Application of the hottest three-level APF in civi

Application of three-level APF in civil low-voltage distribution system

low voltage distribution in China generally adopts three-phase five wire system, with three phase lines, one neutral line and one ground line. In the actual system, there are both three-phase load and single-phase load, and the load type, quantity and proportion are different in different occasions. In the general civil field, three-phase load is less, mainly single-phase load. During the power distribution construction, the three-phase power supply shall be reasonably distributed according to the load conditions to reduce the neutral line current. With the continuous development of electronic technology, all kinds of electronic equipment, energy-saving lamps, fluorescent lamps, frequency conversion appliances are more and more used in civil buildings. While these devices and lamps continue to meet people's life, they are also injecting a large amount of harmonic current into the electricity. In the case of concentrated load, its harmonic content is even more serious than that of industrial enterprises. For a long time, the harmonic problem has only been paid attention to in the industrial field, but rarely in the civil system. The electrical technicians of relevant units seldom understand the knowledge of harmonics and pay little attention to harmonics. It also seems blind when encountering problems caused by harmonics. In this article, Ao Hong, deputy secretary of the Party group and deputy general manager of Chinalco, will attend the ceremony to explain in detail the causes and solutions of harmonics in civil and commercial distribution systems

what equipment generates harmonics

the current generated on the sinusoidal AC voltage active load (resistance, capacitance, inductance, ideal transformer and its combined circuit) is sinusoidal AC current, and its law conforms to Ohm's law of AC circuit. The volt ampere characteristic curve of nonlinear load is not a straight line, and its circuit characteristics do not follow Ohm's law. When the sinusoidal voltage acts on it, the current generated is no longer a standard sinusoidal current, but a distortion occurs. According to the Fourier series theory, the load current can be Fourier integrated to decompose each harmonic based on the fundamental frequency, that is, in addition to the fundamental (50Hz) current, there are also 2 (100Hz), 3 (150Hz),... Harmonic currents in the current. In civil buildings, fluorescent lamps, energy-saving lamps, variable frequency air conditioners, computers, televisions and other common electrical appliances will produce a large number of harmonics. It can be said that almost all of the electricity in civil buildings will produce harmonics. Different equipment produces different harmonics, and the harmonic problem has become a problem that can not be ignored in civil buildings

what is the harm of harmonics

there are many hazards of harmonics. The most obvious problem in civil power distribution is the excessive neutral current. In civil buildings, single-phase load is the main load. For an ideal linear three-phase system, if the three-phase load is balanced, the neutral line current is zero. In practice, if there is no harmonic current and the three-phase load is properly distributed, the neutral current is very small. However, the third harmonic generated by a large number of nonlinear loads is superimposed on the middle line, which cannot be reduced or eliminated by adjusting the three-phase load balance. Through the harmonic test on some occasions, it can be seen that the current on the neutral line may exceed the phase line. According to the waveform measurement, the current on the neutral line is almost all the third harmonic current. The harmonic current on the phase line of the three-phase four wire distribution system is also mainly the third harmonic. The neutral line current caused by the third harmonic will cause the neutral line to heat up, and in serious cases, it will cause the neutral line to be burned and cause a fire. At the same time, the harmonic current acts on the neutral impedance to produce the neutral voltage, and the neutral may also cause electric shock and cause danger. Excessive harmonic current may also lead to misoperation of protection equipment, causing tripping and power failure for unknown reasons. Harmonics will also increase the losses of lines and transformers and waste electric energy

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