| 1. |
- Hassanpoor, Arman
(författare)
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Modulation of Modular Multilevel Converters for HVDC Transmission
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The outstanding features of modular multilevel converters (MMC) have recently gained much attention in the high-voltage direct-current (HVDC) transmission field. Power quality, converter cost and system performance are three crucial aspects of HVDC MMCs which are directly linked to the converter modulation and switching schemes. High power quality and performance require high switching frequency and large cell capacitor whereas low switching frequency and small cell capacitor are needed to reduce the converter cost.The main objective of this thesis is to propose a practical switching method for HVDC MMCs which balances the aforementioned contradictory requirements. A mathematical analysis of the converter switching pattern, against the power quality and converter cost, has been conducted to formulate an optimization problem for MMCs. Different objective functions are studied for the formulated optimization problem such as converter loss minimization, voltage imbalance minimization and computational burden minimization. This thesis proposes three methods to address different objective functions. Ultimately, a real-time simulator has been built to practically verify and investigate the performance of the proposed methods in a realistic point-to-point HVDC link.The most significant outcome of this thesis is the tolerance band-based switching scheme which offers a direct control of the cell capacitor voltage, low power losses, and robust dynamic performance. As a result, the converter switching frequency can reach frequencies as low as 70 Hz (with the proposed cell tolerance band (CTB) method). A modified optimized CTB method is proposed to minimize the converter switching losses and it could reduce the converter switching losses by 60% in comparison to the conventional phase shifted carrier modulation method.It is concluded intelligent utilization of sorting algorithm can enable efficient HVDC station operation by reducing the converter cost.
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| 2. |
- Heinig, Stefanie
(författare)
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Main Circuits, Submodules, and Auxiliary Power Concepts for Converters in HVDC Grids
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In order to enable the massive introduction of renewable energies the need for high-voltage direct current (HVDC) grids is anticipated. Large, globally interconnected HVDC networks will likely be the most cost-efficient means to balance electricity demand and available generation. In a meshed system it is important to ensure reliability, robustness, failure management, and fast protection of equipment. In case of a failure somewhere in the grid, the remaining system must be kept operational. State-of-the-art converter implementations are either not adapted to future system requirements or lead to increased losses, cost, and converter footprint. Therefore, this thesis examines several aspects of how to improve the HVDC converter design and functionality with the ultimate aim of developing reliable, highly efficient, cost-effective, more compact and lightweight converters.Advancements are made on several levels of the converter hardware hierarchy. Main circuits, submodule (SM) topologies, and auxiliary power supply (APS) concepts are investigated and new solutions are proposed. On main-circuit level, different voltage-source converters (VSCs) are evaluated in terms of their energy storage elements. This is useful to compare the physical volume of capacitors required by each topology and, thus, to address the need to develop more compact converter stations. The theoretical analysis indicates that the required energy storage of the alternate arm converter (AAC) is smaller compared to the modular multilevel converter (MMC).On SM level, new topologies are evaluated with the goal to find topologies, which enable efficient handling of dc-side short circuits, reduction of power loss, and lower SM capacitance. The semi-full-bridge (SFB) SM is identified as one of the most promising topologies from this point of view and is investigated in detail. A control concept for capacitor balancing and several options for improved operation of the SFB are presented. Furthermore, a novel SM cluster topology is proposed which features low conduction losses and increased protection against explosion.The availability of a reliable APS system is crucial for equipment in future HVDC grids. Therefore, APS solutions are investigated considering design complexity, reliable performance, and power consumption. This thesis presents a novel combined optical power and data transmission concept which is tailored to the specific requirements of HVDC converters employing high-voltage (HV) silicon carbide (SiC) devices. The proposed concept offers a robust solution for isolated APS and signal transmission across any voltage barrier.
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| 3. |
- Ilves, Kalle
(författare)
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Modeling and Design of Modular Multilevel Converters for Grid Applications
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis aims to bring clarity to the dimensioning aspects and limiting factors of the modular multilevel converter (MMC). Special consideration is given to the dc capacitors in the submodules as they are a driving factor for the size and weight of the converter. It is found that if the capacitor voltages are allowed to increase by 10% the stored energy must be 21 kJ/MW in order to compensate the capacitor voltage ripple. The maximum possible output power can, however, be increased by injecting a second-order harmonic in the circulating current.A great advantage of cascaded converters is the possibility to achieve excellent harmonic performance at low switching frequencies. Therefore, this thesis also considers the relation between switching harmonics, capacitor voltage ripple, and arm quantities. It is shown that despite subharmonics in the capacitor voltages, it is still possible to achieve periodic arm quantities. The balancing of the capacitor voltages is also considered in further detail. It is found that it is possible to balance the capacitor voltages even at fundamental switching frequency although this will lead to a comparably large capacitor voltage ripple. Therefore, in order to limit the peak-to-peak voltage ripple, it is shown that a predictive algorithm can be used in which the resulting switching frequency is approximately 2–3 times the fundamental frequency.This thesis also presents two new submodule concepts. The first submodule simply improves the trade-off between the switching frequency and capacitor voltage balancing. The second submodule includes the possibility to insert negative voltages which allows higher modulation indices compared to half-bridge submodules.A brief comparison of cascaded converters for ac-ac applications is also presented. It is concluded that the MMC appears to be well suited for ac-ac applications where input and output frequencies are close or equal, such as in interconnection of ac grids. In low-frequency applications such as low-speed drives, however, the difficulties with handling the energy variations in the converter arms are much more severe in the MMC compared to the other considered topologies.
