Optimal Hierarchical Voltage Control for VSC-MTDC Distribution Network

This paper proposes an effective optimal hierarchical voltage control for VSC-MTDC distribution network. In this scheme the lower control layer used droop control strategy which enables quick response to power fluctuation and ensures a stable DC voltage, and the upper control layer used optimal power flow (OPF) based hybrid particle swarm optimization (HPSO) to efficiently find the optimal reference values of the dc voltage and active power for the voltage-regulating converters in order to minimize the losses during the operation of the flexible MTDC distribution network. In this control structure, the N-1 constraint is added to the upper control layer in order to avoid the DC voltage of a converter station to exceed its DC voltage operating limits. In case of variation in load or generation of the grid, a new stable operating point is achieved based on the proposed control strategy for optimal operation of the MTDC grid. A five terminals VSC distribution network is designed and simulated in PSCAD. First, the steady state analysis is conducted, and then dynamic simulations results are analyzed. The analysis shows that the proposed control strategy achieves dc voltage stability, power balance and minimum losses in a safe manner.