## 1 Introduction  
Most protection installed in China 10 kV distribution networks is mainly based on the simple three stage overcurrent protection. As more and more distributed new energy sources are connected to the distribution networks, the conventional radial distribution network is becoming an active distribution network. It has the characteristics of multi-source, multi-branch, bidirectional power and fault current flow, as well as weak infeed. It is very difficult or impossible to apply the conventional overcurrent protection in the distribution networks with distributed generations, so adaptive overcurrent protection has been developed for distributed generation [1; 2; 3; 4]. As this type of protection is still basically an overcurrent protection, it is very difficult to provide a sensitive, fully satisfactory protection for distribution networks with distributed generations under all operation conditions. Therefore, current differential protection has been considered for application in distribution networks with distributed generations [5; 6; 7; 8; 9]. Compared with the traditional overcurrent protection, the principle of line current differential protection is simple. As it is a unit protection, the protection zone is defined, it does not require time grading. Its protection performance is also not affected by distributed generation connected in the distribution networks. It can be applied for fast main protection or fault location in the distribution networks. Current differential protection requires reliable communication. With the development of 4G LTE (Long-Term Evolution) wireless communication, it is possible to apply the current differential current protection using the 4G LTE wireless communication in the distribution networks. In recent years, research has been carried out on the application of current differential protection for distribution networks with distributed generations. Reference [5] analyzed the protection problem of distributed energy resources (DER) access to distribution network and proposed a differential protection scheme for active distribution network. The scheme uses centralized line differential protection and decentralized bus differential protection. DTU (Distribution Terminal Unit) uses built-in Ethernet port technology and dedicated fiber-optic communication channels for data transmission and simultaneous sampling. Reference [6] proposed a unit-type protection scheme that utilizes the principle of differential current protection in urban distribution networks. Multifunction line differential relays provide basic in-zone unit protection, local and remote standby overcurrent protection, and dedicated ground fault protection for downstream power systems to improve line-to-ground fault sensitivity. Reference [7] proposed a current differential protection scheme which uses positive-sequence fault component for the protection. Reference [8] proposed a pilot protection scheme which compares the phase angle variation of positive-sequence fault current; this scheme only needs to transmit numerical value of phase variation, therefore, communication channel of high synchronization is not required. Reference [9] derived a differential criterion of current amplitude; the criterion just needs current amplitude information without strict synchronization of data sampling. Reference [10] proposed an integrated protection and control system for 10 kV feeder applications based on PTN (Packet Transport Network) communication. The system uses centralized line differential protection and decentralized bus differential protection. It can be seen from these publications that these current differential protections for application in distribution networks cannot adaptively change their settings automatically if the topology of the primary distribution networks change.