What is the operating temperature of a dry-type transformer

Date: June 16, 2025 08:06:23

In the power system, dry-type transformers play a vital role in providing stable and reliable power support for various types of electrical equipment. Its operating temperature as a key parameter, not only directly affects the performance of the transformer, but also closely linked to the service life and safety of the equipment. Understanding the operating temperature of dry-type transformer related knowledge, to ensure the efficient and stable operation of the power system is of great significance.​
Normal operating temperature range of dry-type transformer​
The normal operating temperature range of the dry-type transformer is not fixed, but by a variety of factors, including the heat-resistant grade of the insulation material plays a decisive role. Different heat-resistant grade of insulation materials, the maximum temperature can withstand different. At present, in the field of dry-type transformer manufacturing, common insulation material heat-resistant class A, E, B, F, H and C. The insulation material heat-resistant class A, E, B, F, H and C is the most common.​
  1. Class A insulation: The limiting operating temperature is 105°C and the maximum temperature rise should be less than 60 K. This means that in actual operation, when the ambient temperature is 25°C, the temperature of the transformer windings should not exceed 85°C (25°C + 60 K).​
  1. Class E insulationThe limiting operating temperature is 120°C and the maximum temperature rise should be less than 75 K. Assuming that the ambient temperature is also 25°C, the winding temperature should not exceed 100°C (25°C + 75 K).​
  1. Class B insulationThe limiting working temperature is 130°C and the maximum temperature rise should be less than 80 K. Corresponding to the above ambient temperature, the winding temperature should be controlled below 105°C (25°C + 80 K).​
  1. Class F insulationThe limiting operating temperature is 155°C and the maximum temperature rise should be less than 100 K. In this case, the winding temperature should not be higher than 125°C at 25°C ambient temperature (25°C + 100 K).​
  1. Class H insulationThe maximum temperature rise should be less than 125 K. If the ambient temperature is 25°C, the winding temperature should be kept within 150°C (25°C + 125 K).​
  1. Class C insulationThe maximum temperature rise should be less than 150 K. When the ambient temperature is 25°C, the winding temperature can be allowed to reach 175°C (25°C + 150 K).​
In practice, most dry-type transformers are insulated with class F or H insulation. Therefore, in general, the normal operating temperature of dry-type transformers should normally be kept below 155°C (class F) or 180°C (class H). However, from the point of view of prolonging the service life of the transformer and ensuring operational stability, it is more desirable to maintain the operating temperature at a relatively low level. For example, for dry-type transformers with class H insulation materials, controlling the temperature between 120℃ - 130℃ during normal operation can effectively slow down the aging of the insulation materials and improve the reliability of the equipment.​
Factors affecting the operating temperature of dry-type transformers​
  1. Load size: Load is one of the key factors affecting the operating temperature of dry-type transformers. When the load on the transformer increases, the current through the winding increases accordingly. According to Joule's law, heat is generated when current passes through a conductor (Q = I²Rt, where Q is the heat, I is the current, R is the resistance and t is the time). As the current increases, more heat is generated in the windings, resulting in an increase in transformer temperature. For example, when a transformer is operating at full load, its temperature is typically 20°C - 30°C higher than when it is operating at light load. If overload operation for a long time, the transformer temperature will continue to climb, and in serious cases, it may exceed the tolerance limit of the insulation material, accelerate the aging of the insulation, and even cause failure.​
  1. environmental temperature: The ambient temperature in which the transformer operates has a direct impact on its temperature. In a high temperature environment, the heat dissipation conditions of the transformer become worse. Due to the high ambient temperature, the temperature difference between the transformer and the environment decreases, the heat dissipation efficiency decreases, so that the heat generated inside the transformer is difficult to effectively radiate out, resulting in increased temperature. In summer, for example, when the ambient temperature reaches 35 ℃ - 40 ℃, dry-type transformer operating temperature compared to the spring and fall seasons (ambient temperature of about 20 ℃ - 25 ℃) will rise significantly. If the ambient temperature is too high and lasts for a long time, it may be necessary to take additional cooling measures, such as strengthening ventilation, installation of air conditioning, etc., to ensure that the transformer operates within the normal temperature range.​
  1. thermal condition: Good heat dissipation conditions are essential for controlling the operating temperature of dry-type transformers. Transformer heat dissipation is mainly realized through natural convection, radiation and forced air cooling and other ways. If the transformer installation location is not well ventilated, such as placed in a small closed space, the air can not effectively circulate, natural convection cooling effect will be greatly reduced. In addition, the surface of the transformer dust, dirt accumulated too much, will also affect its radiation heat dissipation ability. For some large-capacity dry-type transformer, usually equipped with forced air cooling system, when the temperature exceeds a certain threshold, the fan automatically starts, by accelerating the air flow to take away the heat. However, if the fan fails or the air duct is blocked, the effect of forced air cooling will not be able to play normally, resulting in higher transformer temperature.​
Hazards of high operating temperatures on dry-type transformers​
  1. Accelerated insulation aging: Insulation materials are an important part of dry-type transformers, and their performance is directly related to the safe operation of the transformer. When the operating temperature is too high, the molecular structure of the insulation material will change, resulting in its physical and chemical properties deterioration, accelerate the insulation aging process. Studies have shown that the dry-type transformer operating temperature rises 8 ℃ - 10 ℃, the aging of the insulation material will be accelerated by about double. Insulation aging will make the insulation properties of insulation materials, such as insulation resistance reduction, dielectric strength weakening, etc., increasing the winding short circuit, grounding and other failure probability, seriously affecting the service life of the transformer. For example, a normal operating temperature can be used for 20 years of dry-type transformer, if long-term operation in high temperature environment, its service life may be shortened to 10 years or even shorter.​
