With the rapid development of power electronics and computer technology, AC speed regulation has become the replacement of DC speed regulation, a growing trend. Variable frequency speed regulation (VFD) is widely recognized both domestically and internationally as the most promising speed regulation method due to its superior speed regulation and starting/braking performance. VFD technology is the core technology of AC speed regulation, while power electronics and computer technology are the core of VFD 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 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. The 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 primarily the silicon controlled rectifier (SCR), which was promoted nationwide in my country as an energy-saving technology in the 1970s. However, the SCR is ultimately a semi-controlled switching device, only capable of controlling its conduction and not its turn-off, limiting its application in AC drives and frequency converters. Following the 1970s, various transistors were invented, including the power transistor (GTR), gate turn-off thyristor (GTO), power MOSFET, insulated-gate transistor (IGBT), static induction transistor (SIT), and static induction thyristor (SITH). These devices share the common characteristic of being fully controllable switching devices, capable of controlling both conduction and turn-off. Because they do not require a commutation circuit, their size and weight are significantly reduced compared to SCRs. Currently, the IGBT has become the mainstream device due to its superior characteristics, while the high-capacity GTO also holds a significant position.
Many countries are working hard to develop high-capacity devices, and foreign countries have already produced 6000V IGBTs. IEGT (injection enhanced gate thyristor) is a new type of device that combines the advantages of IGBT and GTO, and 1000A/4500V samples have been launched. IGCT (integrated gate 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 little 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. High-power trench-gate IGBT modules, IEG TRTs, MOS-gated thyristors, high-voltage gallium arsenide high-frequency rectifier diodes, and silicon carbide (SiC) are among the newest power devices that have seen recent developments abroad. It is reasonable to believe that using new semiconductor materials such as GaAs and SiC to fabricate power devices, thus realizing the 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 undoubtedly bring more opportunities and challenges to the development of new power electronics and frequency conversion technologies worldwide in the 21st century.
II. The Development Process of Variable Frequency Technology
Variable frequency drive (VFD) technology arose to meet the need for stepless speed regulation of AC motors. Advances in power electronic devices have spurred power conversion...
The continuous development of technology. Initially, frequency conversion technology was limited to frequency conversion and could not convert 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 problem of PWM mode, which is the core of frequency conversion technology, attracted great interest and led to many optimization modes, such as: longitudinal segmentation of the modulation wave, in-phase carrier PWM technology, phase-shifted carrier PWM technology, and simultaneous phase-shifted carrier modulation wave PWM technology.
VVVF frequency converters are relatively simple to control and have 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 smaller output voltage, resulting in a reduction in the maximum output torque.
The method of vector control variable frequency speed regulation is as follows: the stator AC currents Ia, Ib, and Ic of the asynchronous motor in the three-phase coordinate system are transformed into DC currents Iml and Itl in the synchronous rotating coordinate system through three-phase to two-phase transformation. Then, the control quantity of the DC motor is obtained by imitating the control method of the DC motor. After the corresponding coordinate inverse transformation, the control of the asynchronous motor is realized.
Direct torque control analyzes the mathematical model of the AC motor directly in the stator coordinate system, controlling the motor's flux linkage and torque. It does not require converting the AC motor into an equivalent DC motor, thus eliminating many complex calculations in vector rotation transformation; it does not require mimicking the control of a DC motor, nor does it require simplifying the mathematical model of the AC motor for decoupling.
VVVF frequency converters, vector control frequency converters, and direct torque control frequency converters are all types of AC-DC-AC frequency converters. Their common drawbacks include low input power factor, large harmonic current, a need for large energy storage capacitors in the DC circuit, and the inability to feed regenerated energy back to the grid, meaning they cannot operate in four quadrants. Therefore, matrix AC-AC frequency converters were developed to address these issues.
III. Variable Frequency Technology and Home Appliances
In the 1970s, household appliances began to gradually become frequency-inverted, and electromagnetic cookers, frequency-inverted lighting appliances, frequency-inverted air conditioners, frequency-inverted microwave ovens, frequency-inverted refrigerators, IH (induction heating) rice cookers, frequency-inverted washing machines, etc. appeared[4].
In the late 20th century, household appliances relied on inverter technology, primarily targeting high functionality and energy saving.
