What are the types of transformer temperature sensors?

I. Why is transformer temperature monitoring so important?

Transformer is the core equipment of the power system, long-term operation in high voltage, high current environment, heat accumulation is the primary factor causing failure. Studies have shown that the insulation material is warmed up every 6 ° C, the service life will be shortened by about half. Once the temperature is out of control, the light is to accelerate the aging of insulation, shorten the life of the equipment, the heavy is to trigger a short circuit winding, transformer combustion and even large-scale power outages.

Because of this, the transformer temperature sensor has become an indispensable "health guard" for each transformer. It real-time collection of winding, core and oil temperature, temperature control system to provide data basis, automatically triggering the air-cooled start and stop, over-temperature alarm and even emergency tripping.

This article will systematically introduce the current mainstream six categories of transformer temperature sensors, focusing on the analysis of the higher technical threshold of the fiber optic temperature measurement program, and to provide practical advice on selection, to help engineers and procurement staff to quickly find the right product for their projects.

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II. Overview of the six mainstream types

According to the different working principles and applicable scenarios, transformer temperature sensors can be divided into the following six categories: PT100 platinum RTDs, thermocouples, fiber-optic temperature sensors, bimetal/expansion thermometers (BWY/BWR), NTC/PTC thermistors, and RFID wireless passive temperature sensors. The following is an in-depth analysis of each of them.

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Third, PT100 platinum RTD temperature sensors

Working Principle

The PT100 is one of the most classic industrial temperature measuring elements. Its core is a platinum wire wound resistor - the resistance of platinum increases linearly with temperature, 0 ° C resistance value of exactly 100 Ω, thus the name "PT100". Temperature controller through the precision measurement of the resistance value of the change, can be accurately converted to the current temperature.

Main features

High measurement accuracy, good linearity, error is usually within ±0.3°C; strong stability, long-term use of small drift; in line with the international common standards (IEC 60751), good compatibility with various brands of temperature controllers; moderate price, easy to install, is the standard temperature measurement program for dry-type transformers.

Typical Application Scenarios

PT100 sensors are widely embedded inside the three-phase windings of dry-type transformers and are connected via wires to the BWDK Series temperature controller (such as BWDK-S201, BWDK-Q201, etc.), real-time acquisition of winding temperature, display and multi-level alarm control. Usually each transformer configuration 3 road (three-phase all the way), some high-end program to increase the core temperature measurement point total 4 ~ 6 road.

💡 dry-type transformer factory, PT100 sensors are usually pre-embedded by the manufacturer in the winding casting body, the user side only need to correctly connect the temperature controller terminals.

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IV. Thermocouple temperature sensors

Working Principle

Thermocouples are based on the "Seebeck effect": the two ends of two different metal conductors are connected separately, when there is a temperature difference between the two points of contact, the circuit will produce a weak thermal electromotive force, the voltage signal is proportional to the difference in temperature, which is converted by the transmitter to obtain the temperature value. Common specifications are K-type (Nichrome-Nickel-Silicon) and T-type (Copper-Copper-Nickel).

Main features

Measurement range is very wide, up to 1000°C or more; fast response time, suitable for dynamic temperature monitoring; simple structure, low cost; accuracy is slightly lower than PT100, in the low temperature section (0 ~ 200°C) non-linear error is relatively obvious.

Typical Application Scenarios

In the field of transformers, thermocouples are mainly used for the auxiliary measurement of oil temperature in the top layer of oil-immersed transformers, or to replace PT100 in special high-temperature working conditions. for the hot spot monitoring of the windings, which requires high temperature measurement accuracy, it has been gradually replaced by fiber-optic sensors.

💡 The biggest difference between a thermocouple and a PT100 is that the thermocouple outputs a voltage signal, which is susceptible to strong electromagnetic field interference, so special attention needs to be paid to shielding when used inside the transformer body windings.

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V. Fiber Optic Temperature Sensors (Highlighted)

▶ This is currently recognized as the optimal temperature measurement solution for large transformers and precision applications.

What is fiber optic temperature measurement of transformer windings?

