In the operation and management of transformers, temperature is a core indicator for measuring the "health status" of the equipment. Among all temperature parameters, hot spot temperature is particularly critical—it refers to the temperature at the hottest spot inside the transformer and directly determines the aging rate of insulating materials and the service life of the equipment.

I. What is Hot Spot Temperature?

Hot spot temperature is the temperature at the point with the highest temperature inside a transformer. Due to uneven current distribution, differences in heat dissipation conditions and other factors, the temperature of each part of the transformer is not uniform.

Why is hot spot temperature so important? According to the classic 8-degree Rule—for every 8℃ increase in the operating temperature of insulating paper, its thermal aging rate doubles, and the service life is halved accordingly. Therefore, controlling the hot spot temperature is equivalent to controlling the service life of the transformer.

Hot spots usually appear at the following positions:

- Inside the winding: high current density and long heat dissipation path

- At the winding ends: concentrated leakage flux and poor heat dissipation conditions

- Local parts of the iron core: areas near the iron core joints or structural components

II. Differences Between Hot Spot Temperature and Top Oil Temperature

In daily operation and maintenance, these two indicators are often confused, but they are essentially different:

Indicator	Hot Spot Temperature	Top Oil Temperature
Definition	The maximum temperature inside the winding or iron core	The temperature of insulating oil at the top of the oil tank
Measurement Method	Indirect calculation or direct measurement with optical fibers	Direct measurement with a thermometer
Application	Evaluate insulation aging and control overload	Daily operation monitoring and alarm threshold setting

Comparison of Hot Spot Temperature and Top Oil Temperature

Key point: Top oil temperature is easy to monitor, but it cannot fully reflect the actual thermal state inside the transformer. Hot spot temperature is the real temperature that determines the insulation life.

III. How to Monitor Hot Spot Temperature?

1. Indirect Calculation Method

Estimate the hot spot temperature by measuring the top oil temperature and load current based on the thermal circuit model. It has the advantage of low cost but the disadvantage of limited accuracy.

2. Optical Fiber Temperature Measurement

Embed optical fiber sensors inside the winding to directly measure the actual temperature. It features high accuracy and real-time monitoring, and is mainly applied in large transformers or high-reliability scenarios.

3. Infrared Thermal Imaging

Detect the surface temperature distribution of oil tanks, radiators and other components from the outside to assist in judging internal overheating. It is suitable for regular patrol inspection and external defect detection, but cannot directly measure the internal hot spot temperature.

4. Intelligent Monitoring System

Integrate load monitoring, oil temperature monitoring, hot spot calculation and early warning functions to realize continuous online monitoring.

IV. How to Control Hot Spot Temperature?

1. Design Phase

- Reasonably distribute current density to avoid local concentration

- Optimize the design of oil ducts to ensure cooling oil flows fully through heating parts

- Adopt transposed conductors to suppress additional losses

- Strengthen the cooling of end insulation

2. Selection Phase

- Select the capacity reasonably to avoid long-term overload

- Choose an appropriate cooling method (ONAN, ONAF, etc.) according to environmental conditions

- Adopt enhanced heat dissipation configuration for high-temperature or poorly ventilated places

3. Operation and Management

- Avoid long-term overload operation and arrange production load reasonably

- Record oil temperature regularly and set reasonable alarm thresholds (alarm at 85℃ for top oil temperature, trip at 95℃)

- Ensure good ventilation in the transformer room and prevent direct sunlight on outdoor transformers

- Keep the surface of radiators clean

4. Maintenance

- Regularly clean dust accumulation on radiators

- Check the oil level to ensure normal oil circulation

- Inspect the operation status of cooling equipment such as fans and oil pumps

- Test oil quality regularly and filter or replace oil when necessary

V. The Relationship Between Hot Spot Temperature and Transformer Service Life

Insulating paper, the main insulating material of transformers, undergoes thermal cracking at high temperatures, leading to a decrease in mechanical strength—a non-reversible process.

Under normal operating conditions (hot spot temperature not exceeding 98℃), the designed service life of a transformer is 20 to 30 years. Sustained high hot spot temperature will significantly shorten the service life:

Hot Spot Temperature	Relative Service Life
98℃	100%
106℃	Approx. 50%
114℃	Approx. 25%
122℃	Approx. 12.5%

Relationship Between Hot Spot Temperature and Relative Service Life

VI. Conclusion

Hot spot temperature is a barometer of the transformer's operating state, which is directly related to the equipment's service life and operational reliability. Every link, from design optimization and capacity matching to load management and cooling system maintenance, affects the thermal state of the transformer.

With the application of technologies such as optical fiber temperature measurement and intelligent diagnosis, our control over the internal temperature of transformers is becoming increasingly precise. Attaching importance to hot spot temperature, conducting scientific monitoring, issuing timely early warnings and implementing reasonable control are the keys to ensuring the long-cycle safe operation of transformers and maximizing economic benefits.