The differences between voltage type and current type inverter are mainly manifested in:
(1) the load of the buffer inverter of reactive power is an asynchronous motor, which is an inductive load, and in addition to transmitting active power between the intermediate DC link and the motor, there is also the exchange of reactive power. Because power electronic switching devices cannot store energy, the non-functional amount can only be buffered by energy storage elements (filters). The difference between the two types of inverters lies in what energy storage elements are used to buffer non-functional.
(2) There is a big difference between regenerative braking.
(3) The DC voltage Ua of the dynamic echo current inverter during speed regulation can quickly change the size and direction, so the dynamic response of the speed regulation system powered by it is relatively fast. In contrast, the voltage type inverter is slow to change the Ud by the charging and discharging effect of the dry filter capacitor, so its dynamic response is also slow.
(4) Scope of application: The voltage inverter is suitable for the irreversible speed regulation system and does not need to be often accelerated and decelerated, and is suitable for multi-motor transmission. The current type inverter is suitable for applications that require fast braking and reversible operation.
For voltage type inverters, the output voltage can be filtered through a large capacitor. The structure of the rectifier and the inverter are exactly the same. Its operating principle is: using the PWM control method, through the control of the input current of the rectifier, the waveform of the transmission current is approximately sinusoidal and in phase with the input voltage. When it is operating as a rectifier, it converts communication electrical energy into direct current electrical energy and provides it to the load; It can also be used as an active inverter, that is, the energy fed back by the load is converted into communication power and sent to the grid, where we call it a chopping rectifier or PWM rectifier.
It controls the size and phase of the communication current, on the one hand, so that the input communication current is close to the sine wave, and on the other hand, the power factor of the system is close to 1, and then greatly reduces the reactive weight and harmonic disturbance of the power grid. At the same time, when the motor decelerates and brakes from the inverter to return the regenerative power to make the DC voltage rise, the phase of the communication transmission current can be reversed from the phase of the power supply voltage, complete the regenerative operation and feed back the regenerative power to the communication power grid, and keep the DC voltage at a certain stable value. In this case, the chop-controlled rectifier operates in the active inverter state.
The primary effect of the inductance at the rectifier transmission end is to filter out the high-frequency current, and its inductance value is not large, so the voltage drop of the power frequency fundamental current on the inductor is very small relative to the grid voltage, when the load changes, in the process of power conditioning, the rectifier input fundamental voltage value and the grid voltage value are very close, and the change range of the modulation ratio is very small. Therefore, regarding the change of load, the DC terminal voltage stability can be completed by using the control method of constant modulation ratio and adjusting the phase angle to adjust the input power.
For voltage type inverters, the output voltage can be filtered through a large capacitor. Because the capacitance is relatively large, electrolytic capacitors are generally used. In order to obtain the required withstand voltage value and capacitance, the capacitors are often used in series and parallel according to the requirements of the inverter capacity. For current inverters, the DC intermediate circuit filters the output current through an inductor.
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