Observability Analysis For AC-DC System Considering The Decomposition Of DC Systems

With the rapid development and construction of LCC-HVDC and VSC-HVDC systems in China, large scale AC-DC hybrid power systems have been established in recent years. The issue of state estimation and observability analysis for AC-DC system draws extensive attention.This dissertation focuses on the observability analysis of LCC-HVDC, VSC-HVDC and AC-DC system.The observability of LCC-HVDC, VSC-HVDC and AC-DC systems are analyzed based on the solvable condition of power flow. The DC system is decoupled and every converter station in DC system is considered as a unit to overcome the difficulty in merging and extending observable islands, when DC system is not observable as a whole but some converter stations of it still are observable. It is pointed out in this dissertation that AC voltage magnitude and injection measurements in the boundary nodes of AC system and DC system are coupling variables. After discussing the interaction between AC system and DC system, the conception of effective converter is presented. Combining with the DC correlation coefficient and the disposing rules presented in this paper, a new algorithm for AC-DC system observability analysis is proposed. Numeric test shows that the proposed algorithm is valid and efficient, and the influence of DC line on the convergence of AC-DC system state estimation is also presented.