The many years of operating experience of the power system shows that in the power grid powered by power cables, more than 70% of the faults occur at the middle of the cable, and the cable connector is the weakest link in the entire system. The presence of contact resistance, poor performance of insulating materials, or imperfect manufacturing processes are the main causes of frequent failures of cable connectors.
The various types of faults that occur at the cable joints are generally not a sudden process, but a gradual and quantitative change from qualitative to qualitative process. The beginning is due to the increase in the contact resistance to increase the temperature at the joint, so that the gradual aging of insulation, leakage current is gradually increased, will reach a certain degree of breakdown will occur. Therefore, by constantly and continuously monitoring the temperature changes at the cable joints, you can understand and master its operating status. When you find that the temperature at a joint is too high, and the temperature is very different from the ambient temperature, or changes quickly, it shows here Insulation is relatively weak, and continuous operation may cause serious failures. In this case, an alarm signal is issued in a timely manner to enable the on-duty personnel to deal with it in time, which can effectively avoid the occurrence of serious faults and ensure the system to work safely and reliably.
According to the operating characteristics and requirements of cable joints, a collection and distribution power cable connector temperature monitoring system integrating measurement, display, printing, alarm, and remote communication and other functions was designed.
Unified relay protection and automatic monitoring. Wang Xinchao, male, master, associate professor, born in 1962, graduated from Shandong University of Technology in 1995, research direction: power system relay protection and automatic monitoring. Wang Hui, female, college, inspection system.
The system is equipped with a temperature sensor at the middle of each cable connector. The output current signal is sent to the front temperature measurement unit through the auxiliary cable. According to the geographical distribution of cable connectors, each front unit can be connected with 8 to 48 sensors. Connection, the detection result of the front unit can be displayed on site to print, limit alarm, and can also be transmitted to the monitoring host installed in the main control room or dispatch center through RS-485 or other types of communication networks. The host can also be connected to other computers through a local area network or the Internet, so that all personnel can understand the working status of the cable joints of the entire system through the network.
3 The design of the front monitoring unit is suitable for the needs of different detection points. The front monitoring unit adopts a modular structure. Its functional block diagram is as shown.
It is mainly composed of 1~6 acquisition and processing boards, a human-machine interface management board and a power plug-in. The plug-ins communicate with each other through a common base.
4 Selection of sensors The selection of temperature sensors is an important part of the design of cable head temperature detection systems. The different types of sensors, detection methods and circuit structure of the detection equipment will also be very different, the merits of the sensor characteristics, have a great impact on the performance of the entire device.
The commonly used temperature sensing elements include infrared sensors, thermocouples, thermistors, semiconductor PN junctions, temperature-controlled thyristors, and integrated circuit type temperature sensors. The biggest advantage of infrared is non-contact measurement, which has a good safety, the disadvantages of which are complex structure, poor anti-interference ability, high environmental requirements, and should not be used; thermocouple transmission signal requires a special compensation line, and the transmission distance is not appropriate Too long to adapt to the wide distribution of the cable head; the thermistor is usually a platinum resistance, generally requires three-wire transmission, balanced bridge output, the transmission distance should not be too long, and the ability to resist interference is poor Semiconductor PN junction type and temperature-controlled thyristor type sensors also have such problems, which are not suitable for the detection of cable head temperature; there are many types of temperature measuring elements of integrated circuit type, among which current output type elements have great Internal resistance (>10M), suitable for long-distance transmission, strong anti-interference ability, usually small size, can be sealed at the measured point with anti-corrosion moisture-proof and high-temperature resistant thermal silica gel, and the external wiring is led by two lines. No need for working power, more suitable for cable head temperature measurement needs. After repeated comparisons, the current output type integrated circuit temperature measurement module AD590 is selected as a temperature sensor.
The program's menu area provides nine main menus, which are: File, Settings, Query, Adjustment, Writer, Print, OLE, Attachments, and Help. They are briefly described as follows: The file menu mainly completes the temperature data (in this program The formation of a specific data file operation processing, such as open data files, save data files, delete data files, etc.
The setup menu mainly completes the setting work, such as setting the number of connectors, connector name, alarm temperature, form color, and communication parameter setting of 71994-2015 China AcademicJournalElectronic. The communication parameters include serial port, baud rate, parity, stop bit and communication. cycle.
The query menu corresponds to the setup menu and is used to inquire about the setting contents completed in the setup menu such as the road name, the number of routes, the alarm temperature, and the communication cycle. In addition, the menu can also query the temperature value, fault report, and disk. The type and space of the disk. Click the temperature submenu, the temperature value query form appears, and the temperature of each joint can be queried through the warm value query form, and whether the temperature can be checked or not is limited. Click on the fault report submenu. The fault report window appears. The fault report window is similar to the main window but it only shows the details of the over temperature limit connector.
The adjustment menu is used to adjust some of the setting information. Since the new cable is often added in real life and the old cable is removed, the menu can be adjusted by adding cable connectors, deleting cable connectors, and changing the name. The corresponding sub-menu is to add nodes, delete nodes, modify the name.
5 Conclusion Monitoring and measuring the temperature of the cable connector can find out the hidden troubles in time and effectively reduce the number of failures. It is an important technical measure to improve the reliability of power supply. The distributed temperature monitoring system designed in this paper has features such as multiple measuring points, strong functions, high precision, and easy use. The products have been operating for many years in units such as Qilu Petrochemical Company with good results.
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