Design of infrared online temperature measurement system for 20kV gas-insulated metal-enclosed switchgear (GIS)

    For the power industry, strengthening its own energy conservation and consumption reduction is of great significance to the development of circular economy, the rapid construction of a resource-saving society, and the promotion of economic development. Therefore, while the country is developing 1000kV and above ultra-high voltage, it is also actively researching and optimizing the medium voltage level and developing the 20kV voltage level.

     Compared with 10kV voltage, under the same cross-section and current density of different lines, when transmitting the same electric power, the reasonable transmission distance of 20kV voltage can be nearly doubled, and the economic load of the line can be increased by about four times. Under the premise of transmitting the same distance and the same power, the power loss is reduced by 50%, and the power loss is reduced by 75%, which can reduce the number of distribution outlets in the substation by half, and avoid the difficulties such as excessive poisonous lines in the densely loaded areas. Under the condition that the rated current and short-circuit current are the same, the use of 20kV voltage can double the capacity of the substation compared to 10kV voltage, and the number of step-down substations in the same area can be reduced by half. Therefore, Jiangsu Power Supply Company promotes 20kV voltage level power supply in the company's system medium-voltage distribution network, and actively adopts 20kV gas insulated metal enclosed switch (GIS) equipment to further reduce the loss of conventional cables and improve the power supply capacity. In response to the new situation of economic development and power grid development in our province, it improves the power supply capacity and adaptability of the distribution network, reduces the loss and power supply cost of the distribution network, and saves land resources.

2    Characteristics of 20kV gas-insulated metal-enclosed switchgear

     Gas-insulated metal-enclosed switchgear (GIS) is a system in which high-voltage components such as circuit breakers, isolating switches, grounding switches, and busbars are concentrated in a sealed metal box. Low-pressure SR gas is used as the insulating medium inside the box, making the entire high-voltage system completely closed and unaffected by external environmental conditions.

     The technical content of 20kVGIS is high. It is the product of organic combination of SF6 insulation technology, sealing technology and air-insulated metal-enclosed switchgear manufacturing technology. SF6 is a colorless, odorless, non-toxic, non-flammable inert gas with excellent arc cooling characteristics. It is widely used in millisecond electrical equipment as a cooling and arc extinguishing medium. 20kV GIS uses low-pressure SF6 gas, solid insulating materials and specific insulating structures to seal high-voltage conductors and high-voltage components to meet the requirements of rated insulation levels; through the sealed shell

The reserved socket holes are connected to the wires through plug-in cable terminals to ensure safe and efficient transmission of electric energy. The vacuum breaking technology is used to control and protect the load current and short-circuit current of the line. It is particularly suitable for places with high requirements on size and reliability and relatively harsh natural conditions, such as industrial and mining enterprises or urban subway substations with limited reliability requirements and small distance from the base.

3 Necessity of developing GIS infrared online temperature measurement system

3.1 The importance of GIS temperature measurement and the problems existing in current temperature measurement methods

     When the contact of GIS equipment is in poor contact, the contact resistance becomes larger, and heat will be generated when the current flows through. Overheating of contacts and busbars will cause insulation aging or even breakdown, resulting in short circuits and major accidents. Therefore, detecting and monitoring the temperature of GIS equipment contacts, busbars and disconnecting cable joints is of great significance to the safe and reliable operation of GIS. According to incomplete statistics, GIS used by many domestic power generation companies and power companies have experienced abnormal temperature changes in closed busbars, disconnectors, cable heads and other components due to insulation aging or poor contact to varying degrees, which in turn caused accidents. For this reason, the power grid company has formulated special technical specifications, requiring temperature monitoring of overheated parts of GIS equipment to achieve temperature limit alarms to ensure the safe production of power equipment and the reliability of equipment operation.

     GIS is a fully enclosed high-voltage live equipment, and it is inconvenient to measure the internal part of the equipment at the heating point. At present, the main method is to open a monitoring window on the GIS cabinet, and then use a thermal imager or a point thermometer to conduct regular inspections of the equipment. However, thermal imagers or point thermometers can only reflect the heating conditions of the contacts at the time of inspection, and cannot achieve online real-time temperature measurement, let alone timely discover the continuous changes in the temperature field, which is not very helpful for timely discovering equipment hidden dangers. In addition, there is also a method of temperature monitoring through optical fiber transmission, but since this method requires the temperature sensor to be close to the surface of the object to be measured, it belongs to contact temperature measurement, which will undoubtedly cause certain hidden dangers to the safe operation of GIS.

3.2 Characteristics of GIS infrared temperature measurement system

(1) Principle of infrared temperature measurement

     All objects in nature with a temperature higher than absolute zero are constantly and spontaneously radiating infrared rays. Infrared radiation is an electromagnetic wave with a wavelength between 0.76 and 1000 lxm. The physical essence of infrared radiation is thermal radiation, and its thermal effect is much stronger than that of visible light, but human eyes are not sensitive to infrared rays, so it must be received by an infrared detector that is sensitive to infrared rays. The infrared radiation emitted outward transmits energy at the speed of light, and the size of the radiation energy and the distribution of its wavelength are closely related to its surface temperature. For black body materials, the energy of the radiation completely determines the temperature of the object. The higher the temperature of the object, the greater the energy of the infrared radiation emitted, and as long as the temperature changes slightly, it can cause a significant change in the radiation energy. Therefore, by measuring the infrared energy wall radiated by the object itself, the surface temperature of the object can be accurately determined.

