Existing reliability evaluation methods are mainly concerned with the medium voltage distribution network and transmission network but unsuitable for the high voltage distribution network since in general only its first-order fault is considered. For the medium voltage distribution network

multi-faults involving long execution time for the transmission network have to be taken into account. The state space truncation is studied from three aspects: the fault order

the network topology structure and the outage time ratio. Firstly

based on the reliability impact analysis of multi-faults from a high voltage distribution network

it is pointed out that the reasonable maximum fault order for the reliability evaluation of a high voltage distribution network is the second-order. Then

a network topology search method is proposed to quickly determine the valid branch combination for the second-order fault

and a component combination tree is created to determine the valid component combination for each valid branch combination. Finally

if the outage time ratio for a valid state of second-order fault is less than the threshold

the state can be ignored. In addition

a method is proposed to quickly obtain the isolation scope of a second-order fault based on the derivation of the corresponding first-order faults' isolation scopes. The proposed method can effectively cut off the state space and avoid the redundancy topology search when searching for the isolation scope

which is of great significance to the effective reliability evaluation of the high voltage distribution network.