Design of Power Quality Monitoring System Based on Network

0 Introduction

With the systematization, intelligence, automation and networking of power system operation and management, remote real-time monitoring and automatic debugging of power grids are the inevitable trend of power system development. In recent years, with the continuous growth of people's demand for electric energy, power electronic equipment has been more and more widely used. The commissioning of a large number of nonlinear loads and impact loads has caused a large number of harmonic interferences, voltage waveform distortion, voltage fluctuations and three-phase imbalance in the public power grid, and the power quality has continued to deteriorate. In order to realize real-time monitoring and accurate dispatch of the power system, it is very important to fully grasp the power quality status in the power grid and conduct rapid and accurate testing of power parameters. This paper proposes a network-based power quality monitoring system (hereinafter referred to as the "monitoring system"), which can not only realize real-time collection and analysis of field data, but also remote monitoring and control through the network, which helps to solve the problem of difficult to conduct accurate testing on site due to harsh field environment.

1 Overall design of monitoring system

The system is used to monitor various power parameters in the power supply system line. According to the characteristics of the measured parameters, the corresponding sensors are used to collect the voltage and current signals of the measured points through the data acquisition circuit, and then the voltage, current, frequency, harmonic content, distortion coefficient, crest factor and other power parameters of the AC power supply system are calculated based on the collected measured point data. The monitoring system mainly includes management computers, monitoring computers, PXI data acquisition systems, signal conditioning, industrial Ethernet switches and printers, etc. The overall structure of the system is shown in Figure 1.

2 System Hardware Design

The management computer is mainly used to manage the system's operating status and data, coordinate the system's operation, provide users with on-site real-time monitoring conditions, and realize unified monitoring and data management. The on-site monitoring computer mainly monitors the signals of the measured points of the power supply system, performs data collection and data analysis; the PXI data acquisition system mainly realizes the acquisition of real-time data of voltage and current from the measured lines. According to the monitoring requirements of the measured site, two NI PXI data acquisition cards PXI-6254 are selected. Each board has 32 analog input channels, with a total of 64 analog input channels. The power supply on-site monitoring computer can analyze the data collected by the PXI data acquisition system, generate reports, and upload them to the distribution system management computer through the Ethernet switch according to user needs; the distribution system management computer has a fault alarm function, and can also receive data from the on-site computer through Ethernet for data analysis, replay the monitoring fault signal, and use it to find the cause and solve the problem.

The data acquisition module is mainly composed of signal conditioning circuit, hardware synchronization circuit, A/D conversion circuit and other parts. The signal conditioning circuit mainly completes the function of conditioning the voltage and current signals of the measured point to the range that the acquisition card can bear. The conditioning circuit is mainly composed of voltage transformer, current transformer, operational amplifier circuit, follower circuit, etc., which realizes the electrical isolation of external power high voltage and large current signals from the system, reduces external electromagnetic interference, improves electrical safety performance and reliability, and performs linear transformation on the input signal. The signal conditioning circuit is shown in Figure 2. The current conditioning uses precision resistors to convert the current signal into a voltage signal, and then sends it to the input end of the conditioning circuit. Finally, the software module in the system is used to collect data on the conditioned signal.

3 System Software Design

3.1 Introduction to LabVIEW Software

In this test system, NI's LabVIEW7.1 is used to develop system monitoring application software. LabVIEW is an efficient graphical application development environment. It combines the advantages of easy-to-use graphical development methods and flexible and powerful programming languages. It is easy to achieve seamless connection between software and hardware. After completing data acquisition, you can use the powerful data analysis module provided by LabVIEW to complete data processing and display, and you can efficiently create stable software code. Based on the previous single-machine test, secondary development was carried out, some special processing modules were developed, and the network communication technology of LabVIEW was successfully used to enable the management computer and the test computer to exchange network data.

3.2 Monitoring system software design

In order to save memory resources, in addition to using DataSocket technology to share and publish real-time data, the system status monitoring software also uses Lab-VIEW's unique VI Server technology, using the library function programming it provides to achieve dynamic control of the program, ensuring that the VI is loaded into memory only when needed, reducing system memory usage and improving operating efficiency.

The monitoring system application software is divided into two parts: management computer application software and on-site monitoring computer application software. The management computer application software runs on the management computer and mainly completes the coordination and management functions of the power supply system, and can realize functions such as fault alarm; the on-site monitoring computer application software runs on the on-site monitoring computer and can complete functions such as real-time monitoring, data storage, historical playback, report generation and data information upload.

The management computer application software is mainly composed of four modules, and the program structure diagram is shown in Figure 3. The function of the system configuration module is to set the RS485 bus communication parameters and Ethernet parameters; the power parameter analysis module is mainly used to retrieve the relevant data collected by the on-site monitoring computer through Ethernet for analysis when a fault alarm occurs on the on-site monitoring computer, and then perform fault diagnosis based on the analysis results, thereby eliminating system faults and ensuring power supply; the system status monitoring module is mainly used to display the operating status of the monitoring system.

The field monitoring computer application software consists of four main modules: data acquisition module, data analysis module, fault alarm module and local waveform monitoring module. In order to realize the monitoring of the monitoring line in the power supply system, the field monitoring computer must first collect the signal of the monitored line through the data acquisition module, and can use the data analysis module for analysis, such as analyzing the steady-state/transient parameters of voltage and current, and can realize harmonic analysis, distortion analysis and power factor analysis of the power grid, etc., and then automatically judge whether there is a fault according to the analysis results. If there is a fault, the fault information is uploaded to the management computer through the fault alarm module, and the relevant data is saved at the same time; if there is no fault, the data is abandoned. For the convenience of monitoring, the software can also record data first, and then analyze the characteristic parameters, or directly enter a single module to analyze the parameters directly. The local waveform monitoring module synchronously collects the waveforms of multiple channels in real time, and can send the field monitoring results to the management computer through the network for users to monitor at the remote terminal. The data acquisition module, data analysis module and local waveform monitoring module all include functions such as data storage, playback, report generation and printing. The program flow of the field monitoring computer is shown in Figure 4.

4 Application of remote communication technology in monitoring system

Remote monitoring of power supply systems is mainly achieved by using network communication technology. Remote panels and DataSocket communication are data communication methods developed by LabVIEW to adapt to the development of industrial Ethernet. Remote panels support direct operation of VI front panels located on remote computers, while DataSocket can be used to publish test data in real time at high speed.

1) Remote panel technology

In order to realize real-time monitoring of the status in the remote site, the system adopts remote panel technology. When performing remote monitoring, LabVIEW's remote panel technology allows users to monitor the situation on site through the network in the office computer. Remote panel technology can realize remote monitoring of the power supply system by setting relevant parameters in LabVIEW. This technology allows users to directly open and operate the VI front panel located on the remote computer on the client.

2) DataSocket Technology

With the continuous development of modern science and technology, using Web Server to spread the dynamic images of VI on the Internet can achieve satisfactory results. Lab-VIEW provides a method for real-time high-speed data exchange on the Internet - DataSocker, which is mainly oriented to measurement and real-time high-speed data exchange on the Internet. It can be used for data exchange between multiple applications in a computer or in the network, thus meeting the needs of remote field monitoring.

5 Conclusion

The network-based power quality monitoring system proposed in this paper can not only complete the test of the electrical parameters of the power supply system, but also enable users to monitor and control the site at the remote terminal. Through experiments, it is found that the monitoring system has the characteristics of stable operation, high measurement accuracy, good real-time performance, simple operation, and easy maintenance. The system can meet the requirements of remote monitoring and control of the power supply system.