Unified load flow calculation for flexible interconnected AC-MTDC distribution system considering control strategy switching with high penetration of DGs

Abstract A new unified algorithm is proposed in this paper to solve the power flow problem of AC distribution systems incorporating voltage-sourced converter (VSC) based multi-terminal direct current (MTDC) grids. In this method, the VSC modulation indices, mode changes, power losses as well as MTDC control strategy switching are all considered. Active power mismatch equations of VSC boundary buses in AC side have been modified to eliminate VSC power injection items. Various VSC control strategies are demonstrated in both active power function of DC boundary buses and reactive power function of AC boundary buses. In this way, only few modifications are required for existing AC load flow to incorporate the MTDC models. Moreover, the change of VSC operation mode and the switch of MTDC control strategies can be easily realized. The power flow calculation of AC and DC subgrids is solved simultaneously by Newton-Raphson (NR) method using a modified Jacobian matrix. Two case studies have been presented to demonstrate that the proposed algorithm is fast, flexible and effective in solving power flow of AC-MTDC distribution systems with various control strategies and can easily accommodate any changes in VSC control modes with high convergence performance.