Winding Temperature Indicator (WTI)

 Winding Temperature Indicator (WTI)

📌 What is a WTI?

It is a device used in oil-immersed transformers to indirectly estimate the temperature of the windings. Installing a physical temperature sensor directly inside the windings is impractical due to strict electrical insulation requirements, design complexities, and maintenance challenges.

⚙️ How Does it Work?

The WTI operates on the principle of Thermal Imaging (Thermal Simulation) by combining two factors:

The temperature of the top oil.

The load current flowing through the windings.

🔍 The Mechanism: Current is drawn from a Current Transformer (CT) and fed into a small heating element inside the WTI device. This element simulates the temperature rise of the windings above the oil temperature, providing a reading that closely matches the actual Hot Spot temperature inside the transformer.

🛠️ Core Functions

🌡️ Temperature Estimation: Accurately estimates winding temperature.

🌬️ Fan Control: Automatically switches cooling fans ON and OFF.

💧 Pump Control: Controls oil pumps in forced cooling systems (OFAF/ODAF).

⚠️ Alarm Generation: Triggers an early warning signal when a specific temperature threshold is breached.

🚨 Tripping Mechanism: Issues a command to trip (disconnect) the transformer to prevent catastrophic failure at dangerous temperatures.

📋 Example of Standard Settings

🌬️ Fans ON: \approx 60°C

💧 Oil Pumps ON: \approx 75°C

⚠️ Alarm Trigger: \approx 110°C

🚨 Emergency Trip: \approx 120°C

(Note: These values are not fixed; they vary from one transformer to another based on its MVA rating, insulation class, and manufacturer recommendations).

🧠 Advanced Engineering Insights

🔍 Dual Monitoring: Some transformers utilize two independent indicators: one for the High Voltage windings (HV WTI) and another for the Low Voltage windings (LV WTI).

🔴 Maximum Pointer: The gauge typically features a red slave pointer that records the maximum peak temperature reached. It stays at that peak until it is manually reset by maintenance personnel during inspections.

🔄 OTI vs. WTI: The Difference

🛢️ Oil Temperature Indicator (OTI): Measures the temperature of the top oil layer (Top Oil) directly, relying solely on the temperature sensor bulb immersed in the oil.

⚡ Winding Temperature Indicator (WTI): Estimates the winding temperature indirectly. It doesn't just measure the oil; it combines the sensor bulb reading with the current value coming from the Current Transformer (CT) to simulate the true Hot Spot heat.

🏢 Distribution Transformers vs. Power Transformers

📦 Distribution Transformers (Small): Generally do not include a WTI. They rely solely on an OTI because they use natural cooling and require a more economical protection scheme.

🏭 Power Transformers (Medium & Large): A WTI is mandatory and critical for controlling fans/pumps and protecting a multi-million dollar strategic asset.