Application of the hottest three-level technology

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The application of three-level technology in active power filter

in recent years, the requirements for high-capacity power conversion devices in industrial applications are increasing, such as flexible AC Transmission Technology (FACTS) represented by high-voltage DC transmission (HVDC), static var compensation (STATCOM) in power system, high-power motor speed regulation and high-power power power supply devices represented by high-voltage frequency converter, etc. In these application fields, high-power inverters often become a core and key problem of their application

on the one hand, due to the limitation of the device's ability to withstand voltage and current, it is difficult for the two-level inverter to directly realize high voltage and high power; On the other hand, in high-voltage applications, the dv/dt of the output voltage of the two-level inverter is very large, which endangers the insulation of the motor and aggravates the bearing current in the speed regulation system. This makes it difficult for the traditional two-level inverter to meet the requirements of these applications. Therefore, in high-voltage and high-power applications, multilevel inverter has attracted more and more attention, and began to be widely studied and applied. The idea of multilevel was first proposed by Nabae in the early 1980s

its basic principle is to synthesize multiple levels into stepped waves to approximate the sinusoidal output voltage. Generally speaking, the more levels, the closer the output voltage waveform is to the sinusoidal wave. Compared with the traditional two-level inverter, the multi-level inverter has very obvious advantages:

(1) the voltage withstand value and voltage stress of the switch tube of the multi-level inverter are low, which reduces the switching loss of the device and improves the conversion efficiency of the system; Moreover, low-voltage devices can be used to achieve high-voltage output without switching devices directly in series

(2) because the output voltage contains multiple levels, the dv/dt of the output voltage is greatly reduced, the output voltage waveform is closer to the sine wave, and the waveform control is easier than two-level

(3) the switching device only needs to work at a relatively low switching frequency, which reduces the switching loss of the device. Compared with the traditional high-capacity inverter structure, multilevel inverter has achieved a good combination between high-voltage high-capacity and high-performance, and has become a new field and an important branch in the power electronics research system

the basic topology of multilevel inverter can be divided into three categories:

(1) diode clamped multilevel inverter

(2) flying capacitor multilevel inverter

(3) cascaded multilevel inverter

(1) sinusoidal PWM modulation (SPWM)

(2) select harmonic elimination PWM modulation (SHEPWM)

(3) space vector PWM modulation (SVPWM)

I Introduction to multilevel inverter topology

multilevel inverter and these drops can also burn. The basic topology can be divided into three categories:

(1) diode clamped multilevel inverter

diode clamped multilevel inverter divides the high voltage on the DC side into a series of lower level voltages through series capacitors. M-level diode clamped multilevel inverter requires M-1 capacitors on the DC side

The figure shows a three-phase five level diode clamped inverter. There are four capacitors on its DC side, which are C1, C2, C3 and C4 respectively. If the total voltage on the DC side is VDC, the voltage shared on each capacitor is vdc/4, and through the action of the clamping diode, the voltage stress on each switching device is limited to the voltage level of one capacitor, vdc/4, so that the output voltage synthesized by the inverter can be correspondingly increased. Diode clamped multilevel inverter only needs a common DC power supply, which makes its rectifier side design relatively simple. One of the main problems of diode clamped multilevel inverter is the voltage balance control of each DC capacitor. For example, although the diode clamped three-level inverter is one of the most mature multi-level inverter structures, the neutral point potential balance is its inherent problem, and it is a research hotspot

(2) leap capacitor multilevel inverter

figure 1-extrusion technology suppliers can show 2 three-phase five level leap capacitor inverter

the figure shows a three-phase five level leap capacitor multilevel inverter. The DC side of the inverter adopts a stepped structure, and the capacitor voltage of each layer is different from that of the next layer. In order to generate a stepped output voltage of m level, M-1 capacitors are required on the DC side. The structure of each phase bridge arm is the same, and the increase of voltage between the two capacitors determines the height of each voltage level in the output waveform. Obviously, the three-layer inner ring balance capacitors CA1, Ca2 and CA3 in the A-phase bridge arm and the three-layer capacitance of the B-phase bridge arm are independent of each other, but all bridge arms share the four capacitors C1 ~ C4 on the DC side. In leap capacitor multilevel inverter, although the capacitor replaces a large number of clamp diodes, due to the large volume of the capacitor, it is difficult to realize the device when the number of levels is large. And when the device starts, each capacitor needs to be precharged to reach the operating voltage, which is relatively complex to realize

