Analysis of voltage dynamics within current control time-scale in a VSC connected to a weak AC grid via series compensated AC line

In this paper, an analytical approach is proposed to analyse the small-signal stability associated with the voltage dynamics of three paths within current control of a VSC connected to a weak AC grid via series compensated AC line: the PLL, active damping and virtual conductance loops. The basic control structure of the VSC is a cascade type based on PI controllers, where DC voltage and reactive current setpoints are used to drive the VSC terminal voltage change. Besides, active damping (AD) and virtual conductance (VC) filters are included to damp high and low frequency oscillations of terminal voltage, respectively. Different series compensation level (SCL) and AC grid stiffness values are considered while performing the analysis. The main design implications obtained through this analysis are the proposal of adequate values for PLL bandwidth according to a decreasing level of AC grid stiffness and suitable ratio values between AD and VC filter gains to ensure stability and good cross-impact, given a SCL value. The analysis is validated by means of simulations conducted in MATLAB® Simulink. The proposed analytical framework serves as a contribution to support a robust terminal voltage change mechanism within current control time-scale.