Keywords

1 Introduction

With the establishment of the goal of “double carbon”, the country for the first time put forward the new concept of “new power system with new energy as the main body” of the future grid blueprint [1]. The wide area measurement system can monitor the distribution network status in real time by using synchronous phase measurement technology, which provides a new scheme for the safe operation and stable control of the high proportion of new energy distribution network in the future [2-3]. The data measured by WAMS has three characteristics: time synchronization, spatial wide area and direct measurement of phase angle data, which provides data for the good control of power system [4]. Reference [5] analyzes the development of synchronous measurement technology at home and abroad and the future development direction of distribution network. In Reference [6], a new PDC with blade structure is designed to make it extensible. For Phasor Measurement Unit (PMU), intelligent substation platforms have applicability, low energy consumption, strong storage capacity, strong communication makes WAMS system more reliable. Reference [7] analyzes the communication mode and existing problems of the existing distribution network communication network, and proposes a communication scheme of hybrid optical fiber and power line carrier network. This paper will discuss WAMS communication network and access layer communication mode, and design a PDC that can process data from multiple channels. Finally, the PDC hybrid network experimental environment was built for verification.

2 WAMS Network

WAMS system is mainly composed of communication network, PMU, GPS, PDC and data center station [8]. WAMS collects phasor data through GPS and aggregates data from the entire power system through a communication network. In this way, the dynamic information of the power grid can be obtained to achieve the role of the monitoring system and improve the security and stability of the power grid. GPS synchronous clock provides a unified high precision clock signal for power system. PMU can unify the state quantity of different nodes and lines, and establish a connection with the dispatch center through the communication network, and save and transmit data in real time to ensure the synchronization of data of the whole network.

Distribution network WAMS communication network generally includes access layer and backbone layer communication. The backbone layer communication is the communication between the main station and the PDC, and the communication mode is mainly Synchronous Digital Hierarchy (SDH) fiber. Access layer communication is PDC to multiple PMUs of communication, there are fiber optic, PLC, wireless network and other communication methods mixed [9]. Most of the PMUs in the distribution network are installed on the lines and important nodes, a distribution network main station will connect a large number of PMUs, a single main station cannot process a large number of communication messages in a timely manner, will make the sent message conflict. The double-layer communication structure of master station connecting PDC and PDC connecting PMU can greatly reduce the communication pressure of master station and ensure the stability and reliability of data transmission.

3 Access Layer Communication Network Analysis

Compared with the backbone layer communication network, the coverage of access layer communication network is obviously insufficient. This is due to the restriction of economic and technical level, the degree of distribution network construction in different places is very different. Access layer communication mode can be divided into wired and wireless mode, wired communication mainly includes power line carrier, optical fiber, field bus. Wireless communication mainly includes 230 MHz wireless private network, wireless public network, 4G, 5G. Optical fiber communication is suitable for distribution network backbone communication or pre-buried lines, high transmission bandwidth, simple network is less affected by the environment, high reliability. However, the cost of fiber optic construction is large, and the construction and installation of old urban areas and economically backward areas is difficult. PLC communications can be transmitted using existing power lines without laying additional lines, and the installation is convenient and secure, saving costs, but real-time, reliability is not high. 230 MHz wireless network communication can save line investment, construction facilities and a wide range of applications, but low bandwidth coverage is small, real-time cannot be guaranteed. Therefore, a single means of communication cannot meet the existing distribution network communication needs. Only in the access network using a hybrid network, a variety of communication methods complement each other, and further improve the quality of communication.

4 Distribution Network PDC Software Design

The PDC needs to have up-and-down communication as an intermediate device between the primary and PMUs. PDC communication needs to meet the main and sub-station interaction processes specified in G.BT 26865.2-2011.There are two kinds of communication between master station and sub-station: real-time communication and offline communication. There are four data formats for real-time distribution network communication: data frame, head frame, configuration frame, and command frame [10]. The data frame contains information such as switching quantity, analog quantity, amplitude and phase angle. The head frame uses the ASCII code to represent information such as synchronous phase measurement devices, data sources, etc. The configuration frames are divided into CFG-1 and CFG-2, representing the output and configuration of the sub-stations respectively. The command frame is responsible for transmitting the instructions sent.

