Introduction
Transformers are critical assets in electrical grids, and their operational reliability directly impacts both supply security and economic performance. Among the various stresses they face, temperature rise—caused by load variations, ambient conditions, and internal faults—is the most common and consequential. The lifespan of transformer insulation follows an exponential relationship with temperature: an increase of just 6–8 °C can double the rate of aging. Therefore, accurate, continuous temperature monitoring is not merely a maintenance aid; it is an essential safeguard against sudden insulation failure and catastrophic equipment damage.
In practice, transformer thermal monitoring relies on two complementary instruments: the Oil Temperature Indicator (OTI) and the Winding Temperature Indicator (WTI). The OTI measures the temperature of the top insulating oil, providing a general indication of the transformer’s overall heat load and enabling control of cooling systems and alarms. The WTI, by contrast, indirectly estimates the hot‑spot temperature of the windings using load current and oil temperature, offering a more direct measure of the thermal stress that the windings themselves experience. Together, they form the “dual core” of transformer thermal protection.
This article provides a systematic review of both devices, covering their construction, working principles, key features, and functional differences. A comparative analysis is also presented to help operators and maintenance engineers select, install, and interpret data from these instruments effectively, ensuring safe and prolonged transformer service life.
Oil Temperature Indicator (OTI) |  |
Introduction
Oil Temperature Indicators (OTI) are essential devices used to monitor the temperature of the insulating oil within a transformer’s tank. Oil Temperature Indicator can indicate the oil temperature of the transformer and operates the alarm, trip, and cooler control contacts. They play a critical role in preventing overheating, which can result from factors such as variations in load, changes in ambient temperature, and internal faults. By continuously monitoring oil temperature, OTI helps manage the transformer’s cooling system effectively.
When the oil temperature exceeds preset safe limits, cooling mechanisms such as fans can be activated to regulate temperature. By tracking temperature trends over time, maintenance personnel can proactively identify potential issues and schedule necessary maintenance activities. Additionally, monitoring oil temperature aids in maintaining the quality of the insulating oil, as elevated temperatures can accelerate oil degradation, leading to reduced insulating properties.
Functions
● Ensure Normal Operation: Maintains the oil temperature within a safe range, thus ensuring the efficiency and safety of the transformer.
● Prevent Faults: High temperatures can degrade the oil, compromising the insulation system and leading to electrical failures.
● Provide Maintenance Data: Continuous monitoring allows for better planning of transformer maintenance and repairs.
Construction
The OTI consists of:
● Sensing Element: Typically a capillary-type sensor that detects temperature changes through fluid expansion and contraction.
● Dial Display: A mechanical dial that shows the current oil temperature using a pointer.
● Alarm Setting Device: Includes adjustable alarm and trip set points, which trigger protective actions when temperature rises beyond safe levels.
● Electrical Contacts: Activate alarm systems or automatically disconnect power when temperature limits are breached.
Working Principle
● The capillary sensor within the OTI reacts to changes in oil temperature, causing fluid expansion inside the sensor.
● This expansion drives a mechanical transmission system that moves the pointer on the dial to display real-time temperature.
● When the temperature exceeds preset limits, electrical contacts close and activate alarms or initiate circuit disconnection.
Key Features
● Resettable Maximum Temperature Pointer: The OTI is equipped with a resettable maximum temperature pointer that allows users to easily identify the highest temperature reached, ensuring effective monitoring during operation.
● Versatile Alarm and Control Functions: Designed with up to two embedded switches, the OTI can integrate seamlessly into various alarm and control systems, providing flexibility in operation (including normally open and changeover options).
● Durable and Corrosion-Resistant Components: All components of the OTI are surface-treated and designed to withstand harsh operating conditions, ensuring longevity and reliability in challenging environments.
● High Visibility Dials: The OTI features high-contrast dials, available in both analog and glass variations, promoting quick and accurate reading of temperature levels at a glance.
● Extensive Dial Range: With a generous 260-degree dial deflection, users can easily observe and interpret temperature readings, enhancing usability and operational efficiency.
● Robust Environmental Protection: The indicator is housed in robust enclosures rated at IP55 or IP65, making it suitable for a wide range of installations, including those exposed to extreme temperatures as low as -60°C.
● Flexible Configuration Options: The OTI can be customized according to specific user requirements, providing numerous mounting configurations and functionalities to suit diverse applications.
Winding Temperature Indicator (WTI) |  |
Introduction
Winding Temperature Indicators (WTI) are essential instruments employed in power and distribution transformers to accurately monitor and report the temperature of transformer windings. These indicators are critical because the windings represent the primary conductive elements that carry electrical currents. Unlike Oil Temperature Indicators, which measure the temperature of the surrounding oil, WTIs focus on the actual temperature of the windings themselves, which typically operate at higher temperatures than the oil. This capability allows WTIs to provide a more precise understanding of the thermal stress experienced by the transformer and its proximity to critical temperature thresholds.
During normal operations, heat is generated in the windings as electrical current flows through them. Effective temperature monitoring offered by WTIs plays a vital role in asset management by enhancing maintenance scheduling and extending the operational lifespan of transformers. These indicators are designed to trigger alarms or activate trips when temperature readings exceed established safe limits, thereby protecting the equipment from overheating.
The windings are inherently the hottest components within the transformer and are subject to the most rapid increases in temperature as electrical loads vary. Therefore, accurately measuring winding temperature is crucial for managing the thermal parameters of transformers. The WTI works in conjunction with devices that monitor the temperature of the transformer oil, ensuring a comprehensive approach to thermal management.
