VSC-HVDC联接弱交流系统下的新型附加频率阻尼控制方法

A Novel Supplementary Frequency-based Damping Control for VSC-HVDC Station Connected to a Weak AC Grid

电压源型换流器高压直流输电（voltage source converter based HVDC，VSC．HVDC）联接弱交流系统时，很容易受到锁相环（phase．10ckedloop，PLL）特性的影响而使传输功率受限甚至出现系统不稳定的现象。该文从传统交流发电机的运行原理角度出发，提出了一种简单有效且适用于VSC-HVDC联接弱交流系统时的附加频率阻尼控制（supplementary frequency-based damping control，SFDC）方法，该方法基于PLL的输出频率特征量同时引入阻尼系数仇，产生了有功功率的附加阻尼分量来抑制或消除由于PLL引起的系统失稳现象。首先，建立了基于传统电流矢量控制的VSC系统的小信号模型，采用特征根分析和参与因子分析方法，研究了PLL参数对系统小干扰稳定性的影响及不同短路比（short circuit ratio，SCR）下PLL参数的可行域。然后，建立了含有SFDC控制的VSC系统联接弱受端交流系统时的小信号模型，并通过详细电磁暂态仿真验证了模型的正确性；基于特征根分析方法研究了阻尼系数仇对VSC系统小干扰稳定性的影响及其参数可行域。最后，对比研究了SFDC控制闭锁和投入时，VSC系统在不同SCR下的最大传输有功功率及不同PLL参数下所允许的最小短路比的变化规律，并提出了一种基于阻尼系数Dk的临界短路比裕度增加量指标，来定量评价所提出的SFDC控制对VSC系统稳定裕度的提升能力。结果表明，所提出的SFDC方法不仅可以有效抑制或消除弱交流电网下由于PLL引起的VSC系统失稳现象，而且可以提升VSC系统的最大传输功率及临界稳定裕度。

The dynamics of phase-locked loop （PLL） has major impact on the voltage source converter based high voltage direct current （VSC-HVDC） under weak AC grid condition, and potentially limit the power transfer capability and even results in system instability. From the operation concept of the traditional generator, this paper proposed a simple and effective control approach i.e., supplementary frequency-based damping control （SFDC）, for VSC-HVDC under weak AC grid. Based on the tracked frequency from PLL, the SFDC approach introduced a damping coefficient Dk and generates an additional damping power-component to suppress/eliminate the system instability due to the PLL dynamics. Firstly, a small-signal model of VSC system adopting the traditional vector current control was developed, and the impact of PLL gains on the small-signal stability of the system and the feasible region of the PLL gains under different short circuit ratio （SCR） were investigated by eigenvalue analysis and participation-factor analysis method. Then, a small-signal model of VSC system with the proposed SFDC approach under weak AC grid was also developed, and was validated by the detailed electromagnetic transient （EMT） simulation. From the eigenvalue analysis, the impact of the damping coefficient Dk on the small-signal stability of VSC system and the feasible region of Dk were also studied. Finally, this paper investigated the maximum available power （MAP） under different SCR values and the acceptable minimum SCR, i.e., SCRmin, under different PLL gains for VSC system with orwithout the SFDC approach, and proposes an ＂margin increment in SCRr, an＂ or MI_SCRmin index, based on damping factor Dk, to quantitatively evaluate the improved stability margin of VSC system by the SFDC approach. The results show that the proposed SFDC approach can effectively mitigate or eliminate the instability issue induced by the PLL, and improve the maximum power transfer capability and the stability margin for the VSC system under weak AC system.