1. Introduction Variable frequency technology has been widely applied in variable frequency air conditioners, variable frequency refrigerators, variable frequency microwave ovens, variable frequency washing machines, variable frequency induction cookers, variable frequency motors, servo motors, etc. These are all public benefits brought to the public by variable frequency technology, and more importantly, they can bring us real benefits. Variable frequency technology plays a huge role in energy saving, improving product quality, improving automatic control, increasing equipment lifespan, and increasing environmental comfort. However, it is not without its drawbacks and has also produced some negative effects, such as pollution or adverse effects on the power grid in various industries. 2. Harmonics Generated by Variable Frequency Speed ​​Controllers [align=center] Figure 1.1 Main Circuit of Variable Frequency Drive [/align] The most representative of variable frequency technology is the "variable frequency speed controller," also known as an AC variable frequency speed controller, used for AC motors. It adjusts the output voltage proportionally while adjusting the output frequency, thereby changing the motor speed to achieve the purpose of AC motor speed regulation. Variable frequency drives (VFDs) come in two types: AC-AC and AC-DC-AC. The main circuit of domestically produced VFDs is generally composed of AC-DC-AC. This article mainly discusses AC-DC-AC VFDs, whose basic structure is shown in Figure 1.1. An AC-DC-AC VFD first converts the industrial frequency AC power into DC power through a three-phase rectifier (the DC link can be an inductor or a capacitor), and then converts the DC power back into AC power with adjustable frequency and voltage. The main circuit includes a rectifier, a DC link, and an inverter. Because the VFD uses a circuit structure of "rectifier—capacitor/inductor—inverter," both the rectifier and the inverter have nonlinear characteristics, thus generating high-order harmonics. These high-order harmonics distort the voltage and current waveforms of the input power supply. If effective suppression measures are not taken, they will have varying degrees of impact on various electrical equipment, automation devices, computers, measuring instruments, and communication systems. For power supply lines, the effects of high-order harmonics deteriorate power grid quality indicators, reduce power grid reliability, increase power grid losses, and shorten the lifespan of electrical equipment. 3. Generation of Harmonics and Reactive Power 3.1 Harmonic Generation Mechanism on the Input Side [align=center] Figure 1.2 Three-phase bridge rectifier circuit and input side waveform[/align] Currently, the application of power electronic devices in China is becoming increasingly widespread, making them the largest source of harmonics, including frequency converters. On the input side of frequency converters, the rectifier circuit accounts for the largest proportion, as shown in Figure 1.2 (B). Currently, the rectifier circuits commonly used in domestic frequency converters almost all adopt thyristor phase-controlled rectifier circuits or diode rectifier circuits, with three-phase bridge and single-phase bridge rectifier circuits being the most common. The harmonic pollution and power factor lag generated by rectifier circuits with resistive-inductive loads are well known. Diode rectifier circuits using capacitor or inductor filtering on the DC side of frequency converters are also serious sources of harmonic pollution. The fundamental component of the input current in this circuit is roughly in phase with the power supply voltage, so the fundamental power factor is close to 1. However, its input current is non-sinusoidal, as shown in Figure 1.2 (A), which contains abundant high-order harmonic components, causing serious pollution to the power grid and reducing the overall power factor. Variable frequency speed control devices are nonlinear devices and also consume reactive power. For example, when a three-phase bridge rectifier circuit adjusts the voltage, the fundamental current lags behind the grid voltage during operation, consuming a large amount of reactive power. In addition, these devices also generate a large amount of harmonic current, and harmonic sources all consume reactive power. The fundamental current phase of the diode rectifier circuit is roughly in phase with the grid voltage, so it basically does not consume fundamental reactive power. However, it also generates a large amount of harmonic current, thus consuming a certain amount of reactive power. 3.2 Harmonic generation mechanism on the output side [align=center] Figure 1.3 Inverter circuit and output side waveform[/align] In the inverter output circuit, both the output voltage and output current have harmonics. As shown in Figure 1.3 (C), the frequency converter uses a DSP to generate six adjustable pulse width PWM waves to control the on/off of six sets of power components in three phases, thus forming a three-phase output voltage with adjustable voltage and frequency. Its output voltage and current are generated by the intersection of the PWM wave and the triangular carrier wave, and are not standard sine waves. For example, in a voltage-type frequency converter, the output voltage waveform is a square wave, as shown in Figure 1.3 (A). Fourier series decomposition of the voltage square wave and the current sinusoidal sawtooth wave reveals strong high-order harmonic components. These high-order harmonics cause strong interference to equipment, even rendering it unusable and distorting signals from surrounding instruments. The fundamental cause of harmonic generation is nonlinear loads. The output current also contains high-frequency high-order harmonic components, as shown in Figure 1.3 (B). When the current flows through the load, it is not linearly related to the applied voltage, forming a non-sinusoidal current, thus generating harmonics. The frequency of harmonics is related to the modulation frequency of the inverter. When the modulation frequency is low (1-2KHz), the electromagnetic noise (screaming sound) generated by high-order harmonics can be heard by the human ear; when the modulation frequency is high (such as 20KHz for IGBT inverters), it cannot be heard by the human ear, but the high-frequency signal objectively exists. The fundamental wave of the power square wave and the positive wave of the current and other harmonics, and the high-order harmonic current directly interferes with the load. In addition, the high-harmonic current also radiates into space through the cable, interfering with nearby electrical equipment. 