| 1. |
- Bessegato, Luca, 1989-, et al.
(författare)
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Control and Admittance Modeling of an AC/AC Modular Multilevel Converter for Railway Supplies
- 2020
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Ingår i: IEEE transactions on power electronics. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0885-8993 .- 1941-0107. ; 35:3, s. 2411-2423
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Tidskriftsartikel (refereegranskat)abstract
- Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies 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. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC.
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| 2. |
- 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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| 3. |
- Zhang, Hongyang, et al.
(författare)
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Loop-at-a-Time Stability Analysis for Grid-Connected Voltage-Source Converters
- 2021
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Ingår i: IEEE Journal of Emerging and Selected Topics in Power Electronics. - : Institute of Electrical and Electronics Engineers (IEEE). - 2168-6777 .- 2168-6785. ; 9:5, s. 5807-5821
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Tidskriftsartikel (refereegranskat)abstract
- The instability phenomena caused by converter–grid interactions can be prevented by designing controllers with adequate stability margins. Yet, the multiple-input–multiple-output (MIMO) dynamics of grid-connected voltage-source converters (VSCs) complicate the stability analysis for the controller design. To tackle this challenge, this article presents a loop-at-a-time stability analysis for grid-connected VSCs, which not only shows close correlations with the generalized Nyquist criterion for MIMO systems but also enables to quantify the stability margins of individual closed loops. Moreover, the interactions between the closed loops can be analyzed. Test cases with numerical sensitivity analysis, simulations, and field measurements of a converter validate the theory.
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| 4. |
- 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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| 5. |
- Östlund, Stefan, 1961-, et al.
(författare)
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Responsible internationalisation : Guidelines for reflection on international academic collaboration
- 2020
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Rapport (populärvet., debatt m.m.)abstract
- Premise– Internationalisation has a number of positive effects on research and higher education.– Today, researchers at higher education institutions (HEIs) in Sweden collaborate to a greater extent with a larger number of countries than earlier.– Collaborations with researchers from countries whose scientific and political systems, culture and laws differ significantly from ours entail other and sometimes greater challenges.– There is an increased need for informed assessments of the contents and consequences of international collaborations before, during and after a project.– This document has been drawn up in collaboration between STINT, Karolinska Institutet, KTH Royal Institute of Technology and Lund University. The background to this work is the growing debate on values and ethical considerations in international collaborations in connection to HEIs in several countries.– The document is intended to serve as support for reflection and as the basis for discussion of strategic decisions on internationalisation. The purpose is to aid researchers, research directors, department heads, and university administration in assessing collaborations and structuring discussions on how the HEI, department or research group should approach international collaboration.
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