Distinction of weak and strong AC grids for emerging hierarchical-infeed LCC-UHVDC systems is important for planning and operation departments. However, accuracy of earlier distinction methods is limited as they were ...Distinction of weak and strong AC grids for emerging hierarchical-infeed LCC-UHVDC systems is important for planning and operation departments. However, accuracy of earlier distinction methods is limited as they were developed by empirical reasoning without rigorous theoretical analysis. Hence in this letter, hierarchical-infeed interactive effective short-circuit ratio (HIESCR) index is first used for strength evaluation of HIDC systems with complex inter-inverter interactions considered. Boundary HIESCR (BHIESCR) is also introduced in the proposed distinction method of weak and strong AC grids. That is, weak (or strong) AC grids are, respectively, identified when HIESCR is less (or greater) than BHIESCR. Second, it is shown BHIESCR remains almost unchanged as 3.0 versus various system parameters and rated operation variables based on rigorous theoretical analysis. This salient feature makes the proposed method more accurate than earlier methods. Finally, the proposed method is validated by simulations based on the PSCAD/EMTDC program.展开更多
新能源汇集经柔性直流输电(voltage source converter based high voltage direct current,VSC-HVDC)技术送出是促进新能源消纳的有效方式。但新能源渗透率的持续增加导致电网强度不断下降,采用传统跟网型(grid-following,GFL)换流技术...新能源汇集经柔性直流输电(voltage source converter based high voltage direct current,VSC-HVDC)技术送出是促进新能源消纳的有效方式。但新能源渗透率的持续增加导致电网强度不断下降,采用传统跟网型(grid-following,GFL)换流技术已无法满足系统稳定运行需求。为提高系统弱电网适应性,满足大规模新能源接入弱同步支撑柔直系统应用场景需求,提出在柔直系统送端换流站采用VSG控制策略。首先,建立整流侧控制小信号数学模型,利用根轨迹法深入研究虚拟阻抗对系统稳定性的影响。其次,提出利用交流电压变化率及电压差值等电气量构建虚拟电抗自适应调整项的改进VSG控制算法,在保证系统等效阻抗呈感性的同时,可提高送端交流系统的等效短路比,达到改善系统整体性能的效果。最后,通过PSCAD/EMTDC电磁暂态仿真验证所提控制策略的有效性。展开更多
模块化多电平换流器高压直流输电(modular multilevel converter based high voltage direct current,MMC-HVDC)由于具备自换相能力,尤其适用于向弱交流电网供电。交流系统强度降低时,会制约系统的直流功率传输能力,甚至导致系统失稳。...模块化多电平换流器高压直流输电(modular multilevel converter based high voltage direct current,MMC-HVDC)由于具备自换相能力,尤其适用于向弱交流电网供电。交流系统强度降低时,会制约系统的直流功率传输能力,甚至导致系统失稳。针对联接弱交流电网时MMC-HVDC系统功率传输受限的问题,建立了状态空间与直流阻抗模型,从时域、频域两方面研究了交流系统强度对直流功率传输能力的影响,明确了弱交流电网工况下功率传输受限的原因。基于参与因子定位结果,提出了在定直流电压控制环节引入直流电流反馈的功率传输能力提升方法,从时域、频域两方面对控制策略的提升作用进行了机理分析,并定量得出了控制参数的可行域及功率传输能力的最大提升水平。该方法在避免稳态误差的前提下,有效提升了MMC-HVDC系统的直流功率传输能力。展开更多
A small-signal model of photovoltaic(PV)generation connected to weak AC grid is established based on a detailed model of the structure and connection of a PV generation system. An eigenvalue analysis is then employed ...A small-signal model of photovoltaic(PV)generation connected to weak AC grid is established based on a detailed model of the structure and connection of a PV generation system. An eigenvalue analysis is then employed to study the stability of PV generation for different grid strengths and control parameters in a phaselocked loop(PLL) controller in the voltage source converter. The transfer function of the power control loop in the dq rotation frame is developed to reveal the influence mechanism of PLL gains on the small-signal