Thermistors are the most important sensors in hydropower plants. The operation of temperature-measuring resistors in hydropower plants directly affects the safe operation of generator sets. In the hydropower industry, unstable performance of temperature measuring resistors and poor reliability are very common problems. Because of unstable performance, false alarms of temperature signals have plagued power plant operators and maintenance personnel. In severe cases, this can cause accident shutdown of the unit. The life of the unit and the safety of the power grid will have an inestimable impact. Therefore, it is a very urgent task to analyze the cause of temperature measurement resistance and improve the long-term stability and reliability of temperature measurement resistance.
In the process of serving a large number of power plants, we often encountered various problems on the site. At first, we thought that these problems existed in small and medium-sized hydropower plants. Later, we discovered that these problems existed in large foreign generating units such as the Three Gorges, Guangzhou and Xiaolangdi. Therefore, these issues have certain commonalities in the industry. Here we have listed these issues and analyzed them and tried to solve them.
The special features of hydropower plants
For temperature resistance, the operating environment of the hydropower plant is very special. This is different from other industrial fields. If you take the temperature resistance commonly used in the industry to the hydropower plant, you must have problems. These particularities manifest as:
1, long running time, difficult to maintain. Watt temperature measuring resistors are installed in places where space is not suitable for maintenance and replacement of sensors. Generally, they have the opportunity to maintain temperature measuring resistors during overhaul. Now, due to technological progress, the overhaul period is getting longer and longer, which requires the stable operation of the temperature measuring resistor for a long time.
2, a high degree of importance. Thrust bearing is one of the key devices of the generator set. The temperature measurement resistance is the only means to monitor the operating status of the thrust bearing. In addition, the resistance of the thrust tile temperature measuring resistor is generally required to be protected. The importance is self-evident. The general industrial field does not have such high importance.
3, bad operating environment. The temperature of the resistors is also taken as an example. The sensors and their wires are immersed in the turbine oil with high temperature for a long time, and they are subjected to the impact of the oil flow and the vibration of the unit at all times. In such an environment, few sensors and wires can withstand up to five years of testing.
4, the intensity of electromagnetic interference is quite large. Generally, the power of the hydropower generators is very large. The strong electric field generated by the generator, especially the strong magnetic field generated by the magnetic flux leakage, greatly interferes with the temperature resistance of the upper guide shoe and the thrust shoe. This places high demands on the immunity of sensors and their wires.
Ubiquitous problems
Because of the particularity of the use environment of temperature measuring resistors in hydropower plants, the temperature measuring resistors in hydropower plants generally have the following problems.
1, poor long-term stability, low reliability. In fact, the accuracy requirement for temperature measuring resistors in hydropower plants is not high, but the long-term stability and reliability of the sensors are very high. Many power plants have adopted long-term unstable temperature measurement resistors. After a few years of running the unit, there will be a large number of false alarms, jumps, and no readings, making it difficult for engineers to determine whether the fault is caused by the unit itself. The temperature resistance is still a problem. If the above-mentioned problems occur in the temperature resistance of the thrust watts, it will cause trips and cause serious accidents.
2, the cable breaks or the skin cracks. The phenomenon of cable breakage at the root is present in almost every power plant. The cable is immersed in the turbine oil for a long time. If no special treatment is performed, the wire will be broken at the root of the sensor for a long time. The failure of the root breakage accounts for about half of the temperature measurement resistance faults and should be taken seriously. In addition, the cable sheath can also crack in high temperature and corrosive turbine oil environments.
3, sensor and wire are not shielded, or shielded but not connected. Many power plants do not effectively shield the temperature measuring resistor, so that the generator's strong electric field and strong magnetic field interfere with the temperature measurement resistor and the interference signal is introduced into the temperature measurement loop, resulting in inaccurate temperature measurement. We have seen that the thrust tile temperature sensing resistance induced leakage signal reaches 110V. This makes the measured value meaningless and can also cause damage to other devices in the loop. The temperature measuring resistor and the entire temperature measuring circuit have many and long wires, and there are many connection links. The shielding requirements must be reliably shielded in the whole process. If there is a problem in one link, the shielding will be invalid.
4, sensor installation is not standardized. Generally, it is required to rigidly connect the sensor with the tile body when installing the Watt temperature resistor. It is preferable that the sensor is connected with the tile body, and the wire in the tile should also be reliably fixed, especially the root wire should be fixed on the same rigid body with the sensor. However, we have seen that some power plants are simply placed inside the hole when installing the temperature resistance of the bearing, and some are potted with epoxy resin in the hole. These are all non-standard installation methods. Such installation methods cannot effectively protect the wire roots.
