Ⅰ. Harmonic

1. What are harmonics?

The root cause of harmonics in power systems is due to non-linear loads. When the current flows through the load, it does not show a linear relationship with the applied voltage, and a non-sinusoidal current is formed, that is, harmonics are generated in the circuit. The harmonic frequency is an integral multiple of the fundamental frequency. According to the French mathematician Fourier (M. Fourier) analysis principle, any repetitive waveform can be decomposed into a sine wave containing the fundamental frequency and a series of harmonics that are multiples of the fundamental. Harmonics are sine waves, and each harmonic has a different frequency, amplitude and phase angle. Harmonics can be divided into even-order and odd-order. The 3rd, 5th, and 7th times are odd-order harmonics, while 2, 4, 6, 8 and so on are even-order harmonics. For example, when the fundamental wave is 50Hz, 2 The sub-harmonic is 100Hz, and the third harmonic is 150Hz. Generally speaking, odd harmonics cause more harm than even harmonics. In a balanced three-phase system, due to the symmetry relationship, even harmonics have been eliminated, and only odd harmonics exist. For a three-phase rectified load, the harmonic currents appearing are 6n±1 harmonics, such as 5, 7, 11, 13, 17, 19, etc. The frequency converter mainly produces 5 and 7 harmonics.

The term "harmonic" originated from acoustics. The mathematical analysis of harmonics has laid a good foundation in the 18th and 19th centuries. The harmonic analysis method proposed by Fourier et al. 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. The paper on converter harmonics published by J.C. Read in 1945 is an early classic paper on harmonics research.

In the 1950s and 1960s, due to the development of high-voltage direct current transmission technology, a large number of papers on the harmonic problems of power systems caused by converters were published. Since the 1970s, due to the rapid development of power electronic technology, various power electronic devices have been widely used in power systems, industry, transportation and homes, and the harm caused by harmonics has become increasingly serious. Countries all over the world are fully and concerned about the harmonic problem. Many academic conferences on harmonic issues have been held internationally, 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 that morality is because the harm of harmonics is very serious. Harmonics reduce the efficiency of the production, transmission and utilization of electric energy, cause electrical equipment to overheat, generate vibration and noise, and cause insulation aging, shorten the service life, and even malfunction or burn. Harmonics can cause local parallel resonance or series resonance in the power system, amplify the harmonic content, and cause the burning of capacitors and other equipment. Harmonics can also cause malfunctions of relay protection and automatic devices, causing confusion in electric energy measurement. For the outside of 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 a harmonic compensation device to compensate for harmonics, which is applicable to all kinds of harmonic sources; the other is The power electronic device itself is modified so that it does not produce harmonics and the power factor can be controlled to 1. This of course only applies to power electronic devices that are the main source of harmonics.

The traditional method of installing harmonic compensation devices is to use LC tuned filters. This method can not only compensate for harmonics, but also for reactive power, and has a simple structure and has been widely used. The main disadvantage of this method is that the compensation characteristics are affected by the grid impedance and operating conditions, and it is easy to have parallel resonance with the system, which leads to harmonic amplification, and overloads or even burns the LC filter. In addition, it can only compensate the harmonics of a fixed frequency, and the compensation effect is not ideal.

3. Reactive power compensation

It is very easy for people to understand active power, but it is not easy to have a deep understanding of reactive power. In the sinusoidal circuit, the concept of reactive power is clear, and when there are harmonics, there is no recognized definition of reactive power yet. However, the importance of the concept of reactive power and the understanding of the importance of reactive power compensation are consistent. Reactive power compensation should include compensation for fundamental reactive power and compensation for harmonic reactive power.

Reactive power is very important to the operation of the power supply system and the load. The impedance of power system network components is mainly inductive. Therefore, roughly speaking, in order to deliver active power, it requires a phase difference between the voltages at the transmitting end and the receiving end, which can be achieved in a wide range; and in order to transmit reactive power, the voltage at both ends is required to have a magnitude. Poor, this can only be achieved within a very narrow range. Not only do most network components consume reactive power, most loads also need to consume reactive power. The reactive power required by network components and loads must be obtained from somewhere in the network. Obviously, it is unreasonable for these reactive powers to be provided by generators and transmitted over long distances, and it is usually impossible. A reasonable method should be to generate reactive power where reactive power needs to be consumed. This is reactive power compensation.

The main functions of reactive power compensation are as follows:

(1) Improve the power factor of the power supply system and load, reduce equipment capacity, and reduce power loss.
 

