A WTI is a dual-function gauge that monitors and displays the internal winding temperature of a transformer. By combining oil temperature data with a simulated heating effect from the transformer’s load current, it provides a real-time estimate of the hottest point within the windings. Its primary role is to trigger cooling fans, sound alarms, or trip the circuit breaker if temperatures reach levels that could permanently damage the insulation.
The WTI is a critical component of transformer asset management because excessive heat is the leading cause of insulation aging and equipment failure. [
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- Operating Principle (Thermal Simulation):
- The WTI uses a sensing bulb placed in an oil-filled pocket (well) at the top of the transformer tank to measure the top oil temperature.
- A heating element inside the relay housing is fed by a current proportional to the transformer's load, usually via a dedicated bushing current transformer (CT).
- The additional heat generated by this element is added to the oil temperature, effectively creating a "thermal model" or replicated temperature that matches the actual winding hot-spot.
- Key Components:
- Sensor Bulb & Capillary: A liquid-filled bulb connected to the dial via a capillary tube. As the simulated temperature rises, the liquid expands and moves the dial pointer.
- Maximum Temperature Pointer: A secondary (usually red) needle that stays at the highest temperature reached during the day to help operators track peak loads.
- Mercury/Microswitches: Typically, four switches are used to automate protection:
- Switch 1: Controls cooling fans (ON/OFF).
- Switch 2: Controls oil pumps (for forced cooling).
- Switch 3: Triggers a High Temp Alarm (typically set at 85°C).
- Switch 4: Triggers a High Temp Trip (typically set at 95°C) to disconnect the transformer.
