In short, the AC-AC converter needs too many components, which is difficult to control, while the AC-DC-AC converter uses few components and has simple control. Therefore, at present, the AC-DC-AC converter is mostly used.

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

The working principle of AC-AC frequency converter is to directly generate the required variable voltage variable frequency power supply through the control of several groups of phase controlled switches. Its advantages are high efficiency and the energy can be easily returned to the power grid. Its biggest disadvantage is that the maximum frequency of output must be less than 1 / 3 or 1 / 2 of the frequency of input power supply, otherwise the output waveform is too poor and the motor jitters and cannot work. Therefore, the AC-AC frequency converter has been limited to low-speed speed speed regulation, which greatly limits its application range.

Matrix converter is an AC-AC direct converter, which is composed of nine switch arrays directly connected between three-phase input and output. The matrix converter has no intermediate DC link, the output is composed 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 not allowed to turn on or off two switches at the same time in its commutation process, which is difficult to realize. The low maximum output voltage capability of matrix converter and the high withstand voltage of devices are also a great disadvantage of this kind of converter. In wind power generation, because the input and output of matrix converter are not decoupled, that is, the asymmetry of load or power side will affect the other side. In addition, the input end of the matrix converter must be connected with the filter capacitor. Although the capacity of the capacitor is smaller than that of the AC-DC-AC intermediate energy storage capacitor, because they are AC capacitors, they have to bear the AC current of the switching frequency, and their volume is not small.

AC-AC frequency conversion is direct frequency conversion, one link is missing, but a large number of devices are used. 36 thyristors are required for three-phase, and the control is complex. Our teacher joked that whoever adjusted 36 pipes could graduate immediately. In addition, AC-AC frequency conversion can only adjust the frequency to the power frequency, which is generally adjusted to 1 / 3-1 / 2 of the power frequency, almost 20Hz.

2. We change this AC to DC and then AC. this frequency converter is called AC DC AC frequency converter, which is divided into two types: AC DC AC voltage type and AC DC AC current type. The former is widely used, and the current general frequency converter adopts this topology.

Its characteristics are as follows: the middle part 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 (the 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, very superior control characteristics can be obtained.

The AC-DC-AC converter first rectifies the AC into DC through the rectifier, and the DC intermediate circuit smoothly filters the output of the rectifier circuit, and then turns the DC current into AC with variable frequency and voltage through the inverter.

AC-DC-AC converter 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 source AC-DC-AC frequency converter uses the natural commutation thyristor as the power switch, its DC side inductance is relatively expensive, and it is applied to the doubly fed speed regulation. The converter circuit is required when the synchronization speed is over, and its performance is relatively poor under the condition of low slip frequency. It is not widely used in the doubly fed asynchronous wind power generation. Using voltage type AC-DC-AC frequency converter, this rectifier frequency converter has the excellent characteristics of simple structure, less harmonic content and adjustable stator and rotor power factor. It can significantly improve the operation state and output power quality of doubly fed generator, and the structure completely realizes the separation of grid side and rotor side through the DC bus side capacitance. The stator field oriented vector control system of doubly fed generator based on voltage source AC-DC-AC frequency converter realizes the decoupling control of generator active and reactive power based on the tracking of fan maximum power point. It is a representative direction of variable speed constant frequency wind power generation at present.

In addition, there is a parallel AC-DC-AC inverter topology. The main idea of this structure is that an AC-DC-AC current source inverter and an AC-DC-AC voltage source inverter are connected in parallel. The current source inverter is responsible for power transmission as the main inverter and the voltage source inverter is responsible for compensating the harmonics of the current source inverter as the auxiliary inverter. In this structure, the main inverter has lower switching frequency and the auxiliary inverter has lower switching current. Compared with the AC-DC-AC voltage source inverter mentioned above, the topology has low switching loss and high efficiency of the whole system. Its disadvantages are also obvious. The use of a large number of power electronic devices leads to the rise of cost and more complex control algorithms. In addition, the voltage utilization of this structure is relatively low.

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

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 be mature;

With IGCT (or GTO) and iect converters, the protection of direct short circuit caused by device failure is still a difficult problem; If a direct short circuit occurs in the power side converter, it will cause a short circuit in the power grid, so the converter must adopt a high leakage reactance input transformer, which is generally required to be 15% or even 20%;

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

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

PWM modulation of AC-DC-AC frequency converter will produce harmonic, noise, shaft current and other problems.