Variable frequency drive (VFD) systems are widely used in our company. These systems have advantages such as high power factor, wide speed range, significant energy saving, convenient adjustment, simple maintenance, and networking. However, the interference caused by their nonlinear, impulsive power consumption is also a major concern. 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. 1. 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 is a reversible circuit with two sets of thyristor rectifiers connected in antiparallel. The two sets switch back and forth at a certain period, resulting in an alternating output voltage U0 at the load. The amplitude of U0 depends on the control angle of each rectifier, and the frequency depends on the switching frequency of the two sets of rectifiers. Currently, indirect frequency converters are the most widely used. There are three different structural forms of indirect frequency converters: First, a controlled rectifier transforms the voltage, and an inverter converts the frequency; voltage and frequency regulation are performed in two separate stages, requiring coordination in the control circuit. Second, an uncontrolled rectifier and chopper transform the voltage, and an inverter converts the frequency; this type of converter uses a chopper in the rectification stage and pulse width modulation (PWM) for voltage regulation. Third, an uncontrolled rectifier rectifies the voltage, and a PWM inverter simultaneously converts the frequency; this type of converter only outputs a very realistic sine wave when using fully controllable devices with controllable shutdown (such as IGBTs). Regardless of the type of frequency converter... They all use a large number of nonlinear power electronic components such as thyristors. Regardless of the rectification method used, the way the frequency converter draws energy from the grid is not a continuous sine wave, but a pulsating and intermittent current. This pulsating current and the grid's impedance along the way together form a pulsating voltage drop that is superimposed on the grid voltage, 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 a frequency greater than the fundamental wave frequency. 2. Harmful Effects of Harmonics Generally speaking, the impact of frequency converters on large-capacity power systems is not very significant. However, for small-capacity systems, the interference generated by harmonics cannot be ignored. It pollutes the public power grid, causing additional harmonic losses in its components, reducing the utilization rate of power generation, transmission, and consumption equipment. Large amounts of third harmonics flowing through the neutral line can cause overheating and even fires. It also affects the normal operation of various electrical components, causing additional losses, mechanical vibration, noise, and overcurrent, leading to overheating of capacitors, cables, and other equipment, insulation aging, shortened lifespan, and even damage. Furthermore, it causes localized parallel and series resonances in the public power grid, amplifying harmonics and greatly increasing the aforementioned hazards, potentially leading to serious accidents. Finally, it interferes with nearby communication systems, reducing communication quality, even causing information loss, and rendering the communication system unable to function properly. 3. Harmonic Suppression While frequency converters are utilized, they inject a large amount of harmonics and reactive power into the power grid, causing a continuous deterioration in power quality. On the other hand, with the widespread application of sensitive devices such as computers, people have increasingly higher requirements for the power quality of public power grids, and many countries and regions have formulated their own harmonic standards. China also passed the "Regulations on Harmonic Management of Power Systems" in 1984 and the "GB/T-14549-93 Standard" in 1993, respectively, to limit harmonic pollution from power supply systems and electrical equipment. The basic approach to harmonic suppression is threefold: installing harmonic compensation devices; modifying the power electronic devices themselves to prevent harmonic generation and control the power factor to 1; and adopting appropriate measures in the mains power network to suppress harmonics. 3.1 Installing Appropriate Reactors: The input power factor of the frequency converter depends on the internal AC-DC conversion circuit system. A parallel power factor correction DC reactor and a series AC reactor on the power supply side can be used to reduce the THDv of the input current by approximately 30-50%, which is about half of the harmonic current without reactors. 3.2 Installing Active Power Filters: Besides traditional LC filter adjustments, a significant trend in harmonic suppression is the use of active power filters. These filters are connected in series or parallel to the main circuit, detecting harmonic currents in real time from the compensation target. A compensation device generates a compensation current equal in magnitude but opposite in direction to the harmonic current, thus ensuring the grid current contains only the fundamental component. This type of filter can track and compensate for harmonics with varying frequency and amplitude. Its characteristics are unaffected by the system and there is no risk of harmonic amplification, making it highly sought after and widely used in countries like Japan. 3.3 Using Multiphase Pulse Rectification: When conditions permit or harmonics need to be limited, multiphase rectification can be used. The THDV of a 12-phase pulse rectifier is approximately 10-15, and that of an 18-phase pulse rectifier is approximately 3-8, meeting international standards. The disadvantages are the need for a dedicated transformer, which hinders equipment modification, and the higher price. 3.4 Using Filter Modules Currently, there are many filter modules or components on the market specifically designed for resisting conducted interference. These filters have strong anti-interference capabilities and also prevent interference from the appliance itself from being conducted to the power supply. Some also have peak voltage absorption capabilities, offering numerous benefits to various electrical devices. 3.5 Developing New Types of Converters The main method for reducing harmonics in large-capacity converters is the use of multiplexing technology. High power factor rectifiers ranging from several kilowatts to several hundred kilowatts mainly use PWM inverters, which can be configured as four-quadrant AC speed control frequency converters. These frequency converters not only have sinusoidal output voltage and current, but also sinusoidal input current, and a power factor of 1. They can also achieve bidirectional energy transfer, representing the development direction of this technology. 3.6 For three-phase distribution transformers using the D-YNl1 connection group, the high-voltage winding is connected in a delta configuration, and the low-voltage winding is in a star configuration with the neutral point connected to "l1" to ensure that the phase electromotive force (EMF) is close to a sinusoidal shape, thus avoiding the influence of phase EMF waveform distortion. In this case, for 220V loads supplied by the local low-voltage power grid, the line current will not exceed 30A, and 220V single-phase power supply can be used; otherwise, 220/380V three-phase four-wire power supply should be used. Other methods to reduce or weaken the harmonics of frequency converters include adding an AC reactor between the frequency converter and the motor to reduce electromagnetic radiation during transmission; using a transformer with a spacer layer to isolate most of the conducted interference before the transformer; using a dual-integral A/D converter with a certain degree of high-frequency interference elimination; selecting low-voltage electrical appliances such as instruments with switching power supplies; separating signal lines from power lines and using twisted-pair cables as much as possible to reduce common-mode interference; and appropriately adding software filtering for detection signals and output control parts when programming software in control systems using microcontrollers, PLCs, etc., to enhance the anti-interference capability of the system itself. 4 Conclusion The use of frequency converters has brought convenience and huge benefits to people and will inevitably be used more widely. However, due to its unique working mode, it has caused certain damage to the public power grid and become one of the sources of harmonic pollution in the power grid. Therefore, analyzing and studying methods to suppress harmonics will become a very important topic. [References] [1] Han Anrong. General Frequency Converters and Their Applications. Beijing: China Machine Press, 2000, 09. [2] User manual for Mitsubishi FR-A500 variable frequency drive. [3] User manual for Mitsubishi FR-F500 variable frequency drive.