The word "harmonic" comes from acoustics. The mathematical analysis of harmonics has laid a good foundation in18th century and19th century. Harmonic analysis proposed by Fourier et al.
This method is still widely used. 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.
In 1950s and 1960s, due to the development of HVDC transmission technology, a large number of papers were published on harmonic problems caused by converters in power systems. Since 1970s, due to the rapid development of power electronic technology, various power electronic devices have been widely used in power system, industry, transportation and home, and the harm caused by harmonics has become increasingly serious. All countries in the world attach great importance to harmonic problems. Many international academic conferences on harmonics have been held, and many countries and international academic organizations have formulated standards and regulations to limit harmonics in power systems and electrical equipment.
The significance of harmonic research is moral, because the harm of harmonic 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.
2. Harmonic suppression
In order to solve the problem of harmonic pollution of power electronic devices and other harmonic sources, there are two basic ideas: one is to install harmonic compensation devices to compensate harmonics, which are suitable for all kinds of harmonic sources; The other is to transform the power electronic device itself, so that no harmonics are generated during the period, and the power factor can be controlled at 1, which is of course only applicable to the power electronic device as the main harmonic source.
The traditional method of installing harmonic compensation device is to use LC tuning filter. This method can compensate both harmonics and reactive power, and its structure is simple, so it has been widely used. The main disadvantage of this method is that the compensation characteristics are affected by the impedance of the power grid and the operating state, and it is easy to have parallel resonance with the system, which leads to harmonic amplification and LC filter overload or even burning. In addition, it can only compensate harmonics with fixed frequency, and the compensation effect is not ideal.
3. Reactive power compensation also
It is very easy for people to understand active power, but it is not easy to deeply understand reactive power. In sinusoidal circuits, the concept of reactive power is clear, but when it contains harmonics, there is no accepted definition of reactive power. However, the concept of reactive power is as important as reactive power compensation. Reactive power compensation should include fundamental reactive power compensation and harmonic reactive power compensation.
Reactive power is very important to the operation of power supply system and load. The impedance of power system network elements is mainly inductive. Therefore, roughly speaking, in order to transmit active power, it is required that there is a phase difference between the terminal voltage of power transmission and the terminal voltage of power reception, which can be realized in a fairly wide range; In order to transmit reactive power, it is required that the voltage at both ends have amplitude difference, which can only be realized in a very narrow range. Not only do most network elements consume reactive power, but most loads also need to consume reactive power. The reactive power required by network elements and loads must be obtained from somewhere in the network. Obviously, it is unreasonable and usually impossible for these reactive powers to be provided by generators and transmitted over a long distance. The reasonable method should be to generate reactive power where it needs to be consumed, which is reactive power compensation.
The functions of reactive power compensation mainly include the following points:
(1) Improve the power factor of power supply system and load, reduce equipment capacity and reduce power consumption.
(2) Stabilize the voltage of the receiver and the power grid, and improve the quality of power supply. Setting dynamic reactive power compensation devices at appropriate positions of long-distance transmission lines can also improve the stability of transmission systems and transmission capacity.
(3) When the three-phase load is unbalanced, such as electrified railways, the active and reactive loads of the three-phase can be balanced by appropriate reactive power compensation.
Second, the generation of harmonics and reactive power
In the industrial and domestic electricity load, inductive load accounts for a large proportion. Asynchronous motor, transformer, fluorescent lamp, etc. Are typical perceptual loads. The reactive power consumed by asynchronous motors and transformers accounts for a large proportion of the reactive power provided by the power system. Reactors and overhead lines in the power system will also consume some reactive power. Resistive load must absorb reactive power to work normally, which is determined by its own nature.
Nonlinear devices such as power electronic devices also consume reactive power, especially various phased devices. Such as phase-controlled rectifier, phase-controlled AC power regulating circuit, periodic converter, etc., when working, the fundamental current lags behind the grid voltage and consumes a lot of reactive power. In addition, these devices will also generate a lot of harmonic current, and the harmonic source will consume reactive power. The fundamental current phase of diode rectifier circuit is almost the same as the grid voltage, so it basically does not consume fundamental reactive power. But it also produces a lot of harmonic current, so it also consumes a certain amount of reactive power.
In recent 30 years, the application of power electronic devices has been increasingly extensive, which also makes power electronic devices the largest harmonic source. Among all kinds of power electronic devices, rectifier devices account for the largest proportion. At present, almost all commonly used rectifier circuits adopt thyristor phase-controlled rectifier circuits or diode rectifier circuits, among which three-phase bridge rectifier circuits and single-phase bridge rectifier circuits are the most. Harmonic pollution and power factor lag caused by resistive load rectifier circuit are well known. The diode rectifier circuit with capacitor filter on DC side is also a serious harmonic pollution source. The phase of the fundamental component of the input current in this circuit is almost the same as that of the power supply voltage, so the fundamental power factor is close to 1. But the harmonic component of its input current is very large, which causes serious pollution to the power grid and makes the total power factor very low. In addition, power electronic devices such as AC power regulating circuit and periodic converter with phase control mode will also produce a large number of harmonic currents on the input side.
Third, the influence of reactive power and the harm of harmonics
1. Influence of reactive power
(1) The increase of reactive power will lead to the increase of current and apparent power, thus increasing the capacity and conductor capacity of electrical equipment such as generators and transformers.
. At the same time, the size and specifications of starting and control equipment and measuring instruments for power users should also be increased.
(2) With the increase of reactive power, the total current increases, thus increasing the loss of equipment and lines, which is obvious.
(3) Increase the voltage drop of lines and transformers. If it is an impact reactive load, the voltage will fluctuate violently, seriously reducing the quality of power supply.
2. Harm of harmonics
The voltage provided by the 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 is deteriorating day by day, and it is also widely used in the surrounding power electronic equipment. Before it was widely used, people had done some research on harmonics and its harm, and had a certain understanding, but at that time, harmonic pollution still needed to be severely punished, which did not attract enough attention. In recent thirty or forty years, the rapid development of various power electronic devices has been made public. The harmonic pollution of power grid is becoming more and more serious, and various faults and accidents caused by harmonics are also happening constantly. The seriousness of harmonic harm has attracted great attention. Harm of harmonics to public power grid and other systems is roughly as follows.
(1) harmonics will cause additional harmonic losses to components in the public power grid, thus reducing the efficiency of power generation, transmission and electrical equipment. When a large number of third harmonics flow through the neutral line, it will overheat the line and even cause a fire.
(2) Harmonics affect the normal work of various electrical equipment. The influence of harmonics on the motor will not only cause extra losses, but also produce mechanical vibration, noise and overvoltage, which will cause local overheating of the transformer in severe cases. Harmonics make capacitors, cables and other equipment overheat, insulation aging, life shortening and even damage.
(3) Harmonics will cause local parallel resonance and series resonance in public power grid, thus amplifying harmonics, greatly increasing the hazards of (1) and (2) above, and even causing serious accidents.
(4) Harmonics will lead to misoperation of relay protection and automatic devices, making the measurement of electrical measuring instruments inaccurate.
(5) 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.
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