This thesis deals with the problem of control system design for power electronic controllers when high performance is desired despite unaccounted for internal and external conditions. Factors such as parameter variations, operating condition changes, and filtering and measurements delays, may adversely impact the performance of a circuit whose controller design is not immune to external and internal disturbances. The thesis explores the method of internal model design as a viable approach for designing controllers with superior performance despite system variations. Following a presentation of the theoretical background of the internal model design, the thesis considers two examples of state variable models, improving the stability of a voltage source converter and speed control of an induction motor. Conclusions show the new control system is more stable and offers better controllability despite unexpected system variations, compared to classical control system.

Voltage Source Converter (VSC) applications include but are not limited to HVDC, Flexible AC Transmission System (FACTS) devices such as STATCOM, SSSC, UPFC and Wind generators and active filters. The VSC based HVDC system is a feasible option for bulk power transmission over long or short distances and the grid integration of renewable energy sources in existing transmission and distribution systems. The main requirement in a power transmission system is the precise control of active and reactive power flow to maintain the system voltage stability. The VSC operating with the specified vector control strategy can perform independent control of active/reactive power at both ends. This ability of VSC makes it suitable for connection to weak AC networks or even dead networks i.e. without local voltage sources. For power reversal, the DC voltage polarity remains the same for VSC based transmission system and the power transfer depends only on the direction of the DC current. This is advantageous when compared to the conventional Current Source Converter (CSC) based HVDC system. Furthermore, in case of VSC, the reactive power flow can be bi-directional depending on the AC network operating conditions. In this thesis, a 3-phase, 2-level, 6-switch VSC connected to an active but weak AC system at both ends of the HVDC link is developed using EMTP-RV. The VSC-HVDC transmission system model is developed using both direct control and vector control techniques. The direct control method is an approximate method in which the active power, AC voltages at both ends of HVDC link and DC link voltage are controlled directly by using PI-controllers. In vector control method, closed loop feed-forward control system is used to control the active power, reactive power at both ends and DC voltage. By comparing the simulation results, it is concluded that the vector control method is superior to the direct control because of the removal of the coupling between control variables to achieve the independent control of active and reactive powers at both ends of the HVDC link. / UOIT

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This dissertation is dedicated to a comprehensive study of static synchronous compensator (STATCOM) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the STATCOM has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the STATCOM applications. The controls for the CMC-based STATCOM were, however, very complicated. The intricate control design was begun without well-defined system transfer functions. The control compensators were, therefore, randomly selected. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. To be able to operate in a high-voltage application, a large number of DC capacitors are utilized in a CMC-based STATCOM. All DC capacitor voltages must be balanced in order to avoid over-voltages on any particular link. Not only do these uneven DC voltages introduce voltage stress on the semiconductor switches, but they also lower the quality of the synthesized output waveforms of the converter. Previous researches into DC capacitor voltage-balancing techniques were very straightforward, in that individual voltage compensators were added into the main control loop. However, the compensator design for these individual loops is very problematic because of the complexity of the voltage-loop transfer functions. Basically, the trial and error technique again provides the simplest way to achieve acceptable compensators. Moreover, the greater number of voltage levels, the more complex the control design, and the main controller must perform all of the feedback control procedures. As a result, this approach potentially reduces the reliability of the controller. The goal of this dissertation is to achieve high-performance, reliable, flexible, cost-effective power stages and controllers for the CMC-based STATCOM. Major contributions are addressed as follows: 1) optimized design for the CMC-based STATCOM power stages and passive components, 2) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, 3) an effective decoupling power control technique, 4) DC-link balancing strategies; and 5) improvements in the CMC topology. To enhance the modularity and output voltage of the CMC, the high-switching-frequency, high-power H-bridge building block (HBBB) and the optimized design for its power stage and snubber circuits are first proposed. The high-switching-frequency feature is achieved by utilizing the Virginia Tech-patented emitter turn-off (ETO) thyristor. Three high-power HBBB prototypes were implemented, and their performance was experimentally verified. To simplify the control system design, well-defined models of the CMC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC with any number of voltage levels. The key system transfer functions are achieved and used in the control design processes. To achieve independent power control capability, the control technique, called the decoupling power control, is proposed. By applying this control technique, real and reactive power components can be controlled separately. In order to balance the DC capacitor voltages, a new, effective pulse width modulation (PWM) technique, which is suitable for any number of H-bridge converters, is proposed. The proposed cascaded PWM algorithm can be practically realized into the field programmable gate arrays (FPGA), and its complexity is not affected by the number of voltage levels. In addition, the complexity of the main controller, which is essentially based on the digital signal processor (DSP), is no longer a function of the number of the output voltage levels. The basic structure of the cascaded PWM is modular, which, in general, enhances the modularity of the CMC power stages. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This significantly simplifies and optimizes the control design process. To verify the accuracy of the proposed models and the performance of the control system for the CMC-based STATCOM, a low-power, seven-level cascaded-based STATCOM hardware prototype is implemented. The key control procedures are performed by a main controller, which consists of a DSP and an FPGA. The simulation and experimental results indicate the superior performance of the proposed control system, as well as the precision of the proposed models. / Ph. D.

