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RISS 인기검색어
- 검색키워드
- 저자 : Ilves, Kelli (검색결과 4 건)
- 무료
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Semi-Full-Bridge Submodule for Modular Multilevel Converters
K. Ilves,L. Bessegato,L. Harnefors,S. Norrga,H.-P. Nee 전력전자학회 2015 ICPE(ISPE)논문집 Vol.2015 No.6
The energy variations in each arm of the modular multilevel converter comprises two components. The first component relates to the difference between the instantaneous input and output power of each phase-leg, and the second component relates to the energy which is moved back and forth between the two arms of the phase-leg. The latter component can be reduced or even eliminated if the peak-to-peak amplitude of the alternating voltage is greater than the pole-to-pole voltage of the dc link. This will, however, require submodules which can insert negative voltages. Therefore, a semi-full-bridge submodule which uses less semiconductors than the conventional fullbridge is proposed. Simulation results shows that by using the negative voltage-levels the capacitor voltage ripple can be reduced by up to 59%. Experimental results also shows that a 7-level voltage waveform can be generated using only one semi-full-bridge submodule with two capacitors per arm.
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K. Ilves,A. Antonopoulos,S. Norrga,H-P. Nee 전력전자학회 2011 ICPE(ISPE)논문집 Vol.2011 No.5
This paper presents a new modulation method for the modular multilevel converter. The proposed method is based on a fixed pulse pattern where harmonic elimination methods can be applied. Modulation methods with harmonic elimination based on calculated pulse patterns have been presented for other multilevel topologies. However, similar modulation schemes have not yet been presented for the modular multilevel topology. In the proposed modulation method, the pulse pattern is chosen in such a way that the stored energy in each submodule remains stable. It is shown that this can be done at the fundamental switching frequency without measuring the capacitor voltages or using any other form of feedback control. Such a modulation scheme has not been presented before. The theoretical results are verified by both simulations and experimental results. The simulation results show successful operation at the fundamental switching frequency with a larger number of submodules. When a smaller number of submodules are used, harmonic elimination methods may be applied. This is verified experimentally on a converter with eight submodules per phase leg. The experimental results verify that stable operation can be maintained at the fundamental switching frequency while successfully eliminating the fifth harmonic in the ac-side voltage.
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Trunk Muscle Strength After Lumbar Spine Fusion: A 12-Month Follow-up
Outi Elina Ilves,Marko Henrik Neva,Keijo Häkkinen,Joost Dekker,William J. Kraemer,Sami Tarnanen,Kati Kyrölä,Jari Ylinen,Kirsi Piitulainen,Salme Järvenpää,Tiina Kaistila,Arja Häkkinen 대한척추신경외과학회 2019 Neurospine Vol.16 No.2
Objective: The aim of this study was to investigate changes in trunk muscle strength 12 months after lumbar spine fusion (LSF) compared to preoperative strength. Methods: A total of 194 patients (mean±standard deviation [SD] age, 61±21 years) who underwent LSF participated in this prospective longitudinal study. Physical measurements of the participants were made before surgery and 12 months postoperatively. Isometric trunk extension and flexion strength was measured using a strain-gauge dynamometer in the standing position. Strength changes were calculated. Regression analysis was performed to explore which factors predicted strength levels at 12 months postoperatively. Results: The preoperative mean±SD extension strength was 205±144 N, which increased to 258±142 N (p<0.001) at the 12-month follow-up. Flexion strength increased from 295±172 N to 364±164 N (p<0.001). The preoperative extension/flexion strength ratio was 0.75±0.38 and remained similar (0.73±0.26) at 12 months postoperatively (p=0.39). Conclusion: Although trunk muscle strength increased by 26% for extension and 23% for flexion at the 12-month postoperative follow-up, both values remained objectively low. In addition, flexion strength remained higher than extension strength, which indicates an imbalance between those muscle groups. Age, severe back pain, and low trunk muscle strength before surgery predicted low trunk muscle strength at 1 year after spinal fusion.
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Concurrent Isolation of 3 Distinct Cardiac Stem Cell Populations From a Single Human Heart Biopsy
Monsanto, Megan M.,White, Kevin S.,Kim, Taeyong,Wang, Bingyan J.,Fisher, Kristina,Ilves, Kelli,Khalafalla, Farid G.,Casillas, Alexandria,Broughton, Kathleen,Mohsin, Sadia,Dembitsky, Walter P.,Sussman, Grune & Stratton 2017 Circulation research Vol.121 No.2
<P><B><U>Rationale:</U></B></P><P>The relative actions and synergism between distinct myocardial-derived stem cell populations remain obscure. Ongoing debates on optimal cell population(s) for treatment of heart failure prompted implementation of a protocol for isolation of multiple stem cell populations from a single myocardial tissue sample to develop new insights for achieving myocardial regeneration.</P><P><B><U>Objective:</U></B></P><P>Establish a robust cardiac stem cell isolation and culture protocol to consistently generate 3 distinct stem cell populations from a single human heart biopsy.</P><P><B><U>Methods and Results:</U></B></P><P>Isolation of 3 endogenous cardiac stem cell populations was performed from human heart samples routinely discarded during implantation of a left ventricular assist device. Tissue explants were mechanically minced into 1 mm<SUP>3</SUP> pieces to minimize time exposure to collagenase digestion and preserve cell viability. Centrifugation removes large cardiomyocytes and tissue debris producing a single cell suspension that is sorted using magnetic-activated cell sorting technology. Initial sorting is based on tyrosine-protein kinase Kit (c-Kit) expression that enriches for 2 c-Kit<SUP>+</SUP> cell populations yielding a mixture of cardiac progenitor cells and endothelial progenitor cells. Flowthrough c-Kit<SUP>−</SUP> mesenchymal stem cells are positively selected by surface expression of markers CD90 and CD105. After 1 week of culture, the c-Kit<SUP>+</SUP> population is further enriched by selection for a CD133<SUP>+</SUP> endothelial progenitor cell population. Persistence of respective cell surface markers in vitro is confirmed both by flow cytometry and immunocytochemistry.</P><P><B><U>Conclusions:</U></B></P><P>Three distinct cardiac cell populations with individualized phenotypic properties consistent with cardiac progenitor cells, endothelial progenitor cells, and mesenchymal stem cells can be successfully concurrently isolated and expanded from a single tissue sample derived from human heart failure patients.</P>
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