Optimized Modulation and Thermal Management for Modular Power Converters

The transition to a more and more decentralized power generation based on renewable energy generation is accompanied by high challenges. Modular power converters play a central role in facing these challenges, not only for grid integration but also to provide flexible services, highly efficient power transmission and safe storage integration. These goals are the key elements in becoming independent from fossil and nuclear power plants in near future. Even if the costs for renewable energy power plants like wind or photovoltaic systems are already competitive to conventional solutions, more flexible operation and further reduction in costs are required for faster global transformation towards sustainable energy systems. The further optimization of modular power converters can be seen as an ideal way to achieve these ambitious goals. It is therefore chosen as the focus of this work

Primary thoracoplasty and pedicle screw instrumentation in thoracic idiopathic scoliosis

Thoracoplasty in combination with spine fusion is an established method to address the rib cage deformity in idiopathic scoliosis. Most reports about thoracoplasty and scoliosis correction focused on Harrington or CD instrumentation. We report a retrospective analysis of 21 consecutive patients, who were treated with pedicle screw instrumentation for idiopathic thoracic scoliosis and concomitant thoracoplasty. Minimal follow up was 24 (24-75) months. Indication for thoracoplasty was clinical rib prominence of more than 15°. In average there was a 44% correction of clinical rib hump, from 18 (15-25°) to 10° (0-18°) (p<0.0001) and a 40% correction of radiological rib hump, from 15 (5-20°) to 9°(2-15°) (p<0.0001). The preoperative pulmonary function, accessed by forced vital capacity (FVC) and one-second forced expiratory volume (FEV1), remained unchanged at the last follow up. The distal end of fusion was the end vertebra of the curve in 83.3% and the end vertebra plus one in 16.7% of the patients. There was a 68% correction of instrumented primary thoracic curves, from 60 (45-85°) to 19°(5-36°) (p<0.0001), and a 45% correction of non-instrumented secondary lumbar curves, from 40 (28-60°) to 22°(8-38°) (p<0.0001). Apical vertebral rotation (AVR) of the thoracic curves improved 54%, from 24 (10-35°) to 11° (5-20°) (p<0.0001). The tilt of lowest instrumented vertebra (LIV) improved 68%, from 28 (20-42°) to 9°(3-20°) (p<0.0001). There was no significant change in sagittal profile of the spine. Analysis with SRS-24 questionnaire showed that the majority of the patients were very satisfied with the outcome. A matched control group (n=21) operated by the same surgeon with the same operation technique but without concomitant thoracoplasty was chosen for comparison. The scoliosis correction in the two groups was comparable. The patients without thoracoplasty had 37% spontaneous improvement of the clinical rib hum

Late implant infections caused by Propionibacterium acnes in scoliosis surgery

One hundred and one consecutive adolescent scoliosis patients instrumented from the posterior between 1995 and 2002, with a minimum follow-up of 24months (maximum 106months), were reviewed for incidence of infection. Stainless steel implants with pedicle screws were used in the thoracic and lumbar spine of all patients. All were operated on by a single surgeon. There were no early infections. Incidence of late infection was 6.9% (seven patients). Clinical indicators for infection were the sudden onset of local pain and swelling without fever after an initial pain-free interval. There were no reliable laboratory parameters. Normal CRP and ESR did not rule out a late infection. Extended cultures were done from intraoperative swabs. Propionibacterium acnes was found in six patients. There were no other organisms identified. No causative organism could be identified in one patient, despite extended cultures. All patients were successfully treated with implant removal and antibiotic therapy for 4-9weeks. No pseudarthrosis was seen on implant removal. Despite bony fusion, loss of correction between 10° and 26° was observed in three patients after implant remova

Short anterior correction of the thoracolumbar/lumbar curve in King 1 idiopathic scoliosis: the behaviour of the instrumented and non-instrumented curves and the trunk balance

