Mathematical modelling and stability analysis for grid-connected cascaded H-bridge converter

When power converters are employed to connect the renewable power resources to the grid, the grid inductance can typically be quite significant because wind farms and solar power stations are located far away from the converters. The non-negligible grid inductance interacts with the Phase Locked Loop (PLL) which is assisting the grid side current controller with the phase angle of grid voltage and furthermore interacts with the DC-link voltage controller. It is known that the inductance of the grid side transmission line would distort the input voltage of PLL at the point of common coupling and this interference can even lead to the instability of the converter operation. However, the accurate prediction of the stability thresholds of the PLL configuration related to the grid inductance was missing which requires advanced modelling techniques. The key technique is a new mathematical approach to calculating the spectra, which is more sophisticated and more flexible than existing methods. The Harmonic Balance Technique is powerful for modelling the steady state of the periodic waveform and is already a sophisticated technique in other fields but occasionally adopted on power electronics. This study aims to adopt this modelling approach to obtain a detailed description of the operation of the converter steady state thus the stability of its steady state can be investigated. The stability analysis is conducted for the periodic steady state employing the Floquet theorem. The stability analysis indicates that for a single-phase grid-connected cascaded H-bridge rectifier, the interactions between control loops and PLL can be affected by grid side inductance, and their interactions can even lead to instability of the system. Notably, the instability boundaries are predicted for PLL bandwidth about three different grid strengths. These thresholds have been verified with both simulation and experimental results

A Research on Community-Based Livestock of Qinghai-Tibet Plateau

Qinghai-Tibet Plateau locates in Southwestern China, covering the whole area of Tibet Autonomous Region, Qinghai Province, Southern part of Gansu Province, Northwestern part of Sichuan Province and Northwestern part of Yunnan Province, with an area of around 139.08 million hectares of natural grassland, accounting for 39% of the total area of natural grassland in China. It is also the largest natural ecozones in China and one of the least disturbed regions by human activities, with its air, water sources, soil, grassland, wildlife in their pristine state. Qinghai-Tibet Plateau is the native home for Tibetan people. Grassland animal husbandry is the foundation of the economy of QTP and the main source of livelihood for local nomadic people. During the long term of concerted evolution with the nature, Tibetan people living on Qinghai-Tibet Plateau have formed a uniquely holistic grassland ecological culture that is compatible with their production system and the ecosystem. The majority of Tibetan people observe Tibetan Buddhism. Their respect for nature and their belief in that all sentient beings are equal take deep root in their traditional culture. Their harmonious co-existence with nature exemplifies the eco-civilization ideas and provides a solid cultural foundation for both ecology conservation and featured animal husbandry development. On Qinghai-Tibet Plateau, national policies and initiatives such as dual contract of livestock and forage, natural grassland vegetation recovery, returning grazing land to grassland, grassland ecosystem subsidy and rewarding mechanism have been implemented, playing an important role in promoting grassland ecosystem conservation and grassland animal husbandry development. However, since grassland animal husbandry is a complex system involving grassland, farm animal, environment, society, economy, culture, etc, there are still many outstanding problems to be solved

Clinicopathological Significance and Prognostic Value of DNA Methyltransferase 1, 3a, and 3b Expressions in Sporadic Epithelial Ovarian Cancer

Altered DNA methylation of tumor suppressor gene promoters plays a role in human carcinogenesis and DNA methyltransferases (DNMTs) are responsible for it. This study aimed to determine aberrant expression of DNMT1, DNMT3a, and DNMT3b in benign and malignant ovarian tumor tissues for their association with clinicopathological significance and prognostic value. A total of 142 ovarian cancers and 44 benign ovarian tumors were recruited for immunohistochemical analysis of their expression. The data showed that expression of DNMT1, DNMT3a, and DNMT3b was observed in 76 (53.5%), 92 (64.8%) and 79 (55.6%) of 142 cases of ovarian cancer tissues, respectively. Of the serious tumors, DNMT3a protein expression was significantly higher than that in benign tumor samples (P = 0.001); DNMT3b was marginally significant down regulated in ovarian cancers compared to that of the benign tumors (P = 0.054); DNMT1 expression has no statistical difference between ovarian cancers and benign tumor tissues (P = 0.837). Of the mucious tumors, the expression of DNMT3a, DNMT3b, and DNMT1 was not different between malignant and benign tumors. Moreover, DNMT1 expression was associated with DNMT3b expression (P = 0.020, r = 0.195). DNMT1 expression was associated with age of the patients, menopause status, and tumor localization, while DNMT3a expression was associated with histological types and serum CA125 levels and DNMT3b expression was associated with lymph node metastasis. In addition, patients with DNMT1 or DNMT3b expression had a trend of better survival than those with negative expression. Co-expression of DNMT1 and DNMT3b was significantly associated with better overall survival (P = 0.014). The data from this study provided the first evidence for differential expression of DNMTs proteins in ovarian cancer tissues and their associations with clinicopathological and survival data in sporadic ovarian cancer patients

