Parametric analysis of the stability of VSC-HVDC converters

This paper proposes a reduced order mathematical model for the VSC converter and its controls by linearizing the combined set of equations of the power system and controller. The reduced order model can preserve the unstable characteristic of the VSC system and provide an explicit mathematical expression for the system eigenvalues. This analytical formula can directly reveal by simple inspection relationships between the system stability and system parameters. Results show that small signal stability of the system is significantly affected by the ac system strength (SCR) and the Phased Locked Loop (PLL) parameters. Also, small signal stability is more sensitive to the ac voltage controller´s gain than to the power controller gains. Compared with the previous developed full order model, the reduced order model can give more insights and constructive suggestions how to adjust the parameters of the control system of VSC. In particular, it is shown that by increasing the value of the voltage controller´s integral gain the system can be made to operate at an SCR which is very close to the theoretical minimum SCR limit.