Research on de-blocking/blocking operation in ±800 kV UHVDC system

Based on the design criteria for blocking/de-blocking of valve groups,this paper mainly studies the de-blocking/ blocking sequence and control strategies for groups under different circumstances.De-blocking the first group of a pole,simultaneously de-blocking both groups of a pole,and blocking the first group of a pole,simultaneously blocking both groups of a pole can be done the same way as that in a conventional bipolar HVDC scheme.The special coordinated sequence and control strategies for de-blocking the second group of a pole and blocking the last group of a pole are designed.In case of without telecom,de-block of second group of a pole is not recommended.In order to avoid unnecessary spill currents into the shutdown pole,the bypass switches of the blocked group is proposed to be left open.