Networked sensors are becoming more and more widely used, and their unparalleled advantages have gradually become the trend and mainstream of technology. Below, we combine the requirements in practical applications, talk about the characteristics of digital, networked sensors, for your reference when selecting.
1. Apply digital and networked temperature sensors to realize online monitoring of cable trenches
1. Why monitor cable trench online
Cables can be seen everywhere in daily production and life. In electric fields, factories, and laboratories, a large number of cables are usually concentrated in cable trenches to facilitate wiring, maintenance, and aesthetics. As people's dependence on electricity grows, there are more and more cables in the cable trench, and the probability of fire accidents also increases accordingly.
Taking the electric field as an example, as the capacity of the unit increases, the level of automation increases accordingly, and the amount of cable used is increasing. A 200MW unit with various cables up to 200 ～ 300Km. Two 500Mw supercritical parameter units in the first-phase project of a power plant with a cable consumption of 3000Km.
Once a cable fire occurs in a thermal power plant, it will cause serious losses. Most of the thermal power plants currently under construction and operation still use flammable cables, so the problem of cable fire protection is particularly prominent.
Domestically, according to relevant statistics, in the past 20 years, there have been more than 140 cable fires in thermal power plants in China, and the total number of city fires reached 75 in the seven years from 1986 to 1992. There were 24 power plants with two or more cable fire accidents, and some power plants had 4 to 6 accidents. The losses caused by more than 70% of cable fires are very serious, of which 2/5 of the fire accidents caused huge losses. From 1975 to 1985, there were 60 major accidents caused by the fire and flame of the cable, causing more than 5 billion yuan in real connection and indirect losses.
The cost of equipment damage and emergency repairs in Shanxi Shenfa Power Plant due to cable trench fire exceeded 10 million yuan.
In 1999, the second power plant of Mudanjiang caused a power outage accident in the whole plant due to a fire in the cable trench. The direct and indirect losses amounted to nearly ten million yuan. In addition to the electric field, there are also fire hazards in the cable trenches of large factories, mines, scientific research institutions, and workshops. In 2000, the positive and negative electron collider monitoring computer room of the Beijing Institute of High Energy Physics was forced to shut down due to the fire in the cable trench, which seriously affected the progress of scientific research.
In the 3285 electrical fire accidents counted in the United States from 1965 to 1975, wire and cable fire accidents accounted for 30.5%, with a direct loss of about 40 million US dollars.
Japan has surveyed factories and enterprises such as power, steel, petrochemicals, and paper manufacturing. 78% of the units have experienced cable fires, of which 40% are accidents with a greater degree of harm.
Through the analysis of the electric field accidents, the direct cause of the fire in the wire trench is the long-term operation of the cable middle leading to aging, oxidation, and increasing contact resistance, which causes the cable head to overheat and burn through the insulation, and finally leads to the fire in the cable trench. .
Example 1: An overheated cable head caused a fire in Liaoning Power Plant. When firefighters were about to leave the site after extinguishing the fire, the cable head broke down, the fire reignited, and several people were burned on the spot, causing a group injury accident.
Example 2: At the Fulaerji Power Plant, when the tester searched for cable faults, the capacitor breakdown method was used in the morning. After noon break, a fire broke out in the cable trench, which caused major accidents. If the online cable monitoring system is installed, it can be avoided. accident.
Example 3: The middle end of the circulating water cable of Hunjiang Power Plant # 2 is overheated, burning all the cables in the ditch and causing a forced shutdown accident. It is understood that some people have smelled the insulation burning in the shaft more than 80 meters away from the middle end of the failed cable in the morning. The smell of coke caused a fire at seven o'clock in the afternoon.
Example 4: The second power plant of Mudanjiang on June 28, 1998, due to the overheating of the middle end of the circulating water cable of one machine ignited all the cables in the cable trench, resulting in seven machines in the whole plant (installed passenger volume of 1.03 million kilowatts) Forced to shut down, a vicious accident of power outage throughout the plant.
