Recommended 5 solutions for temperature monitoring of oil-immersed transformers
Date: December 9, 2025 15:23:23
- Mechanical OTI/WTI: Industry basic standard, through the current simulation calculation of winding temperature, non-direct measurement, there is a response lag.
- Pt100 electronic temperature measurement: Accuracy is better than mechanical, but the signal is prone to distortion in strong electromagnetic environments, and there is a risk of insulation creepage in the metal leads.
- Infrared Thermal Imaging: Only the surface of the tank and the casing joints can be monitored, it is not possible to penetrate the metal casing to monitor the internal core winding temperature.
- Wireless Passive SAW: The problem of high voltage isolation was solved, but the signal transmission was unstable due to the “Faraday cage” effect of the transformer tank.
- Fluorescent fiber optic temperature measurement (recommended) Adopting quartz fiber material, the fiber optic temperature sensing probe is directly pre-embedded into the high-voltage winding, anti-EMI interference, real-time accurate data.
1. Mechanical oil level/winding thermometer (OTI/WTI)
Mechanical thermometers are the most widely used monitoring devices in the history of oil-immersed transformers and usually exist as standard equipment from the factory. Its principle of operation does not rely on electronics, but is based on physical thermal expansion and contraction.
Technical principles and limitations
The oil level thermometer (OTI) drives the pointer by the expansion of the fluid in the temperature-sensitive package. The Winding Thermometer (WTI) is actually a “thermal simulator”. It is based on the OTI and takes a load current from a current transformer (CT), which drives a heating resistor inside the instrument. The temperature displayed by the gauge is not the directly measured winding temperature, but a calculated value of “top oil temperature + simulated temperature rise”.
Why is this approach stretched thin in the modern grid? This is because it is designed based on an ideal thermal model. When oil circuit blockage or dust accumulation in the radiator leading to poor local heat dissipation occurs inside the transformer, the thermal simulation model will be invalidated, and the normal values displayed by the meter may mask the real internal overheating fault.
2. Pt100 platinum RTD electronic monitoring
With the popularity of SCADA systems that require analog transmission to the backend, Pt100 sensors are beginning to be widely used. Utilizing the characteristic of linear change in resistance of platinum metal under temperature change, the temperature is converted into an electrical signal.
Challenges in the electromagnetic environment
Although the Pt100 is highly accurate, in the high EMF environment of an oil-immersed transformer, the metal conductor sensor becomes a receiving antenna for interfering signals. Despite the use of shielded wires, strong magnetic fields can cause the signal to jump. A more serious problem lies in insulation. Putting a metal probe deep inside a high-voltage winding of several hundred kilovolts requires an extremely high level of insulation treatment, and any insulation defect may lead to a breakdown accident, so the Pt100 can usually only be mounted in low-potential areas or on the wall of the oil tank.
3. Infrared Thermography
Infrared technology utilizes the Stefan-Boltzmann law to capture the distribution of infrared radiation on the surface of an object. It is categorized into hand-held patrol and on-line window monitoring.
The Surface-Inside Divide
The transformer case is only 50°C, could the internal windings have reached 90°C? The answer is yes. The biggest shortcoming of infrared technology is its inability to penetrate metal. Oil-immersed transformers have fully enclosed metal tanks that completely block internal radiation. Infrared camera can only be used to detect loose casing joints, heat sink blockage or tank eddy current overheating and other external defects, for the decision of the transformer life of the core indicators - “winding hot spot temperature”, infrared technology can not help.
4. Wireless passive surface acoustic wave (SAW)
SAW technology attempts to solve the problem of high voltage insulation. The sensor is passive and transmits temperature data by generating an echo from the RF signal received from the reader.
Trouble with the Faraday cage effect
The transformer oil tank is a perfect Faraday cage, which has an extremely strong shielding effect on wireless RF signals. Although it can be solved by adding an antenna inside the tank, the complex core and clip structure inside the transformer and the transformer oil medium will produce multipath effects and attenuation on the microwave signals. In addition, the electromagnetic wave spectrum generated by partial discharges inside the transformer may cover the operating frequency band of the SAW, resulting in a sharp decrease in the signal-to-noise ratio and serious data packet loss.
5. Fluorescent fiber optic temperature measurement system (recommended)
Fluorescent fiber optic temperature measurement technology is currently able to safely penetrate into the core of the oil-immersed transformer, to achieve “direct measurement of winding hot spots” technical means. It is based on rare earth fluorescent material afterglow life principle, and light intensity has nothing to do with, only with the temperature.
How to maintain monitoring stability for 30 years under the extreme environment of high pressure, strong magnetic field and chemical corrosion?
- Intrinsically safe insulation: The optical fiber is made of quartz (SiO2), which is intrinsically insulating and has excellent creepage resistance. It can be directly pre-buried between the high-voltage windings of 110kV or even 500kV transformers without fear of insulation breakdown.
- Electromagnetic immunity: The optical signal transmission is not affected by any electromagnetic interference (EMI/RFI) and the temperature data is always stable and reliable, regardless of whether the transformer is under short-circuit shock or lightning overvoltage.
- Resistant to oil corrosion: Using special Teflon (PTFE) or PEEK sheath fiber optic probe, resistant to transformer oil immersion for a long period of time, no aging, no pollution of the oil, the design life and transformer body synchronization.
- Calibration-free: Fluorescence lifetime is a physical property of the material that does not drift over time, requiring no recalibration for life and minimal maintenance.
