Abstract: This paper proposes a method to control the temperature difference between the inside and outside of high-voltage equipment to prevent condensation. This method uses a temperature sensor instead of a condensation sensor, which can greatly improve the reliability of the anti-condensation function of the switchgear and has a good energy-saving effect. Keywords: High-voltage equipment anti-condensation; Temperature difference control; Improved reliability; Energy saving I. Overview The internal space of new high-voltage switchgear is very compact. To ensure the internal insulation level in high-humidity areas and ensure the reliable operation of the equipment, higher requirements are placed on the moisture-proof and anti-condensation functions of the cabinet. China has been using automatic heating and dehumidification controllers to prevent condensation in switchgear for more than ten years. These controllers have played a positive role in resisting moisture and preventing condensation, ensuring the reliable operation of high-voltage equipment. However, some problems have also arisen in some areas. For example, in the plum rain season in the south, the air humidity inside the switchgear is sometimes high, and condensation may even occur in some parts of the switchgear. Because the condensation sensor installed inside the cabinet is not at the point of condensation, or because the sensor is corroded by dust and gas in the air for a long time, the sensor sensitivity is affected, and the condensation controller cannot start the heater in time. This causes condensation control to fail, threatening the safety of switchgear. Based on the mechanism of condensation occurrence and prevention, a multi-channel temperature control device is proposed. This device uses the temperature difference between the inside and outside of the switchgear to prevent condensation. This method can greatly improve the reliability and service life of the switchgear's anti-condensation measures. Replacing the long-term heating dehumidification method with this method has a significant energy-saving effect. Please visit: Power Transmission and Distribution Equipment Network for more information . II. Commonly Used Anti-Condensation and Dehumidification Measures for Switchgear and Existing Problems 2.1 Working Principle of Commonly Used Condensation Sensors Almost all condensation sensors on the market are of the same type, namely, a polymer semiconductor resistive material printed on a ceramic substrate, with two electrodes led out. When the sensor surface is dry, the intermolecular contact resistance is small, and the resistance at both ends of the electrodes is about 1kΩ. However, when the polymer material absorbs moisture, its internal molecular space expands rapidly, the intermolecular contact resistance increases, and the resistivity at both ends of the electrodes increases significantly. The electronic controller senses or predicts whether condensation has occurred by testing the resistance. For example, the HDP-07 condensation sensor is a positive characteristic switching element. It is insensitive to low humidity but sensitive to high humidity. The operating characteristic curve of this type of sensor, with its resistance changing within the range of 70%RH to 100%RH, is shown in Figure 1. From the characteristic curve in Figure 1(A), it can be seen that this type of condensation sensor has two inherent characteristic defects: a) When the relative humidity is not high, the condensation sensor has a discrete region. In this region, it is insensitive to changes in relative humidity, and the changing resistance is not accurate. Its output characteristics are not only related to the relative humidity but also directly related to the dew point temperature. For example, sometimes we feel that the humidity in the air is very high, but the heater does not start. This is because the sensor surface has not reached the dew point temperature (that is, water droplets have not formed on the sensor surface). A. Condensation sensor characteristic curve B. Figure 1 shows the characteristic curves of a linear humidity sensor and a linear humidity sensor. (b) Because the condensation sensor must continuously absorb or release water molecules, its sensitivity changes after being corroded by dust or chemicals in the moisture, leading to deviations in the anti-condensation control point. This can result in situations where the heater hasn't started even when humidity is high and condensation is highly likely, or where the heater doesn't automatically shut off after starting, reducing the anti-condensation sensitivity and reliability. 2.2 Two commonly used anti-condensation methods: Method 1: Install an aluminum alloy heater (typically 150W) in both the switchgear cable compartment and the circuit breaker compartment. The surface temperature of the aluminum alloy heater is 120℃～130℃ during operation. Dehumidification is achieved through air cooling, and the user manually controls the heater's activation and deactivation based on the working environment. Method 2: An automatic dehumidification controller is installed in the switchgear instrument compartment. The condensation sensor is installed inside the switchgear cabinet, and the aluminum alloy heaters are installed in the switchgear cable compartment and circuit breaker compartment respectively. The condensation sensor's operating characteristics are used to automatically activate and deactivate the heaters. 