The primary purpose of the WTI is to continuously indicate the winding temperatures of both high-voltage (HV) and low-voltage (LV) transformer windings. It plays a crucial role in operating safety by managing alarm systems, triggering trips, and controlling cooling mechanisms to maintain optimal operating temperatures. In essence, the WTI is a vital component in ensuring transformer reliability and longevity.
Functions
● Prevent Overheating: Helps prevent overheating of windings, thereby preventing insulation failure and transformer damage.
● Load Impact Detection: Monitors thermal accumulation caused by load changes, identifying potential overload or short-circuit risks.
● Maintenance Strategy: Utilizes temperature data for better load management and maintenance decisions.
Construction
WTI typically includes:
● Hot Spot Simulator: Simulates the hot spot temperature changes of the winding.
● Temperature Detector: Combines Current Transformers (CT) with thermistors to estimate winding temperature.
● Display: Either analog or digital for showing the calculated temperature.
● Setting Device: Allows for setting alarm and trip temperature points.
● Control Contacts: Connected to protective relay systems, triggering alarms or trips.
Working Principle
The Winding Temperature Indicator (WTI) operates on a principle similar to that of the Oil Temperature Indicator (OTI), with a crucial difference in its design and functionality. The WTI measures transformer winding temperature but does so indirectly to maintain safety in high-voltage environments.
The sensing bulb, located on the top cover of the transformer, is surrounded by a heater coil that is powered by current from secondary current transformers associated with the winding. The electric current flowing through this heater coil generates heat, which causes the surrounding oil to warm up. Consequently, the temperature around the bulb rises, leading to an expansion of the liquid inside the bulb. This liquid expansion is then transmitted through a capillary line to an operating mechanism, where it results in movement conveyed by a linked lever system.
This mechanism amplifies the expansion of the liquid, enabling it to drive a pointer. Consequently, as the transformer load increases, both the winding temperature and oil temperature rise, reflecting these changes in the WTI’s readings. Notably, since direct measurement of temperature inside the winding is not feasible, the WTI effectively infers winding temperature based on the temperature of the heater coil and the surrounding oil.
The WTI also features a maximum temperature indicator, calibrated to alert operators when the winding temperature reaches critical thresholds. Typically, alarms are triggered at 85°C, and a trip signal is activated at 95°C to safeguard the transformer against potential overheating and damage.
Key Features
● Six Switch Functionality: Equipped with the ability to control up to six switches, allowing for customizable alarm and control settings.
● Wide Dial Range: Offers a generous 260-degree dial deflection for optimal visibility and easy reading of temperature levels.
● Robust Switching Capability: Designed to handle high switching tasks efficiently without requiring additional components for fan bank management or alarm triggering.
● Diverse Analog Output Options: Supports various outputs, including mA, Pt 100, and Cu 10, catering to different monitoring and control systems.
● Durable Enclosure Ratings: Available in enclosures with IP55 or IP65 ratings, providing protection in a variety of environmental conditions, including extreme temperatures as low as -60°C.
● Adjustable Hysteresis: Features adjustable hysteresis for precise control, minimizing the risk of unnecessary alarms or trips.
Winding Gradient in Power Transformers
Measuring the temperature of the top oil in a transformer is crucial for assessing its overall operational condition. The top oil typically exhibits the highest temperature profile within the transformer, serving as an indirect measure for identifying potential hot spots in the direct windings. While the top oil temperature provides valuable insight, it may not accurately reflect the immediate windings’ thermal state, as it tends to change gradually due to the oil’s excellent insulation properties and substantial thermal mass.
To achieve a more precise understanding of the winding temperature, a comparison between the winding and top oil temperatures is essential. Since heat is primarily generated in the transformer windings, these regions often experience significantly higher temperatures. Elevated temperatures in the windings can lead to accelerated aging and may indicate insulation failure or operational faults.
The method used to determine the winding temperature can vary based on the technology employed. Common practices involve simulating the winding temperature using the Current Transformer (CT) current in a few ways. This can be achieved through internal mechanisms within the device, utilizing heated wells, or thermal plates. Such simulated winding temperature methods are valuable as they can also be applied retroactively to existing transformers, which is not the case with other solutions such as fiber optics.
By integrating measurements from both the top and bottom oil temperatures alongside those from direct windings, one can establish a highly accurate thermal model for the transformer. Studies suggest that even a slight increase in temperature, specifically 6 to 8 degrees, may effectively double the rate of life decay of the transformer. Thus, effective monitoring of thermal conditions is essential to prevent premature failure and ensure the longevity of transformer systems.
Comparative Table
| Feature | OTI (Oil Temperature Indicator) | WTI (Winding Temperature Indicator) |
| Monitoring Target | Transformer oil temperature | Transformer winding temperature |
| Primary Use | Ensure oil temperature remains safe to prevent overheating of transformer oil | Monitor hot spot temperature, indicating overload or overheating of winding |
| Temperature Principle | Direct measurement of oil temperature | Indirect estimation of winding hot spot temperature using oil temperature and load current |
| Signal Function | Provides real-time temperature data, alarm, and trip signals | Provides data on winding temperature, alarms, and trip signals |
| Structural Complexity | Simpler structure | More complex, requiring hot spot simulators and CTs |
| Application Scope | For monitoring the overall operational temperature of transformers | For analyzing transformer load conditions, especially in high-power or varying load scenarios |
| Importance | Initial protection for oil cooling systems | Critical for insulation protection of windings, has stronger preventative significance |