Fiber optic temperature measurement is a special fiber optic sensing probe directly embedded in the transformer winding turns or coil inside, the use of optical signals (rather than electrical signals) to transmit temperature data of the new temperature measurement technology. As the fiber itself is not conductive, not affected by electromagnetic fields, can be safe and reliable work in a high-voltage strong magnetic field environment, is the traditional electrical sensors can not be compared.

Working Principle

Fluorescent fiber optic temperature measurement: A special fluorescent material is coated on the end of the fiber optic probe. When the excitation light is irradiated, the fluorescence decay time changes with temperature. The pyrometer accurately calculates the temperature of the temperature measurement point by measuring the duration of the fluorescent afterglow. This method has high precision and fast response, and is the mainstream solution for hot spot temperature measurement of transformer windings.

Six core advantages of fiber optic temperature measurement

Intrinsically safe insulation: fiber optic is non-conductive material, can be safely embedded inside the high-voltage winding, no risk of electric shock; strong anti-electromagnetic interference: purely optical signal transmission, completely unaffected by the strong magnetic field of the transformer; hotspot temperature direct measurement: probe can be placed near the hottest point of the winding (between the turns), measured the real hotspot temperature, rather than the average temperature; multi-point synchronous monitoring: a set of hosts can be accessed at the same time multi-channel probe, covering the three-phase windings Multi-point synchronous monitoring: one set of host can simultaneously access multiple probes, covering three-phase windings and iron cores; good long-term stability: no metal oxidation, no electrical drift, and the life span is synchronized with that of the transformer; support intelligent integration: data can be uploaded to the intelligent monitoring platform via RS485/Modbus or Ethernet.

Typical Application Scenarios

220kV and above large power main transformer; wind farms, photovoltaic power station box-type transformer (high load fluctuation occasions); subway, high-speed rail traction transformer; offshore platforms, data centers and other occasions that require very high reliability.

💡 Inotera PHM-300U Transformer Intelligent Monitoring Device supports fiber-optic winding temperature measurement access, can simultaneously monitor the three-phase winding temperature, core temperature, ambient temperature and humidity and vibration data, and supports Modbus RTU remote transmission, which is the ideal choice for the intelligent transformation of large transformers.

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VI. Bimetal / Expansion Thermometers (BWY/BWR Series)

Working Principle

This type of sensor has a long history, the principle is intuitive: the use of temperature-sensitive package of special liquids (or bimetal) with the temperature change and expansion and contraction, through the capillary tube to promote the pointer deflection, directly on the dial to indicate the current temperature. Some models are also built-in microswitch, the temperature exceeds the set value when the contact action, the output alarm signal.

Main features

No external power supply, simple structure, intuitive and reliable; suitable for oil-immersed transformer top oil surface temperature and winding temperature monitoring; some models (such as the BWR series) built-in PT100 components, taking into account the pointer display and telecommunication function; weather resistance, suitable for outdoor substation use.

Main Product Models

BWY-802/803A, BWY2-803A: oil level thermostat, suitable for oil-immersed transformer top oil temperature measurement; BWR-04J/BWR-06: winding thermometer, built-in PT100, support temperature telecommunication; BWY-804/BWY-806: with telecommunication signal output, can be used with online monitoring system.

💡 BWY series mainly reflects the oil surface temperature (indirectly projected winding temperature rise), BWR series is closer to the actual temperature of the winding through the simulation of the thermal model, precision occasions are recommended to give priority to the BWR series or fiber optic solutions.

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VII. NTC/PTC Thermistor Sensors

Working Principle

NTC (Negative Temperature Coefficient) thermistor: resistance value decreases rapidly when the temperature rises, very high sensitivity.PTC (Positive Temperature Coefficient) thermistor: resistance value rises sharply when the temperature exceeds the Curie point, often used as an over-temperature protection switching element.

Main features

Extremely small size, low cost, fast response speed; NTC is suitable for continuous temperature monitoring of small transformers; PTC is mostly used for over-temperature protection triggering, which is not suitable for accurate temperature measurement; strong non-linear characteristics, usually need software to do linearization compensation.

Typical Application Scenarios

It is mainly used for simple over-temperature protection in small and medium-sized dry-type transformers, distribution boxes, or embedded in cost-sensitive applications such as motor windings. In large and medium-sized power transformers, it is usually not used alone, but as an auxiliary protection element with PT100 or fiber optic system linkage.