This is a retrospective clinical, radiological and patient outcome assessment of 21 consecutive patients with King 1 idiopathic adolescent scoliosis treated by short anterior selective fusion of the major thoracolumbar/lumbar (TL/L) curve. Three-dimensional changes of both curves, changes in trunk balance and rib hump were evaluated. The minimal follow-up was 24months (max. 83). The Cobb angle of the TL/L curve was 52° (45-67°) with a flexibility of 72% (40-100%). The average length of the main curve was 5 (3-8) segments. An average of 3 (2-4) segments was fused using rigid single rod implants with side-loading screws. The Cobb angle of the thoracic curve was 33° (18-50°) with a flexibility of 69% (29-100%). The thoracic curve in bending was less than 20° in 17 patients, and 20-25° in 4 patients. In the TL/L curve there was an improvement of the Cobb angle of 67%, of the apex vertebral rotation of 51% and of the apex vertebral translation of 74%. The Cobb angle of the thoracic curve improved 29% spontaneously. Shoulder balance improved significantly from an average preoperative imbalance of 14.5-3.1mm at the last follow-up. Seventy-five percent of the patients with preoperative positive shoulder imbalance (higher on the side of the thoracic curve) had levelled shoulders at the last follow-up. C7 offset improved from a preoperative 19.8 (0-40)to 4.8 (0-18) mm at the last follow-up. There were no significant changes in rotation, translation of the thoracic curve and the clinical rib hump. There were no significant changes in thoracic kyphosis or lumbar lordosis. The average score of the SRS-24 questionnaire at the last follow-up was 91 points (max. 120). We conclude that short anterior selective fusion of the TL/L curve in King 1 scoliosis with a thoracic curve bending to 25° or less (Type 5 according to Lenke classification) results in a satisfactory correction and a balanced spine. Short fusions leave enough mobile lumbar segments for the establishment of global spinal balance. A positive shoulder imbalance is not a contraindication for this procedure. Structural interbody grafts are not necessary to maintain lumbar lordosi

Enhanced Current Capability for Modular Multilevel Converters by a Combined Sorting Algorithm for Capacitor Voltages and Semiconductor Losses

The modular multilevel converter (MMC) has become very attractive for high- and medium-voltage applications, generating excellent waveforms at very high efficiencies. One of the main challenges is the appropriate selection of inserted submodules (SMs), commonly done by capacitor voltage balancing algorithms. However, the semiconductor stress can only be balanced up to a certain degree by conventional algorithms, since the stress is not directly monitored. An uneven stress distribution between the SMs does not only result in different lifetime expectations, but also in increased maximum temperatures, for which each SM needs to be designed. With the goal of more effective utilization of chip area, a new balancing approach is introduced for monitoring and balancing not only the capacitor voltages but also the average power losses in each SM. In this way, the MMC current capability is significantly increased only by software without deteriorating the system performance and efficiency

Impact of Partial Power Processing Dual-Active Bridge Converter on Li-ion Battery Storage Systems

Partial Power Processing converters (PPPCs) provide economical and highly efficient solutions to integrate battery energy storages systems (BSSs) into DC grids. Beside these significant advantages, direct connection of the batteries by PPPCs to DC grids imposes challenges on the operation of batteries which need more evaluation. DC grid imperfections such as harmonics and faults may directly propagate into BSS and result in battery capacity fade acceleration and/or battery failure. In this paper, low voltage (LV) grid imperfection impacts are compared for conventional dual-active bridge (DAB) and Partial Power Processing DAB (PPP-DAB) converters. Both theoretical and experimental studies are carried out. The obtained results show that PPP-DAB exposes the BSS to high amount of current harmonics and safety issues

Anterior short correction in thoracic adolescent idiopathic scoliosis with mini-open thoracotomy approach: prospective clinical, radiological and pulmonary function results

