The evolution and working principle of frequency converter

Simply put, AC-AC inverters require too many components and are difficult to control, while AC-DC inverters use fewer components and are easier to control, so most inverters currently use AC-DC-AC structures.

1. The development of frequency converters also has to go through a gradual process. The initial frequency converters did not adopt the AC-DC-AC topology: AC to DC and then to AC, but directly AC-AC without an intermediate DC link. This type of frequency converter is called AC-AC frequency converter. Currently, this type of frequency converter is used in ultra-high 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. At that time, there was no IGBT, only SCR, so the application range was limited.

The working principle of AC-AC inverter is to directly generate the required voltage-converter power supply through several groups of phase-controlled switches to control the three-phase industrial frequency power supply. Its advantages are high efficiency and convenient return of energy to the power grid. Its biggest disadvantage is that the maximum output frequency 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 fail to work. Therefore, AC-AC inverter is still limited to low-speed speed regulation occasions, which greatly limits its scope of use.

The matrix inverter is a direct AC-AC inverter, which consists of nine switch arrays directly connected between the three-phase input and output. The matrix converter has no intermediate DC link, and the output consists of three levels with relatively small harmonic content; its power circuit is simple and compact, and can output a 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 the matrix converter has many advantages, it is difficult to implement because it does not allow two switches to be 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 voltage withstand of the device are also a major disadvantage of this type of converter. In wind power generation, since the input and output of the matrix converter are not decoupled, the asymmetry of the load or 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 its capacitor is smaller than that of the intermediate energy storage capacitor of AC, DC and AC, they are AC capacitors and have to withstand the AC current of the switching frequency, so their size is not small.

AC-AC frequency conversion is direct frequency conversion, which reduces one link, but uses a lot of components. Three-phase requires 36 thyristors, which is complex to control. Our teacher joked that whoever can turn on 36 tubes can graduate immediately. In addition, AC-AC frequency conversion can only adjust the frequency to the power frequency, usually to 1/3-1/2 of the power frequency, about 20Hz.

2. We call this inverter that converts AC to DC and then back to AC an AC-DC-AC inverter. There are two types: AC-DC-AC voltage type and AC-DC-AC current type. The former is widely used, and the current general-purpose inverter adopts this topology.

Its characteristics are: the middle is an electrolytic capacitor to store the bus voltage, the front stage uses diode uncontrolled rectification, which is simple and reliable, and the inverter uses three-phase PWM modulation (the current modulation algorithm is space voltage vector). Due to the use of electrolytic capacitors of 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 controlled well, very superior control characteristics can be obtained.

The AC-DC-AC inverter first rectifies the AC power into DC power through a rectifier, then the DC intermediate circuit smoothes and filters the output of the rectifier circuit, and then the inverter converts the DC current into AC power with variable frequency and voltage.

AC-DC-AC inverters can be divided into two types: voltage type and current type. Due to various factors such as control methods and hardware design, voltage type inverters are widely used. Traditional current type AC-DC-AC inverters use naturally commutated thyristors as power switches. Their DC side inductors are relatively expensive. In addition, they are used in doubly fed speed regulation. When they are over-synchronous, they require commutation circuits. Their performance is also relatively poor under low slip frequency conditions. They are not widely used in doubly fed asynchronous wind power generation. The voltage type AC-DC-AC inverter, a rectifier and frequency conversion device, has excellent characteristics such as simple structure, low harmonic content, and adjustable stator and rotor power factor. It can significantly improve the operating state and output power quality of the doubly fed generator. In addition, this structure completely realizes the separation of the grid side and the rotor side through the DC bus side capacitor. The stator field oriented vector control system of the doubly fed generator of the voltage type AC-DC-AC inverter realizes the decoupling control of the active and reactive power of the generator based on the maximum power point tracking of the wind turbine, which 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-AC current type and an AC-DC-AC voltage type inverter in parallel. The current type inverter is responsible for power transmission as the main inverter, and the voltage type inverter is responsible for compensating the harmonics of the current type inverter as the auxiliary inverter. In this structure, the main inverter has a lower switching frequency and the auxiliary inverter has a lower switching current. Compared with the AC-DC-AC voltage type inverter mentioned above, this topology has low switching losses and the efficiency of the entire system is relatively high. Its disadvantages are also obvious. The use of a large number of power electronic devices leads to an increase in cost and a more complex 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:

Currently, 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;

For converters using IGCT (or GTO) and IECT, protection against direct short circuit caused by device failure is still a problem; if a direct short circuit occurs in the converter on the power supply side, it will cause a short circuit in the power grid, so the converter must use a high leakage reactance input transformer, generally requiring 15% or even up to 20%;

The overload capacity of the AC-DC-AC inverter is reduced when it runs at low frequency. Generally, the overload capacity of the inverter is halved when it runs below 5Hz.

The voltage change rate du/dt of the PWM modulated voltage waveform output by the AC-DC-AC inverter is very high, which can easily cause insulation fatigue damage to motors and electrical appliances; when the output wire is long, the common-mode reflected voltage will generate a very high voltage on the motor side. If it is a two-level converter, the peak value of this voltage is twice the DC voltage, and 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 inverter will produce problems such as harmonics, noise, shaft current, etc.