Several space vector modulation schemes have been analyzed for three-leg and four-leg voltage source inverters. The analysis is performed with respect to a) switching losses, b) total harmonic distortion, c) peak-to-peak ripple in the line current and d) the ease of digital implementation. The analysis is performed over the entire range of modulation index and for varying load power factors (leading and lagging) under both balanced and unbalanced load conditions. The analysis shows that the performance of four-leg inverters is similar to three-leg inverters for various space vector modulation schemes. The analysis also verifies the fact that a modulation scheme with good harmonic performance usually has high switching losses and vice-versa. The analysis is verified using simulation and experiments. A novel algorithm for the calculation of total harmonic distortion of PWM signals has been proposed. / Master of Science

This dissertation covers three levels of issues and solutions dealing with unbalanced and/or nonlinear situations in power electronic systems, namely power converter level, power converter system level, and large-scale power electronics system level. At power converter level, after review of traditional PWM methods, especially two-dimensional space vector modulation schemes, three-dimensional space vector modulation schemes are proposed for four-legged voltage source converters, including inverters and rectifiers. The four-legged power converters with three-dimensional space vector modulation schemes have a better DC link voltage utilization and result in a low distortion. It is an effective solution to provide the neutral point for a three-phase four-wire system and to handle the neutral current due to unbalanced load or source and nonlinear loads. Comprehensive design guidelines for a four-legged inverter are presented. The four-legged rectifier is also presented which allows not only fault tolerant operation, but also provides the flexibility of equal resistance, equal current, or equal power operation under unbalanced source. Average large-signal models of four-legged power converters in both the a-b-c and d-q-o coordinates are derived. Small signal models are obtained in the d-q-o rotating coordinates. Voltage control loops are designed in the d-q-o coordinates for a high power utility power supply. Performance is studied under various load conditions. At the power converter system level, the load conditioner concept is proposed for high power applications. A power converter system structure is proposed which consists of a high-power low-switching frequency main inverter and a low-power high-switching frequency load conditioner. The load conditioner performs multiple functions, such as active filtering, active damping, and active decoupling with a high current control bandwidth. This hybrid approach allows the overall system to achieve high performance with high power and highly nonlinear loads. At the large-scale power electronics system level, the nonlinear loading effect of load converters is analyzed for a DC distribution system. Two solutions to the nonlinear loading effect are presented. One is to confine the nonlinear load effect with the sub-converter system, the other is to use a DC bus conditioner. The DC bus conditioner is the extension of the load conditioner concept. / Ph. D.

Three-phase voltage source inverters are increasingly employed in power systems and industrial applications. Various pulse width modulation strategies have been applied to control the voltage source inverters. This dissertation presents a hybrid 2D-3D space vector modulation algorithm for three-phase voltage source inverters with both three-wire and four-wire topologies. The voltage magnitude and phase angle of the inverters fundamental output phase voltage are precisely controlled under either balanced or unbalanced load conditions, and hence, the space vector algorithm offers synchronization controllability over generation control in distributed generation systems. The numerical efficiency and simplicity of the proposed algorithm are validated through conducting MATLAB/Simulink simulations and hardware experiments. Mathematical description and harmonic analyses of output phase voltages of three-phase voltage source inverter which employs a hybrid 2D-3D SVM are presented in this dissertation. Explicit time domain representation of the harmonic components in addition to the total harmonic distortion of the output phase voltages are given in terms of system and switching parameters. The dissertation also investigates the harmonic characteristics and low total harmonic distortion performance against the linearity of modulation region which helps in the harmonic performance and design studies of such inverters employing the hybrid 2D-3D SVM. Experimental results are used to validate these analyses. In addition, the performance and the harmonic contents of the inverter output phase voltage when applying the proposed hybrid 2D-3D SVM are compared to that obtained from conventional 2D SVM and 3D SVM. As a result, the proposed new algorithm shows advantages in terms of low total harmonic distortion and reduced harmonic contents in both three-wire and four-wire systems.

Permanent magnet synchronous motors are finding ever increasing use in hybrid and electric vehicles. This thesis develops a new control strategy for Permanent Magnet Synchronous Motors (PMSMs) to reduce the motor and inverter losses compared to conventional control techniques. The strategy utilises three common control modes for PMSMs; brushless DC with 120°E conduction, brushless DC with 180°E conduction, and brushless AC control. The torque and power output for each control mode is determined for an example motor system using a three phase axial flux YASA motor and an IGBT inverter. The loss components for the motor and inverter are also estimated using a combination of analytical and simulation techniques and results are then validated against experimental measurements. Efficiency maps for each control mode have been used to determine an optimal mode utilisation strategy, which minimises the total system losses and maximises the available motor torque output. The proposed control strategy switches between the three control modes without interruption of motor torque to maximise the system efficiency for the instantaneous operating speed and demanded torque output. The benefits of the new strategy are demonstrated using an example vehicle over a simulated drive cycle. This yields a 10% reduction in losses compared to conventional brushless AC control.

