Understanding Pressure, Resistance, And Fiber Optic Thermometers in One Article

The reliable operation of an oil-immersed transformer largely depends on the stability of its internal insulating oil and winding temperatures. Overheating is a primary cause of accelerated insulation aging, performance degradation, and ultimately, failures. Therefore, temperature monitoring is one of the most fundamental and critical aspects of transformer operation and maintenance. From traditional mechanical dials to modern intelligent fiber optic systems, the history of thermometer development is an evolution of transformer monitoring technology from passive observation to active early warning.

This article will systematically outline the common types of thermometers used on oil-immersed transformers and provide an in-depth analysis of their working principles and application scenarios.

Chapter 1: The "Family Tree" of Thermometers – A Detailed Look at Three Main Types

Based on measurement principles and installation location, thermometers for oil-immersed transformers are primarily divided into the following three categories. Together, they form a three-dimensional monitoring network from top oil temperature to winding hot-spots.

1. Pressure-Type Thermometer (Remote Reading Thermometer)

• Working Principle: This is a classic mechanical instrument based on thermal expansion/contraction and liquid/gas pressure transmission. The system consists of three parts:

1. Temperature Bulb (Sensor): Inserted into the oil at the top of the transformer tank, filled with a temperature-sensitive medium (e.g., liquid, gas, or low-boiling-point liquid).

2. Capillary Tube: A long, thin metal tube connecting the bulb to the gauge head, filled with a pressure-transmitting medium.

3. Gauge Head (Indicator): Mounted on the transformer tank wall or control cabinet, potentially meters away from the bulb. Its core is a Bourdon tube – a curved, elastic metal tube. When the bulb heats up, the internal pressure change is transmitted via the capillary to the Bourdon tube, causing it to deform. This deformation moves a pointer through a linkage mechanism, displaying the temperature.

• Key Characteristics:

• Purely mechanical, requires no external power, excellent immunity to electromagnetic interference, very high reliability.

• Gauge head can be remotely mounted for convenient local reading.

• Typically equipped with 1-2 adjustable contacts for over-temperature alarm and trip functions.

• Accuracy and response speed are relatively slower compared to electronic types, and the capillary tube is susceptible to mechanical damage.

• Typical Application: The primary monitoring and alarm device for top-oil temperature, a near-standard feature on all oil-immersed transformers.

2. Resistance Temperature Detector (RTD, e.g., PT100)

• Working Principle: Based on the property that a conductor's resistance changes with temperature. The most common sensing element is a platinum resistance thermometer, with PT100 denoting a resistance of 100 ohms at 0°C. Its resistance changes precisely and linearly with temperature.

• System Components:

1. Platinum RTD Probe: Installed in a thermometer well at the top of the transformer, immersed in oil.

2. Measuring Bridge & Transmitter: Often integrated into an intelligent control unit. Precise circuitry measures the PT100's resistance and converts it to a standard 4-20mA current signal or digital signal.

• Key Characteristics:

• High measurement accuracy, signals can be transmitted over long distances, good noise immunity.

• Output is a standard electrical signal, easily integrated with automation platforms like SCADA (Supervisory Control and Data Acquisition) and DCS (Distributed Control Systems) for remote centralized monitoring.

• Often installed alongside the pressure-type thermometer, serving as a redundant or higher-precision means for remote monitoring and logging of oil temperature.

• Typical Application: Used for remote transmission and digital monitoring of top-oil temperature, the cornerstone of modern automated, unattended substations.

3. Fiber Optic Winding Temperature Measurement System (Most Advanced Direct "Hot-Spot" Measurement)

• Working Principle: This is currently the most direct and advanced technology for winding temperature monitoring. It is based on the physics of Fiber Bragg Gratings.

1. Fiber Bragg Grating (FBG) Sensor: A periodic variation in the refractive index (a grating) is written into a segment of special optical fiber using a laser. Its key property: Light of a specific wavelength (Bragg wavelength) is reflected, and this reflected wavelength shifts linearly with changes in temperature (or strain) at the grating's location.

2. Measurement Process: A flexible fiber optic cable embedded with multiple FBG sensors is directly pre-embedded between the insulation layers of the high-voltage windings at the predicted hottest spots during transformer manufacturing. The system emits broadband light, and by analyzing the specific wavelength reflected from each grating, it can accurately and in real-time obtain the absolute temperature at different points within the winding.

• Key Characteristics:

• Direct measurement of winding hot-spot temperature, not indirect estimation. Data is most authentic and reliable.

• Intrinsically safe: Optical fiber is made of silica, insulating, high-voltage resistant, and immune to electromagnetic interference, operating stably in strong EM fields.

• Distributed measurement: A single fiber can host dozens of sensing points, enabling a complete thermal map of the winding.

• Key enabler for transformer "Dynamic Rating" and lifetime assessment.

• Typical Application: Large, critical transformers (e.g., EHV, converter transformers), smart substations requiring load capability management.

Chapter 2: Key Concept Clarification – Top-Oil Temperature vs. Winding Temperature

This is a crucial concept and the starting point for selecting thermometer types.

• Top-Oil Temperature: Measures the temperature of the oil at the top of the tank. It reflects the transformer's overall thermal load but has a thermal lag. When load changes, winding temperature changes fastest, followed by oil temperature. Pressure-type and RTD thermometers measure this.

• Winding Hot-Spot Temperature: Refers to the hottest point in the entire transformer, typically located in the upper part of the low-voltage winding. It is the most critical parameter determining insulation aging rate and load capability. Traditional methods cannot measure it directly, relying instead on a Winding Temperature Indicator (WTI) that simulates/estimates it using "top-oil temperature + current correction." Fiber optic measurement is the only technology that can directly and accurately measure it.

Chapter 3: Selection and Application Guide

Conclusion & Trend:
A modern, high-performance oil-immersed transformer often features a combined configuration of these thermometers:

• The Pressure-Type Thermometer serves as the safety baseline, providing the most reliable local over-temperature protection.

• The Resistance Temperature Detector (PT100) acts as the data bridge, enabling digitization and remote monitoring of oil temperature.

• The Fiber Optic Measurement System is a value-added option, providing indispensable core data for condition-based maintenance, intelligent operation, and unlocking the equipment's full potential.

The evolution from monitoring "body temperature" to understanding "organ health" epitomizes the journey of transformer intelligence. Choosing the right temperature monitoring solution is not just about meeting standards, but about achieving the leap from "time-based maintenance" to "predictive maintenance," thereby maximizing the transformer's safety and economic value.