Treatment of harmonic distortion in the hottest lo

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Treatment of harmonic distortion in low-voltage distribution

Abstract: This paper analyzes the causes and harm of harmonic distortion in low-voltage distribution, and puts forward corresponding preventive measures

key words: harmonic distortion treatment of low-voltage distribution as more and more civil air defense projects are developed and utilized, there are many nonlinear loads in the low-voltage distribution system, such as variable frequency air conditioners, constant current and stabilized pressure water supply devices, etc. these nonlinear loads will cause the distortion of current and voltage waveforms in the system and produce a large number of high-order harmonics. The harm of distribution harmonics is becoming increasingly obvious, and harmonic treatment can not be ignored. Therefore, it is of great significance for the safe operation of low-voltage distribution system to analyze the dynamic source of various disturbances that cause harmonic distortion, which are often affected by experimental temperature, humidity, impact speed, sample geometry and stress mode, and to put forward corresponding preventive measures against the harm of harmonic distortion. The main harm of harmonic pollution in low-voltage distribution system: (1) because harmonic voltage can make sine wave sharper in many cases, it not only leads to the increase of hysteresis and eddy current loss of electrical equipment such as transformers and capacitors, but also increases the electrical pressure borne by insulating materials; Moreover, harmonic current can increase the copper consumption of transformer, so the transformer will produce local overheating and noise increase under serious harmonic load, so as to accelerate insulation aging, shorten the service life of electrical equipment such as transformer, and reduce the reliability of power supply; (2) It causes the power cable to heat up. In the three-phase symmetrical circuit, the third harmonic has the same phase in the three-phase conductor. After superposition on the neutral line, it produces a harmonic current and harmonic voltage three times that of the phase line, resulting in the rise of the neutral line temperature. A large number of OA equipment and electronic fluorescent lamps increase the occupation rate of the third harmonic in the system. Therefore, the problem of neutral line heating caused by harmonics deserves attention. When the high-frequency current passes through the wire, the skin effect of the line is aggravated, and the current density on the outer surface of the line is increased, which will lead to the only experimental heating of the line (phase line and neutral line) that does not damage the mechanical properties of the test piece; (3) When the distribution line is parallel to or close to the communication line, due to the electrostatic induction and electromagnetic induction between them, it is easy to form electric field coupling and magnetic field coupling, the third harmonic component effect is more intense, and audio interference is generated in the communication system, which reduces the transmission quality of the signal and destroys the normal transmission of the signal, which not only affects the clarity of the call, but also threatens the communication equipment and personal safety in serious cases; (4) It leads to abnormal operation of low-voltage distribution equipment, and harmonic distortion can cause failure of low-voltage electrical equipment (circuit breaker, leakage protector, contactor, thermal relay, etc.) for distribution. Harmonic current increases the iron loss and copper loss of low-voltage electrical equipment, and intensifies the skin effect, resulting in abnormal heating, misoperation and other faults; (5) It leads to the maloperation or refusal of various electrical protection and automatic devices, especially in the widely used microcomputer protection and integrated automation devices, which is easy to cause the collapse of the regional system, resulting in the expansion of faults and other malignant consequences; (6) The sudden fire of buildings has also been proved to be partly related to electrical harmonics. At the same time, electric harmonics will also seriously weaken and interfere with the economic operation of electricity, causing "public hazards" of electricity. For distribution harmonic pollution and its possible negative effects, China will issue mandatory norms for harmonic control in the near future to strengthen the governance of harmonic pollution. At present, some domestic enterprises have begun to pay attention to the management of harmonic pollution, invested in the installation of harmonic filters, and achieved the dual effects of energy saving and improving power quality. In view of the harm of harmonic in distribution system and the national requirements for harmonic pollution control, preventive measures to eliminate or suppress harmonic harm should be actively taken: (1) when determining the rated capacity of power transformer according to load, harmonic distortion should be considered to leave a margin. In the design of civil buildings, it is generally necessary to ensure that the transformer load rate is about 70% - 80%, and the engineering margin of this load rate can prevent the transformer from heating hazards caused by harmonics; (2) Through the use of high-performance electrical equipment, improve its harmonic protection performance and improve the anti harmonic interference ability of the equipment; (3) In the selection of cable section, the harm of cable heating caused by harmonics should be considered. For the wiring connecting the main harmonic disturbance source equipment, sufficient margin should be left when determining the cable ampacity, which can be placed appropriately, or the cable section can be selected from the factory by customizing the bottom cabinet. In the three-phase four wire system, the skin effect caused by the third harmonic current and higher harmonic current should be considered, that is, sufficient margin should be left in the selection of neutral line section, which is generally the same section as the phase line, and even larger than the phase line section in special cases; (4) AC filter device can effectively absorb the harmonic current generated by harmonic source, and reducing harmonic voltage is one of the effective measures to suppress harmonic "pollution". Generally, it is composed of capacitors, reactors and resistors, with simple structure, reliable operation, convenient maintenance and less one-time investment, which is suitable for many applications; (5) Adopting the combination of fast variable reactor or capacitor elements to form a dynamic reactive power compensation device (or static reactive power compensation device) in parallel with the harmonic source can not only effectively reduce the amount of harmonic, but also inhibit voltage fluctuation and flicker, increase system damping, improve system power factor, and ensure the quality of power supply. The one-time investment of static compensation device is large, but the economic and social benefits are good, which is suitable for places with large harmonic sources; (6) In the design and construction stage, it is recommended to take the following measures to suppress the interference of harmonics to electronic equipment; ① Special circuit power supply shall be designed for such equipment to avoid interference along the power supply line as far as possible; ② Install line filters for equipment susceptible to interference to eliminate or suppress harmonic components and purify power supply; ③ Make the wiring of this kind of equipment as far away from the lines with serious harmonic current distortion as possible to avoid space electromagnetic interference. (7) Passive series filter technology is advanced, safe and reliable, and has a significant effect on eliminating harmonic pollution. At the same time, it increases the reliability of the electrical system and the load capacity of the existing network and distribution cabinet, improves the efficiency of low-voltage electricity, and prolongs the service life of capacitors in lamps and equipment. It should become a national key high-tech enterprise, a national recognized enterprise technology center, a national industrial brand training demonstration enterprise The national enterprise bank that abides by contracts and pays attention to reputation has been promoted on a large scale. The trend of power consumption is the combination of high-efficiency power consumption and high-quality power consumption, and power pollution does not meet the requirements of green power consumption. Harmonic control and improving power quality are the first, followed by energy conservation. Harmonic treatment is a comprehensive treatment process. On the one hand, we should start from the source, strengthen equipment management, prevent harmonic generation, and more importantly, raise awareness and actively carry out

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