Pulse-width modulation methods are widely used for the synthesis of ac voltages at the terminals of a voltage-sourced converter (VSC). Traditionally sinusoidal pulsewidth modulation (SPWM) has been used. A powerful alternative for this purpose is space-vector modulation (SVM), in which the converter is placed in a finite number of states in order to best approximate the reference voltage. This method offers better utilization of the dc bus voltage and provides several degrees of freedom for enhancement of the harmonic spectrum as well as switching losses. This thesis studies the SVM method for two- and three-level VSCs. A model for implementation of SVM in the electromagnetic transients simulation program PSCAD/EMTDC is developed. The model is able to generate firing pulses in linear as well as overmodulation range and is used to study the performance of different SVM strategies in terms of their harmonic spectra and associated converter and harmonic losses. The model is also used to demonstrate the suitability of the method for network applications. The thesis also employs genetic algorithms to find an optimized SVM sequence for improved harmonic performance. An objective function is defined that seeks to minimize the most significant harmonic components of the generated waveform, while keeping the other harmonic components within the acceptable range outlined in the available standards. The obtained sequence shows great improvement over the conventionally-used SVM sequence. / October 2007

Sliding mode controllers (SMC) with time delay and a rotor position observer are designed for the sensorless speed control of permanent magnet synchronous motor (PMSM) are proposed in this paper. Based on field-oriented principle, a flux SMC is designed to achieve quick flux control. And then a speed SMC with time delay is presented and compared with PI controller in the direct torque control framework. The effectiveness of the proposed control scheme under the load disturbance and parameter uncertainties is verified by simulation results.

Pulse-width modulation methods are widely used for the synthesis of ac voltages at the terminals of a voltage-sourced converter (VSC). Traditionally sinusoidal pulsewidth modulation (SPWM) has been used. A powerful alternative for this purpose is space-vector modulation (SVM), in which the converter is placed in a finite number of states in order to best approximate the reference voltage. This method offers better utilization of the dc bus voltage and provides several degrees of freedom for enhancement of the harmonic spectrum as well as switching losses. This thesis studies the SVM method for two- and three-level VSCs. A model for implementation of SVM in the electromagnetic transients simulation program PSCAD/EMTDC is developed. The model is able to generate firing pulses in linear as well as overmodulation range and is used to study the performance of different SVM strategies in terms of their harmonic spectra and associated converter and harmonic losses. The model is also used to demonstrate the suitability of the method for network applications. The thesis also employs genetic algorithms to find an optimized SVM sequence for improved harmonic performance. An objective function is defined that seeks to minimize the most significant harmonic components of the generated waveform, while keeping the other harmonic components within the acceptable range outlined in the available standards. The obtained sequence shows great improvement over the conventionally-used SVM sequence.

Pulse-width modulation methods are widely used for the synthesis of ac voltages at the terminals of a voltage-sourced converter (VSC). Traditionally sinusoidal pulsewidth modulation (SPWM) has been used. A powerful alternative for this purpose is space-vector modulation (SVM), in which the converter is placed in a finite number of states in order to best approximate the reference voltage. This method offers better utilization of the dc bus voltage and provides several degrees of freedom for enhancement of the harmonic spectrum as well as switching losses. This thesis studies the SVM method for two- and three-level VSCs. A model for implementation of SVM in the electromagnetic transients simulation program PSCAD/EMTDC is developed. The model is able to generate firing pulses in linear as well as overmodulation range and is used to study the performance of different SVM strategies in terms of their harmonic spectra and associated converter and harmonic losses. The model is also used to demonstrate the suitability of the method for network applications. The thesis also employs genetic algorithms to find an optimized SVM sequence for improved harmonic performance. An objective function is defined that seeks to minimize the most significant harmonic components of the generated waveform, while keeping the other harmonic components within the acceptable range outlined in the available standards. The obtained sequence shows great improvement over the conventionally-used SVM sequence.

