The structural principle of frequency converter and the development process of frequency converter

by:V&T Technologies     2019-11-09
It can be simply said that the inverter needs to use too many components, which is not easy to control, and the components used in the direct AC are few and the control is simple, so most of the inverters with the direct ac structure are currently used. 1. The development of frequency converter also needs to go through a gradual process. The original frequency converter did not adopt this topology of AC to DC and then AC, but direct delivery, no intermediate DC link. This type of inverter is called AC Inverter. At present, this type of inverter is used in ultra-large power and low speed regulation. Its output frequency range is: 0-17 (1/2-1/3 input voltage frequency) Therefore, it could not meet the requirements of many applications, and there was no IGBT at the time, only SCR, so the application range was limited. The working principle of the inverter is that the three-phase power frequency power supply is directly controlled by several sets of phase control switches to directly generate the required variable voltage inverter power supply. Its advantages are high efficiency and energy can be easily returned to the power grid, its biggest disadvantage is that the highest output frequency must be less than 1/3 or 1/2 of the input power frequency, otherwise the output waveform is too poor and the motor will shake and cannot work. Therefore, the inverter has been limited to low-speed regulation, which greatly limits its scope of use. Matrix inverter is a kind of AC-ac direct inverter, which consists of nine switching arrays directly connected between three-phase input and output. The matrix converter has no intermediate DC link, the output consists of three levels, and the harmonic content is relatively small; Its power circuit is simple and compact, and can output sinusoidal load voltage with controllable frequency, amplitude and phase; The input power factor of the matrix converter is controllable and can work in four quadrants. Although matrix converter has many advantages, it is difficult to implement the phenomenon that two switches are turned on or off at the same time during its converter process. Matrix converters have low maximum output voltage capability and high device withstand voltage, which is also a great disadvantage of such converters. Applied in wind power generation, because the input and output of the matrix converter are not decoupled, that is, the asymmetry of both the load and the Power side will affect the other side. In addition, the input end of the matrix converter must be connected to the filter capacitor. Although the capacity of the capacitor is smaller than that of the intermediate energy storage capacitor of the AC-AC, because they are AC capacitors, they have to withstand the AC current of the switching frequency, its volume is not small. Frequency conversion is direct frequency conversion, one link is missing, but the number of devices used is large, and the three-phase requires 36 thyristor, which is complicated to control. Our teacher joked that whoever transferred 36 pipes can graduate immediately. There are also exchanges- AC frequency conversion can only adjust the frequency to the power frequency once, generally adjusted to 1/3 of the power frequency-1/2, almost 20Hz. 2. We call this type of AC to DC and then to AC. This type of inverter is called AC direct AC inverter, which is divided into two types, one is AC direct AC voltage type, the other is the AC direct current type. The former is widely used, and the current universal inverter uses this topology. It is characterized by: The Middle provides bus voltage for electrolytic capacitor storage, the front stage adopts diode without control rectification, is simple and reliable, and the inverter adopts three-phase PWM Modulation (The current modulation algorithm is a spatial voltage vector). Due to the adoption of a certain capacity of electrolytic capacitor, the DC bus voltage is stable, so long as the switching sequence of inverter IGBT is controlled (Output phase sequence, frequency)And duty cycle (Output voltage size), You can get very superior control characteristics. Jiao-Straight- The AC converter first rectifies the AC power to DC power through the rectifier, and the DC intermediate circuit smoothly filters the output of the rectifier circuit, then the DC current is turned into AC power with variable frequency and voltage through the inverter. AC direct AC inverter can be divided into voltage type and current type. Due to various factors such as control method and hardware design, voltage type inverter is widely used. The traditional current type AC direct AC inverter uses the thyristor of natural converter as the power switch, and its DC side inductance is relatively expensive, and it is used in double-fed speed regulation. The converter circuit is required at the over-synchronous speed, under the condition of low slip frequency, the performance is also relatively poor, and it is not widely used in doubly fed asynchronous wind power generation. The rectifier frequency conversion device, which adopts voltage type AC direct AC frequency converter, has the excellent characteristics of simple structure, low harmonic content, adjustable power factor of stator and rotor, etc, the running state and output power quality of the doubly-fed generator can be obviously improved, and the structure completely realizes the separation of the network side and the rotor side through the DC bus side capacitor. The stator magnetic field directional vector control system of the doubly-fed generator of the voltage-type AC-DC inverter realizes the decoupling control of active and reactive power of the generator based on the maximum power point tracking of the fan, it is a representative direction of variable speed constant frequency wind power generation at present. In addition, there is a parallel AC-AC inverter topology. The main idea of this structure is to connect a AC-DC current type and a AC-DC voltage type inverter in parallel, and the current type inverter is responsible for power transmission as the main inverter, as an auxiliary inverter, the voltage inverter is responsible for compensating the harmonics of the current inverter. The main inverter of this structure has lower switching frequency and the auxiliary inverter has lower switching current. Compared with the AC-DC voltage inverter mentioned above, the topology has low switching loss and the efficiency of the whole system is relatively high. The shortcomings are also obvious. The use of a large number of power electronic devices leads to higher costs and more complex control algorithms. In addition, the voltage utilization rate of this structure is relatively low.
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