What is the maximum temperature that a dry-type transformer can reach?

Generally speaking, the allowable temperature of a dry-type transformer is related to the heat resistance level of the insulation material it uses.

Generally, when dry-type transformers use Class F and Class H insulation materials, the allowable temperature rise for Class F is 100K, with a maximum allowable temperature of 155℃, while the allowable temperature rise for Class H is 125K, with a maximum allowable temperature of 180℃.

The thermal class of the insulation system is a decisive factor in determining the allowable temperature of the transformer

The insulation materials used in electrical equipment insulation systems primarily include multiple levels such as Class A, Class E, Class B, Class F, Class H, and Class C. The main differences in heat resistance levels are as follows:

1) When using Class A insulation material, its maximum operating temperature is 105℃, and the temperature rise should be less than 60K;

2) When using Class E insulation material, its maximum operating temperature is 120℃, and the temperature rise should be less than 75K;

3) When using Class B insulation material, its maximum operating temperature is 130℃, and the temperature rise should be less than 80K;

4) When using Class F insulation material, its maximum operating temperature is 155℃, and the temperature rise should be less than 100K;

5) When using H-grade insulation material, its maximum operating temperature is 180℃, and the temperature rise should be less than 125K;

6) When using Class C insulation material, its maximum operating temperature is 220℃, and the temperature rise should be less than 150K.

Since the coil insulation of dry-type transformers typically employs Class F and Class H insulation materials, their normal temperature limits are not allowed to exceed 155°C and 180°C, respectively. If the winding temperature of a dry-type transformer is too high, it will accelerate insulation aging, which in turn will affect its service life or cause faults such as short circuits and fires. Therefore, from the perspective of hot spot temperature alone, the lower the operating temperature of the transformer, the better. To prevent abnormal operating temperatures, strict inspection or monitoring is required during use.

The temperature rise of winding 02 is an important indicator for judging whether the transformer temperature is too high.

In fact, the operating ambient temperature of dry-type transformers varies throughout the year. Therefore, under the same load conditions, the temperature in summer tends to be higher, while the temperature in winter tends to be lower. In terms of management, on the one hand, attention should be paid to whether the temperature value of its windings exceeds the standard (especially in summer). On the other hand, another important temperature indicator should also be considered, which is whether the temperature rise is abnormal.

From the perspective of the temperature rise limit of dry-type transformers, the temperature rise is not allowed to exceed 100K when using Class F insulation materials, and it is not allowed to exceed 125K when using Class H insulation materials. For specific dry-type transformers, the relevant specified values of rated temperature rise can be found, which are related to the ambient temperature. Rated temperature rise refers to a certain part of the transformer when it operates under rated load

1. Whether the installation of the transformer is complete, and whether there are any signs of skew, local deformation, and vibration.   

2. Check whether the transformer casing is intact, whether the connections are loose, and whether there are any signs of damage or impact.   

3. Check whether the transformer is clean, whether there is water accumulation, and whether there are any other items that may hinder safe operation.   

4. Check whether the transformer leads are secure, whether their positions are normal, and whether there is any change in the insulation distance around the leads.   

5. Check whether the connecting bolts of the transformer are loose, whether the electrical connections at the bushing and terminal block are loose or locally overheated, whether there are any damages to the porcelain parts, and then remove the dirt and dust on the ferrules.   

6. Check whether the coil pressing plate is tightly pressing the coil, and whether the cushion blocks at various locations are loose.   

7. Whether the transformer grounding is reliable.   

8. The cabinet body should be free of holes and gaps that could allow small animals to enter and pose a safety hazard. The temperature difference between the component temperature and the external cooling medium temperature (cooling air temperature or cooling water temperature) is measured in K. For example, if a dry-type transformer has a rated temperature rise of 90℃, under rated load and an ambient temperature of 40℃ (during hot summer), its *temperature is 130℃ (90℃+40℃); if the ambient temperature is lower (during cold winter), such as 10℃, then the transformer’s *temperature is 100℃ (90℃+10℃). When an excessive temperature rise occurs, it is necessary to activate the cooling system for temperature reduction, or implement appropriate load reduction management. For persistent abnormal temperature rises, the cause of the abnormal temperature rise should be investigated and addressed accordingly.

II. Internal Testing

1. Measure the insulation resistance of the coil:

Under normal conditions (temperature ranging from 20 to 40℃, humidity at 90%), the insulation resistance between high voltage and low voltage or ground should be ≥300MΩ, and between low voltage and high voltage or ground should be ≥100MΩ (measured with a 2500V megohmmeter). However, if the transformer experiences abnormal moisture and condensation, regardless of its insulation resistance, it must undergo drying treatment before undergoing voltage withstand tests or being put into operation.

2. Test of iron core insulation resistance:

Under normal conditions (temperature 20~40℃, humidity 90%), a 2500V megohmmeter is used for measurement. The insulation resistance between the iron core and the clamping piece as well as to ground should be ≥1MΩ, and the insulation resistance between the through-core screw and the iron core as well as to ground should also be ≥1MΩ. Similarly, in relatively humid environments, this value may decrease, but as long as the resistance is ≥0.1MΩ, it can still operate.

