Flexibility of transmission networks is influenced by the coupling of node flexibility and network transmission flexibility

and the balance between flexibility supply and demand dynamically changes with the fluctuation of sources and loads. Therefore

there is an urgent need for research into the coordinated planning of diverse flexibility resources in transmission networks. In this paper

considering the characteristics of dynamic changes in power flow within transmission networks

an index of network transmission flexibility is established

which reflects the dynamic matching degree between the transmission demand and the network's transmission capacity. Subsequently

a two-stage coordinated optimization planning model for diverse flexibility resources in transmission networks

incorporating energy storage devices and UPFC

is formulated. Finally

case studies verify that the proposed network transmission flexibility index can quantitatively reflect the dynamic changes in network transmission flexibility with source-load fluctuations. The two-stage planning model

optimized for network transmission flexibility

enables a fuller utilization of the existing grid's transmission capacity

thereby demonstrating better adaptability to higher levels of source-load fluctuations. The proposed flexibility importance index can provide guidance for the initial siting of flexibility resources.