In-depth comparison of the technical parameters of the 5 monitoring programs
| comparison dimension | Mechanical OTI/WTI | Pt100 Electronic Resistors | infrared thermography | Wireless Passive SAW | Fluorescent Fiber (Recommended) |
|---|---|---|---|---|---|
| Measuring principle | Liquid expansion/thermal simulation | the resistive thermal effect (physics) | radiographic imaging | acoustic frequency shift | Fluorescence afterglow lifetime |
| measuring position | Top oil/simulation calculations | Tank wall/top oil | External surfaces | Internal Surfaces | Hot spots inside the winding |
| Insulation safety | Low (with metals) | Low (lead risk) | High (non-contact) | center | Very high (all media) |
| anti-interference capability | center | differ from | excellent | differ from | Excellent (fully immunized) |
| Authenticity of data | Calculation of the presumed value | off-spot value | surface temperature | local point value | direct and true value |
Is it possible to determine the health of a transformer by relying on the top oil temperature alone?
Apparently not. According to IEC 60076-7, the aging rate of a transformer is mainly determined by the temperature of the hottest point of the winding. There is a temperature difference between the top oil temperature and the winding hot spot, and this temperature difference varies non-linearly with load. Monitoring only the oil temperature is like a doctor taking the temperature of the body without looking at the CT scan, it is very easy to miss the internal localized overheating lesions.
Why is “hot spot” monitoring the key to improving transformer load capacity?
Many transformers run at their nominal capacity of 60%-70% for long periods of time, resulting in huge idle assets. This is because the operation and maintenance personnel can not accurately grasp the true internal temperature, do not dare to increase the load. If a fluorescent fiber optic temperature measurement system is installed, which can see the real hot spot data of 95℃ or 105℃ in real time, the operation and maintenance personnel will be able to boldly carry out dynamic capacity increase within the safety range, tap the potential of the equipment and create direct economic benefits.
Frequently Asked Questions (FAQ)
Q1: Will installing a fluorescent fiber optic probe inside a transformer affect the insulation performance?
A: No. The fiber optic probes supplied by Inotera are made of high purity quartz and oil-resistant Teflon/PEEK materials, with extremely high electrical insulation strength and creepage resistance, and have undergone rigorous high-voltage localized discharge testing to fully meet the requirements of high-voltage transformer internal installations.
Q2: How long can a fiber optic temperature measurement system be used in oil?
A: Our fiber optic sheaths are specially treated to resist transformer oil corrosion. Designed to last more than 30 years, it is synchronized with the full life cycle of the transformer and does not need to be replaced during that time.
Q3: Do I need to cut a hole in the transformer tank to install fluorescent fiber?
A: There needs to be a location to install the penetrator. Usually we reserve or modify a flange port in the tank wall and lead the fiber through a special sealed penetrator to ensure the tank is absolutely sealed without the risk of leakage.
Q4: What communication protocol does Fluorescent Fiber Controller support?
A: Support standard Modbus RTU (RS485) protocol, also optional IEC 61850 protocol, can be easily accessed to the substation automation system or SCADA background.
Q5: Can old transformers be retrofitted with fluorescent fiber optic temperature measurement?
A: Yes, but it is usually recommended to do this when overhauling the lifting shroud as the probe needs to be secured to the winding. For new transformers from the factory, pre-burial during the manufacturing phase is the best time.
Q6: What is the biggest advantage of fiber optic temperature measurement over wireless temperature measurement?
A: The biggest advantage is “stability” and “accessibility”. Wireless signals are highly attenuated and susceptible to interference inside the tank, whereas fiber optic transmission has very low loss, is highly resistant to interference, and can physically reach hot spots deep inside the windings.
Q7: What is the response speed of fluorescent fiber optic temperature measurement?
A: Extremely fast. Sampling times are typically in the seconds per channel, capturing rapid temperature rises due to sudden load changes in real time.
Q8: Does the probe break easily?
A: Modern industrial grade fiber optic probes have a multi-layer reinforced jacket (e.g. Kevlar reinforced) over a quartz fiber core, which provides strong tensile and compressive strength and can adapt to the vibration environment inside the transformer.
Q9: How many temperature measurement points are usually installed in a transformer?
A: A configuration of 3-6 points is recommended. Typically distributed at the top of the three-phase windings (the hottest area) as well as at the top of the core to provide all-around coverage of potential hot spots.
Q10: Does the device need to be calibrated periodically?
A: Not required. Fluorescence afterglow life is an inherent physical property of the material that does not drift over time, so the system is calibration-free for life.
Advantages of choosing Inno Tongda
As a leader in fiber optic temperature measurement, Inno Tongda focuses on providing highly reliable monitoring solutions for power assets. Our fluorescent fiber optic temperature measurement systems have been widely used in the State Grid, Southern Power Grid and many industrial rectifier transformer projects.
Our core competencies:
- Customized probe process: Customized PEEK/PTFE probes are available for different voltage levels and oil immersion environments to ensure zero local discharge and zero leakage.
- Cost-effective program: Relying on self-developed core algorithms and production capacity, we offer highly competitive prices to break the monopoly of high prices of imported equipment.
- Rapid response delivery: Standing stock, support for expedited orders, provide full-process technical support from installation guidance to commissioning and acceptance.
- Real-life case validation: With many successful operation cases of 110kV and above main transformers, the data is stable and reliable, trusted by users.
Need a customized oil-immersed transformer temperature measurement solution or to get an up-to-date quote?
Contact us today for one-on-one professional selection consultation with an Innotekda engineer.