(2) Stabilize the voltage of the power receiving terminal and the power grid to improve the quality of power supply. Setting up a dynamic reactive power compensation device at a suitable location in a long-distance transmission line can also improve the stability of the power transmission system and increase the power transmission capacity.
(3) In the case of unbalanced three-phase loads such as electrified railways, the three-phase active and reactive loads can be balanced through appropriate reactive power.

Ⅱ. Generation of harmonics and reactive power

In industrial and domestic electrical loads, resistive inductive loads account for a large proportion. Asynchronous motors, transformers, fluorescent lamps, etc. are all typical resistive loads. The reactive power consumed by asynchronous motors and transformers occupies a high proportion of the reactive power provided by the power system. Reactors and overhead lines in the power system also consume some reactive power. The resistive load must absorb reactive power to work normally, which is determined by its own nature.

Non-linear devices such as power electronic devices also consume reactive power, especially various phase control devices. For example, phase-controlled rectifiers, phase-controlled AC power adjustment circuits, and cycle converters, the fundamental current lags behind the grid voltage during operation, and consumes a lot of reactive power. In addition, these devices also generate a large amount of harmonic currents, and harmonic sources consume reactive power. The phase of the fundamental wave current of the diode rectifier circuit is roughly the same as the phase of the grid voltage, so fundamental wave reactive power is basically not consumed. But it also generates a lot of harmonic currents, so it also consumes a certain amount of reactive power.

In the past 30 years, the application of power electronic devices has become increasingly widespread, making power electronic devices the largest source of harmonics. Among various power electronic devices, the rectifier device accounts for the largest proportion. At present, almost all commonly used rectifier circuits use thyristor phase-controlled rectifier circuits or diode rectifier circuits, of which three-phase bridge and single-phase bridge rectifier circuits are the most. The harmonic pollution and power factor lag produced by the rectifier circuit with resistive load are familiar to people. The diode rectifier circuit that uses capacitor filtering on the DC side is also a source of severe harmonic pollution. The phase of the fundamental wave component of the input current of this circuit is roughly the same as the phase of the power supply voltage, so the fundamental wave power factor is close to 1. However, the harmonic components of its input current are very large, causing serious pollution to the power grid and making the total power factor very low. In addition, power electronic devices such as phase-controlled AC power adjustment circuits and cycloconverters will also generate a large amount of harmonic currents on the input side.

Ⅲ. The influence of reactive power and the harm of harmonics

1. The influence of reactive power

(1) The increase in reactive power will lead to an increase in current and increase in apparent power, thereby increasing the capacity of generators, transformers and other electrical equipment and wire capacity. At the same time, the size and specifications of starting and control equipment and measuring instruments for electric power users must be increased.

(2) It is obvious that the increase of reactive power increases the total current and therefore increases the loss of equipment and lines.

(3) Increase the voltage drop of the line and the transformer. If it is an impulsive reactive power load, it will also cause the voltage to fluctuate sharply, which will seriously reduce the quality of the power supply.

2. The harm of harmonics

The voltage provided by the ideal public grid should be a single and fixed frequency and a prescribed voltage amplitude. The emergence of harmonic current and harmonic voltage is a kind of pollution to the public grid. The environment in which it uses electrical equipment is deteriorating, and it is also widely used in the surrounding capable power electronic equipment. People have done harm to harmonics and their hazards. Some research, and there is a certain understanding, but at that time harmonic pollution has not attracted enough attention. In the past 30 to 40 years, the rapid development of various power electronic devices has made the harmonic pollution of the public grid increasingly serious. Various faults and accidents caused by harmonics have also occurred continuously. The severity of harmonic harm has attracted people's attention. The harm of harmonics to public power grids and other systems roughly has the following aspects.

(1) Harmonics cause additional harmonic losses to the components in the public grid, which reduces the efficiency of power generation, transmission and electrical equipment. When a large number of third harmonics flow through the neutral line, the line will overheat and even cause a fire. .

(2) Harmonics affect the normal operation of various electrical equipment. The influence of harmonics on the motor will not only cause additional loss, but also produce mechanical vibration, noise and overvoltage, which will cause severe local overheating of the transformer. Harmonics make capacitors, cables and other equipment overheating, aging of insulation, shortening their lifespan, and even damage.

(3) Harmonics will cause local parallel resonance and series resonance in the public grid, thereby amplifying the harmonics, which greatly increases the harm of (1) and (2) above, and even causes serious accidents.

(4) Harmonics will cause the malfunction of relay protection and automatic devices, and will make the measurement of electrical measuring instruments inaccurate.

(5) Harmonic will cause interference to the adjacent communication system. The lighter ones will produce noise and reduce the communication quality; the more serious ones will cause the loss of residence and make the communication system unable to work normally.