多时间尺度控制下跟网型变换器的同步稳定性分析与改进控制

Synchronization Stability Analysis and Enhanced Control of Grid- Following Converters under Multi-Timescale Control

直流电压时间尺度与交流电流时间尺度控制耦合可能会导致跟网型变换器阻尼下降,引发相应的同步失稳问题。为此,该文计及多环控制的多时间尺度特性,建立跟网型变换器的小信号同步模型,揭示各时间尺度控制的耦合路径与阻尼分布规律;采用灵敏度分析法,量化控制参数对阻尼特性的影响,筛选出引发系统同步失稳的主导因子;同时,考虑多场景电网工况对控制耦合的影响,从阻尼的角度直观地分析不同工况下系统失稳的机理。针对直流电压时间尺度内部dq耦合及多时间尺度控制耦合对系统阻尼的影响,提出一种基于端电压前馈的阻尼重塑改进控制策略,削弱控制耦合带来的负阻尼效应,增强变换器的同步稳定性。仿真与实验验证了理论分析的正确性和改进控制方案的有效性。

In the new power systems with increasing penetration of renewable energy and power electronic devices,grid-following(GFL)converters synchronized by a phase-locked loop(PLL)suffer from inadequate damping due to complex control coupling,leading to potential synchronization instability issues.The control loops dominating the dynamic characteristics of the converter can be categorized into DC voltage and AC current timescales,possessing multi-timescale(MTS)features that exhibit strong coupling effects on system damping.However,traditional synchronization stability analysis methods focus on the impact of control on system damping at only one timescale.Thus,the analysis of system synchronous dynamics still needs to be completed.This paper proposes a synchronization stability analysis model,considering the multi-loop control's MTS characteristics.Through sensitivity analysis of system damping,the impact of different operating conditions and control parameters on system synchronization stability is intuitively analyzed.Firstly,a synchronization stability analysis model that considers multiple timescales for GFL converters is established.The coupling paths and damping distribution patterns of each timescale control are revealed.Secondly,the influence of control parameters on damping characteristics is quantified using sensitivity analysis.The dominant factors causing synchronization instability in the system are identified,and their impact on system damping is visually represented through a three-dimensional graph.Additionally,according to the influence of grid impedance change and grid voltage drop on control coupling,the mechanism of system instability under various conditions is analyzed from a damping perspective.Finally,an improved control strategy based on terminal voltage feedforward for damping reshaping is proposed to mitigate the adverse damping effects of MTS control coupling.Simulation and experimental results demonstrate that under weak grid conditions,considering the effects of MTS coupling,step changes in active power reference values,an increase in grid impedance,and voltage drops in the grid can lead to sub-synchronous oscillations in the GFL converter system.The proposed improved control can suppress such oscillations,improving system synchronization stability.The following conclusions can be drawn from theoretical analysis and experiments.(1)The essential nature of the multi-loop control coupling in GFL converters is the frequency band overlap in the timescale,manifesting as MTS characteristics,significantly impacting the synchronization stability of the system.(2)Among the multi-loop control parameters,the dominant factors significantly affecting the system's synchronous characteristics are k_(p_pll) and k_(p_tvc).Reducing k_(p_pll) or increasing kp_tvc can mitigate adverse damping effects,enhance total system damping,and improve system synchronization performance.(3)Operating conditions,such as an increase in grid impedance,voltage drops in the grid,and internal dq coupling in DC voltage timescales,may exacerbate negative damping effects,potentially causing system synchronization instability.(4)The proposed improved control scheme effectively eliminates the impact of internal dq coupling in DC voltage timescales while significantly reducing adverse damping characteristics,thereby enhancing the synchronization stability of the MTS control system.