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The winding temperature indicator of a transformer works through a combination of direct oil temperature detection and simulated heating that reflects winding conditions. Its thermal bulb is inserted into the upper oil hole of the transformer tank. When the transformer carries no load, the indicator simply shows the oil temperature at the top layer. Once the transformer is loaded, the current transformer extracts a signal proportional to the load current. After adjustment through a converter, this current flows through an embedded heating element placed inside a bellows system. The heating element releases heat, causing an elastic component to expand.
The magnitude of this expansion is determined by two key factors: the top oil temperature and the magnitude of the transformer load current. Therefore, the winding temperature indicator reflects a composite value—the oil temperature at the top of the tank plus the additional heating caused by the windings’ electrical losses. This combination aims to approximate the hottest spot in the transformer winding. Under normal circumstances, the winding temperature should always be higher than the oil temperature. However, anomalies sometimes occur where the main transformer’s top oil temperature surpasses the winding temperature, pointing to several potential causes.
Both oil and winding temperature measurements rely on instrumentation that is prone to failure. The oil temperature is gauged via thermal bulbs or sensors, while the winding temperature is indirectly derived through simulated heating. If any part of this chain malfunctions—such as a damaged PT100 resistance sensor, wiring issues, or a defective transmitter—the readings may no longer represent reality. For instance, when the resistance-to-temperature curve of a PT100 sensor shifts, the system might falsely indicate that the oil is hotter than the winding. These errors create the illusion of abnormal thermal behavior, even when the transformer itself is operating correctly.
The oil temperature measurement is localized at the very top of the oil tank, while the winding temperature is an artificial representation influenced by load current and oil heating. These two parameters inherently reflect different aspects of the system. Top oil temperature is highly sensitive to environmental fluctuations, particularly ambient heat conditions. If the surrounding climate is exceptionally hot, the top oil sensor may record a value disproportionally elevated. Meanwhile, the windings’ internal heat distribution is uneven, and the simulated heating model cannot perfectly capture their thermal gradient. This disparity between the two measuring methods may cause occasional instances where the oil temperature appears higher than the winding temperature.
The transformer’s core is designed to minimize magnetic losses, but defects or abnormal conditions can alter this balance. When core losses become excessive—perhaps due to short-circuited laminations or deteriorated insulation—heat production within the core escalates. This surplus heat does not primarily raise the winding temperature; instead, it radiates outward into the surrounding oil. The result is a significant rise in oil temperature, which can sometimes exceed the apparent winding temperature. Such a condition is a red flag, often indicative of deeper structural issues within the magnetic core.
Improper winding design or misaligned assembly can intensify leakage flux. Excessive stray flux induces heating in metallic structures such as clamping plates, tank walls, or other non-magnetic components. This localized heating readily transfers to the transformer oil, raising its temperature more sharply than that of the windings. In extreme cases, this phenomenon can cause persistent oil overheating while winding indicators remain deceptively stable.
The cooling system plays a decisive role in maintaining thermal equilibrium. Malfunctions here directly skew the balance between oil and winding temperatures. If an oil pump fails, circulation slows, and the heat from the core and windings accumulates in the upper oil layer. If fans stop functioning, air cooling efficiency plummets, leaving the oil hotter than usual. Blocked or fouled radiator tubes reduce heat dissipation capacity, while closed or malfunctioning valves can obstruct oil flow to external coolers. Each of these conditions leads to uneven cooling: oil heats rapidly while the winding simulation system, tied to current input rather than actual cooling performance, may not reflect the same escalation. The consequence is a scenario where the oil temperature surpasses the winding temperature.
The phenomenon of a transformer’s top oil temperature exceeding the winding temperature should not be dismissed as trivial. It often stems from measurement inaccuracies, but it can also signal underlying operational irregularities such as excessive core loss, stray flux heating, or cooling system malfunctions. Identifying the root cause requires careful examination of both instrumentation and the physical state of the transformer. Ultimately, precise temperature monitoring and prompt investigation are essential to prevent minor anomalies from escalating into serious operational failures.
