Principle and sensing process of Distributed Optical Fiber Temperature Measuring System

Principle and sensing process of Distributed Optical Fiber Temperature Measuring System

The Distributed Optical Fiber Temperature measuring system can be divided into three types based on the backward scattering principle: Based on Fourier scattering, based on Raman scattering, and based on Bayesian scattering. At present, the development is relatively mature, and some products are applied to engineering Distributed Optical Fiber Temperature Measurement System Based on Raman scattering. Its sensing principle is mainly based on the optical time-domain reflection (OTDR) Principle of the optical fiber and the temperature effect of backward Raman scattering of the optical fiber.

With the development of China's economy, the power system is moving towards ultra-high voltage, large power grids, large capacity, and automation. Once an accident occurs, it will cause huge losses to the national economy. How to monitor running power equipment online and analyze the Safety Forecast and temperature change trend? Through real-time data, how can we scientifically analyze the device quality, operating environment, operating methods, aging equipment, and load imbalance? These are all urgent problems in the power system. Traditional infrared thermometers, infrared imagers, temperature-sensing cables, and thermal resistance temperature measuring systems can only measure the temperature at the local location of the power system, which cannot provide scientific basis for safe, economical operation and efficient maintenance. The Distributed Optical Fiber Temperature Measurement System can achieve multi-point and online distributed measurement, and realize real-time online monitoring of operating equipment, it effectively solves the problem of long-standing emergency incidents such as high temperature, combustion, explosion and fire. In the power system, this optical fiber temperature measurement technology is used in the temperature point sensing scenarios of high-voltage power cables and electrical equipment for heating areas, cable interlayer, cable channels, large generator stator, large transformers, boilers, and other facilities caused by poor contact. it has broad application prospects.

Basic Principles of Distributed Optical Fiber Temperature Measurement

1. Distributed Optical Fiber Temperature Measuring System Based on backward scattering principles can be divided into three types: Based on Fourier scattering, based on Raman scattering, and based on Bayesian scattering. At present, the development is relatively mature, and some products are applied to engineering Distributed Optical Fiber Temperature Measurement System Based on Raman scattering. Its sensing principle is mainly based on the optical time-domain reflection (OTDR) Principle of the optical fiber and the temperature effect of backward Raman scattering of the optical fiber.

(1) optical time domain reflection (OTDR) Principle

When a laser pulse is transmitted in the optical fiber, the optical fiber will produce scattering due to the microscopic non-uniformity of the refractive index. In the time domain, the time required for returning the incident light to the incident end of the optical fiber through backward scattering is t, and the distance traveled by the laser pulse in the optical fiber is 2L, v indicates the propagation speed of light in the optical fiber, C indicates the speed of light in the vacuum, and n indicates the refractive index of the optical fiber. When t is measured, the distance from L of the light source can be obtained.

(2) Temperature Effect of backward Raman scattering of Optical Fiber

When a laser pulse is injected into the optical fiber from one end, the optical pulse will spread along the optical fiber forward. Because of the elastic collision and non-elastic collision between the optical pulse and the molecules inside the optical fiber, each point of the optical pulse in the propagation generates a reflection, and a small part of the reflection is reflected, the direction is exactly the opposite of that of the incident light (also called backward ). The intensity of the backward reflected light is related to the temperature of the Reflection Point in the light. The higher the temperature of the reflection point (the ambient temperature of the optical fiber at this point), the higher the reflected light intensity. With this phenomenon, if the intensity of backward reflected light can be measured, the temperature of the reflection point can be calculated. This is the basic principle of Temperature Measurement Using optical fiber.

For example, when a laser pulse is transmitted in an optical fiber, it interacts with the optical fiber molecules, which may result in a series of applications, such as red-ray scattering, wide-source scattering, and Raman scattering, among them, Raman scattering is produced by the exchange of energy between the thermal vibration of Optical Fiber molecules and the interaction of photon. If a part of the light energy is converted to a thermal vibration, a longer light than the light source wave is sent, known as the stochastic light. If a part of the thermal vibration is converted to the light energy, A light with a shorter wavelength than the light source will be sent out, which is called the anti-stoks light. According to the Raman scattering theory, the light intensity of the two beams of reflected light is related to the temperature, and their ratio R (T) is:

(1) The sum is the strong and the anti-stochastic light intensity of the stochastic, h is the Plank constant, k is the Boltzmann constant, and T is the absolute temperature. From the formula (1), we can see that R (T) is only related to temperature T. Therefore, we can use the ratio of anti-stochastic to the stochastic light intensity to measure the temperature.

