toulouse, I.N.S.A., 1978. N°: 8.

M.Ing. / This dissertation describes the development and characteristics of a thyristor controlled reactive three phase network which is used for the compensation of reactive power in a power network. The problem of distortion in power networks is illustrated and the development of a thyristor controlled reactive three phase network is motivated. The considerations that were taken into account in the design of the network are explained and the experimental setup is evaluated. A computer simulation was done to verify a single phase model of the reactive power network. The driving function in power networks is a singular harmonic voltage and distortion is defined as any deviation ofthe current from a scaled, in phase version of the voltage. Distortion is caused byloads with a non-linear nature such as arc furnaces and variable speed drives. Reactive power, which forms part of the distortion problem, is caused by loads with a phase shift between the voltage and current. Distortion has several adverse effects on a power network resulting in the degrading of the quality of the supply. At present the state of the art in technology does not permit the economic compensation of distortion with dynamic filters at MVA levels. However, by compensating the reactive power present in a network byusing variable reactive elements, the remaining distortion in the network can be compensated by using a dynamic filter. The combination ofa thyristor controlled reactive network and a dynamic filter are termed a hybrid power compensator. Results are shown where a hybrid power compensator has been implimented to compensate distortion caused by a non-linear load. The thyristor controlled reactive three phase network described in this dissertation is rated at 21 kVAr and comprises the following on a per phase basis: The series combination of a fixed capacitor bank and a filter inductance is connected via an isolation transformer to a reactor and two anti parallel thyristors. The reactive elements have been dimensioned in such a way that reactive power can only be generated and not absorbed.

M.Ing. (Electrical & Electronic Engineering) / The gate turn-off thyristor is discussed as a power switch. A gate-firing circuit for gate turn-off thyristors in the range 10 A - 300 A was developed. The resonant dc-link as snubber for voltage fed inverters is discussed. On considering various factors, the gate turn-off thyristor was chosen as switching component in the inverter. The problems that deve16ped with the use of gate turn-off thyristors in resonant dc-link inverters w,re discussed. Which lead to the development of a storage time compensator for gate turn-off thyristors. Attention was given to the various possible control methods for the resonant dc-link. A study of the possible control strategies results in the development and manufacturing of both a single-phase and a three-phase controller. The operation of the resonant inverter and inverter/load system were evaluated in terms of wave shapes in the time domain.

The Emitter Controlled Thyristor (ECT) is a new MOS-Gated Thyristor (MGT) that combines the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. An ECT is composed of an emitter switch in series with the thyristor, an emitter-short switch in parallel with the emitter junction of the thyristor, a turn-on FET and the main thyristor structure. Numerical analysis shows that the ECT also offers superior high voltage current saturation capability even for high breakdown voltage ratings. Two different ECT structures are investigated in this research from numerical simulations to experimental fabrications. A novel ECT structure that utilizes IGBT compatible fabrication process was proposed. The emitter short FET, emitter switch FET and turn-on FET are all integrated with a high voltage thyristor. Numerical simulation results show that the ECT has a better conductivity modulation than that of the IGBT and at the same time exhibits superior high voltage current saturation capability, superior FBSOA and RBSOA. The technology trade-off between turn-off energy loss and forward voltage drop of the ECT is also better than that of the IGBT because of the stronger conductivity modulation. A novel self-aligned process is developed to fabricate the device. Experimental characteristics of the fabricated ECT devices show that the ECT achieves lower forward voltage drop and superior high voltage current saturation capability. A Hybrid ECT (HECT) structure was also developed in this research work. The HECT uses an external FET to realize the emitter switching function, hence a complicated fabrication issue was separated into two simple one. The cost of the fabrication decreases and the yield increases due to the hybrid integration. Numerical simulations demonstrate the superior on-state voltage drop and high voltage current saturation capability. A novel seven-mask process was developed to fabricate the HECT. Experimental results show that the HECT could achieve the lower forward voltage drop and superior current saturation capability. The resistive switching test was carried out to demonstrate the switching characteristics of the HECT. / Master of Science

The Emitter Controlled Thyristor (ECT) is a new MOS-Gated Thyristor (MGT) that combines the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. An ECT is composed of an emitter switch in series with the thyristor, an emitter-short switch in parallel with the emitter junction of the thyristor, a turn-on FET and the main thyristor structure. Numerical analysis shows that the ECT also offers superior high voltage current saturation capability even for high breakdown voltage ratings. Two different ECT structures are investigated in this research from numerical simulations to experimental fabrications. A novel ECT structure that utilizes IGBT compatible fabrication process was proposed. The emitter short FET, emitter switch FET and turn-on FET are all integrated with a high voltage thyristor. Numerical simulation results show that the ECT has a better conductivity modulation than that of the IGBT and at the same time exhibits superior high voltage current saturation capability, superior FBSOA and RBSOA. The technology trade-off between turn-off energy loss and forward voltage drop of the ECT is also better than that of the IGBT because of the stronger conductivity modulation. A novel self-aligned process is developed to fabricate the device. Experimental characteristics of the fabricated ECT devices show that the ECT achieves lower forward voltage drop and superior high voltage current saturation capability. A Hybrid ECT (HECT) structure was also developed in this research work. The HECT uses an external FET to realize the emitter switching function, hence a complicated fabrication issue was separated into two simple one. The cost of the fabrication decreases and the yield increases due to the hybrid integration. Numerical simulations demonstrate the superior on-state voltage drop and high voltage current saturation capability. A novel seven-mask process was developed to fabricate the HECT. Experimental results show that the HECT could achieve the lower forward voltage drop and superior current saturation capability. The resistive switching test was carried out to demonstrate the switching characteristics of the HECT. / Master of Science