Unified Expression of Power Flow and Feasible Solution for DC Power Network with Multiple Control Strategies

The DC power flow of the AC/DC hybrid system with voltage source converter (VSC) is usually calculated based on power flow equations corresponding to the control strategy of the DC side VSC. Power flow equations differ from the different control strategies. With the development of flexible grids, a hybrid control strategy of the DC system, consisting of multiple single strategies will present with the result of inconsistent power flow expression structures and dimensions. In addition, the control strategy switching in the actual situation brings great inconvenience to the power flow calculation. Based on the analysis of different control strategies of DC-side VSC, this paper deduces the unified expression model of DC grid power flow for various control strategies by defining new DC node types. The model can represent various control modes of DC system and form a unified expression of the model with a unified structure and a fixed dimension. When the DC power grid control mode switches, only a small number of parameters need to be modified. The programming is simple and convenient to implement. The accuracy of the proposed algorithm is verified by an example of VSC DC power grid with 5 terminals and 6 lines. Finally, the algorithm flexibility is verified by the 7terminal VSC-MTDC DC network in the situation of control strategy switching.