Stability of DC-link voltage as affected by phase locked loop in VSC when attached to weak grid

Abstract

This paper analyzes DC-link voltage stability as affected by phase locked loop (PLL) in voltage source converters (VSC) when connected to weak grid. PLL has an impact on power transferred to the grid from VSC due to active/reactive power current regulation aligned to PLL reference frame. DC-link voltage, directly related to power flows, is then affected by PLL dynamics. Interactions between PLL and DC-link voltage control are more serious in weak grid where terminal voltage, as input of PLL, becomes susceptible to power flow variations in VSC. In this paper, a system with VSC connected to infinite bus via grid impedance is studied for operating in inverter mode. Deteriorated DC-link voltage small signal stability is presented with quick PLL response as short circuit ratio (SCR) becomes significantly low. It may be possible for PLL to improve the DC-link voltage large signal stability due to its effect on power transfer.