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- Fjellstedt, Christoffer
(author)
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Studies of the Grid Connection of Offshore Renewable Energy Sources : Technologies and Simulations
- 2024
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Doctoral thesis (other academic/artistic)abstract
- A substantial increase in renewable energy sources connected to the electrical grid is imperative to achieve net-zero emissions from the electricity sector. Marine energy sources, like marine current power and wave power, have the potential to significantly contribute to the increase of electricity from renewable energy sources. A crucial aspect of enabling marine energy utilization is the development of electrical systems for offshore renewable energy. Hence, this thesis addresses challenges regarding the grid connection of offshore renewable energy.Two important questions for offshore renewable energy are how to construct the offshore electrical grid and how to transmit the power to the shore. This thesis provides a review of AC and DC collection grid topologies and compares HVAC and HVDC transmission for offshore applications. It is concluded that HVDC is the preferred technology for transmission distances exceeding 50 to 100 km.Regardless of the configuration of the offshore collection grid, the energy converters must be connected to the collection and distribution grid. Uppsala University has deployed a marine current energy converter in the river Dalälven in Söderfors, Sweden. The grid connection system at the test site is based on a back-to-back converter technology. In the thesis, a simulation model of the grid connection system of the energy converter is presented. The simulation model is used to evaluate MPPT methods for marine current power. An advanced hydrodynamic model based on a two-dimensional free vortex method is utilized for this purpose. Additionally, a low-complexity hydrodynamic model is incorporated into the simulation model to assess electrical grids for marine current energy. One AC and one DC collection grid, each comprising five marine current energy converters, are compared. Furthermore, three DC collection grids, each with ten marine current energy converters, are assessed and compared.The grid connection system at the Söderfors test site includes an LC filter connected to a power transformer. A novel transfer function is derived for this system, and the transfer function is verified with simulations and experimental investigations. It is shown that the derived transfer function accurately captures the frequency response of the experimental system.
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| 2. |
- Mörée, Gustav, et al.
(author)
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Simplified Current-Equivalent Circuit Models of Synchronous Reluctance Machines and Salient Pole Synchronous Machines Considering the Reluctance Torque
- 2024
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In: Energies. - : MDPI. - 1996-1073. ; 17:5
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Journal article (peer-reviewed)abstract
- This paper describes a simplified one-phase equivalent circuit model of three-phase salient pole synchronous machines and synchronous reluctance machines. The model represents the pole saliency as a susceptance, with a magnitude based on the pole saliency and a phase angle based on twice the load angle. The reluctance torque itself is then modeled equivalent to a conductance. The model is made as an extension to Norton equivalents, where the internal inductance is connected as a shunt. This approach shares similarities to the magnetization shunt branch in circuit models of transformers and induction motors. The model is derived using the rotating dq-frame, using Park transform and two-reaction theory. The model is then rewritten to better fit one-phase equivalent circuit models, with the terminal voltage as the angular reference. The resulting two-pole model can then more easily be combined with basic circuit theory, thereby synthesizing dq-theory and phasor circuit models.
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| 3. |
- Parwal, Arvind, et al.
(author)
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A Comparative Analysis of Linear and Nonlinear Control of Wave Energy Converter for a Force Control Application
- 2024
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In: International Marine Energy Journal. - Southampton, UK : International Marine Energy Journal. - 2631-5548.
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Journal article (peer-reviewed)abstract
- The aim of wave energy converters (WECs) is to harvest the energy from the ocean waves and convert into electricity. Optimizing the generator output is a vital point of research. A WEC behaves as a nonlinear system in real ocean waves and a control that approximates the behaviour of the system is required. In order to predict the behaviour of WEC, a controller is implemented with an aim to track the referenced trajectory for a force control application of the WEC. A neural model is implemented for the system identification and control of the nonlinear process with a neural nonlinear autoregressive moving average exogenous (NARMAX) model. The neural model updates the weights to reduce the error by using the Levenberg-Marquardt back-propagation algorithm for a single-input-single-output (SISO) nonlinear system. The performance of the system under the proposed scheme is compared to the same system under a PI-controller scheme, where the PI gains have been tuned accordingly, to verify the control capacity of the proposed controller. The results show a good tracking of dq (direct-quadrature) axes currents by regulating the stator currents, and hence a force control is achieved at different positions of the translator. The dynamic performance of the control is verified in a time domain analysis for the displacement of the translator.
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| 4. |
- Trostén, Torbjörn, 1979-
(author)
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Energy efficient control of motors and inverters
- 2024
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Licentiate thesis (other academic/artistic)abstract
- The purpose of this licentiate thesis is to research a few key concepts which are important in the development of high efficiency electric drives. The selected research method is quantitative, the raw data have been collected from real world measurements, both from powerlab and from train. The data is analysed and the results are presented in the thesis and associated papers.The paper in this thesis describes and evaluates the impact of SiC MOSFET inverters on the traction drive. Furthermore, the papers go into depth about losses within the traction drive and how the electromagnetic noise depends on both the inverter switching frequency and pulse width modulation method. In the papers, results from low fidelity models of both losses and noise are presented, these models offer greater insight into the mechanism behind both losses and electromagnetic noise. Understanding how the motor harmonic losses depends on the current distribution in conductors, can contribute to improved designs of both motors and inverters. An understanding of how electromagnetic noise increases when voltage harmonics coincide with the mechanical modes of resonance of the electric motor, can further contribute to improved designs of both inverters and motors.The thesis concludes that when SiC based MOSFETs inverters makes its debut into everyday trains, we can expect significant increases in energy efficiency as well as significantly reductions in electromagnetic noise.
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