  1. Reduced operational efficiency: As the operating temperature of the transformer increases, the resistance of the windings will increase. According to Ohm's law (I = U / R, where I is the current, U is the voltage, and R is the resistance), an increase in resistance will result in a decrease in current at a constant voltage. The output power of the transformer P = UI (U is the voltage, I is the current), a decrease in current means a decrease in output power, which results in a decrease in the operating efficiency of the transformer. At the same time, due to the increase in resistance, the heat generated by the winding further increases, forming a vicious circle. This not only causes energy waste, but also may affect the power supply quality of the power system and fail to meet the load's demand for power.​
  1. create a potential safety hazard: Excessive operating temperatures can cause a number of safety problems. When the temperature continues to rise and exceeds the tolerance limit of the insulation material, the insulation may be punctured, leading to a short circuit in the winding. The short-circuit current will instantly generate huge heat, which may trigger a fire, posing a serious threat to the safety of the surrounding equipment and personnel. In addition, the high temperature may cause the internal parts of the transformer to expand and deform, leading to poor contact, loosening and other problems, further aggravating the risk of failure. In some important power places, such as substations and data centers, dry-type transformers will cause serious economic losses and social impacts in the event of a safety accident.​
Monitoring and control of dry-type transformer operating temperature​
  1. Temperature monitoring methods​
  • thermometer method: This is a more common and intuitive temperature monitoring method. In the dry-type transformer winding or core and other key parts of the installation of thermometers, such as thermocouple thermometer, RTD thermometer and so on. These thermometers can measure the temperature of the measured part in real time, and convert the temperature signal into electrical signal output, and read the temperature value directly through the display instrument. For example, the thermocouple thermometer utilizes the thermoelectric effect of two different metal materials, when the temperature changes, the thermocouple ends will produce thermal potential, by measuring the size of the thermal potential can be known temperature value.​
  • thermostat method: A thermostat is an intelligent temperature monitoring and control system. It usually consists of temperature sensor, controller and actuator. The temperature sensor is installed inside the transformer and collects temperature data in real time and transmits it to the controller. The controller analyzes and processes the collected data according to the preset temperature threshold, and when the temperature exceeds the set value, the controller issues a command to drive the actuator to take action, such as starting the fan to dissipate heat, sending out alarm signals and so on. At present, the temperature controller on the market is becoming more and more powerful, not only can realize the real-time monitoring and control of temperature, but also has the function of data recording, remote communication, etc., which is convenient for operation and maintenance personnel to remotely monitor and manage the operation status of the transformer.​
  1. Temperature control measures​
  • natural air cooling: For some dry-type transformers with small capacity and relatively low operating temperature, natural air cooling is a simple and effective way to dissipate heat. Through the reasonable design of transformer shape structure and ventilation channel, the natural convection of air will be utilized to emit the heat generated inside the transformer to the surrounding environment. For example, set the heat sink on the transformer shell to increase the heat dissipation area and promote the natural convection of air. At the same time, ensure that the transformer installation location is well ventilated, no obstacles around to block air circulation, in order to improve the cooling effect of natural air cooling.​
  • Forced air coolingWhen the dry-type transformer load is large or the operating temperature is high, the natural air-cooling may not be able to meet the heat dissipation requirements, this time it is necessary to use forced air-cooling system. Forced air cooling system mainly consists of fan, air duct and control system. When the transformer temperature rises to a certain degree (such as the winding temperature reaches 100 ℃ - 110 ℃), the temperature controller issued a command to start the fan. Fan through the duct will be cold air blowing to the transformer winding and core and other hot parts, accelerate the heat dissipation. In the process of forced air cooling, it is necessary to regularly check the operating status of the fan to ensure that the fan operates normally and the air duct is unobstructed. In addition, according to the actual operation of the transformer, the speed of the fan can be adjusted to control the intensity of heat dissipation to achieve energy-saving operation.​
  • Adjustment of the load: Reasonable adjustment of dry-type transformer load is also one of the important measures to control the operating temperature. By monitoring and analyzing the load of the power system, load distribution can be optimized to avoid long-time overload operation of the transformer. For example, during the peak period of electricity consumption, the burden on the current transformer can be reduced by transferring some non-critical loads to other transformers to reduce its operating temperature. Meanwhile, for some seasonal or intermittent loads, the starting and stopping time of the equipment can be reasonably arranged according to the actual demand, so that the transformer can be operated in a high-efficiency and low-temperature state.​
  • Improvement of the operating environment: Improving the operating environment of the dry-type transformer is also important for reducing the temperature. First of all, ensure that the transformer installation place is well ventilated, through the installation of ventilation equipment, such as exhaust fans, ventilators, etc., to enhance air circulation, take away the heat. Secondly, control the temperature and humidity of the operating environment. In the high temperature season, air conditioning and other refrigeration equipment can be used to reduce the ambient temperature; for the environment with high humidity, dehumidification equipment can be installed to keep the environment dry. In addition, regular cleaning of the transformer operating environment to prevent dust, dirt and other accumulation on the surface of the transformer, affecting the effect of heat dissipation.​
Understanding the knowledge related to the operating temperature of dry-type transformers and taking effective monitoring and control measures is the key to ensuring their safe, stable and efficient operation. Through reasonable selection of insulation materials, optimization of heat dissipation conditions, real-time temperature monitoring and timely adjustment of operating parameters and other means, can effectively control the operating temperature of the transformer, extend the service life of the equipment, improve the reliability of the power system. In practice, power operation and maintenance personnel should pay close attention to the dry-type transformer temperature changes, in strict accordance with the relevant standards and norms for operation and maintenance, to ensure the safe and stable operation of the power system.
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