Abstract: This paper discusses the requirements of distribution network automation system for communication system and the composition and characteristics of GPRS communication system. It discusses whether GPRS communication mode can meet the requirements of distribution network automation from three aspects: reliability, real-time performance and bidirectionality. According to the characteristics of distribution network, GPRS and fiber optic communication mode are compared. Keywords: distribution network automation, GPRS, communication 1 Introduction Distribution network automation system is a system that uses modern electronic technology, communication technology, computer and network technology to realize the monitoring, protection, control and distribution management of power distribution system under normal operation and accident conditions. Communication system is an important system in distribution network automation system. Distribution network automation system must use reliable communication means to send control commands from control center (master station) to various actuators or remote terminals, and at the same time upload various information collected by remote monitoring units (RTU, DTU, TTU, FTU, etc.) to control center (master station). The quality of communication system largely determines the quality of distribution network automation system. In order to meet the requirements of distribution network automation, communication system must meet the following requirements: (1) Reliability of communication. It must withstand the test of harsh environments for a long time, such as rain, snow, and strong winds; and be subject to strong electromagnetic interference or noise interference; in addition, it cannot be affected by power outages and faults. (2) Real-time communication. The distribution network automation system is a real-time monitoring system and must meet the real-time requirements. According to the requirements of the distribution system automation planning and design guidelines, the distribution network automation master station system should update all RTU, FTU, and other data within 3-5 seconds. (3) Bidirectional communication. In the distribution network automation system, there is not only data uploading, but also control command issuance. Therefore, the communication system must have bidirectional communication capability. In addition to considering the above three requirements, the construction cost of the communication system, the scalability of the system, and the convenience of use and maintenance should also be considered. At present, the distribution network automation system mostly adopts fiber optic communication. Fiber optic communication fully meets the requirements of the distribution network automation system for reliability, real-time performance, and bidirectional communication. However, the construction cost of fiber optic communication is high, and the scalability and convenience of maintenance of the system are poor. With the development of GPRS wireless communication technology, a new solution has been provided for the communication system of distribution network automation. 2 Overview and Features of GPRS System GPRS is short for General Packet Radio Service, which is a high-efficiency and low-cost mobile data communication service developed on the existing GSM system. It is particularly suitable for intermittent, bursty and frequent small data transmissions, and also suitable for occasional large data transmissions. The GPRS system has the following features: (1) Always online. GPRS provides the real-time online function "always on line". The wireless communication terminal device can automatically attach to the GPRS network as soon as it is turned on and establish a communication link with the data center to send and receive data at any time, with high real-time performance. (2) High-speed transmission. The transmission speed of the GPRS network will reach up to 171.2kbps, which can fully meet the needs of user applications. The speed of the next generation of GPRS services can reach 384kbps. (3) Simple, fast and flexible networking. The GPRS wireless DDN system can build a virtual mobile data communication private network covering the whole country anytime and anywhere through the Internet, providing access convenience for a large number of small and medium-sized users and saving access investment. (4) Billing by traffic. GPRS wireless communication terminal equipment is always online and charges according to the number of data packets received and sent. No charge is charged when there is no data traffic transmission. The billing method is flexible and can adopt the monthly data traffic billing method. (5) The communication link is maintained by professional operators. It eliminates the worries of communication link maintenance. (6) Lightning protection. The transmission power of GPRS wireless communication terminal equipment is very small, the antenna is very short, and it does not need to be erected. It overcomes the disadvantages of wired transmission and radio transmission that are easy to attract lightning and damage the equipment. 3 Composition of GPRS wireless communication system The GPRS wireless communication system consists of three parts: remote terminal, data transmission network, and monitoring center. (1) Remote terminal: The remote terminal is located at the location of equipment such as distribution network connection and sectionalizing switch. It is directly connected to the switch through RS-232/422/485 interface to realize the collection, storage and preprocessing of switch parameters. After being encapsulated and encrypted by TCP/IP protocol, the data is sent to the nearest BSS mobile base station of GPRS network through embedded GPRS module. It is transmitted to the monitoring center through GPRS network to realize the real-time online connection between the equipment and the monitoring center system. Meanwhile, the remote terminal can also transmit the remote control commands sent by the monitoring center to the switch control module to control the switch. (2) Data transmission network: The data collected by the remote terminal is processed and packaged and converted into a format for data transmission on the public network, and transmitted through the GPRS wireless data network. The GPRS network is connected to the GPRS operator router through SGSN (Serving GPRS Support Node) and GGSN (Gate GPRS Support Node). The GPRS operator router is connected to the monitoring center through a DDN digital data network leased line. (3) Monitoring center The monitoring center server applies for and configures a fixed IP address, uses a DDN leased line provided by the GPRS operator, and connects to the GPRS network. After the data is transmitted to the monitoring center, the received data is restored. 4 GPRS can meet the requirements of distribution network automation According to the requirements of distribution network automation for the communication system, the following discusses whether the GPRS wireless communication method can meet the requirements of distribution network automation from the aspects of communication reliability, real-time performance, and bidirectionality. (1) Communication reliability. The GPRS terminal equipment consists of a communication port, a transmission module, and a small antenna. The industrial-grade station equipment can fully meet the requirements for outdoor use. Since 2001, GPRS has been in operation. Operators have invested a lot of manpower, material resources, and financial resources in operation and maintenance, and the network is very reliable. At present, GPRS wireless communication has been widely used in power systems, such as distribution transformer integrated testing systems and load control systems. A large number of practices have shown that the reliability of GPRS wireless communication can fully meet the reliability requirements. (2) Real-time and bidirectional communication GPRS network access speed is fast, supports medium and high speed data transmission, and can provide a data transmission rate of 9.05-171.2 kbit/s (per user). It can recover data retransmission within 0.5-1 second. Each TDMA frame can be allocated 1 to 8 wireless interface time slots. Time slots can be shared by active users, and the allocation of uplink and downlink is independent. The design of GPRS makes it able to support both intermittent burst data transmission and occasional large data transmission. Each terminal device only collects voltage, current, and switch on/off positions. The master station only sends remote signaling, telemetry, remote control, and remote adjustment commands, with each data transmission volume within 10kbps. Theoretically, the GPRS network transmission rate can reach 171.2kbit/s, and the current actual GPRS data transmission rate is around 40kbps, which meets the data transmission rate requirement (≥30kbps). Furthermore, based on the actual system situation, the GPRS operator can allocate more wireless interface time slots to TDMA frames within the APN private network to ensure real-time and smooth data transmission. 5. Comparison of GPRS and Fiber Optic Communication A detailed comparison of GPRS and fiber optic communication methods is shown in the table below. Analysis of the table shows that GPRS has advantages over fiber optic communication in terms of construction costs, construction difficulty, networking methods, and scalability. Due to the numerous and widespread distribution network points and frequent changes, convenient scalability is particularly necessary for the communication system. In this respect, GPRS is far superior to fiber optic communication. 6. Conclusion In summary, the GPRS communication method can fully meet the communication requirements of distribution network automation. Currently, GPRS communication is not yet used in domestic power distribution network automation systems, although it is widely applied in systems such as distribution transformer integrated testing systems and load control systems. With the development of modern wireless communication technology, especially the operation of 3G (third-generation mobile communication system) networks, wireless communication can provide higher communication quality and data transmission rates. Wireless communication will be more widely used in power systems.