低惯量交流系统并网变流器次/超同步振荡分析

Sub/Super Synchronous Oscillation Analysis of Grid-Connected Converter in Low Inertia AC System

低惯量电力系统中跟网型变流器可能会与同步机转子动态产生交互,给系统稳定性分析引入新的问题,仅靠跟网型设备动态和短路比衡量交流系统强度不再准确。为此,该文综合分析系统惯量和短路比对跟网型变流器小干扰同步稳定的影响,提出适用于分析低惯量系统中并网变流器稳定性的方法,所提方法计算量低、物理意义明确。首先,推导变流器和同步机混联系统的闭环模型,并提取出能表征低惯量动态的相互作用系数矩阵,定性分析两类设备间的交互关系。其次,基于模态摄动理论,将原多机系统解耦为稳定性等效的子系统。然后,选取适用于分析小干扰同步稳定问题的相位主导回路,根据该回路提出综合考虑惯量和短路比的稳定性分析方法。最后,基于仿真算例验证了所提低惯量系统中并网变流器小干扰同步稳定性分析方法的有效性。

With the widespread integration of renewable energy sources,the power system is transitioning into a weak AC system characterized by low inertia and a reduced short-circuit ratio.Compared to conventional power systems,the interaction between sources and grids becomes more intricate in weak AC systems,resulting in complex dynamic behaviors.The challenge of oscillation instability in grid-connected inverters is accentuated in low-inertia power systems.In such systems,grid-following inverters may interact with the dynamics of synchronous machine rotors,posing new challenges for stability analysis.Relying solely on the dynamics and short-circuit ratio of grid-following devices is insufficient to assess the strength of the AC system.This paper addresses the small-signal oscillation instability in the sub-synchronous frequency range.The impact of the system's generalized short-circuit ratio and inertia on the small-disturbance synchronous stability of grid-following inverters are analyzed.The proposed stability margin quantification method is designed for grid-connected inverters in low-inertia multi-machine systems with low computational complexity and clear physical significance.This paper initially establishes a closed-loop model for the hybrid system of synchronous machines and inverters.The dynamic coupling of the two types of devices is separated by mathematically transforming the closed-loop model.A unit interaction factor matrix(UIFM)is generated,including the dynamics of synchronous machines and the interconnected network of the two types of devices.This matrix qualitatively describes the interaction between the dynamics of synchronous machines and inverters in low-inertia systems,providing a model foundation for stability analysis.The matrix perturbation theory is employed for the closed-loop model,constructing an isomorphic system equivalent to the original system's stability.Modal decoupling is then applied to break down the constructed equivalent isomorphic system into multiple subsystems.The small-signal circuit model of the dominant subsystem is provided.Key factors affecting the stability of the original complex system are extracted,concretizing the interaction of bottom-layer components in the subsystem.Since the minimal subsystem remains a two-dimensional multiple-input,multiple-output(MIMO)system,this paper selects a phase-dominant loop suitable for analyzing small-disturbance synchronous stability problems.The result in a single-input,single-output(SISO)system is equivalent to the stability of the minimal subsystem.The stability of this system is analyzed using the Nyquist criterion,and stability margin indicators are established.A two-dimensional stable operating region calculation method,considering both system inertia and short-circuit ratio,is provided.Finally,using the Matlab/Simulink platform,the effectiveness of the proposed analysis method is validated using a two-area,four-machine system and a 10-machine 39-bus system.