## 1 Introduction
As the topology of primary distribution networks can change frequently, particularly in China, due to fast development of its distribution networks, this paper derived a method of automatically changing the setting of current differential protection when the topology of primary distribution networks changes. Feeder automation technology is an important mean to improve the standard of fault handling in distribution networks. At present, various types of feeder automation have been used, including voltage-time type [11], closing quick-break type [12], intelligent distribution type, and centralized type [13]. These types are all aimed at the faults with large fault current. To solve the problem existed in the voltage-time feeder automation that the fault processing time is long, the upstream switches of the fault point need to tolerate fault current twice, and the upstream sections need to experience two short-time power outages, [14] proposed a quick fault location and isolation method for distribution network based on adaptive reclosure. However, none of these methods can really provide a fast, sensitive fault location function. They have no adaptive auto-switching function which can automatically change its switching procedure if the topology of the primary distribution networks changes. In the 10 kV distribution networks in China Southern Power Grid, normally circuit breakers are only installed on the outgoing feeders at the 10 kV substations, and load switches with DTU are installed on the ring main unit, as shown in Figure 1. As the load switch is only capable of breaking load current, when a power system fault occurs, the circuit breakers at the 10 kV substation will operate first to break the fault current. After the fault is located, then the load switches which are the nearest to the fault location will open to isolate the fault. After the fault is isolated, the circuit breakers at the 10 kV substation will re-close to restore the power supply to the healthy part of the network. For the traditional automation system, as there is no fast fault location function, after a power system fault is cleared, it takes minutes or even hours to manually locate a fault and isolate a fault; the restoration process is slow after a feeder is tripped out when a power system fault occurs. Therefore, it becomes necessary to find a quick fault location method and automatic closing strategy to restore power supplies to customers after a power system fault. As current differential protection is a unit protection, it can be easily applied for fault location. This paper presents an adaptive current differential protection and a fast auto-closing system developed for the 10 kV distribution networks in China Southern Power Grid. The current differential protection can adaptively change its settings according to the topology change of the primary distribution networks, thus the system effectively reduces the operation and maintenance cost of the power distribution network. As load switches are installed on the 10 kV feeder, the current differential protection function is used for fault location. The fast auto-closing function is implemented in the existing DMS (Distribution Management System). When a fault occurs on the 10 kV feeder, the current differential protection can locate the fault quickly and accurately, and the fault can be isolated quickly. After the fault is isolated, the auto-switching function in the DMS can quickly restore the power supply to the healthy part of 10 kV feeder in less than one second.