Investigation and analysis of line selection devic

2022-08-21
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Investigation and analysis of single-phase grounding fault protection line selection device in distribution system

I. The importance of single-phase grounding protection line selection

in substations (substations), switchyards and power plants, 66 kV, 35 kV, 10 kV, 6 kV and 3 kV distribution lines are the main components of the power system. In these voltage level systems, the neutral points of transformers and generators are ungrounded or grounded through arc suppression coils and resistors. And there are multiple output or input distribution lines connected on the bus of the same voltage level, most of which are led out by aluminum (or copper) bars overhead or high-voltage power cables; The number of lines generally varies from fiveorsix, dozens or twentythree; Each distribution line has many branches, which are erected in a "radial" shape, and then connected with each distribution transformer. After the distribution transformer is reduced to "low voltage", it is supplied to the majority of users. In such distribution lines, faults such as phase to phase short circuit, overcurrent (overload) and single-phase grounding often occur. Among them, the occurrence rate of single-phase grounding is the most frequent, accounting for more than 70% of the total fault rate of the system; Short circuit faults are also mostly formed by the evolution of single-phase grounding into multi-phase grounding

"single phase grounding" refers to any phase of a, B and C on the distribution line where the wire breaks and falls to the ground or contacts trees, buildings or utility poles, and a conductive circuit is formed between the tower falling to the ground and the earth; And the phenomenon that the insulation resistance to ground is obviously too low after the insulation material of distribution equipment is damaged due to atmospheric lightning or other reasons

because the neutral point of the main transformer in the system is not grounded or is grounded through arc suppression coil and high resistance. When there are multiple distribution lines on the same bus, no matter which one is single-phase grounded, it cannot directly form a loop with the winding coil of the main transformer, and there will be no large current phenomena such as short circuit and overload in the line. Only the capacitive current formed between the line and the earth changes, which shows that there will be weak zero sequence current in each line. This current is very small, ranging from a few milliamps to hundreds of milliamps or amperes, which is proportional to the length of the line; Under normal conditions, the overhead line per kilometer is about 15 mA. In the power industry, this power supply system is called "small grounding current system" or "small current grounding system"

in the system, since the primary winding of the voltage transformer (PT) is connected to the bus of the system by Y0 wiring, when any line is single-phase grounded, zero sequence voltage is generated at the triangular opening of the secondary winding, and zero sequence overvoltage alarm can be set; However, you cannot select a certain route. During grounding, the voltage rise of the non grounded phase line to the ground can reach √ 3 times of the phase voltage. When the system is accompanied by ferromagnetic resonance, the phase voltage will increase by 1-5 times, or even higher, forming overvoltage, accelerating the aging of the insulating materials of power equipment, shortening the service life, resulting in the breakdown of insulating equipment, and two or more phases will be grounded at the same time, resulting in short circuit accidents, which increases the damage degree of power equipment. Therefore, in the power system, accidents such as the explosion of voltage transformers and circuit breakers, the burning of distribution transformers, and the breakdown of power cables and porcelain insulators often occur. The "short-circuit protection", "overcurrent protection" and "zero sequence current" protection in the existing relay protection or integrated automation protection devices are all large current starting protection devices; The small current during single-phase grounding cannot drive this kind of protection device to act. Therefore, it cannot act on the tripping of high-voltage switch (circuit breaker), and the fault line and non fault line cannot be isolated

in order to avoid the expansion of the accident, it is necessary to distinguish the fault line from the non fault line in time. In substations (substations), switchyards or power plants, if reliable "single-phase grounding protection line selection device" is not installed, it is necessary to manually switch off and power off one by one to select the fault line. Sometimes, it is even necessary to switch off and power off all distribution lines connected to the bus to find out. This will cause the interruption of the power supply of the trouble free line, resulting in a large area of power failure; At the same time, it also increases the action times of the high-voltage switch (circuit breaker), shortens the service life, and reduces the reliability and quantity of power supply. In order to find the grounding point on the road, it is also necessary to disconnect many branch lines from the main line one by one, and then measure the insulation resistance value of each section or branch to the ground with the insulation resistance instrument, and judge the range of the fault point manually. This process is very complex and heavy workload; For personal safety, we need to set up a variety of safety measures, which will cost a lot of manpower, materials and time, increase the labor intensity of power workers, and at the same time, it also has unsafe hidden dangers to people

the above situation shows that in the system where the neutral point is powered by ungrounded or through arc suppression coil and resistance grounding, although it can prolong the tripping time of fault line when single-phase grounding; However, it has led to the interruption of power supply of many other non fault lines, resulting in a wider range of power outages and hidden dangers to the safety of people's lives and property

at present, the State Electricity Regulatory Commission, power companies and other relevant administrative departments have higher and higher requirements for the reliability of power supply, requiring rural electricity to reach 99.8% and urban electricity to reach 99.9% or more; There are also restrictions on the number and time of switching off and power failure caused by faults in each line every year. In some regions, it is stipulated that if the number of trips exceeds the limit index, the relevant management unit or individual will be fined 200 yuan each time

