The hardware design of the string-type single-phase grid-connected inverter is introduced in three parts, namely the sampling circuit, the drive circuit and the phase lock circuit.
Ⅰ. Sampling circuit design
The sampling circuit of the string-type single-phase grid-connected inverter includes a voltage sampling circuit and a current sampling circuit.
(1) Voltage sampling circuit design
The voltage parameters that need to be collected include grid voltage, inverter output voltage, and DC bus voltage. The grid voltage and inverter output voltage are AC voltages, and the collection circuits are the same.
① AC voltage acquisition circuit The voltage sensor module model used for the acquisition of AC voltage is L.V25-P. Taking the grid voltage acquisition circuit as an example, the AC voltage acquisition circuit is shown in Figure 4.
In the AC voltage acquisition circuit, U0 and V0 are the input terminals of the grid voltage. The grid voltage is used as the input of the voltage sensor LV25-P after the parallel capacitor and series resistance. The output terminal of the LV25-P is output through the comparison circuit to output VAC1, VAC1 is the voltage amplitude The reduced sine wave, VAC1 needs to be input into the DSP through the A/D sampling circuit shown in Figure 5.
In order to ensure that the signal entering the DSP is positive, for the AC signal, it is necessary to add a certain amplitude of the pull-up voltage to convert the AC signal into a positive signal that meets the requirements of the DSP, and then process the collected signal in the program. Restore the signal. In the A/D sampling circuit in Figure 5, a +3.3V pull-up voltage is added to one end of the resistor R: to convert the collected AC voltage into a positive signal that meets the DSP requirements.
② DC bus voltage acquisition circuit The DC bus voltage acquisition circuit is shown in Figure 6.
In the DC bus voltage acquisition circuit, DCH is the DC bus voltage. The voltage divider circuit composed of R22, R78 and R79 outputs a DC voltage DCH-1 with a lower amplitude. Considering that the DC side voltage is about 400V, it is added to the DSP The pin voltage is up to +3.3V, so the resistance value of R22 and R78 is selected as 200kΩ, and the resistance value of R79 is selected as 3kΩ. After DCH-1 passes through the operational amplifier LM324 and the optocoupler HCNR201, the output voltage VDCH. VDCH needs to pass As shown in Figure 7, the A/D sampling circuit is input to the DSP.
Because the DC signals are all positive values, the difference between the DC signal A/D sampling circuit and the AC signal A/D sampling circuit is that the DC signal A/D sampling circuit does not have a pull-up voltage.
(2) Current sampling circuit design
The system needs to collect grid current and load current. The current sensor model used is HKC100BR. The output current signal is output by the follower circuit, and then input to the DSP through the sampling circuit on the control board. The current acquisition circuit is shown in Figure 2-27.
In Figure 8, Hall1 is the output of the current sensor, and IAC1 needs to be input to the DSP through the AC signal A/D sampling circuit.
Ⅱ. Drive circuit design
The inverter module of the string-type single-phase grid-connected inverter uses the intelligent power module IPM, which is a module that integrates IGBTs and peripheral circuits. It is small in size and has peripheral protection circuits. After selection and analysis, the Mitsubishi intelligent power module PM50RLA120 is used as the power module of the inverter, with a maximum current of 50A and a voltage of 1200V, which can meet the requirements of inverter boost and inverter.
In a single-phase inverter, four PWMs are needed to drive the IPM for inversion. Set the four PWM drive signals as Up, UN, Vp, and VN. Take the Up signal as an example. The hardware circuit is shown in Figure 9.
In Figure 9, PWM7 and PWM8 are PWM waves output by DSP, the photocoupler model is 6N137, DCVccl is +15V, and Up is a PWM signal with an output amplitude of +15V.
Ⅲ. Phase lock circuit design
The string-type single-phase grid-connected inverter needs to phase-lock the external grid voltage. Phase-locked control is to first calculate the phase difference between the inverter output voltage and the grid through a mathematical method, and then the inverter output voltage is calculated according to the phase difference The frequency is adjusted.
The phase of the inverter output voltage is detected by a zero-crossing detection circuit, which is implemented by a zero-crossing comparator. The waveform diagram of the sine wave zero-crossing detection circuit is shown in Figure 2-29. The implementation process is as follows: the sinusoidal alternating current passes through the zero-crossing detection circuit and outputs a square wave signal, and the rising or falling edge of the square wave signal is set as the zero-crossing point of the sinusoidal alternating current, and then the DSP microprocessor is used to capture the rising or falling edge. Obtain the zero-crossing point of the sinusoidal alternating current to determine the voltage phase.
Sine curve
The rising edge of the square wave corresponds to the zero-crossing point of the sine wave
Phase detection square wave outlet
The zero-crossing detection circuit is shown in Figure 10. In the figure, the external power grid voltage is first sampled, and the collected external power grid signal passes through the comparison circuit and then outputs a square wave. The rising or falling edge of the square wave signal is set as the zero-crossing point of the sinusoidal alternating current, where Ocrossl is the output square wave Signal.