Abstract : This paper introduces the theory of harmonics in inverters based on the basic structure of inverters, and discusses the harm of harmonics to inverters and electrical equipment. Based on this, common methods for suppressing harmonics are proposed. Keywords: inverter ; harmonics; suppression; 1 Introduction Control systems driven by inverters have been widely used in various fields such as power, machinery, industry, and daily life due to their advantages such as significant energy saving, high precision, reliable operation, convenient adjustment, simple maintenance, and networking. A frequency converter mainly consists of a rectifier circuit, an inverter circuit, and a control circuit. The rectifier and inverter circuits are composed of power electronic devices. Power electronic devices have nonlinear characteristics. When the frequency converter is running, it needs to perform rapid switching actions, which generates high-order harmonics. Simultaneously, the input of the frequency converter itself is a nonlinear rectifier circuit. Especially for medium and high-voltage frequency converters with large capacities and direct connection to the high-voltage power grid, the output waveform of the frequency converter contains a large number of high-order harmonics in addition to the fundamental wave, causing interference to the load and nearby equipment. Harmonics at the input end can also affect the public power grid through the input power line. [sup][1,2][/sup]. 2. Definition of Harmonics The fundamental cause of harmonic generation is due to nonlinear loads. When current flows through the load, it is not linearly related to the applied voltage, forming a non-sinusoidal current, thus generating harmonics. Harmonic frequencies are integer multiples of the fundamental frequency. According to the analysis principle of the French mathematician Fourier, any repeating waveform can be decomposed into sinusoidal components containing the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency. Harmonics are sinusoidal waves, each with a different frequency, amplitude, and phase angle. Harmonics can be classified into even 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, if the fundamental frequency is 50Hz, the 2nd harmonic is 100Hz, and the 5th harmonic is 250Hz. Generally speaking, odd harmonics cause more and greater harm than even harmonics. In a balanced three-phase system, due to symmetry, even harmonics are eliminated, leaving only odd harmonics. For three-phase rectified loads, the resulting harmonic currents are 6n ± 1 harmonics, such as 5th, 7th, 11th, 13th, 17th, and 19th harmonics. Frequency converters mainly generate the 5th and 7th harmonics. A schematic diagram of the definition of harmonics is shown in Figure 1. [sup][1][/sup]. [align=center] Figure 1 Schematic diagram of the definition of harmonics[/align] 3. Harmonic Generation Mechanism 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, thus obtaining an alternating output voltage on the load. The amplitude is determined by the control angle of each rectifier, and the frequency is determined by the switching frequency of the two sets of rectifiers. Currently, indirect frequency converters are more commonly used. There are three different structural forms of indirect frequency converters: one is to use a controllable rectifier for voltage transformation, an inverter for frequency conversion, and voltage and frequency regulation are performed in two separate stages, with the two coordinating in the control circuit; the second is to use an uncontrolled rectifier for rectification and chopper for voltage transformation, and an inverter for frequency conversion. This type of frequency converter uses a chopper in the rectification stage and pulse width modulation for voltage regulation; the third is to use an uncontrolled rectifier for rectification and a PWM inverter for simultaneous frequency conversion. This type of frequency converter can only output a very realistic positive selection wave if it uses a fully controllable device with controllable shutdown. 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 draws current from the power grid in a pulsating discontinuous 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. According to Fourier series analysis, 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. 4 Harmful Effects of Harmonics Generally speaking, the impact of frequency converters on power systems with relatively large capacity is not very obvious, but for systems with small capacity, the interference caused by harmonics cannot be ignored. It is a pollution to the public power grid. The harm of harmonic pollution to the power system is serious, mainly manifested in the following aspects. (1) Harmonics cause additional harmonic losses in public power grid components, reducing the utilization rate of power generation, transmission and consumption equipment. For example, current harmonics will increase the copper loss of transformers. Voltage harmonics will increase iron loss, causing its temperature to rise, affecting insulation capacity, and reducing capacity margin. At the same time, harmonics may also cause resonance between transformer windings and line capacitors. (2) Harmonics affect the normal operation of various electrical components. The effects of inverter output harmonics on motors include: additional heating of the motor, causing the motor to heat up; mechanical vibration, noise, and overcurrent. Harmonics can cause power capacitors to overload, overheat, or even damage the capacitors. When the capacitor and line impedance reach resonance, vibration, short circuit, overcurrent, and noise will occur. Harmonic currents will cause the switching equipment to generate a very high current change rate at the moment of startup, damaging the insulation. (3) Harmonics cause capacitors in the power grid to resonate. At power frequency, the capacitors installed in the system for various purposes are much larger than the inductive reactance in the system and will not resonate. However, when harmonics are generated, the inductive reactance increases by a factor of two while the capacitive reactance decreases by a factor of two, which may cause resonance. Resonance will amplify the harmonic current, causing capacitors and other equipment to burn out. (4) Harmonics 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 liability accidents. (5) Harmonics will cause relay protection and automatic devices to malfunction and cause large errors in instruments and electricity metering; harmonics also have great harm to other systems and power users: such as interfering with nearby communication systems, which may cause noise and reduce communication quality, or even lose information and prevent the communication system from working properly; affecting the working accuracy of electronic equipment and reducing the quality of precision machined products; shortening the life of equipment and deteriorating the working condition of household appliances, etc. [sup][1.2][/sup]. 