Sub/Super Synchronous Oscillation Analysis of Grid-Connected Converter in Low Inertia AC System
Hu Guang1, Zhuang Kehao1, Gao Huisheng1, Xin Huanhai1, Wang Haijiao2, Zhang Yajun3
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China; 2. China Electric Power Research Institute Beijing 100192 China; 3. Electric Power Research Institute State Grid Shanghai Municipal Electric Power Company Shanghai 200437 China
Abstract：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.
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2024, Vol. 39 
