Development and Application Research Based on Power Electronic Devices and Frequency Conversion Technology

The development process of power electronic devices and frequency conversion technology is introduced. The application of DC high voltage generator and frequency conversion technology in household appliances is analyzed. The application of frequency conversion technology also brings about problems such as harmonics, electromagnetic interference and power factor reduction of power system. The related harmonic suppression methods and measures to improve the power factor of the power system are proposed.
Key words: power electronic device; frequency conversion technology; harmonic; power factor
Introduction <br> With the rapid development of power electronics and computer technology, DC high-voltage generator AC speed regulation has replaced DC speed regulation has become a development trend. Frequency control has been recognized as the most promising speed control method at home and abroad for its excellent speed regulation and start and brake performance. Frequency conversion technology is the core technology of AC speed regulation. 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 DC high voltage generator kinds of high-tech rapid development in recent years, is widely used in electrical and mechanical integration, electrical transmission, aerospace and other fields, has become a high-tech race to develop countries. Experts predict that computer technology and power electronics are the two most important technologies in the highly developed field of automation in the 21st century.
First, the development process of power electronic devices <br> The thyristor was introduced in the United States in the late 1950s, marking the birth of power electronics technology. The first generation of power electronic devices was mainly thyristor rectifiers (SCRs), which were listed as energy-saving technologies in China in the 1970s. However, SCR is, after all, a semi-controlled switching device that can only control its conduction and cannot control the shutdown. It is limited in the application of AC DC high voltage generator drive and variable frequency power supply. Power transistors (GTR), gate turn-off thyristors (GTOs), power MOS field effect transistors (Power MOSFETs), insulated gate transistors (IGBTs), electrostatic induction transistors (SITs), and electrostatic induction thyristors (SITH) that were invented after the 1970s. Etc., their common feature is to control both their conduction and control of their turn-off. It is a fully-controlled switching device. Since the converter circuit is not required, the volume and weight are greatly reduced compared with the SCR. At present, IGBT has become a mainstream device with its excellent characteristics, and a GTO with a large capacity also has a certain status.
Many countries are striving to develop large-capacity devices, and 6,000-watt IGBTs have been produced abroad. IEGT (injection enhanced gate thyristor) is a new type of device that combines the advantages of IGBT and GTO. Samples of 1000A/4500V have been introduced. IGCT (integrated gate eommutated thyristor) uses a buffer layer and a transparent emitter on the basis of GTO. It is equivalent to a thyristor when turned on, and is equivalent to a transistor when turned off, thereby effectively coordinating the on-state DC high-voltage generator voltage and blocking voltage. The contradiction, the working frequency can reach several kilohertz [2] [3]. Swiss ABB has launched IGCT up to 4500-6000V, 3000-3500A. MCT retired due to little progress and the development of IGCT has made it an important position in the new pattern of power electronics. Compared with developed countries, China has a greater gap in device manufacturing than in application. New power devices such as high-power trench gate IGBT modules, IEGT, MOS gated thyristors, high-voltage arsenic high-frequency rectifier diodes, and silicon carbide (SIC) have developed in foreign countries. It is believed that the use of new semiconductor materials such as GaAs and SiC to make power devices and realize 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 (PEBB) and integrated power electronics modules (IPEM) are new hotspots in the recent development of power electronics technology in the United States. The fierce competition between GTO and IGCT, IGCT and high-voltage IGBT and other new DC high-voltage generator devices will bring more opportunities and challenges to the development of new power electronics and frequency conversion technologies in the 21st century.
Second, the development process of frequency conversion technology <br> Frequency conversion technology is born in the need of AC motor stepless speed regulation. The update of power electronics has led to the continuous development of power conversion technology. At first, the frequency conversion technology was limited to the variable frequency and could not be transformed. Since the 1970s, the research on pulse width modulation and voltage conversion (PWM-VVVF) speed regulation has attracted people's attention. In the 1980s, the PWM mode optimization problem, which is the core of the inverter technology, attracted people's interest and derived many optimization modes, such as: modulated wave longitudinal segmentation, in-phase carrier PWM technology, phase-shifted carrier PWM technology, carrier modulation. Wave simultaneous phase shift PWM technology.
