Stability Analysis and Control of Grid-connected VSC-HVDC Systems

Abstract

Voltage Source Converter-High Voltage Direct Current (VSC-HVDC) transmission systems are widely employed to transmit wind power over long distances to Alternating Current (AC) grids. Ensuring stable power transmission is essential for the reliable utilisation of wind energy, contributing to the UK’s net-zero goal. This thesis investigates the stability and control of grid-connected VSC-HVDC systems, with a focus on the inverter-side dynamics. To support the analysis, grid-connected VSC-HVDC systems are established. Then dq-domain small-signal state-space models are derived to quantitatively assess system stability using the impedance-based generalized Nyquist criterion. EMT simulation models of the systems are developed in PSCAD for dynamic analysis and theoretical verification. Based on these foundations, the thesis introduces new research in several areas, including: 1. Time delay in VSC systems can degrade control response and even induce instability. This thesis quantifies the impact of time delay on VSC-HVDC system stability (using industrially representative hardware data from a previous PhD) and develops an improved model to analyse the impact of this, bridging the gap between theory and industry practice. 2. Phase-Locked Loops (PLLs) are essential for synchronizing Grid-Following (GFL) VSC-HVDC systems with AC grids. However, the comparative influence of PLLs on system stability remains underexplored. This study systematically investigates the impacts of the conventional Synchronous Reference Frame-PLL (SRF-PLL) and the Inertia PLL (IPLL) on the small-signal stability of grid-connected VSC-HVDC systems. The analysis reveals that both SRF-PLL and IPLL-related system stability is primarily constrained by the PLL bandwidth. 3. Based on these insights, a PLL-less virtual synchronous machine-based Grid-Forming (GFM) VSC-HVDC system is developed. Its small-signal state-space model is constructed, and a corresponding EMT simulation model is established in PSCAD. The performance and stability of GFL and GFM VSC-HVDC systems are compared. The results demonstrate that GFM VSC-HVDC can avoid or improve certain instability issues faced by GFL VSC-HVDC systems.

Date of Award	14 Apr 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robin Preece (Main Supervisor) & Mike Barnes (Co Supervisor)