Example 5: In 2001, the Shanghai Power Supply Bureau caused a cable tunnel fire due to overheating of the middle of the cable, and a large area of ​​the cable was burned, resulting in a large area power outage in the urban area.
In summary, the main cause of fire in the cable trench is due to the heat in the middle of the power cable. According to the analysis of multiple accidents, it is found that from the overheating of the cable head to the occurrence of the accident, its development speed is relatively slow and long. The online monitoring system of the cable can completely prevent and prevent such accidents.
The Jilin Thermal Power Plant summarized this experience many years ago, using manual inspection of the temperature of the cable middle end every day, analyzing its operation status according to the temperature change, which consumed a lot of manpower, but avoided many accidents, so the cable The trench online monitoring system is very important for the safe operation of power plants.
Although most power plants have already spent a lot of money to carry out fire blocking of cable trenches and ordinary fire alarm devices, fires in wire trenches still occur. These measures can only play a role in reducing the scope of the accident after the fire of the wire. There is no fundamental limitation Reduce the occurrence of fire. On-line monitoring of the cable trench is an effective and feasible method to fundamentally limit the occurrence of fire in the cable trench.
2. Why the traditional analog temperature sensor is not suitable for cable trench temperature measurement The structure of a traditional temperature test system is usually as follows: The temperature value of each sensor must enter the system through the above link. Therefore, there are the following deficiencies: a) Hundreds of thousands of signal lines are required (a cable trench usually measures 200 to 300 temperature measurement points) b) Analog voltage signals are easily lost during transmission, which affects the accuracy of the system and the transmission distance Closer. c) Many system links are difficult to maintain. And the accuracy of the system is easily affected by the environment and is not easy to guarantee. d) The price is expensive. 200 ~ 300 points need 100,000 ~ 200,000 3. The application of new technology makes the system more convenient
With the development of technology, digital and networked sensors have become the trend of technology and the mainstream of the market. The DS18B20 digital and networked temperature sensor of American DALLAS Company adopts a unique idea and successfully solves the contradiction between digitalization, network and cost. This makes it possible to establish a monitoring system that is easy to use and economical and reliable.
LTM-8000 digital temperature and humidity environment monitoring system adopts the advanced chip technology of American DALLAS company, combined with the field situation in China, and applies the technology of intelligent field bus. Only digital signals are transmitted in the entire system, and sensors and acquisition modules can be networked , Make the system more reliable, more convenient wiring. The structure is as follows:
The main features of the system based on digital and networked sensors: a) Fewer cables, the sensors can be connected together through the bus, and dozens of sensors only use one 3-core wire. Significantly reduce on-site cables and facilitate on-site wiring. b) Since the output of the sensor is a digital signal, there is no loss of accuracy during transmission, and the system accuracy can be guaranteed. c) There are few system links, and the sensors come directly into the collector. There are few links where the system may fail, which is easy to maintain. d) The use of advanced chip technology, unique concepts, and large-scale production have greatly reduced system costs and improved reliability. Changying Technology's LTM-8000 digital temperature and humidity environment monitoring system has been successfully applied in many fields such as power systems, petroleum, and steel. Such as: Online monitoring of cable trench temperature and fire warning Online monitoring of temperature of high voltage switchgear Online monitoring of temperature of motor and its junction box On-line monitoring of bearing temperature of pump and fan Storage (grain silo, cold storage, oil tank) monitoring Air conditioning and building automatic control
4. System Introduction The system configuration is divided into the following parts: 1.) Upper computer: Function: Data processing and user interface Hardware requirements: Pentium 133 / 32M DRAM / 4.3G or above compatible machine or industrial control machine. Software: optional configuration software or customized program 2) RS232 / 485 communication converter LTM-8520E, with overvoltage, overcurrent, surge, isolation, lightning protection 3) Field acquisition module LTM-8003E function: realize two Liaison between high-level communication networks. One level is for the RS-485 network of the host computer, and the LTM-8003 module is used as a substation; another level is "one-line bus", and the LTM-8003 module is used as a collection center to measure the
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