多光伏发电单元并联接入弱交流系统的稳定性分析

围绕弱交流系统下多光伏发电单元并联运行的稳定性问题,建立了两个光伏发电单元并联小信号模型,分析电网强度、各光伏发电单元运行工况以及各逆变器锁相环控制器参数对其运行稳定性的影响,揭示各光伏发电单元的运行耦合机制。同时,提出了一种基于虚拟端电压的光伏发电运行控制策略,等效地减小电网阻抗,提高多光伏发电单元并联接入弱交流系统的运行稳定性。研究结果表明:随着各光伏阵列输出功率的增加和电网强度的降低,多光伏发电单元易发生振荡现象;一定范围内增加各逆变器锁相环控制器比例系数(暂不考虑积分系数的影响),可增强系统阻尼,易于提高系统的稳定性;相比于单个光伏发电单元接入弱交流系统,多光伏发电单元间功率环路存在耦合项,易受光伏发电单元运行工况与锁相环控制器比例系数的影响,不利于系统稳定运行。基于时域仿真算例验证了理论分析的有效性和控制策略的可行性。

Stability Analysis of Multiple Parallelled Photovoltaic Power Generation Units Connected to Weak AC System

Aiming to alleviate the stability problem for the parallel operation of multiple photovoltaic(PV)power generation units(MPGUs)connected to weak AC system, a small-signal model is developed with two parallel PV units. This model is used to study the influence of power grid strength, the operation states of each PV unit, and the control parameters in each phase-locked loop(PLL)controller on the stability of multiple PV units in parallel operation. Then the operation coupling mechanism of each PV unit is revealed. In addition, a control strategy for PV power generation is proposed based on the virtual terminal voltage, which can equivalently reduce the impedance of the power grid and improve the operation stability of multiple paralleled PV units connected to the weak AC system. The results show that with the increasing of output power of the PV array and the decreasing of power grid strength, the multiple PV units are prone to oscillation; the increasing of proportional coefficient of PLL controller in each inverter can enhance the system damping and improve the stability of the system in a certain range(the influence of integral coefficient is not considered); the power loop of the multiple PV units may have a coupling term in comparison with that of the single one connected to the weak AC system, which is susceptible to the operation state of PV units and the proportional coefficient of PLL controller, hence it is not conducive to the system stability. The effectiveness of theoretical analysis and the feasibility of the control strategy are verified based on the time-domain simulation examples.