Firstly, refrigerators, which operate 24/7, benefit from inverter cooling. The compressor runs at a low speed throughout, completely eliminating noise caused by compressor startup and resulting in significantly improved energy efficiency. Secondly, air conditioners, by adopting inverter technology, expand the compressor's operating range, eliminating the need for intermittent compressor operation to control cooling and heating, 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.
In terms of washing machines, variable frequency drives were used in the past to achieve variable speed control and improve washing performance. In addition to energy saving and quiet operation, the new popular washing machines also introduce new control features to ensure gentle washing of clothes. Electromagnetic cookers use high-frequency induction heating to directly heat the pot, without the incandescent part of gas and electric heating. Therefore, they are not only safe, but also greatly improve heating efficiency. Their operating frequency is higher than that of hearing, thus eliminating the noise caused by the vibration of the rice cooker.
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 hardware circuits subject power electronic devices to significant electrical stress, causing severe electromagnetic interference (EMI) to surrounding electrical equipment and radio waves, and this situation is becoming increasingly serious. Many countries have established national standards for limiting harmonics, and 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 for limiting harmonics.
(I) Countermeasures against 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 install a harmonic compensation device 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. However, their disadvantages include: compensation characteristics are affected by grid impedance and operating conditions; they are prone to parallel resonance with the system, leading to harmonic amplification and potentially overloading or even burning out the LC filter. Furthermore, they can only compensate for harmonics of fixed frequencies, and their effectiveness is not ideal.
With the widespread application of power electronic devices, harmonic compensation using active power filters has become an important direction. The principle is to detect harmonic currents from the object being compensated, and then generate a compensation current of equal magnitude but opposite polarity to the 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 unaffected by grid impedance.
The main method for reducing harmonics in high-capacity converters is to employ multiplexing technology: superimposing multiple square waves to eliminate lower-order harmonics, thus obtaining a stepped wave close to a sine wave. The higher the multiplet, the closer the waveform is to a sine wave, but the more complex the circuit structure. To achieve low harmonics and a high power factor, small-capacity converters generally use diode rectification combined with PWM chopping, commonly referred to as power factor correction (PEC). Typical circuits include boost, buck, and buck-boost types.
2. Electromagnetic interference suppression
The solution to EMI is to overcome the excessive current rise rate di/dt and voltage rise rate du/dt that occur when switching devices are turned on and off. Currently, zero-current switching (ZCS) and zero-voltage switching (ZVS) circuits are attracting considerable attention. The method is:
(1) An inductor is connected in series with the switching device. This can suppress di/dt when the switching device is turned on, so that there is no voltage and current overlap region on the device, and the positive turn-off loss is reduced.
(2) A capacitor is connected in parallel to the switching device to suppress the rise of du/dt when the device is turned off. There is no voltage and current overlap region on the device, which reduces switching losses.
(3) The device has an anti-parallel diode. During the diode conduction period, the switching device is in a zero voltage and zero current state. At this time, the device can be turned on or off to achieve ZVS and ZCS actions.
Currently, commonly used software switching technologies include partial resonant PWM and lossless buffer circuits.
(II) Power Factor Compensation
Early methods employed synchronous condensers, which are synchronous motors specifically designed to generate reactive power. They utilize overexcitation and underexcitation to produce different amounts of capacitive or inductive reactive power. However, as rotating motors, they generate significant noise and losses, are complex to operate and maintain, and have slow response times. Therefore, they are often unsuitable for the requirements of rapid reactive power compensation.
Another method is to use a static var compensator (SVC) with a saturated reactor. It has the advantages of being static and having a fast response speed, but because its core needs to be magnetized to a saturated state, the losses and noise are very large, and there are some special problems with nonlinear circuits. In addition, it cannot adjust the phases separately to compensate for load imbalances, so it has not become the mainstream static var compensator.
With the continuous development of power electronics technology, static var compensators (SVCs), GTOs, and IGBTs have made significant progress, among which static var generators (SVGs) are the most superior. They offer advantages such as fast adjustment speed and wide operating range. Furthermore, by adopting multiplexing, multilevel, or PWM technologies, the harmonic content in the compensation current can be greatly reduced. More importantly, SVGs use small reactors and capacitors, significantly 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 key pillar technologies of the 21st century, and frequency conversion technology occupies an important position in the field of power electronics technology. Its development in medium-voltage variable frequency speed control and electric traction in recent years has been remarkable. With global economic integration and my country's accession to the World Trade Organization, my country's power electronics and frequency conversion technology industries will experience unprecedented development opportunities.
