Abstract: This paper introduces the theory of harmonics of inverters based on the basic structure of inverters, discusses the harm caused by harmonics to inverters and electrical equipment, and finally proposes measures to suppress and reduce harmonics. 1 Introduction In AC motor speed control, control systems driven by inverters are 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. Inverters mainly consist of rectifier circuits, inverter circuits, and control circuits, among which the rectifier circuits and inverter circuits are composed of power electronic devices. Power electronic devices have nonlinear characteristics. When the inverter is running, it needs to perform rapid switching actions, which will generate high-order harmonics. Meanwhile, the input of the frequency converter itself is a nonlinear rectifier circuit. Especially for medium and high voltage frequency converters with large capacity and direct connection to the high-voltage power grid, the output waveform of the frequency converter contains a large number of higher-order harmonics in addition to the fundamental frequency, causing interference to the load and nearby equipment. The harmonics at the input end can also affect the public power grid through the input power line. 2. Basic Concepts of Harmonics The fundamental cause of harmonic generation is due to nonlinear loads. When current flows through a load, its relationship with the applied voltage is not linear, 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, and each harmonic has a different frequency, amplitude, and phase angle. Harmonics can be divided 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 50 Hz, the 2nd harmonic is 100 Hz, and the 5th harmonic is 250 Hz. Generally speaking, odd harmonics cause more and greater damage than even harmonics. In a balanced three-phase system, due to the symmetry, even harmonics are eliminated, and only odd harmonics exist. For a three-phase rectified load, the harmonic currents that appear 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 harmonic definition is shown in Figure 1. [align=center] Figure 1 Schematic diagram of harmonic definition[/align] 3 Harmonic Generation Frequency converters can be divided into two main categories: indirect frequency converters and direct frequency converters. Indirect frequency converters convert the mains frequency current to DC via a rectifier, and then the DC is converted to AC with a controllable frequency via an inverter. Direct frequency converters, on the other hand, convert the mains frequency AC to AC with a controllable frequency without an intermediate DC link. Each phase of a direct frequency converter is a reversible circuit with two sets of thyristor rectifiers connected in anti-parallel. The two sets switch back and forth at a certain period, resulting in an alternating output voltage at the load. The amplitude of this voltage 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 more widely 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. In this type of frequency converter, a chopper is used in the rectification stage, and pulse width modulation is used for voltage regulation; the third is to use an uncontrolled rectifier for rectification and a PWM inverter for simultaneous frequency conversion. Only by using fully controllable devices with controllable shutdown can this type of frequency converter output a very realistic positive selection wave. 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 principle 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 extra; 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 current will cause the switching equipment to generate a very high current change rate at the moment of startup, damaging the insulation. (3) 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 interference to nearby communication systems, which may cause noise and reduce communication quality, or even loss of information and make the communication system unable to work normally; affect the working accuracy of electronic equipment, reduce the quality of precision machined products; shorten equipment life, and worsen the working condition of household appliances, etc. [1, 2] 5 Methods for Suppressing Inverter Harmonies 5.1 Multiplexing of Converter Circuits For high-capacity inverters, a dedicated power input transformer can be installed at the input end of the inverter to divide the power supply side converter into two. This transformer is used to shift the phase of the input current, thereby suppressing high-order harmonics from the inverter to the power supply side through multiplexing. 5.2 Installing AC/DC Reactors Installing reactors actually increases the impedance of the inverter's power supply from the outside. Installing reactors on the AC or DC side of the inverter, or both simultaneously, can suppress harmonic currents. After using AC/DC reactors, as shown in Figure 2, the input current (voltage distortion rate) is reduced by approximately 30% to 50%, which is about half of the harmonic current without reactors. [align=center]Figure 2 Schematic diagram using AC/DC reactors[/align] 5.3 Installing Active Power Filters Besides the traditional LC debugging filters still in use, a significant trend in harmonic suppression is the adoption 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 object. The compensation device generates a compensation current equal in magnitude but opposite in direction to the harmonic current, thus ensuring that the grid current contains only the fundamental current. This type of filter can track and compensate for harmonics with varying frequencies and amplitudes. 