The temperature rise of an oil-immersed transformer refers to the difference between the maximum temperature of its internal components (oil, winding, iron core) and the ambient temperature during rated operation, which is a core indicator to measure its thermal load capacity and insulation life.

　　1. Key Components for Temperature Rise Measurement

　　The temperature rise of oil-immersed transformers focuses on three core parts, each with clear limit standards specified by international standards (e.g., IEC 60076) and national standards.

　　Note: "K" (Kelvin) here is equivalent to "°C" in temperature difference, so a 60K rise means the component is 60°C hotter than the environment.

　　2. Main Causes of Temperature Rise

　　The heat generated inside the transformer comes from three types of losses, which are the root cause of temperature rise:

　　1. Iron Core Loss (No-Load Loss): Generated by magnetic hysteresis and eddy currents in the silicon steel sheet core when the transformer is energized (even without load). This loss is stable and mainly heats the core and the oil in contact with it.

　　2. Winding Copper Loss (Load Loss): Caused by the resistance of the copper (or aluminum) winding when current flows through it. The loss is proportional to the square of the load current—higher load leads to significantly higher copper loss and faster winding temperature rise.

　　3. Additional Losses: Including stray losses in the core clamp, tank, and other metal parts (caused by leakage magnetic flux), and dielectric losses in the insulating oil. These losses are small but contribute to overall temperature rise, especially in large-capacity transformers.

　　3. Heat Dissipation Methods (Controlling Temperature Rise)

　　Oil-immersed transformers rely on "oil circulation + tank heat dissipation" to control temperature rise, with different cooling structures matching different capacities:

　　Natural Oil Circulation + Natural Air Cooling (ONAN):

　　The heated insulating oil rises to the top of the tank due to density differences, while the cold oil at the bottom flows down to the core and windings—forming natural circulation.

　　Heat is dissipated through the tank wall (and radiators) to the air via natural convection.

　　Suitable for small-capacity transformers (usually ≤ 630kVA) with low heat generation.

　　Natural Oil Circulation + Forced Air Cooling (ONAF):

　　Based on ONAN, axial fans are added to blow air onto the radiators, accelerating heat dissipation.

　　Heat dissipation capacity is increased by 30%-50% compared to ONAN, suitable for medium-capacity transformers (1000kVA-10MVA).

　　Forced Oil Circulation + Forced Air Cooling (OFAF):

　　Oil pumps are used to force oil circulation (greatly increasing oil flow rate), and fans are used to cool the oil in the cooler.

　　High heat dissipation efficiency, suitable for large-capacity power transformers (≥ 10MVA) with high load losses.

　　4. Impact of Excessive Temperature Rise

　　Exceeding the rated temperature rise will seriously affect the transformer’s service life and safety:

　　Insulation Aging Acceleration: The insulating paper (main insulation for windings) has a "temperature life coefficient"—for every 8°C increase in temperature above 105°C, its service life is halved (this is the "8°C rule" for transformer insulation).

　　Insulating Oil Deterioration: High temperature causes the oil to oxidize, producing acidic substances and sludge. This reduces the oil’s insulation performance and clogs radiators, further worsening heat dissipation.

　　Increased Failure Risk: Overheating may cause the winding insulation to breakdown (leading to short circuits) or the oil tank to deform, and in severe cases, it may trigger fires or explosions.

　　To help you quickly check the temperature rise standards and cooling methods for transformers of different capacities, do you want me to make a practical reference table that matches capacity ranges, cooling types, and temperature rise limits?