What are harmonics? The generation of harmonics and their hazards[Copy link]
1. What are harmonics?The fundamental reason for the generation of harmonics in power systems is due to nonlinear loads. When current flows through the load, it is not linearly related to the applied voltage, forming a non-sinusoidal current, that is, harmonics are generated in the circuit. The harmonic frequency is an integer multiple of the fundamental frequency. According to the analysis principle of French mathematician Fourier (M. Fourier), any repetitive waveform can be decomposed into a sine wave component 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 and odd. The 3rd, 5th, and 7th harmonics are odd harmonics, while 2nd, 4th, 6th, 8th, etc. are even harmonics. For example, when the fundamental wave is 50Hz, the 2nd harmonic is 100Hz, and the 3rd harmonic is 150Hz.
Generally speaking, odd harmonics cause more and greater harm than even harmonics. In a balanced three-phase system, due to the symmetry, even harmonics have been eliminated, and only odd harmonics exist. For a three-phase rectifier load, the harmonic current that appears is 6n±1 harmonics, such as 5, 7, 11, 13, 17, 19, etc. The inverter mainly generates 5 and 7 harmonics.The word "harmonic" originated from acoustics. Mathematical analysis of harmonics had laid a good foundation in the 18th and 19th centuries. The harmonic analysis method proposed by Fourier et al. is still widely used today. The harmonic problem of power system attracted people's attention as early as the 1920s and 1930s. At that time in Germany, the use of static mercury arc converters caused the distortion of voltage and current waveforms. The paper on converter harmonics published by JC Read in 1945 was a classic paper on early harmonic research.In the 1950s and 1960s, due to the development of high-voltage direct current transmission technology, a large number of papers were published on the harmonic problem of power systems caused by converters. Since the 1970s, due to the rapid development of power electronics technology, various power electronic devices have been increasingly used in power systems, industry, transportation and homes, and the harm caused by harmonics has become increasingly serious. Countries around the world have paid full attention to 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.2. Harmonic suppressionTo solve the harmonic pollution problem of power electronic devices and other harmonic sources, there are two basic ideas: one is to install a harmonic compensation device to compensate for the harmonics, which is applicable to various harmonic sources; the other is to modify the power electronic device itself so that it does not generate harmonics and the power factor can be controlled to 1. This is of course only applicable to power electronic devices that are the main harmonic source.The traditional method of installing harmonic compensation devices is to use LC tuned filters. This method can compensate for both harmonics and reactive power, and has a simple structure, 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 cause parallel resonance with the system, resulting in harmonic amplification, overloading or even burning the LC filter. In addition, it can only compensate for harmonics of fixed frequency, and the compensation effect is not ideal.3. Reactive power compensationIt is very easy for people to understand active power, but it is not easy to have a deep understanding of reactive power. In a sinusoidal circuit, the concept of reactive power is clear, but when it contains harmonics, there is no generally accepted definition of reactive power. However, there is a consensus on the importance of the concept of reactive power and the importance of reactive compensation. Reactive compensation should include compensation for fundamental reactive power and compensation for harmonic reactive power.Reactive power is very important for the operation of power supply systems and loads. The impedance of power system network elements is mainly inductive. Therefore, roughly speaking, in order to transmit active power, the voltages at the power transmission end and the power receiving end must have a phase difference, which can be achieved within a fairly wide range; while in order to transmit reactive power, the voltages at both ends must have an amplitude difference, which can only be achieved within a very narrow range. Not only do most network elements consume reactive power, but most loads also 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 all this reactive power to be provided by generators and transmitted over long distances. The reasonable method should be to generate reactive power where it is needed, which is reactive power compensation.
The main functions of reactive power compensation are as follows:
(1) Improve the power factor of the power supply and load system and load, reduce equipment capacity, and reduce power loss.
(2) Stabilize the voltage at the power receiving end and the power grid, and improve the power supply quality. Setting up dynamic reactive power compensation devices at appropriate locations in long-distance transmission lines can also improve the stability of the transmission system and increase transmission capacity.
(3) In situations where the three-phase load is unbalanced, such as in electrified railways, the three-phase active and reactive loads can be balanced by appropriate reactive compensation. Generation of harmonics and reactive powerAmong the industrial and domestic electrical loads, resistive-inductive loads account for a large proportion. Asynchronous motors, transformers, fluorescent lamps, etc. are typical resistive-inductive loads. The reactive power consumed by asynchronous motors and transformers accounts for 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. Resistive-inductive loads must absorb reactive power to work properly, which is determined by their own nature.
