Thixotropic properties are one of the low temperature rheology behaviors of gelled waxy crudes. In order to study the thixotropic behavior of gelled waxy crudes oil under cyclic loading of linear increasing and decrea...Thixotropic properties are one of the low temperature rheology behaviors of gelled waxy crudes. In order to study the thixotropic behavior of gelled waxy crudes oil under cyclic loading of linear increasing and decreasing of shear rate, three different types of waxy crude oil were studied ex-perimentally by using MARSIII rheometer. It is found that hysteresis loop area could represent strength of thixotropic properties under that loading. With increasing of the rate of share rate sweep, the area of second hysteresis loop is much smaller than first one and began a slow decline from the third hysteresis loop. Areas of each hysteresis loops were decreased with increasing temperature, but it was different in decline rate—the lower the temperature, the greater the rate of decline. The maximum rate of decrease is the first hysteresis loop with exponential decline, and remaining hysteresis hoops approximately decrease linearly. In addition, based on the experi-mental results an empirical correlation is developed to describe the relationship of hysteresis loop area and rate of share rate sweep.展开更多
并网逆变器是分布式发电系统与电网之间的电能变换接口,然而,随着分布式电源的广泛分布,并网的公共耦合点(point of common coupling,PCC)和电网之间含有较长的输电线路、较多的隔离变压器,导致电网和发电系统之间联络变弱,电网等效阻...并网逆变器是分布式发电系统与电网之间的电能变换接口,然而,随着分布式电源的广泛分布,并网的公共耦合点(point of common coupling,PCC)和电网之间含有较长的输电线路、较多的隔离变压器,导致电网和发电系统之间联络变弱,电网等效阻抗增加,电网呈现弱电网特性.并网逆变器与电网弱联络时,并网逆变器的锁相环(phase-locked loop,PLL)、电流控制环的控制作用会在较广的频域内引入负阻尼,可能导致系统在广域范围内失稳.为了解决这一问题,本文建立了考虑PLL影响的并网逆变器广域阻抗模型,提出了基于阻抗分析的广域稳定性判据,并在此基础上通过在PCC点并联有源阻尼来稳定系统的方法设计了一个广域有源阻尼器,将其并联连接到PCC点,使其在PCC点合成一个并联阻尼电阻以抑制系统产生的谐振.当系统工况发生变化时,通过提取PCC点谐振电压并利用谐波含量限制的方法来自适应调节虚拟电阻值,实现广域范围内镇定系统的效果.最后在MATLAB/Simulink中搭建了并网逆变器等值系统模型,通过时域仿真验证了该广域有源阻尼器的正确性和有效性.仿真结果表明,并网逆变器与电网弱联络时,PLL、电流控制环控制作用等会引起广域谐振稳定性问题,所设计的广域有源阻尼器能有效抑制广域范围内的谐振,且当系统联络突然发生变化时,也可以有效实现系统的广域镇定.展开更多
由2篇论文组成的系列论文旨在研究广域闭环控制系统(wide-area closed-loop control system,WACS)中时延的特性。在系列论文第1篇的研究基础上,首先提出估计实际WACS中闭环时延的方法,综合考虑通信时延与操作时延,提出正态分布模型以估...由2篇论文组成的系列论文旨在研究广域闭环控制系统(wide-area closed-loop control system,WACS)中时延的特性。在系列论文第1篇的研究基础上,首先提出估计实际WACS中闭环时延的方法,综合考虑通信时延与操作时延,提出正态分布模型以估计实际系统中的闭环时延分布,并给出确定正态分布模型参数的方法。然后介绍在实际WACS中实测闭环时延的时标对比法。最后,在专用通道与非专用通道的条件下,实测贵州电网WACS中的闭环时延,并将实测结果与使用正态分布模型的估计结果进行对比分析,验证了所提出的正态分布估计模型。该文对时延的分析及测试结果对广域闭环控制的研究及应用具有参考价值。展开更多
文摘Thixotropic properties are one of the low temperature rheology behaviors of gelled waxy crudes. In order to study the thixotropic behavior of gelled waxy crudes oil under cyclic loading of linear increasing and decreasing of shear rate, three different types of waxy crude oil were studied ex-perimentally by using MARSIII rheometer. It is found that hysteresis loop area could represent strength of thixotropic properties under that loading. With increasing of the rate of share rate sweep, the area of second hysteresis loop is much smaller than first one and began a slow decline from the third hysteresis loop. Areas of each hysteresis loops were decreased with increasing temperature, but it was different in decline rate—the lower the temperature, the greater the rate of decline. The maximum rate of decrease is the first hysteresis loop with exponential decline, and remaining hysteresis hoops approximately decrease linearly. In addition, based on the experi-mental results an empirical correlation is developed to describe the relationship of hysteresis loop area and rate of share rate sweep.
文摘并网逆变器是分布式发电系统与电网之间的电能变换接口,然而,随着分布式电源的广泛分布,并网的公共耦合点(point of common coupling,PCC)和电网之间含有较长的输电线路、较多的隔离变压器,导致电网和发电系统之间联络变弱,电网等效阻抗增加,电网呈现弱电网特性.并网逆变器与电网弱联络时,并网逆变器的锁相环(phase-locked loop,PLL)、电流控制环的控制作用会在较广的频域内引入负阻尼,可能导致系统在广域范围内失稳.为了解决这一问题,本文建立了考虑PLL影响的并网逆变器广域阻抗模型,提出了基于阻抗分析的广域稳定性判据,并在此基础上通过在PCC点并联有源阻尼来稳定系统的方法设计了一个广域有源阻尼器,将其并联连接到PCC点,使其在PCC点合成一个并联阻尼电阻以抑制系统产生的谐振.当系统工况发生变化时,通过提取PCC点谐振电压并利用谐波含量限制的方法来自适应调节虚拟电阻值,实现广域范围内镇定系统的效果.最后在MATLAB/Simulink中搭建了并网逆变器等值系统模型,通过时域仿真验证了该广域有源阻尼器的正确性和有效性.仿真结果表明,并网逆变器与电网弱联络时,PLL、电流控制环控制作用等会引起广域谐振稳定性问题,所设计的广域有源阻尼器能有效抑制广域范围内的谐振,且当系统联络突然发生变化时,也可以有效实现系统的广域镇定.
文摘由2篇论文组成的系列论文旨在研究广域闭环控制系统(wide-area closed-loop control system,WACS)中时延的特性。在系列论文第1篇的研究基础上,首先提出估计实际WACS中闭环时延的方法,综合考虑通信时延与操作时延,提出正态分布模型以估计实际系统中的闭环时延分布,并给出确定正态分布模型参数的方法。然后介绍在实际WACS中实测闭环时延的时标对比法。最后,在专用通道与非专用通道的条件下,实测贵州电网WACS中的闭环时延,并将实测结果与使用正态分布模型的估计结果进行对比分析,验证了所提出的正态分布估计模型。该文对时延的分析及测试结果对广域闭环控制的研究及应用具有参考价值。
