An operation-failure of line protection at fault is analyzed in this paper

which has led to the voltage collapse of a 330 kV substation. The results show that under the integration configuration of primary and backup protection

if primary protection fails to operate for reasons of the protection device itself or related equipments when fault occurs

the backup protection is also hardly operable

while only the backup protection in the adjacent line can be relied on to remove faults. Unfortunately

the adjacent line protection is usually unable to operate because of insufficient sensitivity since it is based on the local backup mode

which frequently leads to further expansion of the accident. To solve this problem

the combination of local and remote backup is introduced into the extra high voltage(EHV)transmission line protection. Accordingly

distance section Ⅲ is employed as backup protection for the adjacent line and adequate sensitivity to it is required. Finally

the setting calculation method for distance section Ⅲ is presented and problems for attention in engineering application are discussed as well. This scheme has greatly improved the ability of line protection to protect the power system.