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Eight, RFID wireless passive temperature measurement sensor

Working Principle

Passive wireless temperature measurement is a new technology rapidly popularized in the field of switchgear and transformer terminals in recent years. Sensor tags without built-in batteries, by the reader through the radio frequency signal for its wireless energy supply, the temperature-sensitive chip within the tag to collect data instantly back to the whole process without the need for power lines and signal lines.

Main features

Completely passive: no batteries, no wires, can be installed for up to ten years maintenance-free; installation is very simple: paste or screws can be fixed, the transformation cost is very low; waterproof and dustproof, resistant to harsh environments; moderate temperature measurement accuracy (± 1 ~ 2 ° C), to meet the needs of hot spot monitoring of the terminals; support for multi-point networking, the reader can manage dozens of temperature measurement points at the same time.

Typical Application Scenarios

Transformer high-voltage side, low-voltage side terminals (casing will be military cap), busbar connection points, switchgear contacts, cable intermediate joints and other parts of the electrical line can not go, is the core of RFID wireless temperature measurement application scenarios. Temperature measurement with the transformer body winding (PT100/fiber optic) to form a complementary, together to build a complete hotspot monitoring system.

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IX. Horizontal comparison of the six types

Note: ★ Fiber optic sensors have the best overall performance in the field of hot spot measurement of large transformer windings, but the initial investment cost is high, suitable for high-reliability requirements.

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X. How to choose the right transformer temperature sensor?

There is no absolute right or wrong selection, the key is to match the actual needs.

① Selection by transformer typeDry-type transformer preferred PT100 + BWDK series temperature controller; oil-immersed transformer preferred BWY/BWR series; box-type transformer/new energy station is recommended to add fiber-optic winding temperature measurement + online monitoring platform.

② Selected according to monitoring accuracy requirementsGeneral monitoring (over-temperature alarm, air-cooling control) PT100 or BWY/BWR can be used; precision hot spot monitoring (life assessment, overload management) must choose fiber optic temperature measurement.

③Selected according to whether remote transmission/online monitoring is required.For local display only, choose pointer type or temperature controller with display; for remote transmission/system integration, choose sensor with RS485 Modbus output, or connect to online monitoring device.

④Selected by terminal block/modification occasion: Casing will be military cap, bus and other parts of the line can not be selected RFID wireless passive temperature measurement, zero wiring can be completed the transformation.

💡 Not sure how to select the type? Inotera provides free technical selection consulting, professional team according to the transformer model, the use of occasions and budget to give the optimal program. Contact hotline: 13959168359

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XI, Inotera transformer temperature monitoring product recommendations

Ltd. is a professional supplier in the field of transformer temperature control and intelligent monitoring, covering all mainstream sensor types and supporting systems: dry-type transformer temperature controller (BWDK/IB series), fiber optic temperature measurement of windings (with PHM-300U intelligent monitoring device), oil transformer temperature controller (BWY/BWR series), RFID wireless passive temperature measurement device, temperature and humidity sensors, etc. We have a full range of product models, support OEM/ODM customization, and our customers include State Grid, Southern Power Grid, TBEA, Jinpan Technology and other famous enterprises.

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XII. Frequently Asked Questions (FAQ)

Q: Can the temperature measurement sensors for dry-type transformers and oil-immersed transformers be common?

A: Cannot be directly generalized. Dry-type transformer windings exposed to air, use PT100 platinum RTD with BWDK series temperature controller; oil-immersed transformers need to use BWY/BWR expansion thermometer or special sensor with sealed structure. The two installation interface, working environment and signal type are different, mixed use will lead to measurement inaccuracy or even damage to the equipment.

Q: Fiber optic temperature measurement is much more expensive than PT100, when is it worth the investment?

A: The initial cost of fiber optic temperature measurement is indeed higher than PT100, but the return on investment is significant in the following occasions: ① Transformer long-term high-load operation (such as wind power, data center box variable), the risk of hotspot over-temperature is high; ② Transformer high value of a single unit (220kV and above main transformer), once the loss of damage to the loss of a huge; ③ Need to accurately assess the insulation life of the transformer and overload capacity. On these occasions, the investment of fiber optic temperature measurement can effectively reduce unplanned downtime, and the comprehensive economic benefits are much higher than traditional solutions.

