## 2 Distribution Network Reliability Evaluation Method
### Fault Impact Analysis
(1) Fault location  
The current sensing terminal usually uses the differential analysis method to locate the fault and determine the fault area according to the distribution of fault current. If there is only fault current inflow but no fault current outflow in a region, the fault is determined to be located in the region. The fault current flow direction is from the power supply end to the fault point. The sensing terminal detects the fault information and reports it to the distribution master station. The master station determines the fault area according to the fault information. If the sensing terminal has a communication failure, it may lead to an error in the location of the fault area. Taking Figure 3 as an example, when the line between switches S4 and S5 fails, switches S1, S2, S3, and S4 all flow through the fault circuit, and the sense-control terminal on the switch S5 does not detect the fault current inflow; then, the fault area can be determined to be S4 and S5. At this time, if the sensing terminal installed on the switch S4 fails and the fault information cannot be reported, the fault area will be located between S3 and S4. (2) Fault isolation  
On the basis of fault location, the feeder is divided into fault upstream area, fault downstream area, and fault area by a segmented switch. The power failure time of the upstream area of the fault depends on the type of sensing and control terminal configured by the inlet switch and the inlet switch of the fault area: if the inlet switch of the upstream area of the fault or the fault area is only configured with the TUs without tele-control function terminal, the power failure time is the manually isolated fault time \(t_{2,s}\); if the inlet switch of the fault upstream area and the inlet switch of the fault area are configured with the TUs with tele-control function terminal, the outage time is the automatic fault isolation time \(t_{2,r}\). The outage time of the downstream isolation area is related to the load transfer time, and the specific outage time is analyzed in the power restoration stage. The range of the fault isolation area depends on the result of the fault location. If the fault location is correct, the effective isolation of the fault area can be realized, and the power outage range can be reduced. If the fault location is wrong, it will lead to the expansion of the fault outage area. Taking Figure 4 as an example, if the sense-control terminal on the switches S4 fails, the fault area will be located between the switch S3 and S4, so the switches S3 and S4 are disconnected, and the fault feeder segment is not effectively isolated. Since the fault area and the non-fault area are still connected, when the closed contact switch C1 is used for load transfer, the sense-control terminal on the switch S5 will still detect the fault current. At this time, the switch S5 is disconnected, and the power outage area is expanded to the line range between S3 and S5. The final power restoration area is shown in Figure 5.  
(3) Power restoration  
During power restoration stage, it is necessary to close the contact switch to transfer the load to the downstream area of the fault. The power restoration time depends on the type of the sense-control terminal configured by the inlet switch and the contact switch in the fault downstream area. The outage time of the fault downstream area is the fault isolation time \(t_{2}\) and the load transfer time \(t_{3}\). If the contact switch is not configured with the sense-control terminal or is only configured with the TUs without tele-control function terminal, the load transfer time is the manual on-site operation load transfer time \(t_{3,s}\); if the inlet switch of the fault downstream area is configured with the TUs with tele-control function terminal, the load transfer time is the automatic load transfer time \(t_{3,r}\). Figure 3: Fault location error diagram.