Quasi two-level PWM operation of a nine-arm modular multilevel converter for six-phase medium-voltage motor drives

This paper proposes a hybrid converter for medium-voltage six-phase machine drive systems that mixes the operation of a traditional two-level voltage-source inverter and the modular multilevel converter (MMC) to enable operation over a wide frequency range. Topologically, the proposed converter consists of nine arms resembling two sets of three-phase MMCs with three common arms, yielding a nine-arm MMC with a 25% reduction in the number of employed arms compared to a traditional dual three-phase MMC. The multilevel property of a standard MMC is emulated in the proposed converter, however on a two-level basis, resulting in a stepped two-level output voltage waveform. The proposed converter has a reduced footprint with advantages of small voltage steps, modular structure, and ease of scalability. Further, it is able to drive high-power six-phase machines within low operating frequencies at the rated torque. The operating principle of the converter is elaborated, and its modulation scheme is discussed. The features of the proposed converter are verified through simulations and experimentally

Diversity of inulinase-producing fungi associated with two Asteraceous plants, Pulicaria crispa (Forssk.) and Pluchea dioscoridis (L.) growing in an extreme arid environment

Inulinases are potentially valuable enzymes catalyze the hydrolysis of plant’s inulin into high fructose syrups as sweetening ingredients for food industry and ethanol production. The high demands for inulinase enzymes have promoted interest in microbial inulinases as the most suitable approach for biosynthesis of fructose syrups from inulin. Arid land ecosystem represents a valuable bioresource for soil microbial diversity with unique biochemical and physiological properties. In the present study, we explored the fungi diversity associated with the rhizosphere and rhizoplane of two desert medicinal plants namely Pluchea dioscoridis and Pulicaria crispa growing in the South-Eastern desert of Aswan, Egypt. A total of 180 fungal isolates were screened based on their ability to grow on potato dextrose agar medium supplemented with 1% inulin. The isolated fungal colonies were morphologically identified according to cultural characteristics and spore-bearing structure. In addition, the inulinase activity of the isolated fungi was examined spectrophotometrically. Among these, Aspergillus terreus var. terreus 233, Botrytis cinerea, Aspergillus aegyptiacus, Cochliobolus australiensis 447 and Cochliobolus australiensis exhibited high inulinase activity ranging from 5.05 to 7.26 U/ml. This study provides a promising source of microbial inulinase, which can be scaled up for industrial applications. DOI: http://dx.doi.org/10.5281/zenodo.120564

A modular multilevel converter with ripple-power decoupling channels for three-phase MV adjustable-speed drives

This paper presents a drive system based on a modular multilevel converter (MMC) with high-frequency magnetic channels between adjacent-arm submodules (SMs), suitable for medium-voltage, high-power three-phase variable-speed machines. The configuration employs chains of dual half-bridge (DHB) modules linking adjacent SMs of three-phase symmetrical arms. The DHB modules are operating as power channels enabling energy exchange to restore the power imbalance among the SM capacitors. This allows arms' ripple-powers to be entirely decoupled through bidirectional power transfer between adjacent-arm SMs, resulting in a near ripple-free SM capacitor voltage profile. Therefore, the MMC common problem of wide voltage fluctuation across SM capacitors is comprehensively solved, independent of the operating frequency. Additionally, a significant reduction in the sizing requirement of SM capacitance is achieved. The configuration is able to drive multi-megawatt machines from standstill to the rated speed at the rated torque operating condition. The operating principle of the proposed MMC configuration is explained and necessary mathematical analysis is derived. Features and viability of the proposed drive system are verified through simulation and experimentation

A dual modular multilevel converter with shared capacitor sub-module for MV open-end stator winding machine drives

This paper proposes a new dual modular multilevel converter (MMC) topology as a medium-voltage drive for adjustablespeed applications incorporating open-end stator winding machines. A novel concept of sharing one capacitor between each two adjacent-arm sub-modules (SMs) of MMC phaselegs, operating with out-of-phase modulation, is realized through new SM arrangement. This concept allows the MMC to utilize half the number of the SM capacitors, compared to a traditional MMC topology. Additionally, the sizing requirement of the shared capacitor is diminished, which significantly reduces the volume of the drive system and its stored energy. The switching scheme of the shared capacitor between two oppositely modulated SMs eliminates the problem of capacitor wide voltage fluctuations, independent of the operating frequency. Further, the proposed MMC can efficiently operate at near zero frequency, therefore a machine speed-range from zero speed to the rated speed is possible under rated torque operating condition. The proposed MMC topology is elucidated in detail, and its effective performance is verified through simulation

A modular multilevel converter with integrated shared capacitor sub-module for MV motor drives incorporating symmetrical six-phase machines

