Performance comparison between voltage and current type inverters (I)

The current type and voltage type inverters are both AC-DC-AC inverters, which are composed of a rectifier and an inverter.

Since the load is generally inductive, there must be reactive power transfer between the power supplies. Therefore, in the intermediate DC link, components that buffer reactive power are required.

If a large capacitor is used to buffer the reactive power, it constitutes a voltage source type inverter; if a large reactor is used to buffer the reactive power, a current source type inverter is formed.

The difference between the voltage type inverter and the current type inverter is only in the form of the intermediate DC link filter, but in this case, the performance difference between the two types of inverters is quite large. The main performance list is as follows:

Energy storage components: voltage type inverter - capacitor; current type - reactor. [Brake unit]

The characteristics of the output waveform: the voltage waveform is a rectangular wave current waveform approximate to a sine wave; the current inverter is a current waveform is a rectangular wave voltage waveform is an approximate sine wave

The characteristics of the loop structure, the voltage type has a feedback diode DC power supply parallel high-capacity capacitor (low-impedance voltage source); current-type non-feedback diode DC power supply series large inductance (high-impedance current source) motor four-quadrant operation is easy.

Characteristic characteristics, the voltage type generates overcurrent when the load is short-circuited, and the open-loop motor may also operate stably; the current type can suppress overcurrent when the load is short-circuited, and the motor operation is unstable and requires feedback control.

The current-type inverter adopts a natural commutated thyristor as a power switch, and its DC-side inductance is relatively expensive, and is applied to the doubly-fed speed regulation, and a commutation circuit is required at the over-synchronous speed, and the performance is performed under the condition of low slip frequency. Also relatively poor.

The structural characteristics of the inverter:

1. The DC link of the current-type inverter is named after the inductance component. Its advantage is that it has four-quadrant operation capability, which can easily realize the braking function of the motor. The disadvantage is that the inverter bridge needs to be forced to commutate, the device structure is complicated, and the adjustment is difficult. In addition, since the grid side adopts thyristor phase shift rectification, the input current harmonics are large, and the capacity will have a certain impact on the grid.

2. The voltage type inverter is named after the capacitor element in the DC link of the inverter. It is characterized by the inability to perform four-quadrant operation. When the load motor needs braking, it needs to install the brake circuit separately. When the power is large, the output also needs to add a sine wave filter.

3. High-current inverter It adopts GTO, SCR or IGCT components in series to realize direct high-voltage frequency conversion. The current voltage can reach 10KV. Since the DC link uses an inductive component, it is not sensitive to current, so overcurrent faults are not easy to occur, the inverter works reliably, and the protection performance is good. The input side adopts thyristor phase-controlled rectification, and the input current harmonic is large. When the capacity of the inverter device is large, it is necessary to consider the pollution of the power grid and the interference to the communication electronic equipment. The voltage equalization and snubber circuit is technically complicated and costly. Due to the large number of devices, the device is bulky, and adjustment and maintenance are difficult. The inverter bridge adopts forced commutation, and the heat generation is also relatively large, and it is necessary to solve the heat dissipation problem of the device. It has the advantage of having four quadrant operating capability and can be braked. It should be specially noted that this type of frequency converter needs to install a high-voltage self-healing capacitor on its input and output side due to its low input power factor and high input-output harmonics.

4. The circuit structure of the high-voltage inverter adopts IGBT direct series technology, also called direct device series high-voltage inverter. It uses high-voltage capacitors for filtering and energy storage in the DC link, and the output voltage can reach 6KV. The advantage is that it can use lower-voltage power devices. All the IGBTs on the series bridge arm have the same function, which can be used as a backup or Redundant design. The disadvantage is that the number of levels is low, only two levels, the output voltage dV / dt is also large, the need for a special motor or a high-voltage sine wave filter, the cost will increase a lot. It does not have a four-quadrant operation function, and a brake unit needs to be installed separately for braking. Such a frequency converter also needs to solve the voltage equalization problem of the device, and generally needs to specially design a driving circuit and a buffer circuit. There are also extremely demanding requirements for the delay of the IGBT drive circuit. Once the turn-on and turn-off times of the IGBT are inconsistent, or the slopes of the rising and falling edges are too different, the power device may be damaged.