To put it simply, the AC-AC frequency converter needs to use too many components, which is difficult to control, while the AC-DC-AC frequency converter uses few components and the control is simple, so most of the AC-DC-AC frequency converters are currently used.

1. The development of the frequency converter also has to go through a gradual process. The initial frequency converter did not use this kind of AC-DC-AC topology: AC-to-DC and then to AC-topology, but direct AC-AC without intermediate DC links. This kind of frequency converter is called AC frequency converter. At present, this kind of frequency converter has applications in super-power and low-speed speed regulation. Its output frequency range is: 0-17 (1/2-1/3 input voltage frequency), so it cannot meet the requirements of many applications, and there is no IGBT at that time, only SCR, so the application range is limited.

The working principle of the AC inverter is that the three-phase power frequency power supply is directly controlled by several groups of phase-controlled switches to directly generate the required variable voltage and variable frequency power supply. Its advantages are high efficiency, and the energy can be easily returned to the power grid. It is less than 1/3 or 1/2 of the input power frequency, otherwise the output waveform is too bad, the motor will vibrate and cannot work. Therefore, the AC frequency converter has been limited to low-speed speed regulation occasions, thus greatly limiting its scope of use.

The matrix inverter is an AC-AC direct inverter, which consists of nine switch arrays directly connected between the 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 it can output a sinusoidal load voltage with controllable frequency, amplitude and phase; the input of the matrix converter The power factor is controllable and can work in four quadrants.

Although the matrix converter has many advantages, it is difficult to realize the phenomenon that two switches are turned on or off at the same time during the commutation process. The low maximum output voltage capability of the matrix converter and the high withstand voltage of the device are also a major disadvantage of this type of converter. Applied in wind power generation, since the input and output of the matrix converter are not decoupled, that is, the asymmetry on either the load or the power supply side will affect the other side. In addition, the input end of the matrix converter must be connected to a filter capacitor. Although the capacity of the capacitor is smaller than that of the AC/DC intermediate energy storage capacitor, because they are AC capacitors, they have to withstand the AC current of the switching frequency, and their volume is not small.

AC-AC frequency conversion is direct frequency conversion, which lacks one link, but uses a lot of devices, and requires 36 thyristors for three-phase, and the control is complicated. Our teacher jokes that whoever tunes 36 pipes can graduate immediately. There is also an AC-AC frequency conversion frequency that can only be adjusted to the power frequency, generally adjusted to 1/3-1/2 of the power frequency, about 20Hz.

2. We call this kind of AC inverter into DC and then into AC. This inverter is called AC-DC-AC inverter, which is divided into two types, one is AC-DC-AC voltage type, and the other is AC-DC-AC current type. The former is widely used, and the current general-purpose inverter adopts this topology.

Its characteristics are: the middle provides bus voltage for electrolytic capacitor storage, the front stage adopts diode uncontrolled rectification, which is simple and reliable, and the inverter adopts three-phase PWM modulation (current modulation algorithm is space voltage vector). Due to the use of electrolytic capacitors with a certain capacity, the DC bus voltage is stable. At this time, as long as the switching sequence (output phase sequence, frequency) and duty cycle (output voltage) of the inverter IGBT are well controlled, a very superior control can be obtained. characteristic.

The AC-DC-AC inverter first rectifies the alternating current into direct current through the rectifier, and the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and then passes through the inverter to convert the direct current into alternating current with variable frequency and voltage. .

AC/DC inverters can be divided into two types: voltage type and current type. Due to various factors such as control method and hardware design, voltage type inverters are widely used. Traditional current-mode AC-DC inverters use naturally commutated thyristors as power switches, and their DC-side inductance is relatively expensive, and it is used in doubly-fed speed regulation. The performance is also relatively poor under certain conditions, and it is not widely used in doubly-fed asynchronous wind power generation. The rectification and frequency conversion device using the voltage type AC-DC inverter has the advantages of simple structure, less harmonic content, and adjustable stator and rotor power factor, which can significantly improve the operating state and output power quality of the doubly-fed generator. The separation of the grid side and the rotor side is completely achieved through the DC bus side capacitors. The DFIG stator field-oriented vector control system of the voltage AC/DC inverter realizes the decoupling control of generator active and reactive power based on the maximum power point tracking of the wind turbine, and is a representative direction of variable-speed constant-frequency wind power generation.

In addition, there is a parallel AC-DC-AC inverter topology. The main idea of this structure is to connect an AC, DC, and AC current type inverter in parallel with an AC, DC, and AC voltage type inverter. The current type inverter is used as the main inverter to be responsible for power transmission, and the voltage type inverter is used as the auxiliary inverter to compensate for the current. type inverter harmonics. In this structure, the main inverter has a lower switching frequency, and the auxiliary inverter has a lower switching current. Compared with the above-mentioned AC-DC-AC voltage inverter, this topology has low switching loss, and the efficiency of the whole system is relatively high. Its shortcomings are also obvious. The use of a large number of power Electronic devices leads to an increase in cost and a more complicated control algorithm. In addition, the voltage utilization rate of this structure is relatively low.

3. Although the AC-DC-AC inverter has the advantages of high output frequency and high power factor, there are still many problems to be improved in the AC-DC-AC inverter:

At present, high-power and high-voltage power electronic devices are in the development stage, GTO components are facing elimination, and IGBT and IGCT are yet to mature;

Using IGCT (or GTO) and IECT converters, the protection of direct short-circuit caused by device failure is still a problem; if a direct short-circuit occurs in the power-side converter, it will cause grid short-circuit, so the converter must use a high leakage reactance input transformer. Ask for 15%, or even as high as 20%;

The overload capacity of the AC-DC-AC inverter is reduced during low frequency operation. Generally, the overload capacity of the inverter is reduced by half when it is running below 5Hz;

The voltage change rate du/dt of the output PWM modulation voltage waveform of the AC-DC-AC inverter is very high, which is easy to cause insulation fatigue damage to the motor and electrical appliances; when the output wire is long, the common-mode reflected voltage will produce a high voltage on the motor side. Voltage, in the case of a two-level converter, the peak value of this voltage is twice the DC voltage, and in the case of a three-level converter, the peak value of this voltage is three times the middle half of the voltage;

AC-DC-AC inverter PWM modulation will cause problems such as harmonics, noise, shaft current and so on.

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