Generation of harmonics
The fundamental cause of harmonic generation is due to nonlinear loads. When current flows through a load, it does not have a linear relationship with the applied voltage, forming a non sinusoidal current and generating harmonics. The harmonic frequency is an integer multiple of the fundamental frequency. According to the Fourier analysis principle of French mathematicians, any repeated waveform can be decomposed into a sine wave component containing the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency. Harmonics are sine waves, each with a different frequency, amplitude, and phase angle. Harmonics can be distinguished into even harmonics and odd harmonics. The 3rd, 5th, and 7th harmonics are odd harmonics, while the 2nd, 4th, 6th, and 8th harmonics are even harmonics. For example, when the fundamental frequency is 50Hz, the 2nd harmonic is 100Hz, and the 3rd harmonic is 100Hz. It is 150H2 Generally speaking, odd harmonics cause more and greater harm than even harmonics in a balanced three-phase system. Due to symmetry, even harmonics have been eliminated, and only odd harmonics exist. For three-phase rectified loads, the harmonic current that occurs is 6n harmonics, such as S, 7, 11, 13, 17, 19, etc. The frequency converter mainly generates 5th and 7th harmonics. Structurally, there are two types of frequency converters: indirect frequency converters and direct frequency converters? Indirect frequency converters convert power frequency currents into direct current through rectifiers, and then convert the direct current into controllable frequency alternating current through inverters. On the other hand, direct frequency converters directly convert power frequency currents into controllable frequency alternating current without an intermediate direct current link. Each phase of the inverter is a reversible circuit consisting of two sets of thyristor rectifiers in anti parallel. The positive and negative sets switch with each other at a certain period, and the amplitude of the alternating output voltage U0 is determined by the control angle of each rectifier device, and the frequency is determined by the switching frequency of the two rectifier devices At present, indirect frequency converters are still widely used, and there are three different structural ways of indirect frequency converters:
(1) Using a controllable rectifier for voltage transformation and an inverter for frequency conversion, voltage regulation, and frequency regulation are carried out in two separate stages, which need to be coordinated and coordinated in the control circuit.
(2) Use an uncontrollable rectifier to rectify the chopper for voltage transformation, and use an inverter for frequency conversion. This type of frequency converter uses a chopper for rectification and pulse width modulation for voltage regulation.
(3) Rectify with an uncontrollable rectifier and use a PWM inverter for frequency conversion. Only by using controllable turn off devices (such as IGBT) can the output waveform be very close to a sine wave. Regardless of the type of frequency converter, Daxin uses nonlinear power electronic components such as thyristors. Regardless of the rectification method used, the way the frequency converter extracts energy from the leased grid is not a continuous sine wave, but a pulsating intermittent current drawn from the leased grid. This pulsating current, together with the impedance along the grid, forms a pulsating voltage drop that is superimposed on the voltage of the grid, causing distortion of the historical voltage. Fourier series analysis shows that this asynchronous sine wave current is composed of fundamental waves with the same frequency and harmonics with frequencies greater than the fundamental frequency.