As an important part of the development and utilization of new energy, photovoltaic power generation has far-reaching significance for solving energy and environmental problems. Inverter is an important part of the photovoltaic power generation process. In this paper, the topological structure of the inverter system is studied, and the inverter system with high frequency booster and full bridge inverter is designed, and then combined with active filter, which plays a certain role in the further development and utilization of solar energy.

In the increasingly tense energy today, photovoltaic power generation technology is more and more attention. The direct current generated by solar cells and wind turbines needs to go through the inverter inverter and meet the requirements in order to be connected to the grid, so the design of the inverter is related to the reasonable, efficient and economic operation of the photovoltaic system.

First, the principle structure of photovoltaic inverter

The grid-connected PV inverter is mainly composed of a pre-stage DC/DC converter and a post-stage DC/AC inverter. The basic principle is to convert low-voltage direct current into high-voltage direct current through high-frequency conversion technology, and then get 220V alternating current through the power frequency inverter circuit. This structure has the advantages of simple circuit, small no-load loss of inverter power, large output power, high inverter efficiency, good stability and small distortion.

2. DC/DC control module

The DC/DC module is controlled by SG3525 chip. SG3525 is a double-ended output SPWM pulse width modulation chip, which generates PWM waveform with variable duty cycle and is used to drive the gate of thyristor to control the thyristor on and off, so as to control the output waveform.

As the key module of grid-connected inverter, DC/AC module has higher control requirements. This design uses TMS320F240 of TI company as the main control chip, which is used to collect grid synchronization signal, AC input voltage signal, and adjust the pulse frequency of IGBT gate drive circuit. Through the software phase-locked loop control technology based on DSP chip, Complete the frequency and phase control of grid-connected current, so that the output voltage meets the same frequency and phase relationship with the power grid voltage.

The filter adopts a second-order bandpass filter, which is a kind of active filter, which is used to transmit the signal of useful frequency band and suppress or attenuate the signal of useless frequency band. It can effectively filter out the high-frequency interference waveform generated after the inverter, so that the voltage waveform after the inverter can meet the requirements of grid connection.

SG3525 is a PWM control chip specially designed to drive N-channel power MOSFETs. The output driver of SG3525 is push-pull output form, which can directly drive the MOS tube. Internal contains undervoltage locking circuit, soft start control circuit, PWM latch, with overcurrent protection function, adjustable frequency, while limiting duty cycle. In order to improve the effective control of the push-pull DC/DC high frequency voltage boost process and improve the accuracy of bandwidth modulation, a detection circuit is designed accordingly to detect the output current and voltage, and then feed back to the control chip. The detection circuit includes magnetic bias detection circuit, voltage feedback sampling circuit and current feedback sampling circuit.

Third, DC/AC control module

3.1 TMS320F240 control core

TMS320F240 is a fixed-point digital signal processor chip made by TI company. The hardware architecture takes 16 bits as the basic data processing unit. It integrates high performance DSP core, has rich peripheral functions and has high processing speed. The peripheral circuit of DSP system includes clock circuit, reset circuit, power circuit, etc., with various signal detection circuit, drive circuit, to achieve the inverter system waveform control, pulse width modulation, fault protection and other requirements.

3.2 Voltage and current detection circuit

(1) power grid voltage zero detection circuit

After the inverter, the voltage of AC must be in the same phase and frequency with the grid voltage to be connected to the grid, so the output voltage must be phase-locked control. Since the output voltage signal is a sine wave, and the control chip can only recognize the TTL level signal, a circuit is needed to convert the sine wave signal into the TTL level signal that the control chip can recognize. In this design, LV25P voltage sensor is used to collect the grid voltage and convert it into a low voltage pulse signal equal to the grid voltage. After a group of comparator circuits, a group of low voltage square wave signals in the same phase with the grid voltage can be output. When the detected grid voltage exceeds zero, the output high level.

The square wave signal obtained by the grid voltage crossing detection circuit is sent to the capture pin of the DSP chip through the double Schmidt inverter circuit, and the capture unit triggers the interrupt when detecting the rising edge and performs phase lock.

(2) AC current detection circuit

Ac current detection circuit uses CSM300LT closed-loop current sensor, as shown in Figure 6.CSM300LT is a current sensor that applies the closed-loop principle of Hall effect to measure AC current under the condition of electrical isolation. When the alternating current passes through the sensor, the sensor converts the current signal into a voltage signal and sends it to the signal conditioning circuit, which is processed and input to the pin of the DSP chip. The conditioning circuit is composed of RC filter circuit and two groups of integrated operational amplifier isolation circuit.

Four, auxiliary power supply

The control circuit, signal acquisition circuit and switching tube drive circuit of the inverter need different adaptive power supplies, so an independent power supply is needed to supply power to it. The auxiliary power supply output voltage is +15V, -15V, +5V respectively. The single-ended flyback DC/DC topology is adopted, and the operation is stable and reliable.

5. Active filter

Active filter uses a second-order bandpass filter, which is a frequency-selection network composed of operational amplifiers and resistance-capacitance components to filter out the higher harmonics generated in the inverter process. The principle is to allow a signal in a passband range to pass through by setting circuit parameters, and the signals that are lower than the lower frequency of the passband and higher than the upper frequency are attenuated or suppressed.

Vi. Conclusion

Based on the basic principle and control strategy of grid-connected PV inverter, the structure of grid-connected PV inverter is designed. It adopts the front and back structure of built-in high frequency transformer, that is, the front stage DC/DC high frequency boost and the rear stage DC/AC power frequency inverter. The design mode has the advantages of simple circuit, stable performance and high conversion efficiency.

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