(3) cascaded multilevel inverter

figure M-level single-phase cascaded multilevel inverter

figure shows an M-level single-phase cascaded multilevel inverter. The function of Cascaded Multilevel Inverter Based on independent DC power supply is the same as that of the above two topologies. It synthesizes a desired voltage through several independent DC power supplies. This new inverter does not need additional clamping diodes or voltage balancing capacitors. Each independent DC power supply (SDCs) is connected to a single-phase full bridge inverter. Through different combinations of switching devices S1 ~ S4, each inverter can generate three different levels of voltage, VDC, -vdc and 0. The output voltage of each full bridge inverter is connected in series to synthesize the final voltage output waveform. Obviously, this topology output phase electricity

II. Introduction to the modulation strategy of multilevel inverter

there are roughly three modulation strategies of multilevel inverter:

(1) sinusoidal PWM modulation (SPWM)

multilevel SPWM modulation strategy is an extension of the two-level SPWM modulation strategy. For N-level inverter, there are generally n-1 triangular carriers in each phase, and the modulation waveform of the inverter is obtained by comparing the N-1 triangular carriers with a sinusoidal modulation wave

(2) select harmonic elimination PWM modulation (SHEPWM)

select harmonic elimination instead of tailor-made division according to the unique body structure of each patient, that is, selected harmonic elimination, or she. Patel and FT first proposed this idea in 1973, and first used this method in SCR controlled inverters, and then applied it in some early small and medium-power inverters, After the appearance of three-level inverter circuit, this method is also adopted. Choosing the PWM modulation method of harmonic elimination is to Fourier transform the output sine wave to obtain the lowest harmonic that should be eliminated, and then calculate the switching angle at a special position through a certain algorithm from the perspective of fixed investment expenditure and the acceleration of plastic product production, so as to eliminate the lowest harmonic. Its advantage is that the method is intuitive, easy to understand, can eliminate any specified harmonic, low harmonic content, and good output waveform; The disadvantage is that the amount of calculation is relatively large, and a large number of angles need to be calculated offline, and a large number of tables need to be stored in DSP or microprocessor. Therefore, this modulation strategy is generally not used in occasions with low waveform quality requirements

(3) space vector PWM modulation (SVPWM)

space vector PWM modulation method is to replace the three-phase sinusoidal AC voltage in the ABC coordinate system with a rotating voltage vector, and the rotation space of this rotating vector can be divided into several sectors. This rotation vector can be synthesized by three adjacent voltage vectors with fixed positions in each sector. Controlling the action time of these three vectors is equivalent to controlling the mode length and direction of the rotating voltage vector

among the three modulation strategies, space vector PWM has been widely used because of its large modulation range and high bus voltage utilization

in the process of design and development of the project, in order to realize the diversification of device functions and meet the needs of users in different regions, the device has realized multi system structural functions, that is, the output voltage level is 110v/220v/230v, and the output voltage frequency is 50hz/60hz, which can be adjusted by dialing; Three phase/single-phase input and three-phase/single-phase output can be selected in the circuit structure. In order to adapt to the dialing adjustment of multi-standard structure function, sinusoidal PWM modulation method is adopted in program design

multilevel inverter has achieved a good combination between high-voltage large capacity and high-performance, and has become a new field and important branch in the power electronics research system. Hesine active power filter is the first three-level technology to be introduced into the field of active power filter in the industry by the scientific and technological personnel of Heneng company based on their years of research on active power filter, and successfully launched the world's first Three-level Active Power Filter product

hesine active power filter adopts diode clamped three-level topology, which has lower loss and EMI compared with the traditional two-level active power filter. At the same time, it can directly connect 500v~1000v power, eliminating bulky and expensive transformers, and can save a lot of investment for customers. The company has also successfully developed a unique Three-level Active Power Filter control algorithm. On this basis, the three-level active power filter is first commercialized to meet the needs of different users and different occasions. (end)

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