This paper takes Chenzhou area power system as an example

and analyzes the problem of bad coordination of protection in three modes including multi-level supply

a substation with several 110 k V outgoing lines

T-type connection network structure between two tie lines. Superior power grid can not avoid the low voltage side bus fault causing override trip. To solve the above problems

this paper puts forward the corresponding countermeasures

which provides the basis for the correct coordination of relay protection.