Harmonic is a mathematical or physical concept, which refers to the part of a periodic function or a periodic waveform that cannot be represented by a linear combination of constants, sine functions and cosine functions that are the same as the minimum positive period of the original function. First, the source of harmonics The word "harmonics" comes from acoustics. The harmonic problem of power system has attracted people's attention as early as the 1920s and 1930s. At that time, in Germany, the voltage and current waveforms were distorted due to the use of static mercury arc converters. J.C.Read's paper on converter harmonics published in 1945 is a classic paper on early harmonic research. Harmonic waveform figure 2. Harmonic definition harmonic definition: harmonic refers to the amount of electricity contained in the current whose frequency is an integer multiple of the fundamental wave. Generally, it refers to the electricity generated by Fourier series decomposition of periodic non-sinusoidal electricity, and the rest is greater than the fundamental frequency. Cause: Because the sinusoidal voltage is applied to the nonlinear load, the fundamental current is distorted to produce harmonics. The main nonlinear loads are UPS, switching power supply, rectifier, frequency converter and inverter. Harm of harmonics: reduce the capacity of transformers, circuit breakers, cables and other systems. Accelerate the aging of equipment, shorten the service life of equipment, and even damage the equipment; Endangering production safety and stability; Waste electric energy, etc. Harmonic control: Active power filter is the best product for harmonic control. Thirdly, the generation of harmonics is based on the principle of Fourier analysis, which can decompose non-sinusoidal signals into basic parts and their multiples. In power system, the fundamental cause of harmonic generation is caused by nonlinear load. When the current flows through the load, it is not linear with the applied voltage, forming a non-sinusoidal current, that is, harmonics are generated in the circuit. Due to the switching operation of semiconductor thyristors and the nonlinear characteristics of diodes and semiconductor thyristors, some equipment in power system, such as power converters, will deviate from sinusoidal waveform. The generation of harmonic current is related to the pulse number of power converter. The 6-pulse equipment is only 5, 7, 1 1, 13, 17,19 … n times the power grid frequency. The higher the pulse number of the power converter, the higher the frequency of the lowest harmonic component. Other power-consuming devices, such as the electronic control regulator of fluorescent lamps, will generate strong third harmonic (150 Hz). This non-sinusoidal current leads to a non-sinusoidal voltage drop under the impedance (resistance) of the power supply network. The amplitude of the harmonic voltage generated under the power supply network impedance is equal to the product of the corresponding harmonic current and the power supply network impedance z corresponding to the current frequency. The higher the frequency, the lower the amplitude of harmonic components. As long as there is a harmonic source, there will be harmonics. Harmonic components may also reach the user network through the power supply network. For example, the operation of one user factory in the power supply network may be disturbed by harmonics generated by another adjacent user equipment. The difference between harmonics and overtones is actually physical harmonics, but the definition of numbers is slightly different. Audio with twice the fundamental frequency is called primary overtone, audio with three times the fundamental frequency is called secondary overtone, and so on. Fourth, the classification of harmonics is a sine wave, and each harmonic has a different frequency, amplitude and phase angle. The harmonic frequency is an integer multiple of the fundamental frequency. According to the analysis principle of French mathematician M. Fourier, it is proved that any repeated waveform can be decomposed into sine wave components containing fundamental frequency and a series of harmonics with frequency doubling. According to the different harmonic frequencies, it can be divided into: 4. 1, and the rated frequency of odd harmonics is odd times of the fundamental frequency, which is called "odd harmonics", such as 3rd, 5th and 7th harmonics; 4.2. Even harmonics whose rated frequency is even multiple of the fundamental frequency are called "even harmonics", such as the second, fourth, sixth and eighth harmonics. Generally speaking, odd harmonics are more harmful than even harmonics. In a balanced three-phase system, even harmonics have been eliminated due to symmetry, and only odd harmonics exist. For three-phase rectifier load, the harmonic current is 6n 1 subharmonic, such as 5, 7,1,13, 17, 19, etc. The frequency converter mainly produces 5th and 7th harmonics. 4.3. The component harmonic frequency is not a multiple of the fundamental component of sinusoidal wave. V. Harmonic parameter 5. 1. Harmonic current Harmonic current is a non-sinusoidal characteristic current introduced by equipment or system. Harmonic current is superimposed on the main power supply; 5.2. Harmonic voltage Harmonic voltage is the voltage drop caused by harmonic current and impedance generated on the distribution system; Definition of parameters related to intransitive verbs and harmonics 6. 1 Impedance impedance is the resistance produced at a certain point in the distribution system at a specific frequency. The impedance depends on the transformer and the electrical equipment connected to the system, as well as the cross-sectional area and length of the conductor used. 6.2. Impedance coefficient Impedance coefficient is the ratio of AF (carrier wave) impedance to 50Hz (fundamental wave) impedance. 6.3. The devices that resonate in the power distribution system form a * * vibration circuit with their existing capacitors (cables, compensation capacitors, etc.). ) and inductors (transformers, reactance coils, etc.). ). The latter can be excited by system harmonics and become resonance. One of the causes of harmonics in distribution system is the nonlinear magnetization of transformer core. In this case, the main harmonic is the third harmonic; It has the same length as the single-phase component in all conductors, so it cannot be eliminated at the star point. 6.4. Resonance Frequency Each inductor and capacitor is connected to form a resonance circuit with a specific * * * vibration frequency. A network consisting of several inductors and capacitors has several resonant frequencies. 