Abstract: This paper introduces the development process of power electronic devices and frequency conversion technology, as well as the application of frequency conversion technology in household appliances. It analyzes the problems brought about by the application of frequency conversion technology, such as harmonics, electromagnetic interference, and a decrease in the power factor of the power supply system. Related harmonic suppression methods and measures to improve the power factor of the power supply system are proposed. Keywords: Power electronic devices; Frequency conversion technology; Harmonics; Power factor Introduction With the rapid development of power electronics and computer technology, AC speed regulation has replaced DC speed regulation as a development trend. Frequency conversion speed regulation, with its excellent speed regulation and starting/braking performance, is recognized both domestically and internationally as the most promising speed regulation method. Frequency conversion technology is the core technology of AC speed regulation, while power electronics and computer technology are the core of frequency conversion technology, and power electronic devices are the foundation of power electronics technology. Power electronics technology is a rapidly developing high-tech field in recent years, widely used in mechatronics, motor drives, aerospace, and other fields, and has now become a high-tech field that countries are vying to develop. Experts predict that in the highly developed field of automatic control in the 21st century, computer technology and power electronics technology will be the two most important technologies. I. Development Process of Power Electronic Devices The invention of the thyristor in the United States in the late 1950s marked the birth of power electronics technology. The first generation of power electronic devices was mainly the silicon controlled rectifier (SCR), which was promoted nationwide in China as an energy-saving technology in the 1970s. However, the SCR is ultimately a semi-controlled switching device that can only control its conduction, not its turn-off, limiting its application in AC drives and frequency converters. After the 1970s, power transistors (GTRs), gate turn-off thyristors (GTOs), power MOSFETs, insulated-gate transistors (IGBTs), electrostatic induction transistors (SITs), and electrostatic induction thyristors (SITHs) were invented. Their common feature is that they can control both conduction and turn-off, making them fully controlled switching devices. Because they do not require commutation circuits, their size and weight are significantly reduced compared to SCRs. Currently, IGBTs have become mainstream devices due to their superior characteristics, and high-capacity GTOs also hold a certain position. Many countries are striving to develop high-capacity devices, and 6000V IGBTs are already being produced abroad. IEGT (injection enhanced gate thyristor) is a new type of device that combines the advantages of IGBT and GTO. Samples with a voltage rating of 1000A/4500V have been developed. IGCT (integrated gate eommutated thyristor) adopts a buffer layer and transparent emitter on the basis of GTO. When it is turned on, it is equivalent to a thyristor, and when it is turned off, it is equivalent to a transistor, thus effectively coordinating the contradiction between the on-state voltage and the blocking voltage. The operating frequency can reach several kilohertz [2][3]. The IGCT launched by ABB in Switzerland can reach 4500-6000V and 3000-3500A. MCT has been withdrawn due to limited progress, while the development of IGCT has made it occupy an important position in the new pattern of power electronic devices. Compared with developed countries, my country has a greater gap in device manufacturing than in application. New power devices such as high-power trench gate structure IGBT modules, IEGT, MOS gate thyristors, high-voltage gallium arsenide high-frequency rectifier diodes, and silicon carbide (SiC) have made the latest developments abroad. It is reasonable to believe that using new semiconductor materials such as GaAs and SiC to fabricate power devices, realizing people's pursuit of "ideal devices," will be the main trend in the development of power electronic devices in the 21st century. High-reliability power electronic building blocks (PEBBs) and integrated power electronic modules (IPEMs) are recent hot topics in the development of power electronics technology in the United States. The fierce competition among new power electronic devices such as GTOs and IGCTs, and IGCTs and high-voltage IGBTs, will inevitably bring more opportunities and challenges to the development of new power electronic technologies and frequency conversion technologies worldwide in the 21st century. II. The Development Process of Frequency Conversion Technology Frequency conversion technology was born out of the need for stepless speed regulation of AC motors. The upgrading of power electronic devices has spurred the continuous development of power conversion technology. Initially, frequency conversion technology was limited to frequency conversion and could not transform voltage. Starting in the 1970s, research on pulse width modulation variable voltage frequency conversion (PWM-VVVF) speed regulation attracted great attention. In the 1980s, the optimization of PWM mode, the