Online monitoring and protection of voltage stability for LCC based HVDC system

AC-DC interaction is a common phenomenon whenever HVDC network is weakly connected to AC system. Sometime it leads to severe voltage stability problems on AC bus depending on AC system capability and available amount of VAr compensation. For instance, there are many examples of offshore wind farms connected to onshore grids through HVDC links. When onshore grids may be stressed during contingencies, the volatile wind generation may result in voltage stability problems at the onshore bus. An on-line assessment of proximity to voltage collapse is a critical need of the hour so that commensurate control actions can be taken to prevent it. The work presented here addresses this need by proposing a framework wherein local phasor measurements are used to compute the Voltage Stability Index (VSI). Criterion Index L is also computed for the same. The key idea is to represent the HVDC link as an equivalent AC load. Simulation studies on Single-Infeed and Multi-Infeed systems showcase the effectiveness of the proposed approach. A comparison with existing assessment methods, short circuit ratio based technique and voltage stability factor, computation is also presented. The proposed technique is easy to implement, has low computational complexity and is amenable to inclusion in a general control scheme.