Coefficient Calculation Method of Virtual Resistance Based P-V Droop Control in VSC-MVDC System

The N-1 principle requires that, when one converter in the voltage sourced converter based medium voltage DC distribution system (VSC-MVDC) is out of operation due to failure or overhaul, the remaining system should be able to remain stable. In this case, the DC voltage change will become a serious problem. Droop control brings a feasible solution dealing with this problem. In this paper, a calculation method of P-V droop coefficient based on virtual resistance is proposed to optimize the droop control coefficient and control the DC voltage stability. Firstly, the relationship between the voltage of DC nodes and the voltage of point of common coupling (PCC) is analyzed. Thereafter, the relationship between bridge arm voltage of converter station and DC node voltage is established according to virtual resistance. The calculation method of P-V droop coefficient based on virtual resistance is then derived according to the above analyses. The calculation method was then analyzed at two extreme conditions and a control dead zone was found to exist at one extreme condition. Therefore, a corresponding solution that recalculating the droop coefficient by introducing the error of active power reference is proposed to improve the control strategy. Finally, a three-terminal VSC-MVDC model is built in PSCAD/EMTDC. Simulation results show that the proposed method for P-V droop control has a better performance as compared with general control strategies.