This paper proposes a new modular multilevel converter (MMC) configuration as a medium-voltage drive for variable-speed applications incorporating symmetrical six-phase machines. The proposed topology employs six MMC phase-legs feeding two isolated groups of three-phase machine windings, each with 60° spatial phase-displacement. A novel concept of sharing one capacitor between each pair of adjacent-arm submodules (SMs) of MMC phase-legs, while feeding machine windings in a spatial phase-opposition, is realized through a new integrated SM arrangement. The integrated SM allows the shared capacitor to absorb and release the same energy amount in a consecutive switching scheme, where the capacitor is experiencing both charging and discharging arm currents, one after another. This results in a limited voltage variation across the SM shared capacitor, independent of the operating frequency. Also, the proposed approach allows the MMC to utilize half the number of the SM capacitors, compared to a traditional MMC topology, while further diminishes the SM capacitance requirement, reducing the volume of the MMC system and its stored energy. The proposed configuration can efficiently operate at near zero frequency, therefore a machine speed-range from zero speed to the rated speed is possible under rated torque operating condition. The proposed MMC topology is elucidated in detail, and its effective performance is verified using simulation

A modular multilevel converter with DHB energy balancing channels for medium-voltage adjustable-speed drives

This paper presents a modular multilevel converter (MMC) configuration that utilises energy exchange between submodules (SMs) of upper and lower arms, for energy rebalancing. The configuration is applicable to mediumvoltage high-power variable-speed drives with any number of motor phases, where the traditional MMC topology experiences challenging shortcomings. With the out-of-phase alternation of the fundamental ripple power in upper and lower arms, the proposed MMC configuration decouples this ripple power by employing dual half-bridge modules linking opposite SMs in upper and lower arms of the same MMC-leg. This counter-balances arm ripple-power through bidirectional power transfer between opposite SMs, resulting in a reduction in the SM capacitance and the MMC system stored energy. The proposed MMC configuration solves the problem of wide SM capacitor voltage fluctuation, especially at low operating frequencies, where the SM capacitor voltage ripple profile is almost constant, independent of the operating frequency. Therefore, the configuration is able to drive multi-megawatt machines from stand-still to the rated speed, at rated torque. The operation of the proposed converter topology is elucidated in detail, and its effectiveness is verified through simulation and experimentation

A hybrid nine-arm modular multilevel converter for medium-voltage six-phase machine drives

The nine-arm modular multilevel converter (9A-MMC) has been recently proposed as a reduced MMC topology variation for six-phase drive applications, with 25% reduction in the number of employed arms and associated components, compared to a standard dual three-phase MMC, however with a limited output voltage amplitude. This paper proposes a hybrid 9A-MMC comprised of half-bridge submodules (SMs) in both the upper and lower arms, and full-bridge SMs in the middle arms. By employing the negative-voltage state of the full-bridge SMs, the hybrid 9A-MMC avoids the limitations imposed on the dc-link voltage utilization, while achieving further reduction in the component count, compared to a standard 9A-MMC with identical half-bridge SMs. The operating principles of the proposed hybrid 9A-MMC are illustrated with mathematical analysis, while its characteristics are verified through both simulation and experimentation. An assessment of the proposed topology quantifying its employed components is also provided, in comparison to other MMC-based six-phase machine drives

Elective Cancer Surgery in COVID-19-Free Surgical Pathways During the SARS-CoV-2 Pandemic: An International, Multicenter, Comparative Cohort Study.

PURPOSE: As cancer surgery restarts after the first COVID-19 wave, health care providers urgently require data to determine where elective surgery is best performed. This study aimed to determine whether COVID-19-free surgical pathways were associated with lower postoperative pulmonary complication rates compared with hospitals with no defined pathway. PATIENTS AND METHODS: This international, multicenter cohort study included patients who underwent elective surgery for 10 solid cancer types without preoperative suspicion of SARS-CoV-2. Participating hospitals included patients from local emergence of SARS-CoV-2 until April 19, 2020. At the time of surgery, hospitals were defined as having a COVID-19-free surgical pathway (complete segregation of the operating theater, critical care, and inpatient ward areas) or no defined pathway (incomplete or no segregation, areas shared with patients with COVID-19). The primary outcome was 30-day postoperative pulmonary complications (pneumonia, acute respiratory distress syndrome, unexpected ventilation). RESULTS: Of 9,171 patients from 447 hospitals in 55 countries, 2,481 were operated on in COVID-19-free surgical pathways. Patients who underwent surgery within COVID-19-free surgical pathways were younger with fewer comorbidities than those in hospitals with no defined pathway but with similar proportions of major surgery. After adjustment, pulmonary complication rates were lower with COVID-19-free surgical pathways (2.2% v 4.9%; adjusted odds ratio [aOR], 0.62; 95% CI, 0.44 to 0.86). This was consistent in sensitivity analyses for low-risk patients (American Society of Anesthesiologists grade 1/2), propensity score-matched models, and patients with negative SARS-CoV-2 preoperative tests. The postoperative SARS-CoV-2 infection rate was also lower in COVID-19-free surgical pathways (2.1% v 3.6%; aOR, 0.53; 95% CI, 0.36 to 0.76). CONCLUSION: Within available resources, dedicated COVID-19-free surgical pathways should be established to provide safe elective cancer surgery during current and before future SARS-CoV-2 outbreaks