Transient Stability of GFL Converters Subjected to Switching of Droop-Controlled GFM Converters

TL;DR

This paper analyzes the transient stability risks in grid-following converters caused by switching in grid-forming converters and proposes a local control strategy to improve stability, validated through simulations and hardware tests.

Contribution

It develops a mathematical model for the switched system, derives switching conditions, and introduces a local VFDC strategy to enhance transient stability.

Findings

Stability boundary of the switched system matches that of the CLC subsystem.

The proposed VFDC strategy improves transient stability and convergence to CVC mode.

Validation through simulations and hardware-in-the-loop tests confirms effectiveness.

Abstract

Integrating grid-forming converters (GFMCs) into grid-following converter (GFLC)-dominated power systems enhances the grid strength, but GFMCs' current-limiting characteristic triggers dynamic switching between constant voltage control (CVC) and current limit control (CLC). This switching feature poses critical transient stability risks to GFLCs, requiring urgent investigation. This paper first develops a mathematical model for this switched system. Then, it derives switching conditions for droop-controlled GFMCs, which are separately GFMC angle-dependent and GFLC angle-dependent. On this basis, the stability boundaries of GFLC within each subsystem are analyzed, and the impact of GFMC switching arising from GFLC angle oscillation is investigated. The findings reveal that the switched system's stability boundary coincides with that of the CLC subsystem. To enhance GFLC's transient stability and ensure GFMC converges to the CVC mode, this paper introduces a virtual fixed d-axis control (VFDC) strategy. Compared with existing methods, this method achieves decoupling and self-stabilization using only local state variables from individual converters. The conclusions are validated through simulations and Controller Hardware-in-the-Loop tests.