Study on Block Strategy Optimization of Passive Isolated Island Operation of DXB UHVDC Project

Due to the lack of generators at sending end in the early operation stage of the DXB UHVDC project, if it passively entered into island operation mode, the system will encounter harmonic instability and will cause a great risk to the steady operation. In previous projects, the common solution is to disconnect AC filters and block bipolar DC with a certain time delay when receiving island operation signal. However HVDC system may block valves, cut generation capacity or shed power by error when receiving false island operation signal. Therefore, the blocking strategy at low power under passive island mode has been improved. In DXB UHVDC project, the power on AC tie lines is used as criteria for island operation to prevent false action. The logic and realization of the control strategy are illustrated, then RTDS simulations are carried out and it is verified that by judging the power on AC tie lines, the system can effectively prevent from entering island mode by error. Finally, the field test proved that the proposed improved strategy will not cause rejecting of the existing “island blocking logic”. The proposed strategy can be a reference to other DC projects.