3. Measure the voltage ratio under all taps and connection groups. * The voltage ratio error should be less than 0.5%.

4. For external construction frequency withstand voltage tests where conditions permit, the test voltage should be 80% of the factory test voltage (refer to the national standard GB50150-2006).

Through testing, we can determine whether the transformer meets the conditions for operation, thus eliminating the consequences of phase loss operation, which can lead to excessive current in the remaining phases, increased temperature, abnormal noise from the transformer, and potential damage to the transformer after prolonged use.

III. Power transmission

Generally, the power supply bureau will conduct power transmission 5 times, but there are also cases of 3 times. Before power transmission, they will check the tightness of bolts and whether there are metal foreign objects on the iron core; whether the insulation distance meets the power transmission standards; whether the electrical functions operate normally; whether the wiring is correct; whether the insulation of each component meets the power transmission standards; whether there is condensation on the device body; whether there are loopholes in the shell that small animals can enter (especially at the cable entry point); and whether there is any discharge sound during power transmission.

IV. Monitoring after power-on

The transformer should be switched on and put into operation when it is under no-load conditions. The peak value of the inrush current upon switching on can reach approximately 10 times the rated current. The setting value for the current quick-action protection of the transformer should be greater than the peak value of the inrush current.   

After the transformer is put into operation, the load it carries should be gradually increased from light to heavy, and the product should be checked for any abnormal noise. It is strictly prohibited to blindly apply a large load at once (usually about one-third). The no-load temperature of a dry-type transformer varies depending on its capacity, with higher temperatures for larger capacities.

1. Monitor the load current of the transformer

After the transformer is put into operation, its most economical operating state is when the load capacity reaches 70% to 80% of the rated capacity. However, due to different production conditions of various enterprises, the load capacity of the transformer will vary with seasons, orders, production processes, etc. When it exceeds 80% of its rated capacity, it is necessary to strengthen the monitoring of transformer load current. Usually, if there is an ammeter installed on the low-voltage distribution cabinet, it can be observed directly. If there is no ammeter, a clamp ammeter can be used for measurement. If the transformer operates under overload conditions for a long time, it will seriously shorten the service life of the transformer and increase its loss. While detecting the transformer load current, it is also necessary to ensure that the three-phase load currents are balanced as much as possible. If the three-phase currents differ greatly, it will lead to unbalanced three-phase voltages, resulting in neutral point displacement, making the neutral line charged, causing the shell of the electrical equipment to be charged, posing safety hazards to equipment and personnel.

2. Monitor the operating temperature of the transformer

When the transformer is operating, its temperature should be closely monitored. The temperature of an oil-immersed transformer reflects the temperature of its upper layer of oil. According to the regulations, the temperature of the upper layer of oil should not exceed 95°C, and it is not advisable to operate it frequently at temperatures exceeding 85°C. When the oil temperature increases to 85°C, the oxidation rate will double. Dry-type transformers have different temperature rise requirements for different insulation levels. For example, the temperature rise of a Class A insulation winding should not exceed 60°C, and that of a Class B insulation winding should not exceed 80°C. Since the heat dissipation of dry transformers relies entirely on natural air cooling or forced air cooling, they are more sensitive to temperature. Therefore, when the transformer is in normal operation and its temperature rises due to high ambient temperature, full load, or overload, in addition to closely monitoring its temperature, measures such as load reduction, air blowing, or water cooling can be taken to forcefully cool it down when necessary.         

It is particularly important to note that if the transformer temperature is too high and continues to rise under low load or ambient temperature conditions, it indicates an internal short circuit fault. The transformer should be immediately taken out of service, the cause should be investigated, and the fault should be rectified.

3. Monitor the operating sound of the transformer

During normal operation, a transformer emits a continuous and stable “buzzing” sound. As the load increases and the input voltage rises, the sound will become louder. To determine whether the sound change is normal, it is necessary to compare it with previous normal operating conditions. If the sound increases under the same conditions, or if it is accompanied by other noises, a comprehensive analysis should be conducted by observing whether the three-phase load current and voltage are balanced, as well as the oil level and temperature. If the oil temperature and level do not change significantly and remain stable, the three-phase current increases relatively evenly, and the three-phase voltage is balanced but slightly decreases, it indicates that the increase is caused by an increase in load. If the three-phase current is relatively balanced but slightly decreases, and the three-phase voltage increases significantly, exceeding the rated value by a large margin, it indicates that the cause is an excessively high system voltage. If the sound is abnormally loud, the three-phase load current and voltage are extremely unbalanced, and it persists, it may be caused by a phase-to-phase short circuit or an open circuit on the low-voltage side that has subsequently grounded. In this case, the transformer should be shut down immediately to identify the cause. It can only be put back into operation after the fault is resolved.

4. Check the grounding wire and grounding resistance of the transformer

Both the N-phase and the enclosure on the low-voltage side of the transformer must be reliably grounded. When the grounding connection is poorly contacted, the grounding wire is broken, or the grounding resistance is too high, the grounding becomes ineffective, and the residual current operated protective device will also fail to operate reliably, posing a threat to equipment and personal safety. Therefore, the grounding connection must be in good contact, and the grounding resistance must meet the regulatory requirements. When the grounding resistance is too high, measures such as adding grounding electrodes or extending the grounding body can be taken to reduce the grounding resistance.