Principle of Transformer Winding Temperature Measurement:
The bulb of the winding thermometer is inserted into the top oil hole of the transformer tank. When the transformer load is zero, the winding thermometer reads the oil temperature. When the transformer is loaded, a current proportional to the load is taken from the current transformer, adjusted by a converter, and then flows through a heating element embedded in the bellows. The heat generated by the heating element increases the displacement of the elastic element. Therefore, when the transformer is loaded, the displacement of the elastic element is determined by both the top oil temperature and the load current. The temperature indicated by the winding thermometer is the sum of the top oil temperature and the winding‑to‑oil temperature rise, reflecting the hottest spot temperature of the measured transformer winding.
Under normal conditions, the winding temperature should be higher than the oil temperature. If the main transformer oil temperature is higher than the winding temperature, the following reasons may be responsible:
Faulty temperature measurement device:
The bulb of the winding thermometer is inserted into the top oil hole to measure oil temperature. The winding temperature is measured by taking a load‑proportional current from the current transformer, adjusting it with a converter, and passing it through a heating element inside the bellows. If the device measuring the oil temperature or winding temperature malfunctions—such as a damaged Pt100 sensor for oil temperature, circuit failure, or faulty temperature transmitter—the measured data will be inaccurate, creating a false indication that oil temperature is higher than winding temperature. For example, deviation in the resistance‑temperature relationship of the Pt100 can cause measurement errors.
Difference in measurement locations:
The oil temperature is measured at the top oil layer of the tank, while the winding temperature is measured indirectly via a simulation method. The temperatures measured at these two locations do not fully represent the true average temperatures of the winding and the oil. The top oil temperature is significantly affected by ambient conditions; if the ambient temperature is abnormally high during measurement, the measured oil temperature may be high. Meanwhile, heat transfer inside the winding is non‑uniform, and the selected measuring point can also affect the result, leading to a situation where the oil temperature appears higher than the winding temperature.
Excessive core loss:
If the core loss is too large, excessive heat is generated and transferred to the surrounding oil, raising the oil temperature. For instance, a short circuit in the core can drastically increase core loss, causing the oil temperature to rise rapidly and possibly exceed the winding temperature.
Stray flux issues:
Excessive stray flux can heat structural parts or the tank wall, and this heat is transferred to the oil, increasing its temperature. Improper winding arrangement, for example, can increase stray flux, causing the oil temperature to be higher than normal and even exceed the winding temperature.
Cooling system abnormalities:
An improperly functioning cooling system affects heat dissipation from both the windings and the oil. Examples include a stopped submersible pump (which prevents effective oil circulation), failed fans (which reduce forced‑air cooling), fouled cooling pipes (which lower heat exchange efficiency), or closed radiator valves (which block oil flow through the radiators). Any of these conditions can lead to excessively high oil temperatures. If the oil temperature rises more than the winding temperature, the oil temperature may end up higher than the winding temperature.
To understand the anomaly of “oil temperature higher than winding temperature,” it is necessary to clarify the measurement logic for both parameters:
Top oil temperature measurement:
Typically measured by a Pt100 sensor inserted into the top oil hole of the tank, directly reflecting the temperature of the oil at the top of the tank.
Winding temperature measurement (indirect simulation):
Static part: The thermometer bulb is also placed in the top oil hole; the elastic element (bellows) produces a base displacement corresponding to the oil temperature.
Dynamic part: A current proportional to the load is taken from a current transformer, adjusted by a converter, and passed through a heating element embedded inside the bellows.
Superposition principle: The heat generated by the heating element increases the displacement of the elastic element. Therefore, the winding thermometer indicates top oil temperature + winding‑to‑oil temperature rise, simulating the hot‑spot temperature.
Normal logic: Due to the copper losses in the winding, the winding temperature must be higher than the temperature of the surrounding oil. Thus, under normal operation, winding temperature > oil temperature.
If the oil temperature appears higher than the winding temperature, the cause is usually either a measurement system error or an abnormal internal thermal distribution. The specific reasons are as follows:
Based on Xinhong Electrical’s experience in electrical equipment maintenance, most such alarms originate from measurement circuit faults:
Open circuit in the winding thermometer heating circuit:
The simulated temperature rise of the winding thermometer relies on heating by the electrical heating element. If the current transformer secondary circuit is open, the converter fails, or the heating element burns out, the winding thermometer will only measure the top oil temperature. If at that time the transmitter or pointer has a positive offset, or if the oil temperature actually rises, the displayed oil temperature may appear higher than the displayed winding temperature.
Sensor characteristic drift:
If the Pt100 used for oil measurement ages over time, develops increased contact resistance, or suffers insulation degradation, the resistance‑temperature relationship may become distorted, leading to an erroneously high displayed value.
Calibration mismatch:
If the oil thermometer and winding thermometer are not synchronously calibrated after maintenance, their zero points or slopes may differ, causing inconsistent readings.
When the oil temperature truly rises, heat can be transferred back, causing the oil temperature to approach or even exceed the winding temperature:
Excessive core loss:
Local core short circuits, multiple grounding points, or insulation damage can generate enormous eddy current losses. Because the core is directly immersed in oil, the heat is transferred directly to the oil, causing the top oil temperature to rise sharply.
Stray flux heating of structural parts:
In large transformers, excessive stray flux can cause stray losses in structural parts such as the tank wall and clamping frames. These parts heat the surrounding oil, abnormally raising the oil temperature. Meanwhile, the winding load current may not increase significantly, so the simulated winding temperature rise remains low.
Poor oil circulation:
A stopped submersible pump, closed radiator valves, or blocked oil passages prevent effective oil flow through the radiators. Under these conditions, the oil temperature is generally high. Because the cooling medium flow is insufficient, heat accumulates inside the winding, but the top oil temperature measured may still appear high due to localized heating and stagnation.
Incorrect cooler control logic:
If some coolers are not activated according to the load, the oil temperature may rise faster than the simulated winding temperature rise.
To address the anomaly, the following logical steps are recommended to ensure safe transformer operation:
Verify the measurement circuits:
Check whether the CT circuit for the winding thermometer is open (an open circuit not only causes measurement failure but also poses a high‑voltage hazard).
With power disconnected, perform resistance‑temperature tests on both the oil temperature Pt100 and the winding thermometer (including its heating element) to confirm that the instruments are within specified accuracy.
Analyze operational data:
Compare historical data. If the difference between oil temperature and winding temperature gradually shrinks and eventually reverses, it usually points to core or structural part faults.
Observe the load curve. If the oil temperature does not vary significantly with load but remains high, focus on checking core grounding current and performing dissolved gas analysis (DGA).
Inspect the cooling system:
Verify the status of radiator valves, fan rotation, and submersible pump operation to ensure the cooling system is functioning efficiently.
Under normal conditions, the winding temperature should be the “upper envelope” of the oil temperature. If the top oil temperature appears higher than the winding temperature, a measurement system fault should be the first suspect. If the measurement system is normal, the possibility of abnormal heating in the core or structural parts due to internal faults should be carefully investigated.
