Hierarchical optimal power flow control for loss minimization in hybrid multi-terminal HVDC transmission system

A hierarchical control scheme is proposed for optimal power flow control to minimize loss in a hybrid multiterminal HVDC (hybrid-MTDC) transmission system. In this scheme, the lower level is the droop control, which enables fast response to power fluctuation and ensures a stable DC voltage, and the upper level is power flow optimization control, which minimizes the losses during the operation of hybrid-MTDC and solves the contradiction between minimizing losses and preventing commutation failure. A 6-terminal hybrid-MTDC is also designed and simulated in PSCAD according to the potential demand of power transmission and wind farms integration in China to verify the proposed control strategy. First, the steady state analysis is conducted and then compared with simulation results. The analysis shows that the proposed control scheme achieves the desired minimum losses while at the same time satisfying system constraints. The proposed control scheme also guarantees that the hybrid-MTDC not only has a good dynamic response, but also remains stable during communication failure.