A temperature sensor inside or on a power transformer measures the temperature at its installed location, not automatically the hottest point in the winding. Reliable protection requires the sensor type, placement, thermal coupling, response time, insulation, calibration, and shutdown logic to be coordinated with the expected hot-spot location.
Hot spots are influenced by winding geometry, conductor current density, harmonic content, leakage field, cooling path, nearby heat sources, and airflow. The location can change between normal load, overload, and distorted-current operation.
Common sensing options
- RTDs: offer stable, repeatable resistance-versus-temperature behavior but require measurement wiring and isolation review.
- Thermistors: provide high sensitivity and compact size, though their response is nonlinear and tolerance must be considered.
- Thermocouples: cover a wide range and respond quickly, but low signal level, junction placement, and electrical noise matter.
- Thermal switches: provide simple threshold protection but limited temperature information and reset behavior.
- Winding resistance: estimates average winding temperature from the resistance change before and after loading.
Sensor placement is a thermal design decision
A sensor placed on the outside of a winding may respond slowly and read below the internal hot spot. Placing it deeper improves coupling but can disturb insulation, winding fit, creepage, or electric-field distribution. Leads can also conduct heat away from the sensing element and create a reading bias.
Prototype thermal mapping can compare embedded sensors, surface thermocouples, infrared imaging where visible, and winding-resistance temperature. The goal is to establish a defensible offset between the monitored point and the expected hot spot.
Account for response time
Large transformers change temperature slowly under continuous load, but local conductor heating can rise quickly during overload or fan failure. The sensor, adhesive, insulation layers, and control filter add delay. Protection logic should distinguish short harmless transients from events that can damage insulation, while avoiding a delay that lets the hot spot exceed its limit.
Keep sensor wiring electrically safe
Sensor leads cross magnetic and electric fields. Their insulation rating, routing, shielding, twist, separation, connector, and grounding must be defined. A grounded measurement channel can unintentionally bridge an isolation barrier. The sensor system should be included in dielectric and creepage review, not added after the transformer design is complete.
Validate the complete protection chain
Test sensor tolerance, measurement electronics, calibration, alarm threshold, shutdown threshold, restart behavior, open-circuit and short-circuit detection, and the response to lost cooling. Document whether temperature limits refer to measured sensor temperature, estimated hot spot, or average winding temperature.
BaoHui Tech recommends sharing the load profile, harmonics, ambient temperature, cooling method, insulation class, and required sensor interface when specifying a power transformer. These inputs help the transformer manufacturer reserve the correct sensor location and insulation routing before winding construction is frozen.
Frequently asked questions
Can infrared imaging find the winding hot spot?
It can identify visible surface temperatures, but internal windings, insulation, and enclosures may hide the true hot spot. Use it with other methods.
Why use winding resistance after a heat run?
Resistance provides an estimate of average winding temperature and can reveal a difference between surface sensor readings and internal heating.
Should the sensor be installed by the transformer manufacturer?
Often yes when it must be embedded or insulated within the winding. That allows placement and lead routing to be controlled on the drawing and in production.