| 1. |
- Bitsi, Konstantina
(författare)
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On Electrical Machine Topologies for Electric Vehicle Applications
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The deployment of electric vehicles is considered a viable solution towardsreducing the global greenhouse-gas emissions and fossil-fuel consumption. Inorder to produce highly efficient and economically feasible electrical drivesystems, the selection of suitable electric motor designs is a key step.The target of this work is to identify, analyze and compare suitable elec-trical machine topologies for automotive applications based on highly coupledand conflicting criteria. For this purpose, an evolutionary multi-objective op-timization is developed that can yield a set of Pareto-optimal solutions and,thus, offer different compromises among the considered design-objectives. Theefficacy of the algorithm is demonstrated, rendering it an important tool forthe subsequent analysis.In the first part of this thesis, the investigation of pole-phase changinginduction machines is presented. A special induction machine topology withwound, independently-controlled stator coils (WICSC) is introduced. Thestator-winding configuration in this machine permits the individual energiza-tion and current control of the toroidal coil in each stator slot, thus facilitatingthe real-time change of both phase and pole number. The 2D magnetic and3D thermal finite-element method (FEM) models of this machine are devel-oped, as well as an analytical transient model of the current dynamics. Adetailed investigation of the behavior of pole-phase changing induction ma-chines and the impact of their design on the selection of optimum pole-phaseoperations throughout the entire operating region is performed. Specifically,three WICSC machines that were originally designed with 2, 4 and 6 fixedmagnetic poles are evaluated as pole-phase changing machines with the aim ofdetermining the overall improvement in terms of torque per ampere capabilityand efficiency.The second part of this work is focused on the design and performance ofaxial-flux induction machines (AFIMs). An electromagnetic sizing algorithmfor the design of AFIMs is developed, which adopts a geometrical approachto the design problem, while minimizing the use of empirical factors. Theeffectiveness of this algorithm is experimentally validated, using a commercialdouble-stator AFIM utilized as an integrated starter generator in a hybridelectric vehicle application. Moreover, in order to assess the benefits of pole-changing in axial-flux structures, the optimization of an interior-permanentmagnet synchronous machine, a radial-flux WICSC machine and a double-rotor axial-flux WICSC machine is carried out for a heavy vehicle applicationand the comparison of their Pareto-optimal solutions is presented.
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| 2. |
- Bessegato, Luca, 1989-
(författare)
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Modeling of Modular Multilevel Converters for Stability Analysis
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modular multilevel converters (MMCs) have recently become the state-of-the-art solution for various grid-connected applications, such as high-voltage direct current (HVDC) systems and flexible alternating current transmission systems (FACTS). Modularity, scalability, low power losses, and low harmonic distortion are the outstanding properties that make MMCs a key technology for a sustainable future. The main objective of this thesis is the modeling of grid-connected MMCs for stability analysis. The stability of the interconnected system, formed by the converter and the ac grid, can be assessed by analyzing the converter ac-side admittance in relation to the grid impedance. Therefore, a method for the calculation of the ac-side admittance of MMCs is developed. This method overcomes the nonlinearities of the converter dynamics and it can be easily adapted to different applications. Moreover, the effects of different control schemes on the MMC ac-side admittance are studied, showing how the converter admittance can be reshaped. This is a useful tool for system design, because it shows how control parameters can be selected to avoid undesired grid-converter interactions. This thesis also studies ac/ac MMCs for railway power supplies, which are used in countries with a low-frequency railway grid, such as Germany (16.7 Hz) and Sweden (16 2/3 Hz). A hierarchical control scheme for these converters is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. For computing the insertion indices, an open-loop scheme with sum capacitor voltage estimation is applied to the ac/ac MMC. Lyapunov stability theory is used to prove the asymptotic stability of the converter operated with the proposed control method. This specific open-loop scheme is also adapted to a modular multilevel matrix converter, which performs three-to-three phase direct conversion. Finally, this thesis presents the design of a down-scaled MMC prototype for experimental verification, rated at 10 kW with 30 full-bridge submodules. The hardware and the software are designed to be easily reconfigurable, which makes the converter suitable for different research projects focused on MMCs. Experiments on this down-scaled MMC are used to support and validate the key results presented throughout the thesis.
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| 3. |
- Zhang, Hongyang
(författare)
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On Modeling and Stability Analysis of Grid-Connected Voltage Source Converters
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With increasing deployment of voltage source converters (VSCs) in the grid, instability phenomena due to converter--grid interactions have been observed. Modeling and stability analysis of grid-connected VSCs have become popular research topics. Moreover, the power system is losing its inertia with increased penetration of renewable energy resources. Large and fast frequency deviations tend to vary more frequently compared to the conventional grid. The control and modeling of VSCs in the low-inertia grid is thus a crucial topic. The main objective of this thesis is the small-signal modeling and stability analysis of grid-connected VSCs. As a result of the asymmetrical synchronous reference ($ dq $) frame dynamics, the structure of the dynamical system is multi-input multi-output (MIMO). Conventionally, the small-signal stability analysis for such a system is conducted either by the modal-analysis approach or the impedance based method, which is based on MIMO. In this thesis, methods for modeling and analyzing a MIMO system using a single-input single-output (SISO) approach are developed. Compared to conventional methods, the SISO methods overcome the difficulties of defining the stability margins for individual closed loops. Moreover, the methods provide intuitive solutions to the design of controllers for individual closed loops. The thesis also investigates the control and modeling of grid-connected VSCs connected to low-inertia grids. It is illustrated that, for improvement of network frequency dynamics, a VSC with proper energy storage is capable to emulate inertial response from a synchronous machine. Moreover, other frequency control strategies are shown to be beneficial, compared to the inertial emulation. In addition, the inaccuracies for the $ dq $-frame small-signal model, in the presence of phase jumps and frequency deviations, are investigated. It is concluded that the inaccuracy of the small-signal model always exists under phase jumps. In the presence of frequency deviation, the stability analysis shall be evaluated at the steady-state operating point of the frequency.
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