5 Methods to suppress harmonics of frequency converters (1) Installing AC/DC reactors Installing reactors is actually increasing the impedance of the frequency converter power supply from the outside. Installing reactors on the AC or DC side of the frequency converter or installing them at the same time can suppress harmonic currents. After using an AC/DC reactor, as shown in Figure 2, the THDv (voltage distortion rate) of the input current is reduced by about 30% to 50%, which is about half of the harmonic current without the reactor. [align=center] Figure 2 Schematic diagram of using an AC/DC reactor[/align] (2) The output reactor can also be installed by adding an AC motor between the frequency converter and the motor. The main purpose is to reduce the electromagnetic radiation generated by the line during the energy transmission process of the frequency converter. As shown in Figure 3, the reactor must be installed in the place closest to the frequency converter to minimize the distance between the lead wire and the frequency converter. If armored cable is used as the connection between the frequency converter and the motor, this method can be omitted, but the armor of the cable must be reliably grounded at the frequency converter and motor ends, and the grounded armor must be grounded as is, without twisting into a rope or braid, and without using other wires to extend it. The frequency converter side must be connected to the ground terminal of the frequency converter, and then the frequency converter is grounded. The smaller the output reactor, the larger the internal impedance and the lower the harmonic content; the larger the power supply capacity relative to the inverter capacity, the smaller the internal impedance and the higher the harmonic content. Therefore, when selecting the power supply transformer for the inverter, it is best to choose a transformer with a large short-circuit impedance. (6) Adjust the carrier ratio of the inverter. Increasing the carrier ratio of the inverter can effectively suppress low-order harmonics. As long as the carrier ratio is large enough, low-order harmonics can be effectively suppressed, especially when the reference amplitude and the carrier amplitude are less than 1, odd harmonics below the 13th order will no longer appear. (7) Use an isolation transformer. The main purpose of using an isolation transformer is to deal with conducted interference from the power supply, as shown in Figure 4. Using an isolation transformer with an isolation layer can isolate the conducted interference of the isolated part before the isolation transformer. It can also serve as a power supply voltage conversion function. Isolation transformers are often used in control systems for instruments, PLCs, and other low-voltage, low-power electrical equipment to resist conducted interference. [align=center] Figure 3 Schematic diagram using output reactor[/align] (3) Multiplexing of converter circuit For high-capacity frequency converters, a dedicated power input transformer can be installed at the input end of the frequency converter to divide the power supply side converter into two. The transformer is used to make the phase of the input current staggered, so as to suppress the high-order harmonics from the frequency converter to the power supply side through multiplexing. (4) Installing active power filter In addition to the traditional LC debugging filter, an important trend in suppressing harmonics is to use active power filter. It is connected in series or parallel in the main circuit, and detects the harmonic current from the compensation object in real time. The compensation device 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 current. 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 danger of harmonic amplification. Therefore, it has attracted much attention and has been widely used in countries such as Japan. (5) Increase the internal impedance of the power supply of the frequency converter. Under normal circumstances, the internal impedance of the power supply equipment can buffer the reactive power of the DC filter capacitor of the frequency converter. This internal impedance is the short-circuit impedance of the transformer. The smaller the power supply capacity is relative to the frequency converter capacity, the larger the internal impedance value and the lower the harmonic content; the larger the power supply capacity is relative to the frequency converter capacity, the smaller the internal impedance value and the higher the harmonic content. Therefore, when selecting the power supply transformer for the frequency converter, it is best to choose a transformer with a large short-circuit impedance. (6) Adjust the carrier ratio of the frequency converter. Increasing the carrier ratio of the frequency converter can effectively suppress low-order harmonics. As long as the carrier ratio is large enough, low-order harmonics can be effectively suppressed, especially when the reference amplitude value and the carrier amplitude value are less than 1, odd harmonics below the 13th order will no longer appear. (7) Use an isolation transformer. The main purpose of using an isolation transformer is to deal with conducted interference from the power supply, as shown in Figure 4. Using an isolation transformer with an isolation layer can isolate the conducted interference of the isolated part before the isolation transformer. At the same time, it can also serve as a power supply voltage transformation function. Isolation transformers are often used in control systems for instruments, PLCs, and other low-voltage, low-power electrical equipment to resist conducted interference. [sup][4][/sup]. [align=center] Figure 4 Schematic diagram of using an isolation transformer [/align] (8) Using filter modules and components There are many filter modules or components on the market that are specifically designed to resist conducted interference. These filters have strong anti-interference capabilities and can also prevent interference from the electrical appliances themselves from being conducted to the power supply. Some also have peak voltage absorption functions, which are very beneficial to various electrical equipment. (9) Developing new converters The main method for reducing harmonics in high-capacity converters is to use multiplexing technology. High power factor rectifiers from several kilowatts to several hundred kilowatts mainly use PWM inverters, which can form a four-quadrant AC speed control frequency converter. This type of frequency converter not only outputs a sine wave voltage and current, but also inputs a sine wave current, and has a power factor of 1. It can also realize bidirectional energy transfer, which represents the development direction of this technology. 6 Conclusion The use of frequency converters has brought convenience and huge benefits to people, and it will surely be used more widely. However, due to the inherent structure of frequency converters, the generation of harmonics is unavoidable. In practical applications, we can only try our best to suppress harmonics and reduce their impact on the external environment. This article only mentions some of the effects of frequency converter harmonics and measures to address them. 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