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. 5.4 Increasing the Internal Impedance of the Inverter Power Supply Under normal circumstances, the internal impedance of the power supply equipment can buffer the reactive power of the inverter's DC filter capacitor. This internal impedance is the short-circuit impedance of the transformer. The smaller the power supply capacity relative to the inverter capacity, the larger the internal impedance value and the lower the harmonic content; conversely, the larger the power supply capacity relative to the inverter capacity, the smaller the internal impedance value and the higher the harmonic content. Therefore, when selecting a power supply transformer for a frequency converter, it is best to choose a transformer with a high short-circuit impedance. 5.5 Installing an output reactor can also be achieved 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 energy transmission of the frequency converter's output. As shown in Figure 3, the reactor must be installed as close as possible to the frequency converter, minimizing the distance between the leads and the frequency converter. If armored cables are used as the connection between the frequency converter and the motor, this method can be omitted, but the cable armor must be reliably grounded at both the frequency converter and motor ends. The grounded armor must remain unchanged and not be twisted into a rope or braid, nor extended with other wires. The connection on the frequency converter side should be made to the frequency converter's ground terminal, and then the frequency converter should be grounded. [align=center] Figure 3 Schematic diagram of using an output reactor[/align] 5.6 Adjusting 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, lower harmonics can be effectively suppressed, especially when the reference amplitude and carrier amplitude are less than 1, odd harmonics below the 13th order no longer appear. 5.7 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 form four-quadrant AC speed-regulating frequency converters. This type of frequency converter not only has sinusoidal output voltage and current, but also sinusoidal input current, and a power factor of 1. It can also realize bidirectional energy transfer, representing the development direction of this technology. 6 Methods for Equipment to Reduce Harmonics and Interference 6.1 Using Isolation Transformers The main purpose of using isolation transformers is to deal with conducted interference from the power supply, as shown in Figure 4. Using isolation transformers with isolation layers can isolate conducted interference from the isolated part before the isolation transformer. It can also serve as a power supply voltage transformation function. Isolation transformers are commonly used in instruments, PLCs, and other low-voltage, low-power electrical equipment in control systems to resist conducted interference. [align=center]Figure 4 Schematic diagram using an isolation transformer[/align] 6.2 Using filter modules and components There are many filter modules or components on the market specifically designed for resisting conducted interference. These filters have strong anti-interference capabilities and can also prevent interference from the appliance itself from being conducted to the power supply. Some also have peak voltage absorption functions, which are very beneficial to various electrical equipment. 6.3 Grounding anti-interference Grounding is an important means of suppressing noise and preventing interference. A good grounding method can greatly suppress the coupling of internal noise, prevent the intrusion of external interference, and improve the anti-interference capability of the system. The grounding methods of frequency converters include multi-point grounding, single-point grounding, and grounding through the busbar. The appropriate method should be adopted according to the specific situation. Care should be taken not to cause interference to the equipment due to poor grounding. 6.4 Ensuring interference resistance of signal lines Signal lines undertake the task of transmitting detection and control signals. Undoubtedly, the quality of signal transmission directly affects the accuracy, stability, and reliability of the entire control system. Therefore, it is essential to ensure interference resistance of signal lines. 7 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. There is still much room for discussion regarding frequency converter harmonics. About the author: Wang Ping (1982-), male, from Qingdao, Shandong Province, is a master's student at Shandong University of Light Industry, specializing in complex industrial process control. Mailing address: P.O. Box 245, Shandong University of Light Industry, Changqing District, Shandong Province (250353). Detailed contact information: Author: Wang Ping, Postcode: 250353, Address: P.O. Box 245, Shandong University of Light Industry, Changqing District, Shandong Province, Tel: 13806403609, Email: [email protected]