## 1 Introduction
For example, by placing a PMU device at the grid connection of the microgrid and monitoring both sides of the grid connection point, the problems of grid connection control and island detection can be solved more easily. Additionally, with the development of WAMS technology, the PMU is able to achieve a higher reliability, compact size, and lower cost. It is, thus, promising for PMU to be widely adopted in the distribution network in the future. Therefore, an increased number of scholars have begun to study the application of PMU to the distribution network. Previous studies [12; 13; 14] have discussed the tasks that the PMU in the distribution network can accomplish, including stability analysis and monitoring of the distribution network, protection under the condition of DER introducing bidirectional power flow and phase angle deviation, distribution network status estimation, voltage control, fault location and detection, load rapid prediction, load modeling, harmonic estimation, parameter estimation, closed-loop operation of feeders, etc. Other studies [15; 16] have analyzed the application of PMU in the industrial distribution network, including PMU optimal layout, PDC data collection, and PMU status monitoring of industrial parks. In addition to discussing the tasks of the PMU in the distribution network, for the main network and the distribution network PMU, they all have problems, such as massive data transmission, communication delay control, and network security in the communication process. A previous study [17] proposed a WAMS communication resource allocation method based on optimal dynamic time division multiple access (TDMA) resource allocation. Another study [18] proposed a real-time data compression and adaptedprotocol technology for WAMS, where the compression algorithm exception swing door compression (ESDC) is generated by the combination of exception compression (EC) and the swing door trending (SDT) algorithm. Adaptive protocol technology refers to the transmission of compressed data by improving the format of data frames defined by Institute of Electrical and Electronics Engineers (IEEE) C37.118.2. Another study [19] proposed a method for combating traffic analysis attacks in WAMS. In this study, based on the priority of PDC receiving data packets, the data packets are processed and transmitted to ensure the timely transmission of data. Among the above references, one study [18] improved the data frame structure to compress transmission, reducing traffic, but did not add new frame types to transmit more kinds of information, which caused additional delay. The priority of the packets discussed in another study [19] were based on the remaining time of the packet, without considering the different data inside the packet, and the need for latency may also be different. In order to meet the application requirements of the active distribution network and solve the problems that the existing transmission grid synchronous phasor transmission protocol will encounter in the distribution network, such as an insufficient frame type and different data delay requirements, this paper proposes a priority-based protocol extension method. This method increases the type of frame, determines the priority of the data, and increases the file type, thereby distinguishing the data required for various applications with different delay requirements. The method can also reduce network traffic and storage, and ensure that phasor data is transmitted preferentially. The organization of this work is as follows. Section 2 compares the distribution network SCADA and the transmission network WAMS, and explains the applicable scenarios, functions, and performance requirements of the micro multifunction phasor measurement unit (uMPMU). Section 3 compares National Standard of the People's Republic of China/Recommended (GB/T) 26865.2-2011 and International Electro technical Commission (IEC) 60870-5-101/104 statutes, summarizing their content to meet the needs of active distribution network communication.