This paper proposes an online power system voltage stability index (PVSI_Online) that quantifies power system voltage stability in real time between an AC network and LCC HVDC using only local measurements. Previous methods relied on using telecommunications to inform a change in a predetermined AC network model. The proposed method uses local measurements of voltage and current from the HVDC using a newly developed

numerically robust technique called Moving Window Covariance to detect a change in the AC network in real time. These network parameters are fed to a newly devised index that accurately models and captures the dynamics of the interaction between an LCC HVDC and AC network. The index is used to quantify stability

but also to advise network operation. In an onset of instability due to network degradation

the proposed index can provide a power order value for the HVDC to restore stability. All as a self-contained module that can be implemented within a HVDC scheme.