6.5. The impedance of parallel resonant frequency network reaches the maximum frequency. In the parallel resonant circuit, the current components i 1 and I2C are greater than the total current i.6.6.. The impedance level of the series resonant frequency network reaches the minimum frequency. In the series resonant circuit, the parallel voltages U L and U C are greater than the total voltage u. 6.7. A series resonant circuit is a circuit in which an inductor (reactor) and a capacitor (capacitor) are connected in series. 6.8. The magnetic energy part of reactive power motors and transformers and the power converter used for energy exchange need reactive power Q.. Unlike active power, reactive power does not do work. The unit for measuring reactive power is Var or kvar. 6.9, reactive power compensation power supply department to provide a minimum power factor to avoid waste of electricity. If the power factor of a factory is lower than this minimum value, it will pay reactive power. Otherwise, capacitors should be used to improve the power factor, so it is necessary to install capacitors in parallel on electrical equipment. Harm of Harmonics The voltage provided by an ideal public power grid should be a single fixed frequency and a specified voltage amplitude. The appearance of harmonic current and harmonic voltage is a kind of pollution to the public power grid, and the environment in which it uses electrical equipment deteriorates, which also affects the surrounding capacity. Before the wide application of power electronic equipment, people had some research on harmonics and their harm, and had a certain understanding, but at that time, harmonic pollution had not attracted enough attention. In recent 30 or 40 years, with the rapid development of various power electronic devices, the harmonic pollution of public power grid has become increasingly serious, and various faults and accidents caused by harmonics have also occurred constantly. The seriousness of harmonic harm has attracted great attention. Harm of harmonics is very serious. Harmonics reduce the production, transmission and utilization efficiency of electric energy, make electrical equipment overheat, produce vibration and noise, make insulation aging, shorten service life, and even break down or burn out. Harmonic will cause local parallel resonance or series resonance of power system, amplify harmonic content, and lead to the burning of capacitors and other equipment. Harmonics can also cause misoperation of relay protection and automatic devices, leading to confusion of electric energy measurement. Outside the power system, harmonics will cause serious interference to communication equipment and electronic equipment. Harm of harmonics to public power grid and other systems is as follows: 7. 1. And the power operation cost of the enterprise is increased. Because harmonics can't be eliminated naturally without treatment, a large number of harmonic voltages and currents wander in the power grid and accumulate and overlap, resulting in increased line loss and overheating of power equipment, thus increasing power operation cost and power expenditure. 7.2. The reliability of power supply is reduced. Harmonic voltage will make sine wave sharper in many cases, which 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 stress borne by insulating materials. Harmonic current will increase the copper consumption of transformer, so the transformer will be overheated locally and the noise will increase under severe harmonic load, thus accelerating insulation aging, greatly shortening the service life of transformer and motor, reducing the reliability of power supply, which is likely to cause serious consequences of power failure in the production process. 7.3 The power supply accident power grid contains a large number of harmonic sources (frequency conversion or rectification equipment) and loads such as power capacitors, transformers, cables and motors. These electrical devices are constantly changing, and it is easy to form series or parallel resonance conditions. When the power grid parameters are not well matched, harmonic oscillation will be formed at a certain frequency, resulting in overvoltage or overcurrent, which will endanger the safe operation of the power system. If not handled, it will easily lead to power transmission and distribution accidents. 7.4. The equipment cannot work normally. For rotating generators and motors, harmonic current or harmonic voltage produces additional losses in stator windings, rotor circuits and iron cores, which reduces the efficiency of power generation, transmission and power consumption equipment. What's more, harmonic oscillation is easy to make the turbine generator generate oscillation torque, which may cause mechanical vibration, turbine blade distortion and fatigue cycle, resulting in equipment failure to work normally. 7.5. The automatic relay protection device that causes serious accidents plays a very important role in ensuring the safe operation of the power grid. However, due to the existence of a large number of harmonics, it is easy to make all kinds of protection and automatic devices in the power grid malfunction or refuse to operate, especially in the widely used microcomputer protection and integrated automatic devices, leading to regional (in-plant) power grid collapse and large-scale power outage and other vicious accidents. 7.6. The harmonics of the power grid that lead to short circuit will cause errors of measuring instruments and measuring devices, and the correct indication and measurement will not be achieved (the errors of measuring instruments are mainly reflected in the electric energy meter). When the circuit breaker breaks the current with high harmonic content, the breaking ability of the circuit breaker will be greatly reduced, which will lead to arc reignition, short circuit and even circuit breaker explosion. 7.7, reduce product quality Due to the long-term existence of resonance waves, the operation of motors and other equipment increases vibration, thus increasing production errors and reducing processing accuracy and product quality. 7.8. Affect the normal operation of the communication system When the transmission line and the communication line are parallel or close to each other, due to electrostatic induction and electromagnetic induction between them, electric field coupling and magnetic field coupling will be formed, and harmonic components will cause audio interference to the communication system, thus reducing the transmission quality of the signal and destroying the normal transmission of the signal, which will not only affect the clarity of the call, but also threaten the communication equipment and personal safety in serious cases. Harmonics will interfere with adjacent communication systems, generate noise under light, and reduce communication quality. In the worst case, the residence is lost, which makes the communication system unable to work normally.