Concepts of voltage instability at converter bus in HVDC links

The authors present a general steady state analysis of voltage instability which is applicable to multi-terminal DC (MTDC) systems and also permits the comparison of various controller strategies. In a parallel connected MTDC system of which a two terminal link is a special case, one of the terminals, called the voltage setting terminal (VST), sets the DC voltage reference for all other terminals acting on either constant current or constant power control. The VST can have different control strategies such as constant angle (CAC), constant power factor (CPF), constant reactive power (CRP), constant reactive current (CRC), constant direct voltage (CDV), and constant AC voltage (CAV). A case study of an asynchronous two terminal HVDC link is presented to illustrate the analysis and compare the various novel converter control strategies