the application of "substation integrated automation protection device" in the power system has been very popular, making many substations unattended. Because the solution of "integrated automation" in single-phase grounding protection line selection of small grounding current system is not perfect; Therefore, when single-phase grounding occurs, the grounding fault information cannot be accurately reported to the dispatching and monitoring center in time, delaying the line maintenance personnel to find and repair the line fault point. In some areas, there have been many major accidents caused by the failure to cut off power in time after the single-phase grounding of 10kv High-voltage Distribution Lines, which led to the death of other social personnel living near the grounding point. It has caused great trouble, economic loss and adverse social impact to the power supply department

therefore, according to the requirements of the national standard: GB code for design of relay protection and automatic devices of power devices, in the "small grounding current system", the use of accurate and reliable single-phase grounding protection line selection device is one of the important technical measures to improve the automation level of power supply system

Second, the technical status in this field at home and abroad

the line selection of single-phase grounding protection in small grounding current system has exceeded US $50billion in three years, which is a worldwide problem; It has not been completely solved in the process of power production for more than 100 years. At the beginning of the last century, many power engineering technicians and colleges and universities abroad have conducted a lot of research on this project. Their understanding is not only in-depth, but also more and more technical solutions, and the accuracy is gradually improving. Among them, the representative ones are Bach's "first half wave" theory and Russia's "reactive power direction" theory. The device developed according to these theories has been used in the power system, and the accuracy of line selection can reach about 50%. Since 1980s, China has developed automatic single-phase grounding line selection device for small current grounding system. Although it started late, its development speed is very fast, and it has reached the world advanced level at present. Among them, Beijing electric power academy, Nanjing Electric Power Automation Research Institute, Xuchang relay factory and other units have successively developed various types of devices by setting the action value of zero sequence current for protection and line selection based on the principle of zero sequence current, using sensitive relay and transistor electronic protection technology. After years of use, the accuracy of its line selection has been close to 50%. Later, using the "first half wave" theory and transistor electronic technology, several line selection devices of different specifications were produced, which were popularized and used in China's power system, making the accuracy of line selection higher than the former, up to about 60%. Since the 1990s, single chip computers have been widely used in China. Many high-tech enterprises began to apply this high-tech microcomputer technology to this field; At the same time, it is based on the theory put forward by Professor xuyuanheng of North China Electric Power Institute: "in the power supply system with ungrounded neutral (or grounded through arc suppression coil), the direction of zero sequence harmonic current of fault line is opposite to that of zero sequence harmonic current of non fault line". Many scientific research institutes and enterprises have developed small current grounding line selection devices, especially a group of students graduated from North China Electric Power Institute. Based on this theory, batch production has been carried out in Baoding, Hebei, Beijing and other places, and it has been popularized and applied in the power system, so that the accuracy of protection line selection has reached about 70% on average. Compared with the former, the reliability has been greatly improved; However, it still cannot meet the requirements of power system

after that, Professor sang Zaizhong of Shandong University of technology put forward the theory of "s injection method" through research. Namely: inject an AC current signal of about 250Hz from the low-voltage side to the high-voltage bus through the method of reverse power transmission from the secondary winding of the voltage transformer to the primary winding; Then detect and compare the current signal in each line, and finally judge and select the fault line. According to this theory, several units represented by Shandong University of technology, Huazhong University of science and technology and Chengdu Science and technology company have successively developed relevant series of products. From the surface phenomenon, this method is relatively novel and unique. It has been widely promoted and tried in many places in China. Among them, this device has been installed in the 10kV distribution system of Qiaoying, Shun'an, Gaoxin, Youfang and Qianying substations of Xiangfan Power Supply Bureau and Zaoyang switching station, and has operated for more than 3 years. Through the use, it is found that this kind of device is often misselected, and the substation duty personnel generally reflect that it is unreliable; Once grounding occurs, I don't know what to do; Finally, the device had to be turned off, and the fault line was still selected by the method of successive switching off. Now the device has been out of operation and completely removed

"wavelet analysis method" is another new scheme theory for single-phase grounding protection line selection of small grounding current system in recent years. Its principle refers to the basic principle of "fault recorder" used in power system. By collecting and recording the waveform of high-order harmonic current of each line when single-phase grounding, and then analyzing and comparing with computer software, find out the line whose current waveform is significantly different from that of other lines, and determine it as a fault line. The device developed according to this method has been tested in some substations. But the effect is still not ideal. The accuracy of line selection is less than 50%. It needs to be further tested and improved

in the microcomputer protection device of substation integrated automation, most of the principles used for single-phase grounding protection are the early zero sequence current overcurrent setting principle. That is, the single 1 of the circuit protection device adds zero sequence current overcurrent protection in the line element behind the completion of the main structure; As the principle of short-circuit protection and over-current protection, protection line selection depends on setting the starting value of current. This way, history has already proved to be

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