Supporting Self-Regulated Learning with Visualizations in Online Learning Environments

In this article, we study how visualizations could be used to support students' self-regulation in online learning. We conducted a randomized controlled trial with three groups: one control group without visualization, one treatment group with textual visualization, and one treatment with graphical visualization with information on peers' average achievement. We studied how different visualizations affect students' academic performance and behavior. We focused on four factors; starting, scheduling, earliness and exercise points, where the first three are related to time management and self-regulation. The last factor measures course performance in terms of completed exercises. Our results suggest that the lowest performing students can benefit from a visualization, whereas the highest performing students are not affected by the presence or absence of a visualization. We also found that visualizations that do not provide the means to compare your own performance with others may even be harmful to performance oriented students.Peer reviewe

Tolerance Band Modulation Methods for Modular Multilevel Converters

Abstract Modular multilevel converters (M2C) are increasingly used in the high voltage direct current (HVDC) systems. The efficiency of M2C is highly related to the modulation technique which determines the switching frequency and capacitor voltage ripple in the converter station. A new approach to modulation of M2C is presented in this paper. Tolerance band methods are employed to obtain switching instants and also cell selection. The proposed methods overcome the problem with magnitude modulation for converters with few numbers of cells and also reduce the sorting efforts for cell balancing purposes while maintaining the cell voltage limits. The evaluation is done by timedomain simulation with which the performance of each method is studied in both steady-state and transient cases. It is observed that using tolerance band methods can not only reduces the switching frequency but also handle sever fault cases in a grid connected system. Using this method can reduce the switching losses and also the cell size by optimizing the capacitor design for a fixed ripple

Kõnekeskuse reorganisatsioon perinataalse insuldi korral: haigusjuhu kirjeldus

Artiklis on käsitletud 9aastase lapse haigusjuhtu eesmärgiga tutvustada vasema hemisfääri perinataalse insuldi korral kujunevat ajufunktsioonide reorganisatsiooni. Patsiendil teostati funktsionaalne magnetresonantstomograafia (fMRT) motoorse ja sensoorse kõnekeskuse aktivatsioonitestidega. Mõlemad kõnekeskused lokaliseerusid paremas hemisfääris, kusjuures kliiniliselt lapsel kõnehäiret ei esinenud. Eesti Arst 2009; 88(1):52−5

Modeling and Design of Modular Multilevel Converters for Grid Applications

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.QC 20141010</p

Modeling and Design of Modular MultilevelConverters for Grid Applications

Grid-connected high-power converters are found in high-voltage direct current transmission (HVDC), static compensators (STATCOMs), and supplies for electric railways. Such power converters should have a high reliability, high efficiency, good harmonic performance, low cost, and a small footprint. Cascaded converters are promising solutions for high-voltage high-power converters since they allow the combination of excellent harmonic performance and low switching frequencies. A high reliability can also be achieved by including redundant submodules in the chain of cascaded converters. One of the emerging cascaded converter topologies is the modular multilevel converter (M2C). This thesis aims to bring clarity to the dimensioning aspects and limiting factors of M2Cs. The dc-capacitor in each submodule is a driving factor for the size and weight of the converter. It is found that the voltage variations across the submodule capacitors will distort the voltage waveforms and also induce alternating components in the current that is circulating between the phase-legs. It is, however, shown that it is possible to control the alternating voltage by feed-forward control. It is also shown that if the circulating current is controlled, the injection of a second-order harmonic component can extend the operating region of the converter. The reason for this is that when the converter is operating close to the boundary of overmodulation the phase and amplitude of the second-order harmonic is chosen such that the capacitors are charged prior to the time when a high voltage should be inserted by the submodules. The controller that is used must be able to balance the sbmodule capacitor voltages. Typically, an increased switching frequency will enhance the performance of the balancing control scheme. In this thesis it is shown that the capacitor voltages can be balanced with programmed modulation, even if fundamental switching frequency is used. This will, however, increase the voltage ripple across the aforementioned capacitors. In order to quantify the requirements on the dc-capacitors a general analysis is provided in this thesis which is based on the assumption that the capacitor voltages are well balanced. It is found that for active power transfer, with a 50 Hz sinusoidal voltage reference, the capacitors must be rated for a combined energy storage of 21 kJ/MW if the capacitor voltages are allowed to increase by 10% above their nominal values.QC 20121127</p

Modeling and Design of Modular Multilevel Converters for Grid Applications

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.QC 20141010</p

Effects of Control on the AC-Side Admittance of a Modular Multilevel Converter

The stability of a modular multilevel converter connected to an ac grid can be assessed by analyzing the converter ac-side admittance in relation to the grid impedance. The converter control parameters have a strong impact on the admittance and they can be adjusted for achieving system stability. This paper focuses on the admittance-shaping effect produced by different current-control schemes, either designed on a per-phase basis or in the $dq$ frame using space vectors. A linear analytical model of the converter ac-side admittance is developed, including the different current-control schemes and the phase-locked loop. Different solutions for computing the insertion indices are also analyzed, showing that for a closed-loop scheme a compact expression of the admittance is obtained. The impact of the control parameters on the admittance is discussed and verified experimentally, giving guidelines for designing the system in terms of stability. Moreover, recommendations on whether a simplified admittance expression could be used instead of the detailed model are given. The findings from the admittance-shaping analysis are used to recreate a grid-converter system whose stability is determined by the control parameters. The developed admittance model is then used in this experimental case study, showing that the stability of the interconnected system can be assessed using the Nyquist stability criterion.QC 20181113</p

A Method for the Calculation of the AC-Side Admittance of a Modular Multilevel Converter

Connecting a modular multilevel converter to anac grid may cause stability issues, which can be assessed byanalyzing the converter ac-side admittance in relation to the gridimpedance. This paper presents a method for calculating theac-side admittance of modular multilevel converters, analyzingthe main frequency components of the converter variables individually.Starting from a time-averaged model of the converter,the proposed method performs a linearization in the frequencydomain, which overcomes the inherent nonlinearities of theconverter internal dynamics and the phase-locked loop usedin the control. The ac-side admittance obtained analytically isfirstly validated by simulations against a nonlinear time-averagedmodel of the modular multilevel converter. The tradeoff posedby complexity of the method and the accuracy of the result isdiscussed and the magnitude of the individual frequency componentsis shown. Finally, experiments on a down-scaled prototypeare performed to validate this study and the simplification onwhich it is based.QC 20181113</p