source
The word "harmonic" 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.
definition
harmonic wave
The word "harmonic" 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.
The mathematical analysis of harmonics has laid a good foundation in18th century and19th century. The harmonic analysis method proposed by Fourier et al. is still widely used today.
The main causes of harmonics are as follows: because sinusoidal voltage is applied to nonlinear load, the fundamental current is distorted to produce harmonics. The main nonlinear loads are UPS, switching power supply, rectifier, frequency converter and inverter.
Overtone is a kind of physical harmonic, but the definition of numbers is slightly different. Audio with twice the fundamental frequency is called the first overtone, audio with three times the fundamental frequency is called the second overtone, and so on.
harmonic generation
cause
In an ideal clean power supply system, both current and voltage are sine waves. In a simple circuit with only linear components (resistance, inductance and capacitance), the current flowing is proportional to the applied voltage, and the current flowing is a sine wave.
Using the principle of Fourier analysis, a non-sinusoidal signal can be decomposed into its basic parts and its 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. There are only 5,7, 1 1, 13, 17, 19 pulse devices. N times the 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.
Harmonic waveform
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.