The control of VVVF inverter is relatively simple, and the mechanical characteristics are also good. The DC high-voltage generator can meet the requirements of smooth speed regulation of general transmission, and has been widely used in various fields of the industry. However, in the low frequency mode, since the output voltage is small, the influence of the stator resistance voltage drop is significant, so that the maximum output torque is reduced.
The vector control frequency conversion speed regulation method is: the asynchronous current Ia, Ib, Ic of the asynchronous high-voltage generator in the three-phase coordinate system is passed through the three-phase-two-phase transformation, which is equivalent to the direct current in the synchronous rotating coordinate system. The current Iml, Itl, then imitate the control method of the DC motor, obtain the control amount of the DC motor, and realize the control of the asynchronous motor through the inverse transformation of the corresponding coordinates.
The direct torque control directly analyzes the mathematical model of the AC motor of the DC high voltage generator in the stator coordinate system, and controls the flux linkage and torque of the motor. It does not require the conversion of an AC motor into an equivalent DC motor, thus eliminating many of the complex calculations in vector rotation transformation; it does not require the control of a DC motor, nor does it need to simplify the mathematical model of the AC motor for decoupling.
VVVF frequency conversion, vector control frequency conversion, direct torque control frequency conversion are all one of AC-DC-AC frequency conversion. The common disadvantage is that the input power factor is low, the harmonic current is large, the DC loop requires a large storage capacitor, and the regenerative energy cannot be fed back to the grid, that is, the four-quadrant operation cannot be performed. To this end, matrix-type AC-AC frequency conversion came into being.
Third, frequency conversion technology and household appliances
In the 1970s, household appliances began to gradually change frequency, and electromagnetic cookers, variable frequency lighting appliances, inverter air conditioners, inverter microwave ovens, inverter refrigerators, IH (induction heating) rice cookers, inverter washing machines, etc. [4] appeared.
In the late 20th century, household appliances relied on frequency conversion technology, mainly aimed at high functions and power saving.
The first is the refrigerator. Since it is working all day, the compressor is always in the low-speed running state after the frequency is cooled by the variable DC high-voltage generator , which can completely eliminate the noise caused by the compressor starting, and the energy-saving effect is more obvious. Secondly, the air conditioner uses the frequency conversion to expand the working range of the compressor, and the cold and warm control can be realized without the compressor operating in the intermittent state, thereby reducing the power consumption and eliminating the discomfort caused by the temperature fluctuation. In recent years, the new type of inverter refrigerator has not only reduced power consumption and quietness, but also enables rapid freezing with high-speed operation.
In terms of washing machines, in the past, variable frequency control DC high-voltage generators were used to improve washing performance. In addition to energy saving and quieting, the new popular washing machine also introduced new control contents in ensuring soft washing of clothes; electromagnetic cooking device The high-frequency induction heating causes the pot to directly heat up, without the hot part of gas and electric heating, so it is not only safe, but also greatly improves the heating efficiency, and its working frequency is higher than the hearing, thereby eliminating the noise caused by the vibration of the rice cooker.
Fourth, the harm caused by power electronic devices and countermeasures <br> The phase-controlled rectification and uncontrollable diode rectification in power electronic devices cause serious distortion of the input current waveform, which not only greatly reduces the power factor of the system, but also causes serious harmonics. Wave pollution.
In addition, the sharp changes in voltage and current in the hardware circuit cause the DC electronic device to withstand large electrical stresses and cause severe electromagnetic interference (EM1) to surrounding electrical equipment and electric waves, and the situation is becoming increasingly serious. Many countries have established national standards for limiting harmonics. The International Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC) and the International Conference on Large Power Grids (CIGRE) have introduced their own harmonic standards. The Chinese government has also formulated relevant regulations for limiting harmonics.
