Quantitative Parameter Conditions for Stability and Coupling in GFM-GFL Converter Hybrid Systems from a Small-Signal Synchronous Perspective

TL;DR

This paper develops a small-signal stability model for power systems with GFM and GFL converters, revealing conditions for effective decoupling and stability criteria, validated through simulations.

Contribution

It introduces a small-signal synchronous stability model considering network dynamics and provides analytical criteria for stability and decoupling in GFM-GFL converter systems.

Findings

Decoupling occurs when GFL operates near unity power factor or GFM has large inertia/damping.

Decentralized stability criteria are derived with physical interpretations.

Simulation results validate the theoretical stability conditions.

Abstract

With the development of renewable energy sources, power systems are gradually evolving into a system comprising both grid-forming (GFM) and grid-following (GFL) converters. However, the dynamic interaction between the two types of converters, especially low-inertia GFM converters and GFL converters, remains unclear due to the substantial differences in their synchronization mechanisms. To address this gap, this paper develops a small-signal synchronous stability model for power systems containing GFM and GFL converters, which considers network line dynamics. Based on subspace perturbation theory, we reveal that GFM and GFL subsystems can be effectively decoupled when GFL converters operate near unity power factor or when GFM converters possess sufficiently large inertia or damping, and provide lower bound of control parameters ensuring decoupling. Under the decoupling condition, we propose decentralized and analytical parameter-based stability criteria which have clear physical interpretations: the positive damping of converters compensates for the negative damping of the network. In the case of coupling, we also propose decentralized stability criteria based on the small phase theorem. The effectiveness of the theoretical analysis is validated through simulations in MATLAB/Simulink.