Design example of micro grid-connected inverter

①Design of micro grid-connected inverter
The following introduces a design example based on the series Nuozhen push-pull voltage micro-inverter. The circuit diagram is shown in Figure 1. The series-spectrum push-pull structure is mainly composed of two parts, namely, push-pull DC/DC converter and full-bridge DC/AC inverter, which are first boosted by the converter , And then carry out inverter and grid connection.

Design example of micro grid-connected inverter
Figure 1 Structure diagram of series resonant push-pull DC/DC boost circuit

In the figure, Upv is the voltage of the photovoltaic panel, the positive pole is connected to the middle tap of the transformer, and the negative pole is connected to the two ends of the transformer; the switching tubes S1 and S2 are controlled by PWM to turn on, and the PWM wave complementing the dead zone that controls the conduction of S1 and S2; VD1, VD2, VD3, and VD4 form a bridge rectifier circuit, which is used to rectify the output of the secondary side of the transformer into direct current.

In the example, a 500W micro-inverter is designed with relatively small power, so the inverter function can be realized with MOSFET. Generally, 600V, 30A MOSFET can be used to meet the requirements. The high-voltage DC power is converted into 220V, 50Hz AC power through a full-bridge inverter circuit. The inverter circuit is shown in Figure 2. Four MOS tubes form a full-bridge structure, C2, C3, R1, R2, VD1, VD2 form an RCD buffer circuit to absorb the overshoot voltage component generated when the MOS tube is switched. The buffer capacitor can reduce the loss of the resistance, and the diode suppresses the parasitic oscillation. C1 and C4 ensure the reliability of the switch and can effectively suppress the change of du/dt.

Design example of micro grid-connected inverter
Figure 2 Full-bridge inverter circuit diagram of RCD buffer mode

The entire DC/DC converter part is jointly controlled by a single-chip microcomputer and a PWM controller. The complementary PWM wave with dead zone that controls the conduction of S1 and S2 can be provided by the PWM controller TL494. The single-chip microcomputer provides the sine square of 2 times the grid frequency. The wave signal is used as the current given value in the PWM controller to displace the output current of the booster circuit. The output of the transformer passes through VD1, VD2, VD3, and VD4 to form an H-bridge circuit rectified to obtain a sine half wave of twice the grid frequency. In the inverter link, the single-chip microcomputer first monitors the phase of the external power grid, generates PWM waves according to the phase control of the external power grid, drives the power devices in the full-bridge inverter circuit as power frequency switches, and the inverter outputs 220V AC power with the same frequency and phase as the power grid.

The experimental waveform of the design example is shown in Figure 3. It can be seen from the figure that the grid voltage has obvious harmonics at the peaks and valleys. Due to the disturbance of the grid voltage, the output current waveform of the micro-inverter appears at the peaks and valleys. Obvious distortion, meanwhile, the waveform distortion at the zero crossing point is also more serious. The distortion of the waveform will affect the normal operation of the grid and the quality of the grid, so it is necessary to add feedforward control to the inverter.

Design example of micro grid-connected inverter
Figure 3 The output waveform and grid voltage waveform of the design example

The external power grid voltage is collected by the single-chip microcomputer, and the collected external power grid voltage value is multiplied by the sine coefficient to modulate the sine square wave signal of the given PWM controller. The given current signal is shown in 2 channels as shown in Figure 4.

Design example of micro grid-connected inverter
Figure 4 Given current signal waveform and grid voltage waveform

After adding feedforward compensation, the distortion of the output current waveform of the micro-inverter at the peak and trough is basically solved, and the waveform distortion at the zero-crossing point is also greatly improved. Because feedforward compensation is added, the current loop can quickly follow the grid, so that while observing the static characteristics, it also takes into account the dynamic changes. The waveform is shown in Figure 5.

Design example of micro grid-connected inverter
Figure 5 Output current and grid voltage after adding feedforward compensation

②Design of micro-connected and off-grid inverter
Careful observation will find that the main circuit topology used in the design example is basically the same as the main circuit topology of the push-pull off-grid inverter. Only on the DC output side, the off-grid inverter needs a DC support capacitor to ensure that the DC voltage is not Mutation occurred. The main circuit topology of the push-pull off-grid inverter is shown in Figure 6.

Design example of micro grid-connected inverter
Figure 6 Push-pull off-grid inverter

Although the circuit topology is basically the same, the control methods are quite different. The DC output side of the push-pull off-grid inverter is a stable 400V DC, so the current setting of the PWM controller is a constant value instead of a sine square wave Signal, the driving signal of the power device in the full-bridge inverter circuit is the SPWM wave instead of the power frequency switch. The main circuit of the micro-inverter can be modified by adding DC support capacitors and solid-state relays for switching, and switching between the micro-inverter on-grid and off-grid can be realized through a certain control method. The schematic diagram of the modified circuit is shown in Figure 7.

Design example of micro grid-connected inverter
Figure 7 Modified circuit of micro-inverter

When the micro-inverter is normally connected to the grid, the dimensional appliance RL1 in block 1 is disconnected, the DC support capacitors C1 and C2 are not connected to the main circuit, and the relay RL2 in block 2 is closed and connected to the grid through the interface J2 . If an abnormality of the grid is detected, firstly control the PWM controller to block and stop the inverter through the single-chip microcomputer to stop the entire device, and then control the relay RLI to pull in, so that the DC support capacitors C1 and C2 are connected to the main circuit; RL2 is disconnected to make the inverter output The 220V alternating current is connected to the load through the interface J1, which realizes the off-grid operation of the micro-inverter.