Introduction: This is a prospective study of adolescent patients in whom idiopathic thoracic scoliosis was corrected by short anterior fusion through a mini-open thoracotomy approach. Clinical, radiological and pulmonary function results of minimal 2-year (2-6) follow-up are presented. Materials and methods: Consecutive 62 patients with Lenke 1 and 2 curves, having main thoracic scoliosis of up to 75°, were prospectively included. The shoulder imbalance in Lenke 2 patients was less than 20mm. Thoracic scoliosis was corrected by short anterior fusion. The thoracic spine was exposed by an 8-cm mini-open thoracotomy incision. The operation technique and choosing of fusion levels are thoroughly described. Complete 360° discectomies and convex side vertebral endplates osteotomies are essential for deformity corrections with short fusions. Single-rod 5.5-mm titanium implants were used. The age at the time of operation was mean 15.2 years; 56 patients had a single thoracic curve and 6 patients had a double thoracic curve. There were almost equal numbers of patients with lumbar modifier A, B or C. The average length of fusion was 5.5 (4-7) vertebrae. The average length of fusion was 3.5 (2-6) vertebrae shorter than the average curve length. Results: The instrumented thoracic curves improved by 58.3% at 6 weeks and 56.3% at the last follow-up. Apex thoracic vertebral rotation improved by 73.78% at 6 weeks and 76.24% at the last follow-up. The non-instrumented upper thoracic curve improved by 25% in double thoracic scoliosis, where the mid-thoracic curve was selectively fused, and the non-instrumented lumbar curves improved by 33.9% at the last follow-up. The radiological changes from 6 weeks to the last follow-up were statistically not significant. The clinical rib hump improved by 54% at the last follow-up. There were no significant changes in the pulmonary function. FVC% was 81.04% preoperatively, 76.41% at 6 months and 80.38% at the 2-year follow-up. The results of SRS 24 questionnaire improved from a total of 61.40 points preoperatively to 100.50 points at 6 months and 98.62 points at the 2-year follow-up. There were no neurological or thoracotomy related complications, no pseudarthrosis, no implant pullout or breakage. Conclusion: A good deformity correction without loss of correction or adding on, a good cosmetic result and good patient's satisfaction were achieved through shorter than end-to-end thoracic fusions. The radiological residual deformity is acceptable. Anterior correction of thoracic scoliosis with a short spinal fusion is recommended to keep the large part of the spine mobile. A very short fusion, small thoracotomy incision, low-profile implants and complete closure of parietal pleura are keys to prevent reduction in postoperative lung functio

Scoliosis correction with pedicle screws in Duchenne muscular dystrophy

This report describes the spinal fixation with pedicle-screw-alone constructs for the posterior correction of scoliosis in patients suffering from Duchene muscular dystrophy (DMD). Twenty consecutive patients were prospectively followed up for an average of 5.2years (min 2years). All patients were instrumented from T3/T4 to the pelvis. Pelvic fixation was done with iliac screws similar to Galveston technique. The combination of L5 pedicle screws and iliac screws provided a stable caudal foundation. An average of 16 pedicle screws was used per patient. The mean total blood loss was 3.7l, stay at the intensive care unit was 77h and hospital stay was 19days. Rigid stabilisation allowed immediate mobilisation of the patient in the wheel chair. Cobb angle improved 77% from 44° to 10°, pelvic tilt improved 65% from 14° to 3°. Lumbar lordosis improved significantly from 20° to 49°, thoracic kyphosis remained unchanged. No problems related to iliac fixation, no pseudarthrosis or implant failures were observed. The average percentage of predicted forced vital capacity (%FVC) of the patients was 55% (22-94%) preoperatively and decreased to 44% at the last follow-up. There were no pulmonary complications. One patient with a known cardiomyopathy died intraoperatively due to a sudden cardiac arrest. The rigid primary stability with pedicle screws allowed early mobilisation of the patients, which helped to avoid pulmonary complication

Impact of Modulation Methods on the Trade-Off between Investment and Operation Costs of a Medium-Voltage MMC-based STATCOM

The Modular Multilevel Converter (MMC) has become a preferred topology in HVDC applications due to its full controllability and the huge number of voltage steps. The excellent waveform generation, even at low switching frequencies, makes the MMC also very attractive for medium-voltage applications. In this context, both the converter design and the modulation methods need to be properly studied. Minimum switching frequencies are achieved by appropriate modulation, however, sufficient energy needs to be stored in the capacitors. This is particularly a challenge for STATCOM applications because the stored energy in the system needs to be controlled from the ac grid. In this paper different modulation methods with various capacitor designs are investigated for this application to find an optimum trade-off between capacitor design (investment costs) and switching frequency (operation costs). The appropriate MMC design and operation have been proven by simulation and experimental results showing excellent waveforms without additional filtering