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| 4. |
- Joshi, Siddarth Koduru, et al.
(författare)
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Space QUEST mission proposal : experimentally testing decoherence due to gravity
- 2018
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 20
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Tidskriftsartikel (refereegranskat)abstract
- Models of quantum systems on curved space-times lack sufficient experimental verification. Some speculative theories suggest that quantum correlations, such as entanglement, may exhibit different behavior to purely classical correlations in curved space. By measuring this effect or lack thereof, we can test the hypotheses behind several such models. For instance, as predicted by Ralph et al [5] and Ralph and Pienaar [1], a bipartite entangled system could decohere if each particle traversed through a different gravitational field gradient. We propose to study this effect in a ground to space uplink scenario. We extend the above theoretical predictions of Ralph and coworkers and discuss the scientific consequences of detecting/failing to detect the predicted gravitational decoherence. We present a detailed mission design of the European Space Agency's Space QUEST (Space-Quantum Entanglement Space Test) mission, and study the feasibility of the mission scheme.
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| 5. |
- Kersten, Anton, 1991, et al.
(författare)
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Sensorless Capacitor Voltage Balancing of a Grid-Tied, Single-Phase Hybrid Multilevel Converter with Asymmetric Capacitor Voltages using Dynamic Programming
- 2020
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Ingår i: IECON Proceedings (Industrial Electronics Conference). - 2162-4704 .- 2577-1647. ; 2020-October, s. 4288-4293
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Konferensbidrag (refereegranskat)abstract
- This paper shows a sensorless capacitor voltage balancing control approach for a grid-connected, single-phase hybrid multilevel inverter based on an NPC main stage with a voltage stiff DC-link and an arbitrary number of H-Bridge modules (capacitor modules) with asymmetric capacitor voltages. Using nearest-level control, a model predictive control (MPC) approach with a prediction horizon of one time step is chosen to find an optimal switching-state combination among the redundant switching combinations to balance the capacitor voltages as quick as possible. Using the Lyapunov stability criterion, it is shown that an offline calculated optimal switching-state sequence for each discrete output voltage level can be used to operate the inverter without using any voltage sensors for the capacitor voltages. To validate the stability of the approach, a laboratory inverter with a resistive load is operated with the offline calculated optimal switching-state sequences and it is shown that the capacitor voltages converge to their desired reference voltages.
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| 6. |
- Kuder, Manuel, et al.
(författare)
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Capacitor Voltage Balancing of a Grid-Tied, Cascaded Multilevel Converter with Binary Asymmetric Voltage Levels Using an Optimal One-Step-Ahead Switching-State Combination Approach †
- 2022
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Ingår i: Energies. - : MDPI AG. - 1996-1073 .- 1996-1073. ; 15:2
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a novel capacitor voltage balancing control approach for cascaded multilevel inverters with an arbitrary number of series-connected H-Bridge modules (floating capacitor modules) with asymmetric voltages, tiered by a factor of two (binary asymmetric). Using a nearest-level reference waveform, the balancing approach uses a one-step-ahead approach to find the optimal switching-state combination among all redundant switching-state combinations to balance the capacitor voltages as quickly as possible. Moreover, using a Lyapunov function candidate and considering LaSalle's invariance principle, it is shown that an offline calculated trajectory of optimal switching-state combinations for each discrete output voltage level can be used to operate (asymptotically stable) the inverter without measuring any of the capacitor voltages, achieving a novel sensorless control as well. To verify the stability of the one-step-ahead balancing approach and its sensorless variant, a demonstrator inverter with 33 levels is operated in grid-tied mode. For the chosen 33-level converter, the NPC main-stage and the individual H-bridge modules are operated with an individual switching frequency of about 1 kHz and 2 kHz, respectively. The sensorless approach slightly reduced the dynamic system response and, furthermore, the current THD for the chosen operating point was increased from 3.28% to 4.58% in comparison with that of using the capacitor voltage feedback.
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