The stable temperature control of windings in oil-immersed transformers directly relates to operational safety, reliability, and service life. This guide explains temperature standards, cooling method requirements, temperature correlation logic, and abnormal handling measures – helping you maintain your transformer within safe operating limits.

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1. Core Temperature Standards and Insulation Material Properties

During operation of oil-immersed transformers, temperature control is a key link in ensuring equipment safety. Industry specifications and equipment characteristics define clear temperature limits:

Top Oil Temperature Red Line

• The top oil temperature must not exceed 95°C.
• In daily operation, to prevent long‑term high‑load operation, we typically require the top temperature to stay below 85°C.

These standards balance the thermal stability of transformer oil with the tolerance capacity of winding insulation materials.

Temperature Tolerance of Insulation Materials

Most oil‑immersed transformers use Class A insulation materials for their windings. The maximum allowable operating temperature ranges from 95°C to 105°C. The temperature tolerance of insulation materials directly determines the upper limit of transformer operation. Once the temperature exceeds this range, insulation performance declines rapidly – and irreversible aging damage may occur.

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2. Temperature Correlation Logic and Calculation Basis

The temperature monitoring and control system of oil‑immersed transformers shows a clear correlation between temperatures of various parts. This provides a scientific basis for accurately judging operational status.

Standard Operating Temperature Benchmark

The industry generally adopts 40°C as the standard operating temperature benchmark. This value considers the annual ambient temperature range in most regions worldwide – ensuring the universality and applicability of the standard.

Temperature Correlation Between Various Parts

Under standard operating temperature (40°C ambient):

Component	Temperature Rise Above Ambient
Winding average temperature (including surrounding gas)	65°C
Top oil temperature	55°C
Winding relative to oil (calculated)	10°C

Therefore, we can infer the winding temperature from the top oil temperature:

• When top oil temperature = 85°C → winding temperature = 85°C + 10°C = 95°C (lower limit of Class A insulation tolerance)
• When top oil temperature = 95°C → winding temperature = 95°C + 10°C = 105°C (maximum allowable – equipment is in critical state)

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3. Temperature Control for Different Cooling Methods

Oil‑immersed transformers have different heat dissipation efficiencies and temperature control standards depending on their cooling methods. Therefore, we must develop targeted operation and control measures.

3.1 Forced Oil Circulation Air‑Cooled (OFAF) Transformers

These transformers achieve cooling through forced oil circulation combined with fans – resulting in high heat dissipation efficiency. Consequently, their temperature control standards are more stringent.

Parameter	Limit
Top oil temperature (normal operation)	75°C
Corresponding temperature rise	35°C

Monitoring focus: Check the operating status of oil pumps and fans regularly to ensure the cooling system works properly.

3.2 Natural Oil Circulation Air‑Cooled (ONAN/ONAF) Transformers

These transformers rely on natural convection for heat dissipation – with fans for auxiliary cooling. Additional over‑temperature protection devices (temperature relays, pressure relief valves) are required.

Parameter	Limit
Normal top oil temperature	≤ 85°C
Maximum top oil temperature	95°C
Maximum temperature rise	55°C

Precaution: Natural convection efficiency is greatly affected by ambient temperature and oil flow. Therefore, increase temperature monitoring frequency during high‑temperature seasons or peak load periods.

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4. Hazards of Abnormal Temperature and Emergency Handling

Excessively high temperatures pose significant hazards – shortening service life and potentially causing serious safety accidents.

4.1 Multiple Hazards of High Temperatures

Hazard	Consequence
Insulation aging	Decreased mechanical strength and insulation resistance → winding short‑circuit faults
Oil deterioration	Increased dielectric loss, reduced breakdown voltage → loss of insulation and cooling functions
Shortened service life	20‑30 year design life may reduce to ~10 years before major repair or replacement
Safety accidents	Winding burnout, tank explosion, large‑scale power outages, economic losses

4.2 Abnormal Handling Process

Operation and maintenance personnel must continuously monitor temperature changes through temperature monitoring devices (platinum resistance thermometers, infrared thermometers, etc.).

When any temperature limit is exceeded, follow this emergency process:

1. Report immediately – Notify the production scheduling department. Provide equipment number, location, abnormal temperature value, and current load status.
2. Take load‑limiting measures – Follow scheduling instructions. Reduce electrical load or transfer part of the load to other transformers – reducing heat generation to control temperature.
3. Inspect the system – Assign professional personnel to inspect the cooling system, oil level, and check for leaks – identifying the cause of abnormal temperature.
4. Restore gradually – Only after the equipment returns to normal operation can the load be gradually increased.

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5. Summary of Temperature Limits

Parameter	Limit	Notes
Top oil temperature (normal)	≤ 85°C	Recommended daily limit
Top oil temperature (maximum)	95°C	Absolute limit
Winding temperature (Class A insulation)	95‑105°C	Lower/upper tolerance range
Temperature rise (top oil)	≤ 55°C	Above ambient (40°C baseline)
Temperature rise (winding over oil)	10°C	Calculated value