core of frequency conversion technology, attracted great interest, leading to numerous optimization methods, such as: longitudinal segmentation of the modulation wave, in-phase carrier PWM technology, phase-shifting carrier PWM technology, and simultaneous phase-shifting PWM technology with carrier modulation wave. VVVF frequency converters have relatively simple control and good mechanical characteristics, meeting the smooth speed regulation requirements of general drives, and have been widely used in various industrial fields. However, at low frequencies, this control method is significantly affected by the stator resistance voltage drop due to the small output voltage, resulting in a reduction in the maximum output torque. Vector control frequency conversion speed regulation involves transforming the stator AC currents Ia, Ib, and Ic of the asynchronous motor in the three-phase coordinate system into equivalent DC currents Iml and Itl in the synchronous rotating coordinate system through a three-phase to two-phase transformation. Then, mimicking the control method of a DC motor, the control quantities of the DC motor are obtained, and through corresponding coordinate inverse transformation, the control of the asynchronous motor is achieved. Direct torque control directly analyzes the mathematical model of the AC motor in the stator coordinate system to control the motor's flux linkage and torque. It does not require converting the AC motor into an equivalent DC motor, thus saving many complex calculations in the vector rotation transformation; it does not require imitating the control of the DC motor, nor does it require simplifying the mathematical model of the AC motor for decoupling. VVVF frequency conversion, vector control frequency conversion, and direct torque control frequency conversion are all types of AC-DC-AC frequency conversion. Their common disadvantages are low input power factor, large harmonic current, large energy storage capacitor required for DC circuit, and the inability to feed regenerated energy back to the grid, i.e., they cannot operate in four quadrants. For this reason, matrix AC-AC frequency conversion has emerged. [b]III. Frequency Conversion Technology and Household Appliances[/b] In the 1970s, household appliances began to gradually become frequency conversion, and electromagnetic cookers, frequency conversion lighting fixtures, frequency conversion air conditioners, frequency conversion microwave ovens, frequency conversion refrigerators, IH (induction heating) rice cookers, frequency conversion washing machines, etc. appeared[4]. In the late 20th century, household appliances relied on frequency conversion technology, mainly targeting high functionality and energy saving. Firstly, refrigerators, operating 24/7, benefit from inverter cooling, where the compressor runs at a low speed, completely eliminating noise from compressor startup and resulting in significant energy savings. Secondly, air conditioners using inverter technology expand the compressor's operating range, eliminating the need for intermittent compressor operation for cooling and heating control, thus reducing power consumption and alleviating discomfort caused by temperature fluctuations. In recent years, new inverter cold storage units have not only reduced power consumption and achieved quieter operation but also enabled rapid freezing through high-speed operation. Regarding washing machines, while inverter technology previously enabled variable speed control and improved washing performance, newer models offer energy savings, quieter operation, and gentler washing features. Electromagnetic cookers utilize high-frequency induction heating to directly heat the pot, eliminating the incandescent components of gas and electric heating, making them safer and significantly improving heating efficiency. Their operating frequency is higher than what is audible, thus eliminating noise caused by pot vibrations. IV. Hazards and Countermeasures Caused by Power Electronic Devices Phase-controlled rectification and uncontrolled diode rectification in power electronic devices cause severe distortion of the input current waveform, which not only greatly reduces the power factor of the system but also causes serious harmonic pollution. Furthermore, the rapid changes in voltage and current in the hardware circuits subject power electronic devices to significant electrical stress, causing severe electromagnetic interference (EMI) to surrounding electrical equipment and electromagnetic waves, and the situation is becoming increasingly serious. Many countries have formulated national standards to limit harmonics. The Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC), and the International Conference on Large Electric Systems (CIGRE) have all introduced their own harmonic standards. The Chinese government has also formulated relevant regulations to limit harmonics. (I) Countermeasures for Harmonics and Electromagnetic Interference 1. Harmonic Suppression To suppress harmonics generated by power electronic devices, one method is to perform harmonic compensation, that is, to set up harmonic compensation devices to make the input current a sine wave. Traditional harmonic compensation devices use IC tuned filters, which can compensate for both harmonics and reactive power. Its disadvantages are that the compensation characteristics are affected by