4. Harm of harmonics and their impact on equipment Due to the continuous expansion of variable frequency speed control technology, the impact of harmonic pollution on the power system and surrounding equipment has become increasingly serious, and may even cause other electronic equipment to malfunction. For example: (1) It increases the possibility of resonance in the power grid, thereby causing a high risk of overcurrent or overvoltage and accidents. (2) It increases additional losses, such as current harmonics will increase the copper loss of the transformer. Voltage harmonics will increase the iron loss. (3) It causes electrical equipment (such as motors, capacitors, transformers, etc.) to malfunction, resulting in heat loss, mechanical vibration, noise, and output power loss; thus causing motor overheating, accelerating insulation aging, and shortening their service life. (4) It causes measuring and metering instruments, meters, automatic devices, computer systems, and many electrical devices to malfunction, affecting the measurement accuracy of the equipment, causing malfunctions or errors, and reducing the processing quality of mechanical products. (5) It interferes with communication systems, reduces the quality of signal transmission, disrupts the normal transmission of signals, and even damages communication equipment. (6) In some cases, it not only generates harmonics, but also causes power supply voltage fluctuations and flicker, and even causes three-phase voltage imbalance, which will endanger the safe and economical operation of the power grid and affect the normal use of electrical equipment. 5. Methods to reduce and prevent harmonics In order to reduce the interference generated by higher harmonics, the source of generation (frequency converter) should be suppressed in principle. For small-capacity variable frequency drives (VFDs), high-order harmonics are rare. However, when using large-capacity VFDs, high-order harmonic currents and interference often occur. To address harmonic pollution in VFDs, there are two basic approaches: one is to install harmonic compensation devices, which is applicable to various harmonic sources; the other is to modify the power electronic device itself (e.g., structure, process, control) to prevent harmonic generation and achieve a power factor of 1. This, of course, only applies to power electronic devices that are the primary harmonic source. Therefore, appropriate countermeasures and preventative measures should be taken for high-order harmonics. Specific methods are as follows: 5.1 Add reactors. Reactors are divided into AC reactors (including input and output reactors) and DC reactors, as shown in Figures 1.4 and 1.5. Choosing a suitable reactor to match the frequency converter can achieve the following effects: (1) It can suppress harmonic current, suppress motor noise, and suppress surge current in the input; (2) It can reduce the total amount of harmonics generated by the frequency converter system and improve the power factor; (3) It can suppress the impact of surge current from the power grid on the frequency converter; (4) It can protect the frequency converter and improve the reliable operation of the frequency converter and the motor; (5) It can compensate for the charging current of the long connecting wire, so that the motor can work normally even when the lead wire is long. [align=center] Figure 1.4 AC reactor[/align] [align=center] Figure 1.5 DC reactor[/align] The following issues should be noted when installing the reactor: (1) The reactor must be installed in the place closest to the frequency converter, and the lead wire distance between it and the frequency converter should be shortened as much as possible. (2) The power line should not be twisted into a rope or braid, and should be run separately from the control line as much as possible. (3) In order to avoid damage to the reactor during use and transportation, please tighten the screws during installation to avoid noise and accidents. 5.2 Installing an EMI Filter As shown in Figure 1.6, the function of the filter is to suppress the transmission of wireless signals from wires and metal pipes into the equipment, separate the high-order harmonic components from the inverter from the impedance of the power system, or suppress interference signals from the interference source equipment to the outside through the power line. Here, it is to suppress the transmission of interference signals from the inverter to the power supply or motor through the power line. Installing an EMI filter can also reduce electromagnetic noise and losses. Many people believe that the function of an EMI power filter is to enable the equipment to meet the requirements of electromagnetic compatibility standards for conducted emission and conducted sensitivity, but this is not comprehensive. EMI power filters are also important in suppressing strong radiated interference generated by the equipment. A reasonable explanation is that an EMI power filter can prevent interference generated by the equipment itself from entering the power line, and at the same time prevent interference on the power line from entering the equipment. [align=center] Figure 1.6 EMI Filter[/align] 5.3 Other Measures and Methods (1) In high-frequency impact loads such as welding machines, electroplating power supplies, electrolytic power supplies, etc., it is recommended that users add a reactive power compensation device to improve the power factor and quality of the power grid. (2) In workshops where frequency converters are concentrated, it is recommended to adopt centralized rectification and DC common bus power supply. It is recommended that users adopt pulse rectification mode. The advantages are low harmonics and energy saving, especially suitable for occasions where frequent starting and braking, electric operation and power generation are carried out simultaneously. (3) Adding an active PFC device to the input side of the frequency converter has the best effect, but the cost is high. (4) Improve the structure of the frequency converter to reduce the pollution of harmonics and reactive power injected into the power grid by the frequency converter system. (5) Increasing the carrier frequency of the frequency converter can effectively suppress low-order harmonics. The carrier frequency of PWM output has a great influence on the noise of the motor and also affects the interference of the frequency converter. Therefore, as long as the carrier frequency is high enough, low-order harmonics can be effectively suppressed. (6) Using more suitable control strategies to optimize or improve can reduce the generation of harmonics to the greatest extent; such as the sinusoidal pulse width modulation (SPWM) and specific harmonic elimination method (SHE) commonly used in practical applications. 6. Conclusion Although variable frequency drives have been successfully applied in China and have received praise from users and experts, we are also keenly aware that this is an encouragement. In fact, there is still a long way to go in terms of product performance improvement, long-term reliability, and reducing pollution to the power grid. While improving product competitiveness and brand image, we also need to further optimize control methods, improve structural design, reduce harmonic pollution, and provide better variable frequency equipment for various industries. As long as we take one step at a time to do a good job and strengthen the variable frequency business, the prospects of domestic national brands will definitely get better and better. [i]References (1) Liu Meijun. Application Technology of General Variable Frequency Drives. Fujian Science and Technology Press. 2004 (2) Zhang Yanbin. 460 Questions on Variable Frequency Speed ​​Control. Machinery Industry Press. 2006[/i]