Seizing the window of opportunity to mitigate the impact of climate change on the health of Chinese residents

The health threats posed by climate change in China are increasing rapidly. Each province faces different health risks. Without a timely and adequate response, climate change will impact lives and livelihoods at an accelerated rate and even prevent the achievement of the Healthy and Beautiful China initiatives. The 2021 China Report of the Lancet Countdown on Health and Climate Change is the first annual update of China’s Report of the Lancet Countdown. It comprehensively assesses the impact of climate change on the health of Chinese households and the measures China has taken. Invited by the Lancet committee, Tsinghua University led the writing of the report and cooperated with 25 relevant institutions in and outside of China. The report includes 25 indicators within five major areas (climate change impacts, exposures, and vulnerability; adaptation, planning, and resilience for health; mitigation actions and health co-benefits; economics and finance; and public and political engagement) and a policy brief. This 2021 China policy brief contains the most urgent and relevant indicators focusing on provincial data: The increasing health risks of climate change in China; mixed progress in responding to climate change. In 2020, the heatwave exposures per person in China increased by 4.51 d compared with the 1986–2005 average, resulting in an estimated 92% increase in heatwave-related deaths. The resulting economic cost of the estimated 14500 heatwave-related deaths in 2020 is US$176 million. Increased temperatures also caused a potential 31.5 billion h in lost work time in 2020, which is equivalent to 1.3% of the work hours of the total national workforce, with resulting economic losses estimated at 1.4% of China’s annual gross domestic product. For adaptation efforts, there has been steady progress in local adaptation planning and assessment in 2020, urban green space growth in 2020, and health emergency management in 2019. 12 of 30 provinces reported that they have completed, or were developing, provincial health adaptation plans. Urban green space, which is an important heat adaptation measure, has increased in 18 of 31 provinces in the past decade, and the capacity of China’s health emergency management increased in almost all provinces from 2018 to 2019. As a result of China’s persistent efforts to clean its energy structure and control air pollution, the premature deaths due to exposure to ambient particulate matter of 2.5 μm or less (PM2.5) and the resulting costs continue to decline. However, 98% of China’s cities still have annual average PM2.5 concentrations that are more than the WHO guideline standard of 10 μg/m3. It provides policymakers and the public with up-to-date information on China’s response to climate change and improvements in health outcomes and makes the following policy recommendations. (1) Promote systematic thinking in the related departments and strengthen multi-departmental cooperation. Sectors related to climate and development in China should incorporate health perspectives into their policymaking and actions, demonstrating WHO’s and President Xi Jinping’s so-called health-in-all-policies principle. (2) Include clear goals and timelines for climate-related health impact assessments and health adaptation plans at both the national and the regional levels in the National Climate Change Adaptation Strategy for 2035. (3) Strengthen China’s climate mitigation actions and ensure that health is included in China’s pathway to carbon neutrality. By promoting investments in zero-carbon technologies and reducing fossil fuel subsidies, the current rebounding trend in carbon emissions will be reversed and lead to a healthy, low-carbon future. (4) Increase awareness of the linkages between climate change and health at all levels. Health professionals, the academic community, and traditional and new media should raise the awareness of the public and policymakers on the important linkages between climate change and health.</p

Mathematical modelling and stability analysis for grid-connected cascaded H-bridge converter

When power converters are employed to connect the renewable power resources to the grid, the grid inductance can typically be quite significant because wind farms and solar power stations are located far away from the converters. The non-negligible grid inductance interacts with the Phase Locked Loop (PLL) which is assisting the grid side current controller with the phase angle of grid voltage and furthermore interacts with the DC-link voltage controller. It is known that the inductance of the grid side transmission line would distort the input voltage of PLL at the point of common coupling and this interference can even lead to the instability of the converter operation. However, the accurate prediction of the stability thresholds of the PLL configuration related to the grid inductance was missing which requires advanced modelling techniques. The key technique is a new mathematical approach to calculating the spectra, which is more sophisticated and more flexible than existing methods. The Harmonic Balance Technique is powerful for modelling the steady state of the periodic waveform and is already a sophisticated technique in other fields but occasionally adopted on power electronics. This study aims to adopt this modelling approach to obtain a detailed description of the operation of the converter steady state thus the stability of its steady state can be investigated. The stability analysis is conducted for the periodic steady state employing the Floquet theorem. The stability analysis indicates that for a single-phase grid-connected cascaded H-bridge rectifier, the interactions between control loops and PLL can be affected by grid side inductance, and their interactions can even lead to instability of the system. Notably, the instability boundaries are predicted for PLL bandwidth about three different grid strengths. These thresholds have been verified with both simulation and experimental results