2.3 Problems Method 1 relies on manual operation to put the heater on and off, resulting in poor operability and often leading to two extreme situations. First, the heater may not be turned off after being put on, causing unnecessary power waste. Second, the heater may not be activated in time according to climate changes, leaving the cabinet damp and unable to dehumidify, potentially causing significant losses in the event of creepage or flashover. Method 2 uses a condensation sensor to automatically activate the heater, but it also has two drawbacks. First, this condensation sensor is a passive actuator, meaning it only activates when the air is saturated with water vapor, i.e., when the surface reaches the dew point temperature. From detecting condensation to eliminating it, it's clear that the heater can only be activated when condensation is detected (meaning it must occur). However, for anti-condensation measures, which are crucial for electrical cabinets, activating the heater only when condensation is nearing its edge lacks a preventative process. From a safety and reliability perspective, this method is far from reliable as an anti-accident measure. Secondly, if the condensation sensor is improperly installed, or if its characteristics are altered due to dust or gas corrosion, the condensation controller may fail to activate the heater in time even if condensation has already formed on the cabinet wall, leading to serious consequences. Therefore, to improve reliability, power operators often employ auxiliary measures to ensure the safe operation of electrical cabinets during the rainy season. For example, Shanghai Power Company uses a method of forced continuous heating during the rainy season. While this method strengthens anti-condensation measures, it also results in power loss and waste. III. New Anti-Condensation Scheme for Switchgear and its Advantages To address the problems with commonly used anti-condensation methods for switchgear and to improve the reliability of anti-condensation measures and reduce power consumption, a novel anti-condensation control scheme for switchgear is proposed. 3.1 Basic Concept of "Condensation" and Basis of the New Scheme What is condensation? Condensation refers to the phenomenon where water droplets condense on the inner wall surface when the temperature drops below the dew point temperature. This phenomenon is called condensation. Whether condensation occurs depends on the indoor temperature, cabinet temperature, relative humidity, and dew point temperature. What is dew point temperature? Dew point temperature refers to the maximum amount of water vapor in air at a given temperature, known as the "saturated water vapor content." Air at this point is called "saturated air." When the temperature of saturated air decreases, the water vapor in the air will condense into water droplets. The saturation temperature of air containing water vapor is called the dew point temperature. Please refer to the relevant data table for temperature, humidity, and dew point (Table 1). Table 1: Relevant Data Table for Temperature, Humidity, and Dew Point. Figure 2 is derived from the data in the table. Figure 2: Curves of Ambient Temperature, Dew Point, and Relative Humidity. From the curves in Table 1 and Figure 2, we can see that: ① Under certain temperature conditions, the higher the relative humidity in the air, the closer the dew temperature is to the ambient air temperature. In other words, the closer the ambient temperature is to the dew point temperature, the easier it is for condensation to occur. ② Regardless of the air temperature, the dew point temperature for condensation is always lower than the ambient temperature. For example, when the air temperature is 20 degrees Celsius and the relative humidity is 60%, the dew point temperature is 12 degrees Celsius. From the mechanism of condensation and Table 1 and Figure 2, we can conclude that: ① To prevent condensation, the surface temperature of the part where condensation is not allowed must always be higher than the surrounding ambient temperature. ② For switchgear, to prevent condensation inside the switch cabinet, it is sufficient to keep the internal temperature of the switch cabinet higher than the external ambient temperature. A practical analogy is that if you take a glass of ice water out of a refrigerator in summer, condensation will immediately form on the surface. However, if you take a glass of hot water out of an insulated box at a temperature higher than the ambient temperature, condensation will not form on the surface. Condensation inside switchgear causing creepage and flashover accidents generally occurs in the following situations: ① High humidity and large temperature variations in the region, resulting in damp bottoms of the switchgear, and sometimes even water accumulation in cable trenches; ② Some switchgear is located in basements with high humidity, where the internal temperature, especially near the ground, is lower than the ambient temperature; ③ Some equipment is temporarily out of service, resulting in a lower internal ambient temperature than the surrounding environment, making condensation very easy to form on its surface. In these cases, once power is restored, an accident will occur. 