The important step for transmission network planning is to carry out technical and economic check of the listed schemes

in which the importance of the technical check is whether the planned schemes can meet N-1 criteria.In the common AC power grid

the 《Guide on security and stability for power system》has defined the original copy of needing to meet N-1 criteria:Single element in case of normal operation mode

such as line

generator

transformer ect.In case of checking the planning scheme of power transmission grid containing flexible transmission device

such factors as equipment structure and the mode of connecting to the power grid shall be fully considered and the check range of N-1 check is reasonably defined.In this paper

the issue on how to define the single element in the N-1 criteria is studied on the basis of the feature of flexible transmission device.The suggestion on N-1 check criteria for the flexible device in the planning scheme of transmission grid containing flexible power transmission device is summarized by way specific c alculation verific ation.