Introduction

Temperature is an important operating parameter of power equipment. Obtaining the operating status of power equipment by monitoring the temperature information of power equipment is the research point of power system fault prediction and diagnosis. The research content includes the application of various new temperature sensors, fault prediction and power equipment Diagnostic methods, etc. Among them, the application of optical fiber temperature sensors in power systems has been a hot research topic in recent years, and has been widely used in power plants and substations. The optical fiber sensor has the characteristics of insulation, anti-electromagnetic interference, high voltage resistance, chemical corrosion resistance, and safety. This article summarizes the basic content of power system temperature monitoring, studies the application of current optical fiber temperature sensors in power systems, and looks forward to its development trends.

1 Fiber optic temperature sensor

The optical fiber temperature sensor is a new type of temperature measurement technology developed in the 1970s. It transmits information based on optical signals, and has the advantages of insulation, anti-electromagnetic interference, and high voltage resistance. In foreign countries, the optical fiber temperature sensor has developed rapidly, forming a variety of models of products, and has been applied to many fields, and achieved good results. Domestic research in this area is also in full swing. Many universities, research institutes and companies have cooperated to develop a variety of optical fiber temperature measurement systems and put them into field applications.

According to the working principle, the optical fiber temperature sensor can be divided into functional and transmission type. The optical fiber in the functional temperature sensor serves as the sensor and also the carrier of the optical signal, while the optical fiber in the transmission temperature sensor only transmits the optical signal. At present, the main optical fiber temperature sensors include distributed optical fiber temperature sensors, optical fiber grating temperature sensors, optical fiber fluorescence temperature sensors, and interferometric optical fiber temperature sensors.

2 Application of optical fiber temperature sensor in power system

A large number of equipment in the power system need to detect temperature information to determine the operation of the power equipment, so that the operation dispatcher can take timely measures to eliminate abnormalities and avoid equipment damage and accidents. In the early days, the temperature information of electric power equipment was obtained through temperature-indicating wax tablets, digital temperature sensors, and infrared thermometers. However, the temperature indicating wax and infrared thermometer require manual inspections, which cannot meet the requirements of modern digital power systems. Digital temperature sensors are mostly based on electric power transmission and are greatly affected by electromagnetic fields. They can only measure key points and have certain limitations. The optical fiber temperature sensor overcomes the above shortcomings and shortcomings, and has the advantages of rapid communication, flexible alarm settings, and adaptation to harsh environments. At home and abroad, there are mainly the following application areas.

2.1 Cable tunnel fire monitoring

A large number of high-voltage electrical equipment in power plants and substations are connected by cables, and these cables are laid in cable tunnels under factory buildings or substations. Due to the harsh environment of the cable tunnel and the large number of cables, it is easy to cause large-scale fire accidents due to insulation damage and partial discharge of one cable, causing serious economic losses. In view of the fire-prone situation in cable tunnels, the traditional preventive measures include:

1) Use flame-retardant cable or cable with flame-retardant layer;

2) Fill fire-retardant mud between the compartments;

3) The temperature alarm is realized through the temperature sensing cable in the cable tunnel;

4) Install automatic fire extinguishers with suspended and fixed temperature at fixed distances in cable tunnels.

The above measures cannot detect fire hazards and prevent fire accidents in time. The optical fiber temperature monitoring system is a new technology applied in cable tunnel monitoring in recent years. It can realize the temperature detection of multiple cables and the fire alarm of the entire cable tunnel by laying an optical fiber in the cable tunnel. In general, the advantages of optical fiber temperature measurement system applied to cable tunnel fire monitoring are:

1) Real-time detection of the temperature along the optical fiber, accurate temperature measurement and high resolution. According to the difference between temperature measurement distance and temperature measurement accuracy, generally the shortest temperature update speed for the whole line is about 10s.

2) Store historical temperature data for further analysis.

3) Flexible setting of various indicators such as alarms, which can realize multi-condition alarm settings. There have been many successful application cases, such as Shaoguan Power Plant, Ningxia Shizuishan Power Plant, Nanjing Yangzi Petrochemical Thermal Power Plant, Jinan Iron and Steel, etc.

2.2 High-voltage power cable load safety monitoring

As an important operating parameter of high-voltage power cables, temperature has attracted more and more attention. The digital sensor can only realize the temperature monitoring of important parts such as cable joints. Therefore, in recent years, people have applied optical fiber temperature monitoring systems to some important high-voltage power cables to realize cable temperature monitoring. Cable temperature monitoring can find hot spots and abnormal behavior points within the full length of the cable, including rapid heating points and chronic heating points, etc., and can find the temperature and location information of the bottleneck point that limits the current carrying capacity of the cable, which can prevent cable failure while providing electricity Dispatch provides scientific basis.

