Decoupled AC/DC Power Flow Strategy for Multiterminal HVDC Systems

This paper proposes a decoupled AC/DC power flow approach for multi-terminal HVDC systems. The proposed method simplifies the power flow computation of multi-terminal HVDC systems while accurately reflecting the operation and control characteristics of VSC (voltage source converter) stations in a HVDC network. In the DC network, the power flow calculation is conducted based on a slack DC bus VSC station and power commends issued to other VSC stations from the power system control center. Then, in the AC power flow calculation, VSC stations are treated as special AC generators that can generate and absorb power from the AC grid in active and reactive power or active power and bus voltage control mode. For validation purpose, the conventional unified power flow method for multi-terminal HVDC systems is built. The paper compares the proposed method with the unified power flow method for an 8-bus multi-terminal HVDC system based on MATPOWER. Then, more case studies for different VSC control modes are conducted and evaluated for the 8-bus system. Afterwards, the proposed method is applied to the power flow study of a more practical and complicated multi-terminal HVDC system based on the IEEE 118-bus system.