Abstract: This paper starts with the internal structure of frequency converters, analyzes the causes and hazards of harmonic generation, and proposes common methods for harmonic suppression. Keywords: Frequency converter, harmonic generation, hazard suppression Introduction Frequency converter-driven motor systems are increasingly used due to their significant energy-saving effects, convenient adjustment, simple maintenance, and networking capabilities. However, their nonlinear and impulsive power consumption causes significant interference problems. For a frequency converter, both its input and output terminals generate high-order harmonics. The harmonics at the input terminal can affect the public power grid through the input power line. I. Harmonic Generation Structurally, frequency converters can be divided into two main categories: indirect frequency converters and direct frequency converters. Indirect frequency converters convert the power frequency current into DC through a rectifier, and then convert the DC into AC with a controllable frequency through an inverter. Direct frequency converters convert the power frequency AC into AC with a controllable frequency without an intermediate DC link. Each phase of the direct frequency converter is a reversible circuit with two sets of thyristor rectifiers connected in anti-parallel. The positive and negative groups switch back and forth at a certain period, and the alternating output voltage U0 is obtained on the load. The amplitude of U0 is determined by the control angle of each rectifier, and the frequency is determined by the switching frequency of the two rectifier groups. At present, the indirect frequency converter is more commonly used. There are three different structural forms of indirect frequency converter: (1) Using a controllable rectifier to transform voltage and an inverter to convert frequency. Voltage regulation and frequency regulation are performed in two separate stages, and the two need to be coordinated in the control circuit. (2) Using an uncontrolled rectifier to rectify and a chopper to transform voltage and an inverter to convert frequency. This type of frequency converter uses a chopper in the rectification stage and pulse width modulation for voltage regulation. (3) Using an uncontrolled rectifier to rectify and a PWM inverter to convert frequency at the same time. This type of frequency converter can only output a very realistic sine wave if it uses a fully controlled device with controllable shutdown (such as IGBT). Regardless of the type of frequency converter, nonlinear power electronic components such as thyristors are widely used. Regardless of the rectification method, the frequency converter does not draw energy from the power grid in a continuous sine wave, but rather in a pulsating and intermittent manner. This pulsating current and the impedance along the power grid together form a pulsating voltage drop superimposed on the voltage of the power grid, causing voltage distortion. Fourier analysis shows that this non-synchronous sine wave current is composed of a fundamental wave with the same frequency and harmonics with frequencies higher than the fundamental wave frequency. II. Harmful Effects of Harmonics Generally speaking, the impact of frequency converters on large-capacity power systems is not very obvious, but for systems with small capacity, the interference generated by harmonics cannot be ignored. It is a pollution to the public power grid. The objective harms of harmonics to the public power grid and other systems are roughly as follows: (1) Harmonics cause additional harmonic losses to the components of the public power grid, reducing the utilization rate of power generation, transmission and consumption equipment. When a large amount of third harmonics flows through the neutral line, it can cause the line to overheat or even cause a fire. (2) Harmonics affect the normal operation of various electrical components. In addition to causing additional losses, harmonics also cause mechanical vibration, noise and overcurrent to motors, causing overheating of equipment such as capacitors and cables, insulation aging, shortened lifespan and even damage. (3) Harmonics can cause local parallel resonance and series resonance in the public power grid, thereby amplifying the harmonics, which greatly increases the above-mentioned hazards and may even cause serious accidents. (4) Harmonics can interfere with nearby communication systems, leading to reduced communication quality and even loss of information, making the communication system unable to work normally. III. Harmonic Suppression While frequency converters are convenient, efficient and offer huge benefits, they inject a large amount of harmonics and reactive power into the power grid, causing the power quality to deteriorate continuously. On the other hand, with the widespread application of a large number of sensitive devices such as computers, people have increasingly higher requirements for the power supply quality of the public power grid, and many countries and regions have formulated their own harmonic standards. my country also passed the "Regulations on Harmonic Management of Power Systems" in 1984 and the "GB/T-14549-93 Standard" in 1993 to limit harmonic pollution of power supply systems and electrical equipment. There are three basic ideas for suppressing harmonics. The first is to install harmonic compensation devices to compensate for harmonics. The second is to modify the power electronic devices themselves so that they do not generate harmonics and the power factor can be controlled to 1. The third is to adopt appropriate measures in the mains power network to suppress harmonics. The specific methods are as follows: (1) Install appropriate reactors. The power factor on the input side of the frequency converter depends on the AC-DC conversion circuit system inside the device. The method of parallel power factor correction DC reactors and series AC reactors on the power supply side can be used to reduce the THDV of the incoming current by about 30%-50%, which is about half of the harmonic current without reactors. (2) Besides the traditional LC filter, which is still in use, an important trend in harmonic suppression is the use of active power filters. It is connected in series or parallel to the main circuit, detects the harmonic current from the compensation object in real time, and generates a compensation current that is equal in magnitude and opposite in direction to the harmonic current, so that the grid current contains only the fundamental component. This filter can track and compensate for harmonics with varying frequency and amplitude. Its characteristics are not affected by the system and there is no risk of harmonic amplification. Therefore, it has attracted much attention and has been widely used in countries such as Japan. (3) Using multiphase pulse rectification When conditions permit or when harmonics are required to be limited to a relatively small value, multiphase rectification can be used. The THDV of 12-phase pulse rectification is about 10%-15%, and the THDV of 18-phase pulse rectification is about 3%-8%, which meets the requirements of international standards. The disadvantage is that it requires a special transformer, which is not conducive to equipment modification and has a higher price. (4) Currently, there are many filter modules or components on the market specifically designed for resisting conducted interference. These filters have strong interference capabilities and also prevent interference from the electrical appliances themselves from being conducted to the power supply. Some of them also have peak voltage absorption functions, which are very beneficial to various electrical equipment. (5) Developing new types of converters The main method for reducing harmonics in large-capacity converters is to use multiplexing technology. High power factor rectifiers from several kilowatts to several hundred kilowatts mainly use PWM inverters, which can be used to construct four-quadrant AC speed control frequency converters. This type of frequency converter not only has sine waves for output voltage and current, but also sine waves for input current, and has a power factor of 1. It can also realize bidirectional energy transfer, which represents the development direction of this technology. (6) The three-phase distribution transformer with D-YN11 connection group is selected. In the three-phase transformer, the high-voltage winding is connected in delta, the low-voltage winding is star and the neutral point is connected with "11" to ensure that the phase electromotive force is close to the sinusoidal shape, thereby avoiding the influence of phase electromotive force waveform distortion. At this time, the 220V load supplied by the local low-voltage power grid will not have a line current exceeding 30A. It can be supplied with 220V single-phase power. Otherwise, it should be supplied with 220/380V three-phase four-wire power. Other methods to reduce or weaken the frequency converter include: (1) Adding an AC reactor between the frequency converter and the motor to reduce electromagnetic radiation during transmission. (2) Using a transformer with a spacer layer can isolate most of the conducted interference before the transformer. (3) Using a dual-integral A/D converter with a certain degree of elimination of high-frequency interference. (4) Selecting low-voltage electrical appliances such as instruments with switching power supplies. (5) Separating the signal line and power line wiring and using twisted pair cables as much as possible to reduce common-mode interference. (6) In control systems using microcontrollers, PLCs, etc., appropriate software filtering for detection signals and output control sections should be added during software development to enhance the system's anti-interference capability. IV. Conclusion The use of frequency converters has brought convenience and significant benefits, and their use will undoubtedly become more widespread. However, due to their unique operating mode, they cause some damage to the public power grid, becoming one of the sources of harmonic pollution. Therefore, analyzing and researching methods to suppress harmonics will become a very important topic.