Evaluating Strength of Hybrid Multi-Infeed HVDC Systems for Planning Studies Using Hybrid Multi-Infeed Interactive Effective Short-Circuit Ratio

The strength is an important concept in planning studies for voltage stability analysis of hybrid multi-infeed HVDC (HMIDC) systems, which include both VSC- and LCC- HVDC links. However, the accuracy of earlier strength indices is very limited because they are developed by the empirical reasoning rather than rigorous theoretical derivation. In this paper, a virtual single-infeed model of HMIDC systems is firstly established utilizing the reduced-order equivalent nodal assessment of the inverter bus current-voltage equations. While incorporating the inter-inverter interactions, the virtual model is a strict dual-bus equivalent of the original complicated multi-infeed model. This enables easier and simpler analysis to be implemented. Secondly, the virtual model is employed to propose the hybrid multi-infeed interactive effective short-circuit ratio (HMIESCR) strength index for HMIDC systems by the rigorous theoretical derivation. Compared to earlier indices, the HMIESCR can depict the voltage stability much more accurately. This is because the critical HMIESCR (CHMIESCR) indicative of the stability limit is explicitly and solely evaluated to 1.5 according to the PV sensitivity based stability criterion, while the critical values of earlier indices are ambiguous. Finally, case studies by extensive numerical simulations on the classical hybrid four-infeed HVDC system validate the HMIESCR along with good robustness.