5, wire system and wiring problems. The wire system is the lead wire method of temperature measurement resistance, such as: 4-wire system, 3-wire system and 2-wire system. The wire system determines the influence of the resistance of the sensor wire on the measurement result. Among them, four-wire and three-wire systems can minimize the influence of the wire resistance on the measurement results, while the 2-wire system can. Taking a 20-meter cable as an example, the resistance of the wire is 3 ohms, and the temperature value is 6°C. This error is very large. The three-wire connection, which is also a 20-meter wire, is only 0.1 ohms added to the system, resulting in an error of 0.2°C. This error is acceptable, which means that the wire resistance will hardly affect the measurement results. If four-wire measurement is used, the effect of the wire resistance can be completely ignored. We can see that many power plants use 2-wire temperature-measuring resistors, or connect 3-wire systems to 2-wire systems, or connect 2-wire systems at some point in the middle. In any case, this will produce a big error. Some people may compensate for this on the back-end temperature module, but compensating for different types of conductors of different lengths is not a good method.
6, sensor tail structure problems. The structure of the sensor tail is totally sealed and the connector is different. At present, at least half of the power plants are using the structure of the tail connector. The advantage of this structure is that it can be easily disassembled. Once the sensor is in trouble, it can be used without moving the wire. Replace the sensor. However, such a structure is only suitable for installation in an oil cooler or an air cooler, and the monitoring of the temperature of the bearing bush is not suitable. For example, in the thrust bearing, the sensor is completely immersed in the turbine oil, and the turbine oil is constantly flowing. With the vibration of the bearing bush, the tail connector is very easy to leak oil or the contacts are disconnected, thereby reducing the sensor's Long-term stability. In fact if the sensor itself has long-term stability, it should be little or no maintenance at all.
7. Problems with Pt100 and Cu50. This is a problem of temperature-measuring resistance indexing values. Pt100 and Cu50 are the most commonly used temperature measurement resistors in power plants. Basically 99% of hydropower plants are in use. Pt100 uses platinum material as a sensing element and Cu50 uses copper as a sensing element. The comparison between Cu50 and Pt100 has several disadvantages. Firstly, the resistance of copper is lower than that of platinum, and a long copper wire is needed to make a sensitive element. Platinum is relatively short. Generally, the longer the finer material, the lower the reliability. Second, platinum resistance is the mainstream of temperature measurement resistance. Large manufacturers, especially German manufacturers, produce Pt100 chips using photolithographic sputtering technology, which is very mature and reliable. There is almost no manufacturer of Cu chips, so that if only Cu50 products are used to make coils themselves to make sensitive components, the reliability is greatly reduced. This is why the Cu50 temperature measurement resistors used by some power plants are often bad.
8, very good sensor. In power plants such as the Three Gorges and Xiaolangdi, because of the VOITH and ALSTON units, the sensors are either Swiss or German sensors. The sensor itself is very good, but because it is not a sensor made for a specific use environment, the result is often a problem. Such as: sensor structure problems, disconnection of the wire at the root. Different power plants have different characteristics, and the requirements for temperature measurement resistance are also different. Until now, we have not found any hydropower plants with the same temperature resistance. For a specific power plant, temperature measurement resistors are not designed to be targeted, and even the best sensors will still have problems.
Solution
All efforts are to improve the long-term stability and reliability of temperature measurement resistors. This requires work on all aspects of the manufacture and installation of temperature measurement resistors.
1, using high-quality Pt100 chip. As mentioned earlier, platinum resistance is better than other materials, and the quality of platinum resistance chips is also very different. The Pt100 chip produced by sputter lithography process should be used, and the accuracy requirement should be Class A. The drift of this type of chip is small, long-term stability, and shock and vibration. The pin of the chip is platinum nickel alloy. Because the pin of the chip must be soldered to the wire of the wire or the armored wire, welding can easily cause brittle fracture of the metal material, so this is also a weak link. The use of platinum-nickel alloy on the chip leads guarantees the mechanical properties of the soldered leads and prevents them from breaking in the sensor.