3.2 New Anti-Condensation Technology Solution for Switchgear Based on the above analysis, as long as we maintain the internal temperature of the switchgear higher than the external ambient temperature and minimize the relative humidity inside the cabinet, condensation inside the cabinet can be completely prevented. To ensure that the internal temperature is always higher than the external ambient temperature while also achieving energy conservation, the control instrument in the new anti-condensation solution will eliminate the existing popular condensation sensors. Multiple temperature sensors will be used: one to measure the external temperature of the cabinet; one installed on the cabinet wall to measure the cabinet wall temperature; and the rest to measure the temperature of each compartment inside the cabinet. The CPU intelligently controls the switching of heaters to regulate the internal temperature, ensuring that the temperature of the parts of the switchgear to be protected from condensation is always at least 2℃～3℃ higher than the external ambient temperature, thus preventing condensation. Heating will stop when there is a 2℃～3℃ temperature difference between the inside and outside of the cabinet. The heaters will be activated when the temperature difference is lower than 2℃～3℃. Due to the small volume of the cabinet interior, the heating power required to achieve this temperature difference is related to the volume of the compartments within the cabinet. The following experimental tests simulate the heating/cooling of different internal cabinet volumes: 1. Cabinet volume: 1.1 m³, Heating power: 150 W; 2. Cabinet volume: 0.5 m³, Heating power: 150 W. From the experimental curves above, we can see that: ① For a 10kV switchgear busbar compartment, simulating a 0.5 m³ volume, it takes approximately 8 minutes to heat to a temperature 3°C higher than the outside. After stopping heating, it takes approximately 25 minutes to return to a 1°C temperature difference. If the heater is cyclically activated and deactivated according to this time, maintaining the internal temperature 1-3°C higher than the external temperature, the heating activation time is approximately 1:3. ② The 35kV switchgear busbar compartment volume can be approximated using a 1.1 m³ space. The curves show that maintaining a 1-3°C higher internal temperature than the external temperature requires a heater activation/deactivation time of approximately 1:1. ③ Based on a temperature difference control range of 1-3, the heating time is 1/4 for a 0.5 m³ space, and 1/2 for a 1.1 m³ space. ④ If the electrical cabinet generates some heat during operation, this heat can also be used as part of the heating power, potentially significantly reducing the heating time. Therefore, using a 150W heater and temperature difference control for condensation prevention can save at least 3/4 of the heating power consumption for a 10kV switchgear and at least 1/2 for a 35kV switchgear. To further improve the reliability of moisture and condensation prevention, a linear relative humidity sensor can be added to each condensation control instrument. While monitoring temperature and temperature difference, the CPU can also measure and display the relative humidity inside the cabinet. Based on multiple parameters, the relative humidity inside the cabinet is determined and controlled (heating is activated when it exceeds 65%RH), further improving the dehumidification and condensation prevention capabilities. IV. Conclusion 1. Automatic heating and dehumidification controllers have been used in China's power system for over a decade, playing a positive role in preventing humidity and condensation in the operation of high-voltage equipment. However, in some areas, the application has yielded unsatisfactory results. Analysis shows that this is largely due to the limitations of the condensation sensors and the influence of the operating environment, leading to significant deviations in control sensitivity and accuracy. 2. This new solution effectively addresses the issues of sensitivity, accuracy, lifespan, energy consumption, and reliability inherent in the original automatic heating and dehumidification control scheme, enabling anti-condensation control technology to reach a new level in terms of safety, reliability, and energy efficiency, and demonstrating new effectiveness. 3. By changing the passive anti-condensation method to an active one, the new scheme undoubtedly increases the reliability of switchgear anti-condensation. Furthermore, considering the lifespan of temperature sensors, which is dozens of times longer than that of condensation sensors, the working life and reliability of the anti-condensation control system will be greatly increased.