grid impedance and operating conditions, and it is prone to parallel resonance with the system, leading to harmonic amplification and causing LC filter overload or even burnout. Furthermore, it can only compensate for harmonics of fixed frequencies, and the effect is not ideal. With the widespread application of power electronic devices, the use of active power filters for harmonic compensation has become an important direction. The principle is to detect the harmonic current from the object being compensated, and then generate a compensation current of equal magnitude but opposite polarity to that harmonic current, thus ensuring that the grid current contains only the fundamental component. This type of filter can track and compensate for harmonics with varying frequencies and amplitudes, and its compensation characteristics are not affected by grid impedance. The main method for reducing harmonics in large-capacity converters is to use multiplexing technology: superimposing multiple square waves to eliminate lower-order harmonics, thus obtaining a near-sinusoidal stepped wave. The higher the multiple, the closer the waveform is to a sine wave, but the more complex the circuit structure. Small-capacity converters, in order to achieve low harmonics and high power factor, generally use diode rectification plus PWM chopping, often referred to as power factor correction (PEC). Typical circuits include boost type, buck type, buck-boost type, etc. 2. Electromagnetic interference suppression The measures to solve EMI are to overcome the excessive current rise rate di/dt and voltage rise rate du/dt when the switching device is turned on and off. Currently, the most noteworthy ones are zero current switching (ZCS) and zero voltage switching (ZVS) circuits. The methods are: (1) Connect an inductor in series with the switching device, which can suppress di/dt when the switching device is turned on, so that there is no voltage and current overlap area on the device, reducing positive switching losses; (2) Connect a capacitor in parallel with the switching device, which suppresses the rise of du/dt when the device is turned off, so that there is no voltage and current overlap area on the device, reducing switching losses; (3) Connect a diode in anti-parallel with the device. During the diode conduction period, the switching device is in a zero voltage and zero current state. At this time, driving the device to turn on or off can realize ZVS and ZCS operation. Currently, the more commonly used software switching technologies are partial resonant PWM and lossless buffer circuit. (II) Early methods for power factor compensation included the use of synchronous condensers, which are synchronous motors specifically designed to generate reactive power. These condensers utilize overexcitation and underexcitation to produce capacitive or inductive reactive power of varying magnitudes. However, as rotating motors, they suffer from significant noise and losses, complex operation and maintenance, and slow response. Therefore, they are often unsuitable for rapid reactive power compensation. Another method is the use of static var compensators (SVCs) with saturated reactors. These SVCs offer advantages such as being static and having a fast response, but their cores require saturation, resulting in substantial losses and noise. They also suffer from some unique problems associated with nonlinear circuits and cannot adjust phases to compensate for load imbalances, thus failing to become the mainstream SVC compensator. With the continuous development of power electronics technology, SVCs using SCRs, GTOs, and IGBTs have seen significant advancements, with static var generators (SVRs) being the most superior. They offer advantages such as fast adjustment speed and a wide operating range. Furthermore, by employing multiplexing, multilevel, or PWM technologies, the harmonic content in the compensation current can be greatly reduced. More importantly, the reactors and capacitors used in static var generators are small, greatly reducing the size and cost of the device. Static var generators represent the development direction of dynamic reactive power compensation devices. V. Conclusion We believe that power electronics technology will become one of the important pillar technologies of the 21st century. Variable frequency technology occupies an important position in the field of power electronics technology, and its development in medium-voltage variable frequency speed control and electric traction in recent years has attracted much attention. With global economic integration and China's accession to the World Trade Organization, China's power electronics technology and variable frequency technology industries will experience unprecedented development opportunities. References: [1] Zhou Mingbao. Power Electronics Technology [M]. Beijing: Machinery Industry Press, 1985. [2] Chen Jian. Power Electronics - Power Electronic Conversion and Control Technology. Beijing: Higher Education Press, 2002. [3] Wang Zhaoan and Huang Jun. Power Electronics Technology [M]. Beijing: Machinery Industry Press, 2003. [4] Chen Guocheng and Zhou Qinli. Research on Frequency Conversion Technology [J]. Journal of the School of Automation, Shanghai University, 1995 (6): 23-26. [5] Wang Zhengyuan. Power Electronics Technology and Power Supply Technology Application for the New Century, 2001