With the accelerating pace of global industrialization, pollution and damage to the Earth's environment are intensifying, prompting a global surge in environmental protection efforts. The power system, too, is an "environment" facing pollution; harmonics in public power grids are a serious form of pollution. As the application of industrial frequency converters becomes increasingly widespread, harmonic pollution in factory power distribution systems is becoming more severe. Therefore, harmonic suppression has become a crucial topic in power electronics, electrical automation, and power system research, attracting growing attention. 1. Generation and Propagation of Harmonics in Frequency Converters The main circuit of a frequency converter typically consists of AC/DC and AC inputs. An external 380V/50Hz power supply is uncontrolled rectified into a DC voltage signal by a three-phase bridge circuit, then filtered by a filter capacitor and inverted into a variable-frequency AC signal by high-power transistor switching elements. In the rectifier circuit, the input current waveform is an irregular rectangular wave, which can be decomposed into a fundamental wave and various harmonics according to Fourier series. These harmonics will interfere with the power supply system. In the inverter output circuit, the output current is a pulse waveform modulated by the PWM carrier signal. For GTR high-power inverter elements, the PWM carrier frequency is 2-3kHz; while for IGBT high-power inverter elements, the PWM carrier frequency can reach up to 15kHz. Similarly, the output circuit current can also be decomposed into a fundamental wave containing only a sine wave and other harmonics. Harmonic currents directly interfere with the load. The inverter rectifier module often uses a three-phase uncontrolled bridge rectifier, and the voltage and current waveforms of each phase are shown in Figure 1. The phase current is not a sine wave, but a distorted current wave containing high-order harmonics. According to harmonic theory, the three-phase rectifier of the inverter generates 5th, 7th, 11th, 13th...6k±1 (k-1, 2nd, 3rd, ...) harmonics, and the harmonic content is inversely proportional to the harmonic order. The ratios of the effective values ​​of the major harmonic currents to the effective value of the fundamental current are as follows: 5th harmonic: 0.175; 7th harmonic: 0.110; 11th harmonic: 0.045; 13th harmonic: 0.029… When the frequency converter is operating, it acts as a harmonic source, and the entire factory's power distribution system is the load of this harmonic source. Harmonics will travel along the power supply lines to various electrical devices, affecting their normal operation. [align=center] Figure 1: Input voltage and current diagram for each phase of the rectifier [/align] [b][align=center]For more details, please click: Harmonic Pollution and Control of Frequency Converters in Factory Power Distribution Systems[/align][/b]