The present network security assessment is generally preformed separately at HV distribution network and MV distribution network. Planning the network based on this principle can cause redundant reserve in the two level networks. To solve the problem

a new method is proposed

which combines HV and MV distribution networks with an integrated security assessment. Firstly

the current basic principle of HV and MV distribution networks N-1 security assessment are analyzed. According to the load transfer capability in MV distribution network after HV distribution network N-1 contingencies

the HV and MV distribution network integrated load transfer principle is proposed. Then

the integrated security assessment model is built. Finally

a case is used to test the validity of the proposed method. The results show that the security of power network can be promoted by integrated security assessment in the case of the absence of HV load transfer path or the HV load transfer capability limited. When the traditional HV network N-1 assessment is in failure

the load transfer capability in MV distribution network can ensure user’s power supply. The study can make use of the quick load transfer capability with the MV distribution network automation upgrading currently

and excavate the security potential by the cooperation between two level networks.