2, using a special wire. The long-term immersion of the wire in the oil appears hard and brittle due to the unreasonable selection of the material of the outermost insulating layer of the wire. For example, the outer sheath material of the wire is a PVC material, and its oil resistance and temperature resistance are relatively poor. At higher temperatures, its oil resistance will be greatly reduced, and its service life will be greatly reduced. After prolonged immersion in oils of higher temperatures, the wires become hard and brittle. We use oil-resistant and temperature-resistant wire materials. We have used polytetrafluoroethylene (TEF) abbreviation F-4, a copolymer of tetrafluoroethylene and hexafluoropropylene, and a modified material of PTFE. It has excellent oil resistance, corrosion resistance and heat resistance, and can be used at temperatures from -250 to 250°C for a long period of time. In addition to the high temperature and high pressure of fluorine and molten alkali metal it has a corrosive effect, other such as strong acid (including concentrated nitric acid and aqua regia), strong alkali, strong oxidants, oils, ketones, ethers, alcohols, etc. It does not work for it. In addition, its crack resistance is also very prominent. It can completely solve the problem of cracking of the wire in the oil for a long time.
3. Add a protective device at the junction of the wire and the temperature measurement resistor. It mainly solves the problem of wire breakage at the root of the wire. According to the situation on the site, choose different forms of protection, such as conical spring protection tube, bellows and armored wire extension protection. Here special emphasis should be placed on the armored wire extension protection method. The armor wire inside the sensor is extended all the time, so that all parts of the wires that are impacted by the oil flow are all armored wires. Actually, the armored wire is a stainless steel wire that can be arbitrarily bent. The resistance to corrosion, shock and vibration is very good. This completely solves the problem of the corrosion and impact of the wire on the oil. Longer service life and more reliable performance.
4, temperature measurement resistors and their integrated wire mesh shielding. In theory, the magnetic field is more difficult to shield than the electric field. For strong magnetic field interference, mesh shielding is the most effective solution. Since the thrust and the upper guide bearing are closest to the generator, the net-like shielding of the thrust and the temperature measuring resistor of the upper shoe is particularly important. The temperature measuring system has many conductors and long wires. There are many intermediate links. When wiring, it is necessary to be very careful. In each link, it is required that the shielded wire of the wire be reliably connected to the common ground.
5, three-wire system above the wiring. We recommend the use of three-wire wiring, the sensor itself should be made of three-wire system, the temperature measurement system must be used in all aspects of the three-wire connection, so as to effectively ensure the measurement accuracy. It is particularly emphasized here that we must not use two-wire wiring.
6, the packaging process and structure. The full armored packaging process adopts armored silk produced by a German specialized chemical plant. This armored wire is manufactured through high-purity magnesium oxide filling, high-temperature drying, wire drawing, rolling, and annealing. It is characterized by long service life and response. Fast speed, high mechanical strength and good insulation. The temperature measurement resistors used for Watt temperature and Watt oil temperature must not use the structure of the tail connector.
7, custom temperature thermal resistance. For a specific power plant, temperature measurement resistors must be designed and manufactured in a targeted manner.
8. There are many manufacturers of oil-resistant (oil tank) cables currently, and there are few manufacturers that can achieve the same three-core resistance of oil-resistant and heat-resistant lead wires. Three-wire 20-30 meters of wire, each wire conductor resistance is only 0.01 ohms, three leads. The resistance must be equal to 0.01 ohms. This connection Pt100 thermal resistance can reach A-level standards.
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Oil-resistant and temperature-resistant cables (oil tanks only) KHFRP21 with Pt100 thermal resistors three-core, four-core, six-core shielded silver-plated wire, P100 thermal resistance dedicated extension cord, compensation line.
Thermal resistance index number: CU50.CU100. G53. Pt100. Pt500. Pt1000 Number of cores: 2 cores, 3 cores, 4 cores, 6 cores, 8 cores
The core specifications are 3*7/0.15|3*7/0.20|3*19/0.15|3*19/0.16|3*19/0.18|6*7/0.15|6*7/0.20|6*19/ 0.15|6*19/0.16|6*19/0.18|
Core insulation color: two red and one white or two white and one red or other requirements
Outer diameter insulating layer: white transparent imported oil resistant temperature PTFE, blue imported oil resistant temperature PTFE, red imported oil resistant temperature PTFE, or other colors.
Insulation material: Imported oil-resistant PTFE TEF (260 °C), TEF (200 °C), PVC material (106 °C), silicone rubber cord.
The shielding layer is: silver plating/copper wire/tin plating
Overall outer diameter: 2.8mm-6.5mm
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