(2) Introduction and features of this system

     The operation of GIS is inevitably accompanied by changes in the internal temperature field. Based on the principle of infrared temperature measurement, this system embeds high-precision, non-safe infrared temperature sensors in the air chambers of each GIS, and detects the temperature field changes of each node in the air chamber in real time. Through the corresponding hardware and software design, the detection results are reflected in the main control computer of the substation integrated automation system, thereby realizing online monitoring of GIS temperature changes, making judgments on phenomena such as contact overheating, and issuing different levels of alarms according to the degree of temperature rise to notify the operating personnel to take unconnected measures.

     Compared with the commonly used GIS temperature measurement methods, this system has obvious advantages when applied to 20kVGIS: ① Due to the compact internal structure of 20kVGIS, there are not many contacts that need to be measured, so this system only needs a few infrared temperature sensors to achieve online monitoring of the entire GIS, which simplifies the system structure and greatly reduces costs. ② Use intrinsically safe infrared temperature sensors. The non-contact temperature measurement method will not have any impact on the safe operation of GIS. ③ The infrared sensor has strong anti-interference ability, and can achieve high temperature measurement accuracy when applied to a lower electric field of 20kV, and can simplify the design of the anti-interference system.

4 Basic structure of the system

     This system consists of three parts: signal acquisition unit, data acquisition unit, and data monitoring management unit. The bottom layer of the signal acquisition unit is an infrared temperature sensor. Corresponding infrared temperature sensors are installed on the nodes that need to be monitored in each GIS air chamber, and the temperature data is transmitted to the data acquisition unit through the data line; the data acquisition unit is responsible for summarizing the data, and at the same time, the temperature data and alarm signals are uploaded to the data monitoring management unit through the RS485 bus; the data monitoring management system summarizes the information from the data acquisition unit and leaves a communication port so that the system can be connected to the substation integrated automation system through the RS485 bus according to the standard protocol. The basic structure of the system is shown in Figure 1.

5. Hardware Implementation of the System

5. Hardware implementation of the signal acquisition unit

     The standard IRt/c infrared temperature sensor of EXERGEN is selected as the terminal temperature measuring element. The IRt/c series infrared temperature sensor has the characteristics of high precision, non-contact, no power supply, and thermocouple signal output, and will not cause adverse effects on the normal operation of CIS.

     Considering the measurement requirements and installation methods on site, it is necessary to determine the optical parameters of the infrared temperature sensor. The specific installation diagram of the infrared temperature sensor is shown in Figure 2. The target measurement distance is 10-50cm, the target diameter is less than 10cm, and the field of view angle of the infrared sensor to be selected is 600, and the coefficient (i.e. the ratio of the field of view diameter to the measurement distance) is 5:1.

The temperature information collected by the infrared temperature sensor is converted into the form of temperature difference potential inside the sensor, and is output to the data acquisition unit through its own twisted pair shielded cable. Since the IRt/c infrared sensor adopts packaging technology exceeding NEMA4 level, the signal acquisition unit has a good resistance to electromagnetic interference. [page]

5.2 Hardware Implementation of Data Acquisition Unit

    The temperature signal collected by the signal acquisition unit is transmitted to the data acquisition unit in the form of thermoelectric potential through the RS232 line of the infrared temperature sensor. The AVR single chip is selected as the control core of the data acquisition unit. The amplified thermoelectric potential is converted into a digital signal using its own 10-bit A/D converter. The single chip processes the signal and outputs it through the serial port. After level conversion, the data is sent to the data monitoring management unit through the RS485 bus.

  5.3 Hardware Implementation of Data Monitoring and Management Unit

     INTEL16-bit single-chip microcomputer 80C196KC is selected as the main control chip, and RAM, ROM and FLASH are expanded for data storage, program storage and historical data recording respectively; clock chip DS12C887 is selected to record system time; dot matrix LCD is selected to display the operation interface, temperature data and alarm screen, etc.; 65HVD3082 is used to realize 485 communication; 80C196KC's I/0 port is used to control 5 buttons to realize system reset and related menu operations; IMPT05 chip is selected to realize the watchdog function, which can reset the system when the program fails or the power is too low.

     For the data monitoring management unit, since it needs to respond to the communication of the substation integrated automation system at any time, it is very important to ensure the reliability of communication with the upper layer system. Therefore, the serial port expansion chip ST2250 is selected to expand a serial port to communicate with the upper layer, and the serial port of the microcontroller is responsible for the communication with the lower layer. The hardware structure of the data monitoring management unit is shown in Figure 3.

Software implementation of the system

6.1 Software Implementation of Data Acquisition Unit

    The data acquisition unit program is written in assembly language, and the program flow is shown in Figure 4. The main program loops once every 60 seconds, obtains the data of the infrared temperature sensor in a query mode, responds to the data transmission request of the data monitoring management unit in an interrupt mode, and sends the processed data.

6.2 Software Implementation of Data Monitoring Management Unit

    The main program of the data monitoring management unit cycles once every 60 seconds and processes the data received from the lower-level data acquisition unit. If the temperature field is normal, it will continue to cycle. If there is an abnormality, an alarm screen will pop up and flash to warn. At the same time, it communicates with the substation integrated automation system in an interrupt mode. The program is written in assembly language. The process is shown in Figure 5.

7 Conclusion

    This system is applied to 20kV gas-insulated metal-enclosed switchgear (GIS). It can monitor the changes of the temperature field inside the GIS online and issue different levels of alarms according to the degree of temperature rise. Users can view the status of each monitoring point through the LCD screen and set alarm parameters and environmental parameters through the keyboard to meet the needs of different conditions. The monitoring network can work independently or be connected to the substation integrated automation system according to actual needs. Due to the use of intrinsically safe infrared temperature sensors as terminal temperature measurement elements, compared with other commonly used GIS temperature measurement methods, this system has the advantages of safety, non-contact, high precision, online temperature measurement and remote communication, which is very suitable for unmanned 20kV medium-voltage distribution stations.