How Thermometers Monitor the Operating Status of Oil-Immersed Transformers

Thermometers are core monitoring devices for oil-immersed transformers.They mainly track top oil temperature and winding temperature,reflect internal loss,heat dissipation performance and potential faults in real time,and work together with protection systems to ensure safe operation.

1.Main Types&Monitoring Objects

Two mainstream thermometers are widely used on site:

Oil temperature thermometer:Monitors the top oil temperature,the most intuitive indicator of overall operating temperature.It reflects combined heat generated by the iron core,windings and all internal components.

Winding temperature thermometer:Indirectly measures the hot-spot temperature of windings via thermal simulation.It focuses on the hottest part of coils,which directly determines insulation aging and service life.

2.Working Principle

Oil Temperature Thermometer

Most are gas-filled dial thermometers.The temperature sensing bulb is installed at the top of the oil conservator or main tank.Thermal expansion of internal gas drives the capillary tube and pointer to display real-time oil temperature.Some models integrate remote signal output for background monitoring.

Winding Temperature Thermometer

It adopts the thermal image principle.Based on the top oil temperature,it superimposes the additional temperature rise caused by winding load current.The reading simulates the actual hot-spot temperature of windings,which is always higher than the top oil temperature under load.

3.Judging Normal Operating Status

Refer to standard temperature ranges to evaluate operation:

Under rated load and normal heat dissipation,the top oil temperature stays 50°C~85°C.Steady and slow temperature changes indicate healthy operation.

The winding temperature is normally 10°C~20°C higher than the top oil temperature.A stable temperature difference proves the winding load is normal.

Temperature rises slowly along with load increase and drops gradually after load reduction,matching the thermal inertia of insulating oil.

4.Identifying Abnormal Conditions&Hidden Faults

(1)Continuous over-temperature

Top oil temperature exceeds 85°C or reaches the alarm value:Possible causes include long-term overload,blocked radiators,insufficient oil volume or degraded insulating oil.

Temperature keeps rising rapidly:It may be internal winding short circuit,core multi-point grounding or serious cooling system failure.Emergency inspection is required.

(2)Abnormal temperature difference

The gap between winding temperature and oil temperature becomes excessively large:It usually means local overheating of windings,poor contact of connectors or abnormal partial loss.

Two temperatures are close or nearly equal:The transformer operates under no-load or extremely light load,which is a normal state.

(3)Unusual temperature fluctuation

Temperature rises and falls sharply in a short time without obvious load change:Indicates poor thermometer contact,blocked sensing components or unstable internal operation.

Sudden temperature drop:May be accidental oil leakage or failure of the temperature sensing element.

5.Linkage with Protection&Control Systems

Thermometers are equipped with preset alarm and trip contacts:

When temperature hits the alarm threshold(normally 85°C),it sends out an audible and visual alarm to remind operators of inspection.

When reaching the trip threshold(normally 95°C),it triggers the circuit breaker to cut off power automatically,preventing insulation burnout and equipment scrapping caused by overheating.

For transformers with forced cooling systems,temperature signals control the automatic start and stop of cooling fans and oil pumps to maintain a stable operating temperature.

6.Daily Calibration&Maintenance

Regularly compare thermometer readings with portable temperature testers to check accuracy.

Inspect sensing bulbs,capillary tubes and wiring to avoid extrusion,damage or loose connection.

Clean dust and oil stains on the surface to ensure sensitive temperature collection.Replace damaged thermometers in a timely manner.

References

APA 7th Edition

Li,M.,&Chen,Y.(2023).Temperature monitoring and fault diagnosis technology for oil-immersed power transformers.IEEE Access,11,68215-68223.

MLA 9th Edition

Li,Ming,and Yong Chen."Temperature Monitoring and Fault Diagnosis Technology for Oil-Immersed Power Transformers."IEEE Access,vol.11,2023,pp.68215-68223.

IEEE Style

M.Li and Y.Chen,"Temperature monitoring and fault diagnosis technology for oil-immersed power transformers,"IEEE Access,vol.11,pp.68215-68223,2023.