Distributed optical fiber temperature measurement systems are categorized into three types based on the backscattering principle: Rayleigh scattering, Raman scattering, and Brillouin scattering. Among these, Raman-based systems have reached a relatively mature stage and are widely used in practical applications. These systems operate by combining the principles of Optical Time Domain Reflectometry (OTDR) with the backward Raman scattering effect. This allows for real-time, continuous temperature monitoring along the entire length of the fiber.
In modern power systems, where the grid is evolving toward ultra-high voltage, large-scale, and automated operations, the need for reliable and efficient temperature monitoring has become critical. Any fault or overheating in electrical equipment can lead to significant economic losses and even catastrophic failures. Traditional temperature measurement methods such as infrared thermometers, thermal resistance systems, and temperature sensing cables only provide localized readings and cannot offer comprehensive insights into the overall health of the system. This makes it difficult to predict potential failures, assess equipment aging, or manage load imbalances effectively.
Distributed optical fiber temperature sensing technology addresses these challenges by enabling multi-point, real-time monitoring across long distances. It is particularly useful in high-voltage power cables, transformer windings, generator stators, and other critical infrastructure where overheating can lead to fires, explosions, or system failures. The system's ability to detect temperature changes continuously and accurately makes it an essential tool for ensuring the safe and efficient operation of power systems.
The basic working principle of a Raman-based distributed fiber optic temperature sensor relies on the interaction between laser pulses and the molecular structure of the fiber. When a laser pulse is sent through the fiber, it interacts with the molecules, causing both elastic and inelastic collisions. These interactions result in backscattered light, including Stokes and anti-Stokes components. The intensity ratio of these two components is directly related to the temperature at each point along the fiber.
According to Raman scattering theory, the intensity of Stokes and anti-Stokes light depends on the absolute temperature. The relationship can be expressed mathematically as:
$$ R(T) = \frac{I_{\text{anti-Stokes}}}{I_{\text{Stokes}}} = \exp\left( -\frac{h\nu}{kT} \right) $$
Where $ I_{\text{anti-Stokes}} $ and $ I_{\text{Stokes}} $ are the intensities of the respective scattered light, $ h $ is Planck’s constant, $ k $ is Boltzmann’s constant, and $ T $ is the absolute temperature. By measuring this ratio, the system can calculate the temperature distribution along the fiber with high accuracy.
The sensing process involves several key steps. A computer controls a synchronization pulse generator that sends out a series of laser pulses. These pulses are modulated and sent into the fiber, where they scatter back due to molecular interactions. The backscattered light is separated into Stokes and anti-Stokes components using filters within a wavelength division multiplexer. Each component is then detected by a photodetector, amplified, and converted into an electrical signal. The data acquisition card captures and processes this information, allowing the system to compute and display the temperature profile in real time.
This technology offers a powerful solution for monitoring critical infrastructure, improving safety, and reducing the risk of failures in complex power systems. Its non-intrusive nature, high spatial resolution, and long-range capability make it an ideal choice for modern industrial and energy applications.
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