Power flow analysis of AC-DC networks considering hierarchical connection technique

Abstract Hierarchical Connection (HC) structure is considered as a secure and economical technique for HVDC to simultaneously access multiple interconnected Alternating Current (AC) networks with different voltage grades. In this study, equivalent model of HC is established to generate its static formulations and the equivalent model can be applied in HVDC grids. Then the traditional integrated AC-DC power flow model is improved by introducing the relations and parameters of HC. Moreover, for certain DC control strategy in the case of fixed control variables, equivalent power injection into DC system is expressed as functions of DC parameters and an equivalent method is derived to solve AC-DC power flow. With the equivalent method, quantities of involved power flow equations are effectively reduced, thereby decreasing the computation burden. Power flow calculation and VQ sensitivity analysis based on the improved integrated method and equivalent method are separately carried out. A simple method for calculating VQ sensitivity for systems integrated with HC-HVDC is also proposed.