The electrification of transportation is increasingly of interest to governments around the world as a means of contributing to the achievement of climate change goals. Transportation is a significant source of greenhouse gas emissions, but it is also the backbone of the global economy and local mobility. Electrification is widely seen as a promising pathway to reducing greenhouse gas emissions from transportation while continuing to support economic growth. Multiphase machines have distinctive features that draw attention in the transportation electrification domain due to their features. Recently, powertrains based on the current-source inverter (CSI) are getting more attention to be a more reliable structure for Electric Vehicles (EVs) by replacing the dc-link capacitor with a choke inductor. This thesis combines these two technologies to develop a more reliable, compact powertrain for heavy-duty electric vehicles. First, a survey covers the recent advances in several aspects such as topology, control, and performance to evaluate the possibility and the future of exploiting them more in EV applications. The six-phase drives are extensively covered here because of their inherent structure as a dual three-phase system which eases the production process. The survey presents the different topologies used in dual three-phase drives, the modulation techniques used to operate them, the status of using multiphase drives in traction applications industrially, and the upcoming trends toward promoting this technology. New powertrain configurations for heavy-duty electric vehicles (HDEV) are proposed based on current-source inverters (CSI) and asymmetrical six-phase electric machines. Since the six-phase CSI comprises two three-phase CSIs, multiple configurations can arise based on the connection between the two CSIs. In this context, the proposed powertrain configurations are based on parallel, cascaded, and standalone six-phase CSIs. The standalone topology is based on separating the two three-phase converters by supplying each converter with a dedicated dc-dc converter. A new and straightforward method is proposed to extend the six-phase standalone CSI. The proposed technique employs the vector space decomposition (VSD) to mitigate the inverter current harmonics and extend the linear modulation region by about 8%. For motor drive applications, increasing the fundamental output component can reflect higher torque production capability for the same drive size, given that thermal limits are not exceeded. Moreover, to increase the drive's reliability, space vector modulation (SVM) techniques are developed to operate the six-phase CSI while reducing the common-mode voltage (CMV) content associated with the switching of semiconductors. The SVM techniques select the switching states associated with the minimum CMV value offline to eliminate the need for measurements. Experimental validation of the proposed algorithms is presented to operate a scaled-down six-phase PMSM fed by the proposed powertrain configuration. These proposed techniques make the CSI- based powertrain a promising solution for future HDEVs in terms of cost, performance, and reliability. / Thesis / Candidate in Philosophy

The high penetration of renewable energy and the emerging concept of micro-grid system raises challenges to the high power conversion techniques. Multilevel converter plays the key role in such applications and is studied in detail in the dissertation. The topologies and modulation techniques for multilevel converter are categorized at first by a thorough literature survey. The pros and cons for various multilevel topologies and modulation techniques are discussed. The 3-level neutral point clamped (NPC) topology is selected to build a 200kVA, 20 kHz power conversion system. The modularized phase leg building block of the converter is carefully designed to achieve low loss and stress for high frequency and high power operation. The switching characteristics for all the commutation loops of 3-level phase leg are evaluated by double pulse tests. The switching performance is optimized for loss and stress tradeoff. A detailed loss model is built for system loss distribution and loss breakdown calculation. Loss and stress for the phase leg and 3-phase system are quantified at all power factors. The space vector modulation (SVM) for 3-level NPC converter is investigated to achieve loss reduction, neutral voltage balance and noise reduction. The loss model and simulation model provides a quantitative analysis for loss and neutral voltage ripple tradeoff. An improved SVM method is proposed to reduce NP imbalance and switching loss simultaneously. This method also ensures an evenly distributed device loss in each phase leg and gives a constant system efficiency under different power factors. Based on the improved modulation strategy, a new modulation scheme is then proposed with largely reduced conduction loss and switching stress. Moreover, the device loss and stress distribution on a phase leg is more even. This scheme also features on the simplified implementation. The improved switching characteristics for the proposed method are verified by double pulse tests. Also the system loss breakdown and the phase leg loss distribution analysis shows the loss reduction and redistribution result. The harmonic filter for the grid interface converter is designed with LCL topology. A detailed inductor current ripple analysis derives the maximum inductor current ripple and the ripple distribution in a line cycle. The inverter side inductor is designed with the optimum loss and size trade-off. The grid side inductor is designed based on grid code attenuation requirement. Different damping circuits for LCL filter are evaluated in detail. The filter design is verified by both simulation and hardware experiment. The average model for the 3-level NPC converter and its equivalent circuit is derived with the consideration of damping circuit in both ABC and d-q frame. The modeling and control loop design is verified by transfer function measurement on real hardware. The control loops design is also tested and verified on real hardware. The interleaved DC/DC chopper is introduced at last. The different interleaving methods and their current ripple are analyzed in detail with the coupled and non-coupled inductor. An integrated coupled inductor based on 3-dimentional core structure is proposed to achieve high power density and provide both CM and DM impedance for the inductor current and output current. / Ph. D.