This thesis is dedicated to a comprehensive study of static series synchronous compensator (SSSC) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the SSSC has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the SSSC applications. The control for the CMC-Based SSSC is complicated. The design of the complicated control strategy was begun with well-defined system transfer functions. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. The goal of this thesis is to achieve a reliable controller design for the CMC-based SSSC. Major contributions are addressed as follows: 1) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, and 2) an effective decoupling power control technique. To simplify the control system design, well-defined models of the CMC-Based SSSC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC-Based SSSC focus on only three voltage levels but can be expanded for any number of voltage levels. The key system transfer functions are derived and used in the controller design process. To achieve independent power control capability, the control technique, called the decoupling power control used in the design for the CMC-Based STATCOM is applied. This control technique allows both the real and reactive power components to be independently controlled. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This thesis focuses on the detailed design process needed for a CMC-Based SSSC. / Master of Science

Energy demands are steadily increasing as the world's population continues to grow. Automobiles are primary transportation means in a large portion of the world. The combination of fuel consumption by automobiles along with the shrinking fossil fuel reserves makes the development of new more energy efficient technologies crucial. Electric vehicle technologies have been studied and are still being studied today as a means of improving fuel efficiency. To that end, this work studies the effect of switching frequency on the efficiency of a hybrid electric vehicle traction drive, which contains both an internal combustion engine as well as electric motor. Therefore improving the efficiency of the electric motor and its drive will help improve the viability of alternative vehicle technologies. Automobiles spend the majority of their operational time in the lower speed, lower torque region. This work focuses on efficiency improvements in that region. To estimate the efficiency trend, the system is modeled and then tested both electrically and thermally. The efficiency is shown to increase at lower switching frequencies. The experimental results show that there are some exceptions, but the basic trend is the same. / Master of Science

The thesis describes the development of a model of the three-phase Voltage Source Converter (VSC) in the Harmonic State Space (HSS) domain, a Linear Time Periodic (LTP) extension to the Linear Time Invariant (LTI) state space. The HSS model of the VSC directly captures harmonic coupling effects using harmonic domain modelling concepts, generalised to dynami- cally varying signals. Constructing the model using a reduced-order three-phase harmonic signal representation achieves conceptual simplification, reduced computational load, and direct inte- gration with a synchronous frame vector control scheme. The numerical switching model of the VSC is linearised to gain a small-signal controlled model, which is validated against time-domain PSCAD/EMTDC simulations. The controlled model is evaluated as a STATCOM-type system, exercising closed-loop control over the reactive power flow and dc-side capacitor voltage using a simple linear control scheme. The resulting state- space model is analysed using conventional LTI techniques, giving pole-zero and root-locus analyses which predict the dynamic behaviour of the converter system. Through the ability to independently vary the highest simulated harmonic order, the dependence on the closed-loop response to dynamic harmonic coupling is demonstrated, distinguishing the HSS model from fundamental-only Dynamic Phasor models by its ability to accurately model these dynamics.

Modular multilevel converter type voltage source converter (MMC-VSC) is a potential candidate for present and future HVdc projects. The d-q decoupled control system is widely used to control MMC-VSC systems. Selection of PI-controller parameters for MMC-VSC systems is a challenging task as control loops are not completely decoupled. Since there is no widely accepted method to tune these control loops, the industry practice is to use the trial and error approach that requires a great amount of time. Therefore, it is required to develop a systematic procedure to tune PI-controllers considering necessary system dynamics and also to propose guidelines for control system design. This thesis introduces a systematic procedure to determine PI-controller parameters for the d-q decoupled control system. A linearized state-space model of an MMC-VSC system is developed to calculate the frequency-domain attributes. The control tuning problem is formulated as an optimization problem which is general and any meta-heuristic method can be used to solve the problem. In this thesis, the simulated annealing is applied to solve the problem. The efficacy of the tuned parameters is tested on the electromagnetic transient model of the test system on the real-time digital simulators (RTDS). In addition, it is shown that the proposed method is suitable to tune PI-controller parameters for MMC-VSC systems connected to strong as well as weak ac networks. Further, this thesis investigates the effects of d-q decoupled controller parameters, phase-locked loop (PLL) gains, and measuring delays on the stability and performance of the MMC-VSC test system. It is shown that the converter controllers have greater influence on the system stability and the impact of PLL gains is negligible unless very high PLL gains are used. In addition, the negative impact of measuring delays in instantaneous currents and voltages is also analysed by performing eigenvalue and sensitivity analysis. Finally, a set of guidelines for control design of MMC-VSC systems is summarized. In general, the proposed control tuning procedure would be useful for the industry to tune PI-controllers of MMC-VSC systems. Furthermore, the proposed methodology is generic and can be adapted to tune of any dynamic device in power systems. / February 2017

In this master thesis control systems for a voltage-source converter HVDC connected to weak ac networks are investigated. HVDC stands for high voltage direct current and is a way to transfer power in the electrical power system. A HVDC uses direct current (dc) instead of alternate current (ac) to transfer power, which requires transformation between ac and dc since most power grids are ac networks. The HVDC uses converters to transform ac to dc and dc to ac and the converter requires a control system. A complete control system of a voltage source converter HVDC contains many different parts. The part investigated in this thesis is the active power control. Different structures containing PID controllers have been tested and evaluated with respect to stability and performance using control theory. The impact of weak ac networks has been evaluated in regards to the different control structures. The investigations have been conducted using mainly steady-state simulations. Based on the simulation and analyzes of the simulation results a promising control structure has been obtained and suggested for further investigation.