Strength Evaluation of Multi-Infeed LCC-HVDC Systems Based on the Virtual Impedance Concept

Abstract

Earlier works have reported various strength evaluation indices for static voltage stability assessment of multi-infeed dc (MIDC) systems including multiple LCC-HVDC links. However, the accuracy of earlier indices is limited under disproportional power increasing scenarios and constant current control mode of LCC-HVDC links. Thus, this article aims to propose a new strength index of MIDC systems for the accurate static voltage stability assessment. Firstly, a virtual impedance concept is presented using the Thevenin equivalent to represent the coupling effect of all other inverters on the studied inverter. This concept enables the equivalent single-infeed model of MIDC systems to be developed with inter-inverter interactions considered. Moreover, the equivalent model is used to propose a unified effective short-circuit ratio (UESCR) index for the strength evaluation of MIDC systems. This is achieved by rigorous theoretical analysis of the voltage stability factor (VSF) based static voltage stability criterion. Compared with earlier indices, the UESCR is applicable under any power increasing scenarios and control modes of LCC-HVDC links. Such exclusive advantage enables the UESCR to indicate the static voltage stability much more accurately. Finally, the validity of the UESCR is verified by case study conducted on a modified IEEE 118-bus system including three LCC-HVDC links.