2.3 Transformer winding temperature monitoring

The internal faults of power transformers are mainly overheating faults. The research on the overheating faults of transformers has great practical value. The life of the transformer has a certain relationship with the temperature of the power transformer winding. There are three main methods for measuring transformer winding temperature:

1) Digital sensor temperature measurement based on electrical signals;

2) Infrared temperature sensor measurement;

3) Optical fiber temperature sensor measurement.

The measurement using optical fiber temperature sensor is divided into fluorescence optical fiber thermometer and optical fiber grating temperature sensor. Fiber grating sensor temperature measurement is to paste the fiber grating on the transformer winding to measure the winding temperature. Through continuous monitoring of transformer windings, it is possible to grasp the working status of the transformer and understand the defects and degradation of the equipment.

2.4 Temperature monitoring of high voltage switchgear

The high-voltage switchgear is used to open and close the power line, which is the key equipment to realize the transmission and switching of the electric load, and the withdrawal of the faulty equipment and lines. In the long-term operation process, the switch cabinet contact and the busbar connection are prone to aging, loosening and excessive contact resistance, which may cause serious local heating. If the switch cabinet is overheated, if it is not found in time, it will easily lead to fire accidents and cause large-scale power outages. Realizing online real-time temperature monitoring of the fully enclosed high-voltage switchgear, discovering and eliminating hidden dangers in advance, is of great significance to ensuring safe and reliable power supply. The application of optical fiber temperature sensor to the temperature monitoring of the high-voltage switchgear is a new temperature monitoring method at present, which can avoid the interference of strong electromagnetic fields, and is resistant to high voltage and radiation, which can effectively improve the reliability of the high-voltage switchgear. Optical fiber temperature sensors are more and more widely used in substation switch cabinets, such as Tianjin Tianchen Substation and Shangxinkou Substation.

3 Analysis of the application status of optical fiber temperature sensors in power systems

As a new type of temperature measurement technology, the optical fiber temperature sensor has developed rapidly, and its application has become more and more extensive. The application in the power system has also been well developed, but there are problems in the following aspects:

(1) Some applications of optical fiber temperature sensors in power systems are still in the preliminary stage, and it is still necessary to develop a relationship model between temperature and equipment operating status, so as to make better use of temperature information. At present, there are cable core temperature calculation models and dynamic current-carrying models for cable monitoring.

(2) The price disadvantage of the optical fiber temperature sensor restricts its popularization and application in the power system, and the price is too high to make monitoring in some applications of little practical significance.

(3) It is more difficult to lay optical fibers on certain electrical equipment. It is best to form industry-recognized construction guidance and standard specifications in terms of laying methods and laying techniques.

(4) The current manufacturers of optical fiber temperature sensors basically only have the ability to professionally develop and produce optical fiber temperature sensors, and have little involvement in the field of power systems, and lack the theoretical support and theoretical support for the development of power system fault diagnosis software based on optical fiber temperature sensors. Experience, it is difficult to extend the system functions.

4 Outlook

With the development of the power system, the temperature monitoring of power equipment will be paid more and more attention. The cost reduction of optical fiber temperature measurement system and the improvement of temperature measurement accuracy and other indicators will inevitably promote its wider and deeper application in power systems.

(1) Point-type optical fiber monitoring system construction The plant station temperature monitoring system realizes the temperature monitoring of important equipment in power plants and substations, provides temperature information of important areas of the whole plant (station), detects safety hazards in time, and ensures safe electric power production and stable and reliable operation.

(2) Combining optical fiber temperature measurement system and cable fault diagnosis technology to construct an online cable fault diagnosis system based on optical fiber temperature sensor can not only realize the real-time monitoring of cable core temperature and dynamic current carrying capacity analysis, but also combine the power signal and temperature signal Realize cable electrical fault identification and location.

(3) With the maturity of the monitoring technology of the optical fiber temperature measurement system, combining the stress monitoring function to apply it to the safety monitoring of submarine cables and overhead lines is also a direction of future development.

(4) The optical fiber temperature measurement system is used for temperature monitoring of other power equipment, such as transformers, insulators, fuses, insulating sleeves, capacitors and high-voltage motors, etc., and can develop corresponding condition monitoring and performance evaluation in combination with the characteristics of electrical equipment And fault diagnosis system.