During the operation of an oil-immersed transformer, its temperature is a critical parameter that affects equipment performance, lifespan, and safety. The temperature distribution and control require comprehensive consideration of multiple factors, including windings, iron cores, transformer oil, and the environment. Here is a detailed introduction:

　　I. Temperature Components of Oil-Immersed Transformers

　　Winding Temperature

　　The windings are the parts of the transformer through which current flows, generating heat due to resistive losses (copper losses), with temperatures typically being the highest.

　　During normal operation, the winding temperature is generally 10-15°C higher than the top oil temperature (due to the time required for oil circulation to carry away heat).

　　Prolonged high temperatures can accelerate the aging of insulating materials (such as paper and enameled wire), shortening the transformer's lifespan.

　　Iron Core Temperature

　　The iron core generates heat due to hysteresis losses and eddy current losses, with temperatures slightly lower than those of the windings.

　　Excessive iron core temperatures can cause localized overheating, leading to a decline in insulation performance.

　　Top Oil Temperature

　　Transformer oil cools the windings and iron core through circulation, with the top oil temperature reflecting the efficiency of the oil cooling system.

　　During normal operation, the top oil temperature generally does not exceed 85-95°C (specific limits depend on the insulation class and cooling method).

　　Bottom Oil Temperature

　　The bottom oil temperature is typically 5-10°C lower than the top oil temperature due to differences in oil circulation paths.

　　II. Temperature Limits and Standards

　　Insulation Material Temperature Ratings

　　Oil-immersed transformers commonly use Class A insulation (with a heat resistance temperature of 105°C), allowing the hottest spot temperature in the windings not to exceed 105°C.

　　Modern transformers may adopt higher heat resistance classes (such as Class E or Class B), but the equipment nameplate or manual should be consulted for accuracy.

　　Top Oil Temperature Limits

　　Natural Cooling (ONAN): The top oil temperature generally does not exceed 95°C, with a temperature rise (difference between oil temperature and ambient temperature) not exceeding 55K.

　　Forced Oil Circulation Cooling (OFAF/ODAF): The top oil temperature limit may be relaxed to 105°C, with a temperature rise not exceeding 65K.

　　International Standards (IEC/IEEE): Typically specify that the top oil temperature should not exceed 95-105°C, depending on the design.

　　Winding Hot Spot Temperature

　　The hot spot temperature (the hottest point) in the windings should be controlled within 105-120°C. Prolonged operation beyond this range can significantly accelerate insulation aging.

　　III. Temperature Monitoring and Control

　　Temperature Monitoring Methods

　　Top Oil Temperature Gauge: Directly measures the oil temperature at the top of the tank, reflecting the overall cooling effect.

　　Winding Temperature Indicator: Measures winding temperature through thermal simulation or fiber optic sensors (more accurate but higher cost).

　　Online Monitoring System: Integrates data such as temperature, load, and oil level for remote monitoring and early warning.

　　Cooling System Control

　　Natural Cooling (ONAN): Relies on natural oil convection and radiator cooling, suitable for small-capacity transformers.

　　Forced Oil Circulation Air Cooling (OFAF): Accelerates oil circulation through oil pumps and enhances cooling with fans, suitable for medium- and large-capacity transformers.

　　Forced Oil Circulation Water Cooling (OFWF): Uses water to cool the oil, suitable for extra-high-voltage or large-capacity transformers.

　　Intelligent Temperature Control: Automatically starts and stops cooling fans or oil pumps based on oil temperature to optimize energy efficiency.

　　Over-Temperature Protection

　　Alarm Threshold: Triggers an alarm when the top oil temperature reaches 85-90°C, prompting maintenance personnel to inspect.

　　Trip Threshold: Automatically cuts off power when the top oil temperature reaches 95-105°C or the winding temperature exceeds the limit to prevent equipment damage.

　　IV. Impact and Handling of Temperature Anomalies

　　High-Temperature Hazards

　　Accelerated aging of insulating materials, shortening the transformer's lifespan.

　　Degradation of oil quality (generation of acidic substances and sediment), reducing insulation and cooling performance.

　　Localized overheating may trigger electrical discharge or short circuits, even leading to transformer explosions.

　　Low-Temperature Hazards

　　Increased oil viscosity, reducing circulation efficiency and weakening cooling capacity.

　　Extreme low temperatures may cause oil solidification, affecting transformer startup and operation.

　　Handling Measures

　　High Temperature: Check if the cooling system is functioning properly, clear radiator blockages, reduce load, or enhance ventilation.

　　Low Temperature: Use heating devices to preheat the transformer oil, ensuring the oil temperature is not lower than -20°C before startup (specific limits vary by oil type).

　　Regular Maintenance: Conduct oil sample analysis, insulation testing, and cooling system cleaning to prevent temperature anomalies.

　　V. Practical Operation Recommendations

　　Ambient Temperature: Ensure good ventilation in the transformer room, avoid direct sunlight, and maintain an ambient temperature between -20°C and +40°C.

　　Load Control: Avoid long-term overload operation to prevent exceeding winding temperature limits.

　　Data Recording: Establish temperature history curves and analyze trends to predict potential faults.

　　Standard References: Follow standards such as GB/T 1094 Power Transformers and IEC 60076 Power transformers.