Small Signal Stability In Low Frequency Mode Of PLL-synchronized Weak Grid Connected Voltage Source Converter

Voltage source converters(VSC)based on phase locked loop(PLL)synchronization has been widely used in flexible DC system and renewable energy connected to AC system.Due to the uneven distribution of energy and load in China,VSC will connect to grid through long-distance AC line during large-capacity and long-distance power transmission,which will lead to VSC connected to weak AC grid and cause broadband oscillation instability.This paper focuses on the low frequency mode(LFM)small disturbance stability problem around 1 ～ 10 Hz.The existing papers have not made sufficient research on this kind of small signal stability,and the key influencing factors and stability improvement methods of LFM small signal stability are still lack of clear mechanism disclosure.Therefore,this paper takes the LFM small disturbance stability of weak grid connected VSC(WG-VSC)as the research object,and carries out in-depth research from the following aspects.Aiming at the dual loop vector control WG-VSC based on PLL synchronization(including two-level VSC and modular multilevel converter),an PLL-equivalent model for revealing the small signal stability mechanism of LFM is deduced and established,which can be applied to a variety of typical outer loop control modes.The model consists of the original PLL and the "outer loop weak grid" loop coupling grid strength,VSC operation point and outer loop dynamics.By dividing the latter into the influence of active side outer loop on PLL dynamics and the influence introduced by reactive side outer loop on PLL dynamics,the specific interaction relationship among VSC outer loops,weak grid characteristics and PLL can be clearly displayed.Based on the physical meaning of the equivalent proportional-integral coefficient of this model,the influence mechanism of "outer loop-PLL-weak grid" interaction on small disturbance stability of LFM can be clearly displayed.Detailed electromagnetic transient simulation conditions based on PSCAD/EMTDC verify the effectiveness of the equivalent PLL model.Based on the PLL-equivalent model,the impact of "outer loop-PLL-weak grid" interaction on LFM small disturbance stability of WG-VSC system under different active side and reactive side outer loop control modes can be analyzed in detail,and then the instability mechanism of LFM is revealed.The key conclusions obtained through theoretical analysis include: 1)increase VSC output active power and decrease of AC power grid strength have a consistent impact on LFM;2)When VSC outer loop does not adopt reactive side control,the interaction between active side outer loop and PLL will lead to monotonic instability of LFM of VSC as grid strength decreases,while the introduction of reactive side control will change the LFM instability form into oscillation instability;3)In weak grid condition,when the bandwidth of VSC outer loops is close to that of PLL,the LFM damping is significantly reduced,which will lead to system oscillation and instability;4)The LFM damping of the system with AC voltage control is higher than that with reactive power control.Based on PLL-equivalent model,the mechanism of two typical improved control strategies with additional compensation loop for small signal stability of LFM is analyzed.Then it is pointed out that through a certain additional compensation control design form,the two can have equivalent impact on LFM of system,and the equivalent PLL model can be used as a general model to reveal the improvement mechanism of LFM small disturbance stability with different improved control strategies and evaluate the improvement effect.In addition,from the perspective of increasing the theoretical limit of VSC output power,two improved control strategies are proposed to effectively improve the LFM small disturbance stability of WG-VSC system.