How to Choose a Temperature Controller for Transformer Windings? Principles of Thermal Simulation and Installation Guidelines

• The Unique Value of Winding Thermostats: Normal oil surface temperature does not necessarily indicate that the winding temperature is safe. Winding temperature controllers use thermal simulation technology to estimate the temperature of hot spots in the windings, focusing on the temperature at the most critical locations within the transformer’s insulation. For main transformers rated at 110 kV and above, winding temperature controllers are indispensable protective devices.
• Principles of Thermal Simulation: By tapping into the transformer’s load current via a current transformer, the heat generated by the heating element inside the temperature probe is proportional to the square of the load current (simulating copper losses). By combining this with the surface temperature, the temperature of the winding hot spots is calculated. There is absolutely no need to embed sensors inside the high-voltage windings.
• Relationship with the Oil Temperature Controller: Oil-immersed temperature sensors monitor overall thermal conditions, while winding temperature sensors detect internal hot spots. The relationship between the two is similar to that between a thermometer (which measures surface temperature) and a CT scan (which locates internal lesions)—they complement each other.

1. A Detailed Explanation of Thermal Simulation Technology

The core of the winding temperature controller lies in thermal simulation technology—specifically, how to indirectly calculate the temperature at the hottest point inside the winding without coming into contact with the high-voltage winding. The technical implementation involves placing a heating element inside the temperature sensor, with the heating element drawing power from the low-voltage side bushing leads of the transformer via a current transformer (CT). The secondary current of the CT is proportional to the transformer’s load current, and the heat generated by the heating element is proportional to the square of the current, which precisely simulates the physical principle of winding copper loss (I²R). The temperature rise generated by the heating element is superimposed on the oil surface temperature, and the value displayed on the dial is the estimated temperature of the winding’s hottest spot.

The primary advantage of this indirect measurement method is safety—all temperature-sensing components are installed on the exterior of the transformer tank, completely isolated from the high-voltage windings, eliminating the need to enter the transformer during installation or maintenance. Proven through decades of use, thermal simulation technology delivers excellent accuracy under steady-state loads.

2. Installation Guidelines and Precautions

2.1 CT Installation

Install the current transformer kit on the bushing leads on the low-voltage side of the transformer. The CT ratio must be precisely matched to the parameters of the thermostat’s heating element; otherwise, the simulated temperature rise ratio will be inaccurate. During installation, ensure that the CT is oriented correctly and that the secondary circuit connections are secure—an open circuit on the secondary side of the CT can result in dangerously high voltages.

2.2 Thermocouple Installation

The sensor of the winding temperature controller and the sensor of the oil-level temperature controller are installed in two separate sensor sockets on the top of the transformer tank. Both sensors are immersed in the top layer of oil, but the winding temperature controller’s sensor contains a heating element; care must be taken during installation to protect the capillary tube path.

2.3 Protection of Capillaries

The capillary tube is the most fragile mechanical component of a thermostat. When routing the installation, avoid sharp bends, contact with vibrating parts, and mechanical compression. If necessary, install a metal flexible conduit or plastic corrugated tube to provide mechanical protection.

3. Frequently Asked Questions FAQ

3.1 Q: Is the winding thermostat accurate?

Answer: The thermal simulation method provides good accuracy under steady-state loads and meets the requirements of the vast majority of engineering applications. However, during transient processes with severe load fluctuations, thermal lag occurs—the thermal inertia of the temperature sensor causes the indicated value to lag behind the actual changes in winding temperature. If the load changes significantly within tens of minutes, it takes some time for the thermostat to reflect this on the dial.

3.2 Q: What are the consequences of selecting the wrong CT ratio?

Answer: If the CT ratio is too high, it will result in insufficient heating and low winding temperature readings—operations and maintenance personnel may assume the winding temperature is safe when it is actually overheating. An undersized CT ratio can result in excessive heating and high temperature readings—operations personnel may assume the temperature is too high when it is actually normal, leading to unnecessary derating or shutdowns. When selecting a CT, it is essential that the manufacturer calculates and matches an appropriate CT based on the transformer’s rated parameters.

3.3 Q: Can the capillary tubes for the winding thermostat and the oil level thermostat be shared?

A: No. Each thermostat has its own independent sensor bulb and capillary tube, mounted in separate sensor bulb holders. They are two completely independent temperature measurement systems, each measuring and outputting different temperature data.

Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements.

Looking for a temperature control solution for transformer windings? Contact Innotongda for product selection advice and technical support. Service Hotline: 13959168359 (same number on WeChat).