stability of PV generation. The results can be summarized as follows:(1)oscillation phenomena at a frequency of about 5 Hz may occur when the grid strength is low;(2) the tuning control parameters of the PLL have a noticeable effect on the damping characteristics of the system, and larger proportional gain can improve the system damping;(3)within a frequency range of 4-5 Hz,the PLL controller has positive feedback on the power loop of PV generation. A virtual inductance control strategy is proposed to improve the operational stability of PV generation. Finally, a simulation model of PV generation connected to weak AC grid is built in PSCAD/EMTDC and the simulation results are used to validate the analysis.展开更多
当电压源换流器型直流输电(voltage source converter based high voltage direct current transmission,VSC-HVDC)联接弱交流电网输送功率接近交流电网静态稳定极限时,经典矢量控制方法无法保证换流器稳定运行。文中分析VSC联接于弱交...当电压源换流器型直流输电(voltage source converter based high voltage direct current transmission,VSC-HVDC)联接弱交流电网输送功率接近交流电网静态稳定极限时,经典矢量控制方法无法保证换流器稳定运行。文中分析VSC联接于弱交流电网时的潮流特性,研究经典矢量控制外环在弱交流电网条件下的特性,通过基于小信号模型的极点分析研究导致换流器无法稳定运行的原因,基于VSC联接于弱交流电网时的失稳机理提出一种改进矢量控制方法,在矢量控制外环增加前馈支路,增加暂态无功响应速度,提高弱交流电网条件下VSC在输送功率接近交流电网静态稳定极限时的稳定性。最后,基于小信号建模的极点分析和时域仿真结果验证改进矢量控制方法的有效性。展开更多
通过在电网换相换流器高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)逆变侧交流母线装设静止同步补偿器(static synchronous compensator,STATCOM),来达到抑制换相失败的目的。然而研究发现...通过在电网换相换流器高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)逆变侧交流母线装设静止同步补偿器(static synchronous compensator,STATCOM),来达到抑制换相失败的目的。然而研究发现,在弱交流系统工况下,STATCOM的控制系统(特别是交流电压控制外环)与LCC锁相环之间存在相互作用,会对后者的稳定可行域造成不利影响,进而可能引发整个混合系统的小干扰失稳现象。针对此现象,该文提出一种适用于弱交流系统下含STATCOM的LCC-HVDC系统的新型附加阻尼协调控制(supplementarydampingcoordination control,SDCC),该方法基于LCC锁相环输出频率信息,引入阻尼系数以附加分量形式反馈到STATCOM定交流电压控制器外环输入中。基于Matlab的特征根理论分析及基于PSCAD/EMTDC的详细电磁暂态仿真结果表明,提出的SDCC控制可以有效抑制含STATCOM的LCC-HVDC系统中LCC锁相环增益过大引起的系统小干扰失稳现象,从而提高系统的稳定裕度。展开更多
基金supported in part by the National Natural Science Foundation of China(51907067)in part by the Industrial Research Chair Program of the Natural Sciences and Engineering Research Councilof Canada。
文摘Distinction of weak and strong AC grids for emerging hierarchical-infeed LCC-UHVDC systems is important for planning and operation departments. However, accuracy of earlier distinction methods is limited as they were developed by empirical reasoning without rigorous theoretical analysis. Hence in this letter, hierarchical-infeed interactive effective short-circuit ratio (HIESCR) index is first used for strength evaluation of HIDC systems with complex inter-inverter interactions considered. Boundary HIESCR (BHIESCR) is also introduced in the proposed distinction method of weak and strong AC grids. That is, weak (or strong) AC grids are, respectively, identified when HIESCR is less (or greater) than BHIESCR. Second, it is shown BHIESCR remains almost unchanged as 3.0 versus various system parameters and rated operation variables based on rigorous theoretical analysis. This salient feature makes the proposed method more accurate than earlier methods. Finally, the proposed method is validated by simulations based on the PSCAD/EMTDC program.
