The development of frequency converter has experienced a gradual process. The original inverter did not adopt this topology of AC-DC-AC: AC to DC and then AC to AC, but direct AC to AC without intermediate DC link. This kind of frequency converter is called cycloconverter. At present, this kind of frequency converter has application in super 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. At that time, there was no IGBT, only SCR, so the application range was limited.

The working principle of AC-AC converter is that the three-phase power supply is controlled by several groups of phase control switches to directly generate the required variable voltage variable frequency power supply. 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 the output must be less than 1 / 3 or 1 / 2 of the input power frequency, otherwise the output waveform is too poor, the motor will vibrate and can not work. Therefore, AC frequency converter is limited to low-speed speed regulating occasions, thus greatly limiting its scope of use.

Matrix converter is a kind of 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, and its output consists of three levels with small harmonic content. Its power circuit is simple and compact, and can output sinusoidal load voltage with controllable frequency, amplitude and phase. The input power factor of matrix converter is controllable and can work in four quadrants. Although matrix converter has many advantages, it is not allowed to have two switches on or off at the same time in the commutation process, so it is difficult to realize. A high voltage matrix converter is also a high output voltage converter. In wind power generation, the input and output of matrix converter are not decoupled, that is, the asymmetry of load or power supply side will affect the other side. In addition, the input of matrix converter must be connected with filter capacitor. Although the capacity of the capacitor is smaller than that of AC-DC-AC intermediate energy storage capacitor, because they are AC capacitors, they have to bear the AC current of switching frequency, and their volume is not small.

AC-AC frequency conversion is direct frequency conversion, less a link, but a lot of devices are used. The three-phase needs 36 thyristors, and the control is complex. Our teacher joked that whoever adjusted 36 pipes could graduate immediately. In addition, the frequency of AC-AC frequency conversion can only be adjusted to the power frequency once. Generally, it can be adjusted to 1 / 3-1 / 2 of the power frequency, about 20Hz.

2 AC-DC-AC converter

We change this kind of AC to DC and then to ac. this inverter is called AC-DC-AC converter. The AC-DC-AC converter firstly 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 transforms the DC current into AC with variable frequency and voltage through the inverter.

There are two types:

AC-DC-AC voltage type

AC-DC-AC current type

Among them, the former is widely used, and the current general inverter adopts this topology. It is characterized by the supply of busbar voltage in the middle for the storage of electrolytic capacitors. The former uses diode controlled rectifier, and is simple and reliable. The inverter uses 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 size) of the inverter IGBT are well controlled, very superior control characteristics can be obtained.

Due to various factors such as control method and hardware design, voltage source inverter is widely used. The traditional current source AC-DC-AC converter uses the thyristor with natural commutation as the power switch, and its DC side inductance is relatively expensive. Moreover, when it is used in doubly fed speed regulation, the converter circuit is required when the speed is over synchronous, and the performance is poor under the condition of low slip frequency, so it is not widely used in doubly fed asynchronous wind power generation. Using voltage type AC-DC-AC converter, this rectifier frequency converter has the advantages of simple structure, less harmonic content, adjustable power factor of stator and rotor, which can significantly improve the operation state and output power quality of doubly fed generator, and the structure can completely separate the grid side and rotor side through the DC bus side capacity. The stator field oriented vector control system of doubly fed generator based on voltage type AC-DC-AC converter realizes the decoupling control of active and reactive power of generator based on maximum power point tracking of wind turbine, which is a representative direction of VSCF wind power generation.

In addition, there is a parallel AC-DC-AC inverter topology. In this structure, an AC-DC-AC current source 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 used as the auxiliary inverter to compensate the harmonics of the current mode inverter. 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 obvious. The use of a large number of power electronic devices leads to the rise of cost and more complex control algorithm. In addition, the voltage utilization rate of this structure is relatively low.

Problems of AC-DC-AC converter

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

Current high power and high voltage power electronics