PDC devices should meet the functions of distribution network, dynamic data collection and storage, fault recording data storage, time-to-time and so on. In WAMS system, PDC mainly takes the role of PMU networking, PMU vector data collection and sending to the master station. The data aggregated by PDC mainly includes the configuration information of the underlying PMU, real-time data information and historical data information. Configuration information is generally used only before the PDC aggregates data, and the amount of data is small. Real-time data is continuously uploaded to the PDC at a fixed number of frames per second, data is sent frequently, the amount of data per PMU is small but the real-time requirements of uploading PDC are high. Historical data information is a historical event that records the PMU, is saved as a file, and the amount of data information is large but the upload time is long. Based on LINUX system, this paper uses libuv function based on event-driven asynchronous IO library to implement PDC software operation.

4.1 PDC Up and Down Communication Design

PDC communication is divided into upstream and downstream communication, upstream communication with the dispatching center master station, downstream communication with multiple PMU. PDC needs to build data channels, file channels, and command channels when communicating up and down the line. When communicating upstream, the PDC, as a server, needs to respond to a command request sent by the master and accept the configuration frames sent by the master. The communication flow of the PDC connecting multiple master stations when communicating upstream is shown in Fig. 1. When the PDC communicates uplink with multiple master stations, each master needs to be connected in turn. In the figure, n is the number of connected master stations. The IP and port number parameters are first configured for each master station to be connected to by the PDC through the for loop. The listening is then bound based on the IP and port number of the PDC. When a request for a connection is received and commands, data, and file connections are established, the PDC can communicate with each master.

Fig. 1.
figure 1

Upstream communication.

Full size image
Fig. 2.
figure 2

Downward communication.

Full size image

When communicating downstream, the PDC, as a client, is required to accept real-time data uploaded by multiple PMUs, offline data, and command requests to the PMU. The downstream communication process is shown in Fig. 2. In the figure, npmu is the number of PMUS connected to the PDC. When communicating downstream, the IP, command port number, data port number, file port number, and so on of each PMU to which the PDC is connected are first configured through the for loop. The program connects data, commands, and file channels based on the parameter configuration of each PMU. After the connection is established, the PDC will send command requests to each downstream PMU through the command channel to realize the real-time data upload of each PMU.

For the aggregation of real-time vector data, the libuv network interface API is used to implement. The libuv function used for PDC up-and-down communication is shown in Table 1.

Table 1. Libuv function table.
Full size table

4.2 Software Running Script

When the PDC program stops unexpectedly, it disconnects upstream and downstream traffic, making it impossible for PMU data to be uploaded in real time. The detection of PDC program is very important, and the detection function of the program needs to be realized through the script file. The script is primarily implemented by the ps-ef command in linux, which can view related activity processes. The specific script code is shown in Fig. 3.

Fig. 3.
figure 3

PDC run script.

Full size image

Diagram #! is a special representation, /bin/sh is the shell path to interpret the script, while loop means that the script keeps running. The fourth line in the figure indicates that the number of processes containing ‘pdc’ is viewed and assigned to procnum through the ps-ef command. The fifth line says if pronum equals zero, then proceed down, otherwise re-enter the path of the PDC and run the program. Set to check whether the PDC program is in running state every 10 s. The PDC program is not interrupted and the data is uploaded in real time.

5 PDC Mixed Networking Testing

Build the test environment shown in Fig. 4. Figure 4 synchronous clock device to PMU1, PMU2 to provide time-to-time function, PDC uplink through the network cable connection analog main station. The PDC downlink connects PMU1 and PMU2 via twisted pair cable and PLC. The test begins by simulating commands from the main station, summoning real-time data, and observing the frame rate of data transmission.

Fig. 4.
figure 4

Experimental environment.

Full size image

The communication parameters that simulate the master, PDC, and PMU in the test are shown in Table 2.

Table 2. Device communication parameters.
Full size table
Fig. 5.
figure 5

Master stationdata shows.

Full size image

The test results are shown in Fig. 5. When the master station sends the command correctly, the data channel connection is established to open the real-time data. From the figure, it can be seen that the data of the two PMUs converges in the PDC and is transmitted steadily to the analog master station at 50 frames/s. It is proved that PDC can mix network and carry out stable communication by PLC and twisted pair communication.

6 Conclusion

Based on the data transmission protocol of real-time dynamic monitoring system, this paper introduces the form of WAMS communication network, discusses the feasibility of the access layer hybrid network communication mode. Based on the libuv function, PDC software is developed to realize PDC up and down communication, and the data of multi-channel PMU is pooled and sent to the analog master station in real time, so as to ensure that the operation of the PDC program is not interrupted by script files. The up-and-down communication, twisted pair network cable and PLC networking function of PDC are verified by setting up the test environment of analog main station, PDC and multi-PMU.