(1) Countermeasures for harmonics and electromagnetic interference
1. Harmonic Suppression In order to suppress harmonics generated by power electronic devices, one method is to perform harmonic compensation, that is, to set a harmonic compensation device to make the input current a sine wave.
A conventional IC using harmonic compensation means is tuned filter, which can compensate for the harmonic current high-voltage generator, but also reactive power compensation. The disadvantage is that the compensation characteristics are affected by the impedance and operating state of the power grid, and it is easy to parallel resonance with the system, resulting in harmonic amplification, which causes the LC filter to be overloaded or even burned. In addition, it can only compensate for harmonics at a fixed frequency, and the effect is not ideal.
After the popularization of power electronic devices, the use of active power filters for harmonic compensation has become an important direction. The principle is that the harmonic current is detected from the compensation object, and then a compensation current having the same polarity as the harmonic current DC high voltage generator is generated , so that the grid current only contains the fundamental component. This filter can track and compensate harmonics with varying frequency and amplitude, and the compensation characteristics are not affected by the impedance of the grid.
The main method for reducing harmonics in large-capacity converters is to use multiple techniques: superimpose multiple square waves to eliminate lower harmonics, resulting in a nearly sinusoidal stepped wave. The more the number, the closer the waveform is to the sine, but the more complicated the circuit structure. In order to achieve low harmonics and high power factor, small-capacity converters generally use diode rectification plus PWM chopping, often referred to as power factor correction (PEC). Typical circuits are boost type, step-down type, buck-boost type, and the like.
2. Electromagnetic interference suppression The EMI measures are to overcome the excessive current rise rate di/dt and voltage rise rate du/dt of the DC high voltage generator when the switching device is turned on and off. At present, the comparison is focused on the zero current switch. (ZCS) and zero voltage switching (ZVS) circuits. the way is:
(1) The series inductor on the switching device can suppress the di/dt when the switching device is turned on, so that there is no voltage and current overlapping region on the device, and the positive switching loss is reduced;
(2) Parallel capacitor on the switching device, when the device is turned off, the du/dt is inhibited from rising, and there is no overlap region of voltage and current on the device, which reduces the switching loss;
(3) The anti-parallel diode on the device, during the diode conduction period, the switching device is in a zero voltage, zero current state, at this time the driving device is turned on or off to achieve ZVS, ZCS action.
At present, the more commonly used software switching technology has partial resonant PWM and lossless buffer circuits.
(2) The early method of power factor compensation is to use a synchronous camera, which is a synchronous motor specially used to generate reactive power. Different sizes of capacitive or inductive reactive power are respectively generated by overexcitation and underexcitation. However, due to the DC high voltage generator, it is a rotating motor, which has large noise and loss, complicated operation and maintenance, and slow response. Therefore, in many cases, it has been unable to meet the requirements of fast reactive power compensation.
Another method is to use a static var compensator for a saturable reactor. It has the advantages of static type and fast response speed, but because its core needs to be magnetized to saturation state, the loss and noise are both large DC high voltage generators , and there are some special problems of nonlinear circuits, and it is not possible to adjust the phase to compensate the load. The imbalance is not enough to occupy the mainstream of static reactive power compensation devices.
With the continuous development of power electronics technology, static var compensators using SCR, GTO and IGBT have been greatly developed, among which static var generators are the most advantageous. It has the advantages of fast adjustment speed and wide operating range, and can greatly reduce the harmonic content in the compensation current after taking measures such as multiplexing, multi-level or PWM technology. More importantly, the static var generator uses small actuators and capacitive components, which greatly reduces the size and cost of the device. The static var generator represents the development direction of the dynamic var compensator.
V. Conclusion <br> We believe that power electronics technology will become one of the important pillar technologies in the 21st century. Frequency conversion technology plays an important role in the field of power electronics technology. In recent years, it has been in the field of medium voltage variable frequency speed regulation and electric traction. The development is eye-catching. With the integration of the global economy and China's Canadian World Trade Organization, China's power electronics technology and inverter technology industry will have unprecedented development opportunities.