Sensing process of Distributed Optical Fiber Temperature Measuring System

As shown in 1, the sensing process of the Distributed Optical Fiber Temperature measuring system is as follows: The computer controls the synchronous pulse generator to generate a pulse with a certain repetition frequency. On the one hand, the pulse laser is modulated to generate a series of high-power optical pulses, on the other hand, it provides a synchronous pulse to the high-speed data acquisition card and enters the data collection status. The optical pulse enters the sensing optical fiber through a port of the wavelength division multiplexing, generates backward scattered light at each point in the optical fiber, and returns to the wavelength division multiplexing. The backward scattered light filters out the stoks and anti-stoks through the thin film interference filters in the wavelength division multiplexing, and outputs them through the other two ports of the wavelength division multiplexing, and enters the photoelectric detector (APD) respectively) and the amplifier in the photoelectric conversion and amplification, the signal to the data acquisition card can be collected range. Finally, the data acquisition card stores and processes the data for temperature calculation.

Application of Distributed Optical Fiber Temperature Measuring System in Power System

(1) Temperature Monitoring of Power Cables

In power systems, cable lines serve to transmit high-voltage power. Cables are often insulated and aged due to long-term operation, and may cause local high temperature or even fire due to poor external environment and high internal load current. Therefore, it is necessary to carry out real-time and online monitoring on cables, detect faults in a timely manner, and eliminate accidents in the bud. The Distributed Optical Fiber Temperature Measuring System can monitor the running status of the power cable online to know the running status of the entire line in real time and effectively monitor the heating status of the cable under different loads, improve the management level of cables; monitor and alarm the fire conditions in the cable trench, identify local hot spots of power cables, discover cable faults and give early warnings in advance to prevent accidents; it can optimize the transmission and distribution capital, determine the cable load changes based on the temperature, reasonably configure the load, expand the capacity of the existing cable, and increase the cable service life; external Force damage during cable operation can be found.

(2) substation Temperature Monitoring

Due to its unique advantages, the distributed optical fiber temperature measuring system is widely used in substation temperature monitoring. It can monitor the temperature of the main equipment. Generally, an optical fiber cable with an external sheath is used as the fire monitoring and Alarm System of the main transformer room, the "zero distance" real-time monitoring of transformer casing, GIS wall-Crossing Pipe and wire connection temperature is carried out using the thermal plastic outer sheath optical fiber cable. It can monitor the heat-prone parts in the switchgear in real time, and use it with the ventilation system of the switchgear, so that the temperature in the cabinet can always be kept within the permitted range; winding the optical fiber on the cable joint in the cabinet, a plug-in Arc shield of the circuit breaker car, or the thermal scaling sleeve of the static contact, the temperature can be monitored in real time, before it turns into an accident, detect and take measures as soon as possible.

(3) temperature monitoring of high-voltage distribution devices

The cable connector in the switchgear, the dynamic and static contacts in the 10 kV and 35 kV high voltage switchgear, And the connector of the electrical equipment may be deteriorated due to long-term operation, which is a weak link that is prone to faults. The main reason is that the contact between these parts is poor, the contact resistance is large, and the thermal power is very high in the case of large current, which leads to overheating, increasing the contact oxidation, and further increasing the contact resistance, the formation of a vicious circle, to a certain extent, will cause serious faults, damage the safety and reliability of power supply. The Distributed Optical Fiber Temperature Measuring System can wrap the optical fiber on the joint to monitor its temperature in real time. Before it turns into an accident, it can detect and take countermeasures as soon as possible. For important components such as generator winding and transformer, the optical fiber can be wound on its surface, increasing the length of the optical fiber used to measure the area and improving the measurement accuracy, the high temperature fault point can be quickly found in the temperature curve.

Conclusion

The Distributed Optical Fiber Temperature measuring system integrates multiple technologies, including optical fiber sensing, optical fiber transmission, optical fiber communication, Photoelectric control, and computer, it has the advantages of intrinsic security, long-term reliability, no electromagnetic interference, high temperature measurement accuracy, real-time online, and other advantages. It realizes multi-point, real-time, and online monitoring of power system operating equipment. The system has been widely used and recognized in power systems at home and abroad. Further improving the stability and reliability of the system will make its application prospects in power systems more broad.