Local Load Effect on the DC Voltage Control Timescale Oscillation of Voltage Source Converters

Voltage source converter (VSC)-interfaced renewable power plants are typically far from load centers, the weak grid condition presents significant stability risks. Extensive research has been conducted to analyze oscillations within the DC voltage control (DVC) timescale. However, most studies focus on scenarios where generators are connected to the grid through long transmission lines. More general scenarios, where loads are distributed along these transmission lines, have not received sufficient attention, potentially leaving undiscovered oscillation mechanisms. This paper finds a small-signal stability issue, which is an unexpected oscillation that may occur if local loads are connected near the VSC-based power plants. To comprehensively analyze this phenomenon, this paper takes the phase-locked loop (PLL) as the main object and analyzes the interaction between the terminal voltage control (TVC), DVC, and PLL. Initially, the complex coupling relationship is clarified by applying series expansion to the interaction paths between TVC and DVC. It is found that the load effect contributes to forming a crucial path that introduces the negative damping torque caused by TVC to PLL. Subsequently, analyses show that undamped oscillation risks exist under normal control parameters. A targeted control strategy is proposed for oscillation suppression. Verifications are conducted using MATLAB/Simulink and RT-LAB.