In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalab...In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.展开更多
The present paper investigates how neighborhood effects are connected to chronic poverty. We examine a large sample of groups of households and find that neighborhood effects are significant in a majority of groups, e...The present paper investigates how neighborhood effects are connected to chronic poverty. We examine a large sample of groups of households and find that neighborhood effects are significant in a majority of groups, especially in the poorest groups. People living in poor communities tend to suffer from poverty over time. It is of theoretical and empirical importance to explore how neighborhood effects are interrelated with chronic poverty and the channels through which this occurs. Unlike other econometric analyses, we establish a multilevel econometric model to show that： （i） it is difficult for an individual living in a neighborhood with a high proportion of agricultural labor, low education levels, and poor transport and telecommunication infrastructure to escape from poverty traps; （ii） neighborhood effects dominate in poor communities; and （iii） although poverty is affected by group-level factors, individual factors still play a dominant role in regards to escaping poverty when income surpasses a threshold level Therefore, policy priority should be given to providing social protection and public services, especially in poor rural areas.展开更多
To facilitate rapid analysis of the oscillation stability mechanism in modular multilevel converter-based high voltage direct current(MMC-HVDC)systems and streamline the simulation process for determining MMC impedanc...To facilitate rapid analysis of the oscillation stability mechanism in modular multilevel converter-based high voltage direct current(MMC-HVDC)systems and streamline the simulation process for determining MMC impedance characteristics,a simplified mathematical simulation model for MMC closed-loop impedance is developed using the harmonic state space method.This model considers various control strategies and includes both AC-side and DC-side impedance models.By applying a Nyquist criterion-based impedance analysis method,the stability mechanisms on the AC and DC sides of the MMC are examined.In addition,a data-driven oscillation stability analysis method is also proposed,leveraging a global sensitivity algorithm based on fast model results to identify key parameters influencing MMC oscillation stability.Based on sensitivity analysis results,a parameter adjustment strategy for oscillation suppression is proposed.The simulation results from the MATLAB/Simulinkbased MMC model validate the effectiveness of the proposed method.展开更多
基金Supported by the Science and Technology Program of State Grid Corporation of China(5100-201999330A-0-0-00)。
文摘In the present scenario,modular multilevel converters(MMCs)are considered to be one of the most promising and effective topologies in the family of high-power converters because of their modular design and good scalability;MMCs are extensively used in high-voltage and high-power applications.Based on their unique advantages,MMCs have attracted increasing attention from academic circles over the past years.Several studies have focused on different aspects of MMCs,including submodule topologies,modeling schemes,modulation strategies,control schemes for voltage balancing and circulating currents,fault diagnoses,and fault-tolerant control strategies.To summarize the current research status of MMCs,all the aforementioned research issues with representative research approaches,results and characteristics are systematically overviewed.In the final section,the current research status of MMCs and their future trends are emphasized.
基金financial support of the Key Grant Programs of the National Social Science Foundation of China (Grant Nos.10AZD013 and 11&ZD006)
文摘The present paper investigates how neighborhood effects are connected to chronic poverty. We examine a large sample of groups of households and find that neighborhood effects are significant in a majority of groups, especially in the poorest groups. People living in poor communities tend to suffer from poverty over time. It is of theoretical and empirical importance to explore how neighborhood effects are interrelated with chronic poverty and the channels through which this occurs. Unlike other econometric analyses, we establish a multilevel econometric model to show that： （i） it is difficult for an individual living in a neighborhood with a high proportion of agricultural labor, low education levels, and poor transport and telecommunication infrastructure to escape from poverty traps; （ii） neighborhood effects dominate in poor communities; and （iii） although poverty is affected by group-level factors, individual factors still play a dominant role in regards to escaping poverty when income surpasses a threshold level Therefore, policy priority should be given to providing social protection and public services, especially in poor rural areas.
基金National Natural Science Foundation of China(52307127)State Key Laboratory of Power System Operation and Control(SKLD23KZ07)。
文摘To facilitate rapid analysis of the oscillation stability mechanism in modular multilevel converter-based high voltage direct current(MMC-HVDC)systems and streamline the simulation process for determining MMC impedance characteristics,a simplified mathematical simulation model for MMC closed-loop impedance is developed using the harmonic state space method.This model considers various control strategies and includes both AC-side and DC-side impedance models.By applying a Nyquist criterion-based impedance analysis method,the stability mechanisms on the AC and DC sides of the MMC are examined.In addition,a data-driven oscillation stability analysis method is also proposed,leveraging a global sensitivity algorithm based on fast model results to identify key parameters influencing MMC oscillation stability.Based on sensitivity analysis results,a parameter adjustment strategy for oscillation suppression is proposed.The simulation results from the MATLAB/Simulinkbased MMC model validate the effectiveness of the proposed method.