Nonlinear devices such as power electronic devices also consume reactive power, especially various phase-controlled devices. For example, phase-controlled rectifiers, phase-controlled AC power adjustment circuits, and cycloconverters, 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 large amount of harmonic current, and harmonic sources all consume reactive power. The fundamental current phase of the diode rectifier circuit is roughly the same as the grid voltage phase, so it basically does not consume fundamental reactive power. However, it also generates a large amount of harmonic current, and therefore 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, rectifier devices account for the largest proportion. At present, the commonly used rectifier circuits almost all use thyristor phase-controlled rectifier circuits or diode rectifier circuits, among which three-phase bridge and single-phase bridge rectifier circuits are the most common. The harmonic pollution and power factor lag generated by the rectifier circuit with resistive inductive load are already familiar to people. The diode rectifier circuit with capacitor filtering on the DC side is also a serious source of harmonic pollution. The phase of the fundamental component of the input current of this circuit is roughly the same as the phase of the power supply voltage, so the fundamental power factor is close to 1. However, the harmonic component of its input current is very large, causing serious pollution to the power grid and making the overall power factor very low. In addition, power electronic devices such as AC power adjustment circuits and cycloconverters using phase control will also generate a large amount of harmonic current on the input side. 3. The influence of reactive power and the harm of harmonics1. Impact of reactive power(1) The increase of reactive power will lead to an increase of current and apparent power, thus increasing the capacity of generators, transformers and other electrical equipment and the capacity of wires. At the same time, the size and specifications of the starting and control equipment and measuring instruments of power users should also be increased.
(2) The increase of reactive power will increase the total current, thus increasing the loss of equipment and lines, which is obvious.
(3) It will increase the voltage drop of lines and transformers. If it is an impact reactive power load, it will also cause violent voltage fluctuations, seriously reducing the power supply quality.2. Harm of harmonicsThe voltage provided by an ideal public power grid should be a single and fixed frequency and a specified voltage amplitude. The emergence of harmonic current and harmonic voltage is a kind of pollution to the public power grid. It deteriorates the environment in which the electrical equipment is located and also affects the surrounding capacity. Before the widespread application of power electronic equipment, people have conducted some research on harmonics and their hazards and have a certain understanding of them, but harmonic pollution has not received enough attention at that time. In the past three or four decades, the rapid development of various power electronic devices has made the harmonic pollution of the public power grid increasingly serious. Various faults and accidents caused by harmonics have also occurred continuously. The severity of harmonic hazards has attracted people's high attention. The harm of harmonics to the public power grid and other systems can be roughly divided into the following aspects.
(1) Harmonics cause additional harmonic losses in the components of the public power grid, reducing the efficiency of power generation, transmission and power consumption equipment. When a large amount of third harmonics flows through the neutral line, it will cause the line to overheat or even cause a fire.
(2) Harmonics affect the normal operation of various electrical equipment. In addition to causing additional losses, the impact of harmonics on motors will also generate mechanical vibration, noise and overvoltage, causing serious local overheating of transformers. Harmonics can cause capacitors, cables and other equipment to overheat, age their insulation, shorten their lifespans and even damage them.
(3) Harmonics can cause local parallel resonance and series resonance in the public power grid, thereby amplifying the harmonics, which greatly increases the hazards of (1) and (2) above and even causes serious accidents.
(4) Harmonics can cause malfunctions of relay protection and automatic devices and inaccurate measurements of electrical measuring instruments.
(5) Harmonics can interfere with nearby communication systems. In mild cases, they can generate noise and reduce communication quality; in severe cases, they can cause loss of residence and make the communication system unable to operate normally.
I hope you can share how to make good use of harmonics to improve the working characteristics of motors? For example, in PMSM drive
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Published on 2021-3-20 10:35
There are benefits. To make a frequency source, a crystal oscillator needs harmonics. It is not useless. You can use it. Most of the time, it really interferes with other people's work.
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Published on 2021-3-19 09:17
There are benefits. To make a frequency source, a crystal oscillator needs harmonics. It is not useless. You can use it. Most of the time, it really interferes with other people's work.
sanhuasr posted on 2021-3-20 10:35 I hope you can share how to make good use of harmonics to improve the working characteristics of motors? For example, in the aspect of PMSM drive
The knowledge in this area is relatively limited. I guess you will be disappointed.
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