# Circuit principle of off-grid three-phase inverter

The capacity of single-phase inverter circuit is generally below 10kVA due to the limitation of the capacity of the power switching device, the current of the neutral line (neutral line), the load balance requirements of the power grid and the nature of the electrical load. Most of the large-capacity inverter circuits are in the form of three-phase. Three-phase inverters are classified into three-phase voltage-type inverters and three-phase current-type inverters according to the nature of the DC power supply.

(1) Three-phase voltage inverter

The voltage source inverter is that the input DC energy in the inverter circuit is provided by a stable voltage source. Its characteristic is that the amplitude of the output voltage of the inverter is equal to the amplitude of the voltage source during the pulse width modulation, and the current waveform depends on the actual load impedance. The basic circuit of the three-phase voltage inverter is shown in Figure 1. The circuit is mainly composed of 6 power switching devices, 6 freewheeling diodes and a DC power supply with a neutral point. The loads L and R in the figure represent the phase inductances and phase resistances of the three-phase loads.

Under the action of the control circuit, the power switching devices Q1~Q6, when the control signals are three pulse signals with a difference of 120° from each other, each power switch device can be controlled to be turned on by 180° or 120°, and the turn-on times of two adjacent switch devices are different from each other by 60°. The upper and lower switching devices of the three bridge arms of the inverter are alternately turned on and off at 180° intervals,and Q1~Q6 are turned on and off sequentially with a phase difference of 60°, forming a, b, and c three-phase voltages at the inverter output.

The switch control signal output by the control circuit can be a square wave, a staircase wave, a pulse width modulated square wave, a pulse width modulation triangular wave, a pulse width modulation sawtooth wave, and the like. The last three PWM waveforms all use the fundamental wave as the carrier wave, the sine wave as the modulating wave, and finally output the sine wave waveform. The difference between an ordinary square wave and a square wave modulated by a sine wave is shown in Figure 2. Compared with the ordinary square wave signal, the modulated square wave signal is a series of square wave signals that change according to the sine wave law, that is, the ordinary square wave signal is continuously turned on, and the modulated square wave signal is turned on and off N times within the period of the sine wave modulation.

(2) Three-phase current source inverter

The DC input power supply of the current mode inverter is a constant DC current source, and it is the current that needs to be modulated. If a rectangular current is injected into the load, the voltage waveform is generated under the action of the load impedance. In a current source inverter, there are two different ways to control the magnitude of the fundamental current. One method is the amplitude variation method of the DC current source, which makes the current control of the AC output side relatively simple; the other method is to use the pulse width modulation to control the fundamental current. The basic circuit of the three-phase current source inverter is shown in Figure 3. The circuit is composed of 6 power switching devices, 6 blocking diodes, DC constant current power supply, surge absorbing capacitor, etc. R is the electrical load.

The characteristic of the current-mode inverter is that a large filter inductor is connected to the DC input side. When the load power factor changes, the waveform of the AC output current remains unchanged, that is, the AC output current waveform has nothing to do with the load. From the circuit structure point of view, different from the voltage type inverter, the voltage source inverter connects a freewheeling diode in parallel with each power switching device, while the current source inverter connects a reverse blocking diode in series with each power switching device.

Like the three-phase voltage-type inverter circuit, the three-phase current-type inverter is also composed of three sets of upper and lower pairs of power switching devices, but the switching method is different from that of the voltage type. Since a large inductance L is connected in series on the DC input side, the fluctuation of the DC current is small, and the stability and continuity of the current can be maintained when the power switching device switches and switches. Therefore, one of the upper switching devices Q1, Q3, and Q5 and one of the lower switching devices Q2, Q4, and 06 in the three bridge arms can respectively flow a certain value of current every 1/3 cycle, and the output current waveform is a square wave with a height of 120° of the current value during energization. In addition, in order to prevent the surge voltage caused by the sudden change of the current when the inductive load is connected, a surge absorbing capacitor C is connected in parallel with the output end of the inverter.

The DC power supply of the three-phase current source inverter, that is, the DC current source, is realized by using a variable voltage power supply through current feedback control. However, only using current feedback can not reduce the fluctuation of the inverter input voltage caused by the switching action, and the current fluctuates with it, so a large inductance (reactor) L is connected in series at the power input end.

Current-mode inverters are very suitable for application in grid-connected systems, especially in solar photovoltaic power generation systems, current-mode inverters have unique advantages.

Further reading: Circuit principle of off-grid single-phase inverter.