根据光伏(photovoltaic,PV)系统输出电压-功率(U-P)曲线在最大功率点(maximum power point,MPP)两侧斜率变化的规律有所不同,MPP左侧曲线变化舒缓,右侧相对陡直的特点,在光伏最大功率点跟踪(maximum power point tracking,MPPT)控制方...根据光伏(photovoltaic,PV)系统输出电压-功率(U-P)曲线在最大功率点(maximum power point,MPP)两侧斜率变化的规律有所不同,MPP左侧曲线变化舒缓,右侧相对陡直的特点,在光伏最大功率点跟踪(maximum power point tracking,MPPT)控制方法的研究基础之上,结合现有一些变步长电导增量法,提出了一种在MPP左侧利用指数函数来调整跟踪电压步长;在MPP右侧利用对数函数来调整跟踪电压步长的分段式变步长电导增量优化算法。仿真实验结果表明该方法良好地实现了对光伏系统最大功率点的跟踪,有效地降低了跟踪过程中MPP附近由步长振荡引起的功率损耗,提高了PV系统的功率输出效率。展开更多
局部放电信号在电缆中的传输特性对于放电水平的确定、局部放电定位及干扰区分有着重要的意义。以珠海拱澳线路110 k V电缆为研究对象,在不同长度的电缆上进行单一频率正弦信号的现场实测。运用均匀传输线理论计算得到单一频率正弦信号...局部放电信号在电缆中的传输特性对于放电水平的确定、局部放电定位及干扰区分有着重要的意义。以珠海拱澳线路110 k V电缆为研究对象,在不同长度的电缆上进行单一频率正弦信号的现场实测。运用均匀传输线理论计算得到单一频率正弦信号在该电缆中的单频衰减系数。通过Fourier变换计算高斯函数模拟的局部放电脉冲在电缆本体中的传输衰减特性,对脉冲的幅值、宽度、面积随传播距离变化规律进行了计算,并在不同长度的电缆上对局部放电校正脉冲的传输进行了实验验证。研究结果表明:单一频率高频正弦信号在电缆中的单频衰减系数与频率成正比,高斯函数模拟的局部放电脉冲在电缆本体中的传输服从指数函数与余补误差函数成积的规律,上升时间10 ns的高斯模拟脉冲幅值衰减到10%的传播距离为490 m;脉冲面积在一定范围内为放电量Q与电缆特性阻抗Z的乘积。展开更多
We consider an age-dependent branching process in random environments. The environments are represented by a stationary and ergodic sequence ξ = (ξ 0, ξ 1,…) of random variables. Given an environment ξ, the proce...We consider an age-dependent branching process in random environments. The environments are represented by a stationary and ergodic sequence ξ = (ξ 0, ξ 1,…) of random variables. Given an environment ξ, the process is a non-homogenous Galton-Watson process, whose particles in n-th generation have a life length distribution G(ξ n ) on ?+, and reproduce independently new particles according to a probability law p(ξ n ) on ?. Let Z(t) be the number of particles alive at time t. We first find a characterization of the conditional probability generating function of Z(t) (given the environment ξ) via a functional equation, and obtain a criterion for almost certain extinction of the process by comparing it with an embedded Galton-Watson process. We then get expressions of the conditional mean E ξ Z(t) and the global mean EZ(t), and show their exponential growth rates by studying a renewal equation in random environments.展开更多
文摘根据光伏(photovoltaic,PV)系统输出电压-功率(U-P)曲线在最大功率点(maximum power point,MPP)两侧斜率变化的规律有所不同,MPP左侧曲线变化舒缓,右侧相对陡直的特点,在光伏最大功率点跟踪(maximum power point tracking,MPPT)控制方法的研究基础之上,结合现有一些变步长电导增量法,提出了一种在MPP左侧利用指数函数来调整跟踪电压步长;在MPP右侧利用对数函数来调整跟踪电压步长的分段式变步长电导增量优化算法。仿真实验结果表明该方法良好地实现了对光伏系统最大功率点的跟踪,有效地降低了跟踪过程中MPP附近由步长振荡引起的功率损耗,提高了PV系统的功率输出效率。
文摘局部放电信号在电缆中的传输特性对于放电水平的确定、局部放电定位及干扰区分有着重要的意义。以珠海拱澳线路110 k V电缆为研究对象,在不同长度的电缆上进行单一频率正弦信号的现场实测。运用均匀传输线理论计算得到单一频率正弦信号在该电缆中的单频衰减系数。通过Fourier变换计算高斯函数模拟的局部放电脉冲在电缆本体中的传输衰减特性,对脉冲的幅值、宽度、面积随传播距离变化规律进行了计算,并在不同长度的电缆上对局部放电校正脉冲的传输进行了实验验证。研究结果表明:单一频率高频正弦信号在电缆中的单频衰减系数与频率成正比,高斯函数模拟的局部放电脉冲在电缆本体中的传输服从指数函数与余补误差函数成积的规律,上升时间10 ns的高斯模拟脉冲幅值衰减到10%的传播距离为490 m;脉冲面积在一定范围内为放电量Q与电缆特性阻抗Z的乘积。
基金the National Natural Sciente Foundation of China (Grant Nos. 10771021, 10471012)Scientific Research Foundation for Returned Scholars, Ministry of Education of China (Grant No. [2005]564)
文摘We consider an age-dependent branching process in random environments. The environments are represented by a stationary and ergodic sequence ξ = (ξ 0, ξ 1,…) of random variables. Given an environment ξ, the process is a non-homogenous Galton-Watson process, whose particles in n-th generation have a life length distribution G(ξ n ) on ?+, and reproduce independently new particles according to a probability law p(ξ n ) on ?. Let Z(t) be the number of particles alive at time t. We first find a characterization of the conditional probability generating function of Z(t) (given the environment ξ) via a functional equation, and obtain a criterion for almost certain extinction of the process by comparing it with an embedded Galton-Watson process. We then get expressions of the conditional mean E ξ Z(t) and the global mean EZ(t), and show their exponential growth rates by studying a renewal equation in random environments.