文摘新能源汇集经柔性直流输电(voltage source converter based high voltage direct current,VSC-HVDC)技术送出是促进新能源消纳的有效方式。但新能源渗透率的持续增加导致电网强度不断下降,采用传统跟网型(grid-following,GFL)换流技术已无法满足系统稳定运行需求。为提高系统弱电网适应性,满足大规模新能源接入弱同步支撑柔直系统应用场景需求,提出在柔直系统送端换流站采用VSG控制策略。首先,建立整流侧控制小信号数学模型,利用根轨迹法深入研究虚拟阻抗对系统稳定性的影响。其次,提出利用交流电压变化率及电压差值等电气量构建虚拟电抗自适应调整项的改进VSG控制算法,在保证系统等效阻抗呈感性的同时,可提高送端交流系统的等效短路比,达到改善系统整体性能的效果。最后,通过PSCAD/EMTDC电磁暂态仿真验证所提控制策略的有效性。
文摘模块化多电平换流器高压直流输电(modular multilevel converter based high voltage direct current,MMC-HVDC)由于具备自换相能力,尤其适用于向弱交流电网供电。交流系统强度降低时,会制约系统的直流功率传输能力,甚至导致系统失稳。针对联接弱交流电网时MMC-HVDC系统功率传输受限的问题,建立了状态空间与直流阻抗模型,从时域、频域两方面研究了交流系统强度对直流功率传输能力的影响,明确了弱交流电网工况下功率传输受限的原因。基于参与因子定位结果,提出了在定直流电压控制环节引入直流电流反馈的功率传输能力提升方法,从时域、频域两方面对控制策略的提升作用进行了机理分析,并定量得出了控制参数的可行域及功率传输能力的最大提升水平。该方法在避免稳态误差的前提下,有效提升了MMC-HVDC系统的直流功率传输能力。
基金supported by State Grid Corporation of China ‘‘Study on active frequency and voltage control technologies for second level power disturbance in photovoltaic power plant’’National Natural Science Foundation of China (No. 51277024)
文摘A small-signal model of photovoltaic(PV)generation connected to weak AC grid is established based on a detailed model of the structure and connection of a PV generation system. An eigenvalue analysis is then employed to study the stability of PV generation for different grid strengths and control parameters in a phaselocked loop(PLL) controller in the voltage source converter. The transfer function of the power control loop in the dq rotation frame is developed to reveal the influence mechanism of PLL gains on the small-signal stability of PV generation. The results can be summarized as follows:(1)oscillation phenomena at a frequency of about 5 Hz may occur when the grid strength is low;(2) the tuning control parameters of the PLL have a noticeable effect on the damping characteristics of the system, and larger proportional gain can improve the system damping;(3)within a frequency range of 4-5 Hz,the PLL controller has positive feedback on the power loop of PV generation. A virtual inductance control strategy is proposed to improve the operational stability of PV generation. Finally, a simulation model of PV generation connected to weak AC grid is built in PSCAD/EMTDC and the simulation results are used to validate the analysis.
文摘当电压源换流器型直流输电(voltage source converter based high voltage direct current transmission,VSC-HVDC)联接弱交流电网输送功率接近交流电网静态稳定极限时,经典矢量控制方法无法保证换流器稳定运行。文中分析VSC联接于弱交流电网时的潮流特性,研究经典矢量控制外环在弱交流电网条件下的特性,通过基于小信号模型的极点分析研究导致换流器无法稳定运行的原因,基于VSC联接于弱交流电网时的失稳机理提出一种改进矢量控制方法,在矢量控制外环增加前馈支路,增加暂态无功响应速度,提高弱交流电网条件下VSC在输送功率接近交流电网静态稳定极限时的稳定性。最后,基于小信号建模的极点分析和时域仿真结果验证改进矢量控制方法的有效性。
文摘通过在电网换相换流器高压直流输电(line commutated converter based high voltage direct current,LCC-HVDC)逆变侧交流母线装设静止同步补偿器(static synchronous compensator,STATCOM),来达到抑制换相失败的目的。然而研究发现,在弱交流系统工况下,STATCOM的控制系统(特别是交流电压控制外环)与LCC锁相环之间存在相互作用,会对后者的稳定可行域造成不利影响,进而可能引发整个混合系统的小干扰失稳现象。针对此现象,该文提出一种适用于弱交流系统下含STATCOM的LCC-HVDC系统的新型附加阻尼协调控制(supplementarydampingcoordination control,SDCC),该方法基于LCC锁相环输出频率信息,引入阻尼系数以附加分量形式反馈到STATCOM定交流电压控制器外环输入中。基于Matlab的特征根理论分析及基于PSCAD/EMTDC的详细电磁暂态仿真结果表明,提出的SDCC控制可以有效抑制含STATCOM的LCC-HVDC系统中LCC锁相环增益过大引起的系统小干扰失稳现象,从而提高系统的稳定裕度。