“双碳”目标的确立推动了新能源的快速发展。大规模电力电子设备的接入,使得电网呈现低惯量的特性。基于模块化多电平变换器(modular multilevel converter,MMC)的柔性直流输电系统在连接弱电网的场景下,其换流站采用虚拟同步机(virtua...“双碳”目标的确立推动了新能源的快速发展。大规模电力电子设备的接入,使得电网呈现低惯量的特性。基于模块化多电平变换器(modular multilevel converter,MMC)的柔性直流输电系统在连接弱电网的场景下,其换流站采用虚拟同步机(virtual synchronous generator,VSG)控制来模拟同步发电机的机械特性,可以为新能源系统提供惯量支撑。为满足不同工况,换流站需要在两种控制模式--构网型控制和跟网型控制之间进行切换。文中提出了一种MMC-高压直流输电(high voltage direct current,HVDC)换流站构网型和跟网型控制无缝切换技术,在不改变内环电流控制的前提下,通过构建构网型和跟网型控制统一的外环参考电流,保证换流站输出电压在切换过程中功角和幅值的连续性。仿真结果表明,所提的无缝切换技术可以有效降低控制切换过程对系统造成的过流或者过压影响。展开更多
Climate change and human activities such as overgrazing and rapid development of tourism simultaneously affected the vegetation of the Zoige Plateau.However,the spatiotemporal variations of vegetation and the relative...Climate change and human activities such as overgrazing and rapid development of tourism simultaneously affected the vegetation of the Zoige Plateau.However,the spatiotemporal variations of vegetation and the relative contributions of climate change and human activities to these vegetation dynamics remain unclear.Therefore,clarifying how and why the vegetation on the Zoige Plateau changed can provide a scientific basis for the sustainable development of the region.Here,we investigate NDVI trends using the Normalized Difference Vegetation Index(NDVI)as an indicator of vegetation greenness and distinguish the relative effects of climate changes and human activities on vegetation changes by utilizing residual trend analysis and the Geodetector.We find a tendency of vegetation greening from 2001 to 2020,with significant greening accounting for 21.44%of the entire region.However,browning area expanded rapidly after 2011.Warmer temperatures are the primary driver of vegetation changes in the Zoige Plateau.Climatic variations and human activities were responsible for 65.57%and 34.43%of vegetation greening,and 39.14%and 60.86%of vegetation browning,respectively,with browning concentrated along the Yellow,Black and White Rivers.Compared to 2001-2010,the inhibitory effect of human activity and climate fluctuations on vegetation grew dramatically between 2011 and 2020.展开更多
文摘“双碳”目标的确立推动了新能源的快速发展。大规模电力电子设备的接入,使得电网呈现低惯量的特性。基于模块化多电平变换器(modular multilevel converter,MMC)的柔性直流输电系统在连接弱电网的场景下,其换流站采用虚拟同步机(virtual synchronous generator,VSG)控制来模拟同步发电机的机械特性,可以为新能源系统提供惯量支撑。为满足不同工况,换流站需要在两种控制模式--构网型控制和跟网型控制之间进行切换。文中提出了一种MMC-高压直流输电(high voltage direct current,HVDC)换流站构网型和跟网型控制无缝切换技术,在不改变内环电流控制的前提下,通过构建构网型和跟网型控制统一的外环参考电流,保证换流站输出电压在切换过程中功角和幅值的连续性。仿真结果表明,所提的无缝切换技术可以有效降低控制切换过程对系统造成的过流或者过压影响。
基金partially financed by the National Natural Science Foundation of China(Grant No.42201439)Natural Science Foundation of Sichuan Provincial Department of Science and Technology(Grant No.2022NSFSC1082)Key Laboratory of Smart Earth(No.KF2023YB02-12).
文摘Climate change and human activities such as overgrazing and rapid development of tourism simultaneously affected the vegetation of the Zoige Plateau.However,the spatiotemporal variations of vegetation and the relative contributions of climate change and human activities to these vegetation dynamics remain unclear.Therefore,clarifying how and why the vegetation on the Zoige Plateau changed can provide a scientific basis for the sustainable development of the region.Here,we investigate NDVI trends using the Normalized Difference Vegetation Index(NDVI)as an indicator of vegetation greenness and distinguish the relative effects of climate changes and human activities on vegetation changes by utilizing residual trend analysis and the Geodetector.We find a tendency of vegetation greening from 2001 to 2020,with significant greening accounting for 21.44%of the entire region.However,browning area expanded rapidly after 2011.Warmer temperatures are the primary driver of vegetation changes in the Zoige Plateau.Climatic variations and human activities were responsible for 65.57%and 34.43%of vegetation greening,and 39.14%and 60.86%of vegetation browning,respectively,with browning concentrated along the Yellow,Black and White Rivers.Compared to 2001-2010,the inhibitory effect of human activity and climate fluctuations on vegetation grew dramatically between 2011 and 2020.