Q: How many temperature sensors need to be installed in a transformer?

A: standard configuration, dry-type transformers are usually installed with 3 PT100 (one for each three-phase winding), and some programs increase the core temperature measurement points to a total of 4 to 6; oil-immersed transformers are usually equipped with 1 to 2 oil level thermometers (BWY series) and 1 winding thermometer (BWR series). If the fiber optic temperature measurement or online monitoring program, you can also add multiple hotspot temperature measurement probes as needed to further enhance the monitoring coverage.

Q: Which is more accurate, RFID wireless temperature measurement or wired temperature measurement?

A: accuracy, high quality wired PT100 sensor accuracy of ± 0.3 ° C, slightly better than the RFID wireless temperature measurement (± 1 ~ 2 ° C). But the two application scenarios are not the same: wired temperature measurement is suitable for winding and other fixed temperature measurement points; RFID wireless temperature measurement advantage is deployed in the bolt joints, bushings will be military cap, busbar and other locations that can not be routed at all. In the complete transformer hotspot monitoring system, it is recommended that the two are used in combination, not as a substitute for each other.

Q: The transformer thermostat shows a high temperature, is it a faulty sensor or is it really overheating?

A: There are two possibilities, you need to check step by step. First check the temperature controller shows the three-phase temperature whether a phase is particularly high - if a phase is abnormally high, first check whether the corresponding sensor wiring is loose, short-circuit or disconnection; if the three-phase are high, it is more likely that the real overheating, you need to check theCooling FansWhether it is operating normally and whether the transformer is overloaded. If you need professional support, you can contact Inotera's technical team to assist in judgment remotely.

Q: What special attention needs to be paid to temperature sensors in outdoor substations?

A: Outdoor occasions need to pay attention to: ① protection level of IP54 or above, to prevent the intrusion of rain and dust; ② sensors need to be able to work stably in a wide range of temperatures from -40 ° C to +85 ° C; ③ temperature and humidity controllers and heaters need to be configured in areas with a big difference in temperature between day and night, to prevent the insulation from dropping due to condensation; ④ signal lines need to be well shielded and surge protection, to avoid damage to the temperature controller from lightning strikes.

Q: Can fiber optic temperature measurement probes be added later during transformer retrofit?

A: Fluorescent fiber optic probes usually need to be pre-embedded in the transformer winding casting (dry-type) or winding (oil transformer) stage, it is difficult to retrofit in-service transformers without disassembling the windings after leaving the factory, so it is more suitable for new projects to plan fiber optic temperature measurement solutions. For the transformation and upgrading of in-service transformers, the more common practice is to install RFID wireless temperature measurement in terminals, bushings and other external hotspots, combined with the oil temperature online monitoring device, to realize the upgrade of the overall perceptual ability, without the need for major dismantling and overhaul.

Q: How to judge the quality of the transformer temperature sensor?

A: It can be assessed from the following dimensions: ① whether it is in line with national and industry standards (such as GB/T 1094, IEC 60751, etc.); ② whether it has passed the third-party authoritative testing and certification; ③ the sensor's accuracy level and long-term stability indicators (annual drift); ④ whether the manufacturer provides technical selection support and perfect after-sales service; ⑤ the product's actual application cases in the same type of transformer. Inotera's products have passed the third-party testing and certification, and have established long-term cooperative relationships with mainstream power users such as the State Grid and the Southern Power Grid.

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concluding remarks

Transformer temperature sensor seems to be a small accessory, but directly related to the safety and stability of the entire power supply system. From the mature and reliable PT100 platinum RTD, to the future direction of fiber optic hotspot temperature measurement, to the convenient installation of RFID wireless temperature measurement - each technology has its irreplaceable application value.

There is only one core logic for selection: choose the most matching solution according to the transformer type, operation scenario and monitoring accuracy requirements.

Inotera (Fuzhou) Sales Co. Hotline: 13959168359 Official website:www.innotd.com Specializing in transformer temperature control and intelligent monitoring, providing one-stop solutions