Design of automatic control device for fans and heaters in high voltage room of substation

0 Introduction
    The temperature and humidity of the high-voltage room in the substation have an important impact on the safe operation of the equipment. At present, the Weinan power grid requires that the temperature of the high-voltage room be controlled between 5 and 20°C and the humidity be controlled between 35 and 75%. If the temperature exceeds the range for a long time, it will cause problems such as equipment heating and shortened component life. If the humidity is too low, static electricity is likely to be generated. The accumulation of static electricity may directly damage the electronic components of the secondary equipment and even cause the protection or control components to malfunction. If the humidity is too high, it is conducive to the growth of mold. The acidic substances decomposed by the mold and the long-term interaction with the insulating material will greatly reduce the insulation performance of the equipment. When the humidity reaches the dew point, the wet air will condense into water droplets, causing short circuits or DC grounding faults, which seriously threaten the safe operation of the equipment.

1 Current status of dehumidification in high-voltage room
    The humidity in the 35kV high-voltage room of the 330kV Luofu substation of Weinan Power Supply Bureau is relatively high, especially on rainy days when the surface moisture converges with the cable trench, causing the humidity to often exceed the prescribed upper limit; the busbar and other equipment in the high-voltage room are enclosed in the high-voltage cabinet, and the air circulation is poor. During the peak summer period, the grid load is large and the ambient temperature is high, causing the temperature at the busbar lead connection to be relatively high.

    At present, the Luofu substation uses dehumidifiers, ventilators and heaters to control the temperature and humidity of the high-voltage room, but the effect is not very ideal. This is because the dehumidifier can automatically dehumidify according to the set humidity value, but the ventilator and heater are still manually controlled by the duty officer, which cannot give full play to their dehumidification and cooling effects. The control circuit is shown in Figure 1. This control method also has the following problems:
    (1) The duty officer cannot turn on and off the ventilator and heater in real time according to the changes in indoor and outdoor temperature and humidity;
    (2) If the ventilator is turned on manually and cannot be turned off in time, the ventilator motor will run for a long time and generate serious heat, which will greatly shorten the service life of the ventilator and reduce the reliability of the fan operation. (3)
    With the continuous transformation and operation of unmanned substations, the manual control method of the high-voltage room ventilator and heater cannot meet the operation requirements of existing equipment.

2 Design of ventilator and heater control device
2.1 Principle and control strategy
    This paper designs a control device based on the STC89C52 single-chip microcomputer as the processor. The device consists of a power module, a data acquisition module, a display module, and a fan and heater control module. The principle is shown in Figure 2.

    The data acquisition module uses imported high-precision sensors to collect indoor and outdoor temperature and humidity, and uploads data to the single-chip microcomputer once a second. The single-chip microcomputer performs statistics, analysis, and calculations on the data. The fan and heater are turned on and off according to the calculated results. The control strategy is as follows:
    (1) Fan control. When the indoor humidity value is higher than 40% and the outdoor humidity value is 10% lower than the indoor humidity, the processor issues a command to start the fan. When the indoor and outdoor humidity are the same, the fan is turned off. When the indoor temperature
    is higher than 30℃, the ventilation is turned on, and when it is lower than 10℃, the ventilation is turned off. After the fan has been running for 3 hours, if the indoor humidity has not reached the set shutdown value, the single-chip microcomputer will issue a command to control the fan to stop for 20 minutes, and then start again, and so on, until the indoor humidity value reaches the specified value to avoid damage caused by long-term heating of the motor.
    When the outdoor humidity value is lower than 75%, the ventilation is started at 10 noon every day and automatically turned off after running for 15 minutes. Improving the indoor air of the high-voltage room is conducive to the inspection of the on-duty personnel.
    (2) Heater control. When the indoor temperature is lower than 5℃, the heater is started, and when it is higher than 30℃, the heater is turned off. When the indoor humidity exceeds 75%, the fan is started, and when it is lower than 50%, the fan is turned off.
2.2 Hardware selection and design
    The voltage level of the 330kV substation is high and the electromagnetic interference is strong. Therefore, the processor uses the STC89C52 single-chip microcomputer produced by Hongjing Company, which has the characteristics of high speed, low power consumption, and super strong anti-interference ability. The power supply first uses a transformer for electromagnetic isolation and voltage reduction, and then the low-voltage input power module supplies power to the system, which makes the power supply voltage stable and completely isolates AC and DC, strong power and weak points. The sensor uses the SHT10 single-chip digital temperature and humidity integrated sensor launched by Sensirion, Switzerland. The outdoor sensor uses a double-layer shielded wire to lead from the cable trench of the high-voltage room to collect outdoor temperature and humidity. When the temperature and humidity reach the action value set by the processor, the processor sends a control command through the I/O port to control the opening and closing of the fan. The control circuit is shown in Figure 3. The parameters and information such as temperature, humidity and fan action are displayed through the LCD, and can be modified, set and queried using buttons. The printed circuit board is copper-clad and the leads are widened, making the device highly anti-interference and reliable.

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2.3 Software System Design
    The device uses C language for software programming, with modular design as the idea, which is easy to transplant, expand and maintain. In order to improve the stability and reliability of the device operation, the program continuously monitors the working status of each important peripheral module during initialization and operation. After an abnormality is found, the system can be quickly reset, the data can be backed up and an abnormal alarm can be issued. The software is equipped with watchdog, multiple backup and other measures to ensure that the circuit can automatically correct errors or reset immediately. The workflow is shown in Figure 4.

3 The experimental and analytical
    device was installed in the 35kV I section high-voltage room of Luofu Substation, and connected in parallel to the existing fan control contactor CJ10-10, which does not affect the current manual control of the fan. It meets the document "the dehumidification and heating device should have automatic control and manual switching functions". The automatic control device is shown in Figure 5; Table 1 records the operation of the device in four days. It can be seen from the table that when the indoor and outdoor humidity difference reaches the set value, the fan automatically turns on and off. When the humidity value is lower than 75%, the fan automatically turns on for 15 minutes and then automatically turns off at 10:00 every day, improving the air quality of the high-voltage room and providing a good environment for the on-duty personnel to patrol the high-voltage room at 10:00. During these four days, the heater did not start automatically because the indoor temperature did not reach the set value.

    During the on-site operation of the high-voltage room of the Rove transformer, the functions and parameters of this device basically meet the requirements, and the operation is relatively stable.

4 Conclusion The automatic control device of the fan and heater based on the STC89C52 single-chip microcomputer is designed in
    the language , which can turn on the fan and heater in real time according to the changes in indoor and outdoor temperature and humidity. After the operation of the Rove transformer substation, the device improves the dehumidification and cooling effect of the high-voltage room, reduces the burden of the on-duty personnel, and provides conditions for the further promotion of unmanned substations. Practice has proved that it has certain practicality and promotion, but there are also the following problems that need further research and exploration: 1) There is currently no clear specification for the indoor and outdoor temperature and humidity difference of the fan start-up parameters, and they need to be adjusted and checked continuously during on-site operation. 2) Combined with the operation of the substation, the differences in the temperature and humidity control effects of existing air conditioners, dehumidifiers, fans and heaters at each stage are analyzed through experiments, and a comprehensive optimization control plan is formulated accordingly. Optimal control of the temperature and humidity of the high-voltage room is achieved.