Analytical investigation of poorly damped conditions in VSC-HVDC systems

In this paper, strong AC-grid connected VSC-HVDC systems are studied. Under specific conditions, such systems can suffer from both stability and poor damping related issues, which warrants a stability study. Analytical eigenvalue expressions can directly demonstrate the impact of physical or control parameters on the system stability. However, especially in case of high-order systems, such expressions are challenging to obtain. This paper suggests a method to symbolically represent approximative eigenvalues of two-terminal VSC-HVDC systems, which could also be used to analyze the system dynamics. In addition, by applying symbolic-isolation method, the order of a multi-terminal VSC-HVDC system can be reduced to an equivalent two-terminal VSC-HVDC system, which enables the proposed method to provide symbolic pole expressions. Numerical studies based on Matlab simulations are presented, showing the accuracy of the analytical eigenvalue expressions and providing useful hints on the impact of physical or control parameters.