The reason for the preferred DC mode in the intermediate link of the inverter - Reynolds

The development of the inverter has experienced a slow and gradual process, and the inverter at the beginning of the first century did not choose this kind of AC-DC: communication to DC and then to communication, but direct AC, no central DC link. This kind of inverter is called alternating frequency converter, and now this kind of inverter is used in ultra-high power and low-speed speed regulation. Its output frequency scale is: 0-17 (1/2-1/3 input voltage frequency), so it cannot meet the requirements of many applications, and there was no IGBT at that time, only SCR, so the application scale was limited.

The reason for the preferred DC mode in the intermediate link of the inverter - Reynolds

The reason for the preferred DC mode in the intermediate link of the inverter - Reynolds

The operating principle of the AC inverter is to control the three-phase power frequency power supply through several groups of phased switches to directly generate the required variable voltage variable frequency power supply, its advantage is high power, energy can be convenient back to the power grid, the highest frequency of its largest defect output must be less than the input power frequency 1/3 or 1/2, otherwise the output waveform is too poor, the motor jitter, can not operate. Therefore, the AC inverter has so far limited the low-speed speed regulation occasions, so it has greatly limited its application scale.

A matrix inverter is an alternating direct inverter consisting of nine switching arrays that are directly connected between the three-phase input and output. The matrix converter does not have a central DC link, and the output is composed of three levels, and the harmonic content is relatively small; 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 operate in four quadrants. Although matrix converters have many advantages, they are difficult to achieve because they do not allow two switches to turn on or off at the same time during the commutation process. The maximum output voltage of the matrix converter can be low, and the high acceptance voltage of the equipment is also a big defect of this type of converter. It is used in wind power generation because the input and output of the matrix converter are not decoupled, i.e. the asymmetry of either the load or the power supply side will affect the other side. In addition, it is necessary to connect the filter capacitor at the input end of the matrix converter, although the capacity of the capacitor is smaller than that of the AC and DC center energy storage capacitor, but because they are communication capacitors, they need to accept the communication current of the switching frequency, and their volume is not small.

Alternating - alternating frequency conversion is direct frequency conversion, one link is missing, but the amount of equipment used is a lot, and the three-phase requires 36 thyristors, and the control is complex. Our teacher pranks that whoever has 36 pipes will be able to graduate immediately. There is also an alternating frequency, which can only be adjusted to the power frequency, which is generally adjusted to 1/3-1/2 of the power frequency, almost 20Hz.

2. AC-DC inverter

Let's change this kind of communication to DC and then to communication, this kind of inverter is called AC-DC inverter. The AC-DC-AC inverter first rectifies the communication current into direct current through the rectifier, and the DC central circuit smooths the output of the rectifier circuit, and then turns the DC current into a communication current with variable frequency and voltage through the inverter.

There are two types:

AC-DC voltage type

AC-DC current type

Among them, the former is widely used, and the current general-purpose inverter is selected for this topology. Its characteristics are: the center is stored and supplied with bus voltage for electrolytic capacitors, the diode is selected for the pre-stage and uncontrolled rectification, which is simple and reliable, and the inverter is selected for three-phase PWM modulation (now the modulation algorithm is a space voltage vector). Because of the selection of a certain capacity of electrolytic capacitors, the DC bus voltage is stable, and 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 methods and hardware design, voltage inverters are widely used. The traditional current-type AC-DC inverter selects the natural commutation thyristor as the power switch, and its DC side inductance is more expensive, and is used in doubly-fed speed regulation, and the commutation circuit is required when the oversynchronous speed is over-synchronized, and the function is relatively poor under the condition of low slip frequency, and it is not used much in doubly-fed asynchronous wind power generation. The rectifier frequency conversion device of voltage type AC-DC inverter has the excellent characteristics of simple structure, less harmonic content and adjustable power factor of stator and rotor, which can significantly improve the operating state and output power quality of the doubly-fed generator, and the structure completely realizes the separation of the grid side and the rotor side through the DC bus side capacitor. The stator magnetic field directional vector control system of the voltage AC-DC inverter realizes the decoupling control of the active and reactive power of the generator based on the Z high 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 inverter topology. This structure is mainly connected in parallel through an AC-DC current type and an AC-DC voltage type inverter, the current type inverter is used as the main inverter for power transmission, and the voltage type inverter is used as an auxiliary inverter to compensate for the current type inverter harmonics. This structure has a lower switching frequency for the main inverter and lower switching current for the auxiliary inverter. Compared with the AC-DC voltage inverter mentioned above, this topology has low switching losses and the power of the whole system is relatively high. Its defects are also clear, the use of a large number of power electronic equipment leads to the rise of costs and more complex control algorithms, and the voltage utilization rate of other structures is relatively low.

3 Problems with AC-DC inverters

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

At that time, high-power and high-voltage power electronic equipment was in the development period, GTO components were facing elimination, and IGBT and IGCT were still to be matured;

The selection of IGCT (or GTO) and IECT converters is still a problem for the maintenance of straight-through short circuits caused by equipment failure; If a straight-through short circuit occurs in the power supply side converter, a short circuit will be formed in the power grid, so it is necessary to choose a high leakage resistance input transformer for the converter, which generally requires 15% or even up to 20%;

The overload of the AC-DC-AC inverter can be reduced when it runs at low frequency, and the overload of the inverter can be halved when it runs below 5Hz.

The voltage change rate of the output PWM modulation voltage waveform of the AC-DC-AC inverter is very high, which is easy to cause insulation fatigue damage of motors and electrical appliances. When the output wire is long, the common mode reflected voltage will generate 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 voltage of the central half;

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

The reason for the preferred DC mode in the intermediate link of the inverter - Reynolds

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