Research and design of household multifunctional photovoltaic power generation system

1 Introduction

As environmental pollution, ecological damage and resource depletion become increasingly serious, countries around the world have been competing to implement sustainable energy policies in recent years, among which the use of solar cell power generation has attracted the most attention. Solar cell power generation has the unique advantages of being safe and reliable, pollution-free, fuel-free, without mechanical rotating parts, low failure rate, and easy maintenance. In particular, it can be combined with buildings to form a photovoltaic rooftop power generation system. Therefore, it has been highly valued and strengthened by various countries [5]. With the popularization of applications, there will be more and more household photovoltaic power generation devices. Household photovoltaic power generation devices must not only meet the needs of users, but also be reliable and safe. Based on this, this paper studies and designs the dual functions of independence and grid connection. The experimental results prove the feasibility of the design scheme.

2 System Structure

The inverter system converts direct current into alternating current. Its core is the inverter circuit, which completes the inverter function by turning on and off the power electronic devices. The turning on and off of the power electronic devices requires appropriate control signals. According to the actual needs of the system, the inverter system designed in this paper mainly consists of the main circuit, control circuit, protection circuit, communication circuit, auxiliary power supply, input filter, output filter and other parts. The basic structural block diagram of the inverter system is shown in Figure 1. The control core uses the TMS320F2812 DSP chip of TI.

3 Main circuit structure and parameter design

The main circuit structure of the inverter is varied. According to the control target of this system, a single-phase full-bridge main circuit structure with an industrial frequency isolation transformer is adopted. An anti-reverse diode and a current limiting resistor are added to the input end. The main circuit schematic is shown in Figure 2.

When working in independent inverter mode, LC filtering is used; when the inverter works in grid-connected mode, L filtering is used to reduce the impact of capacitor filtering on the phase, and the capacitor C is disconnected through a switch.

Considering factors such as capacity and frequency, the switch tube of the main circuit of the system is selected as power MOSFET. Among them, the selection of filter inductance should filter out the high-order harmonic components of the modulation wave as much as possible to improve the quality of the output waveform. The high-frequency impedance of the filter inductor cannot be too low compared with the high-frequency impedance of the filter capacitor, that is, the inductance value of the filter inductor cannot be too small. In order to meet the output waveform quality, it is required that the maximum change of the inductor current in a sampling cycle is less than the allowable inductor current ripple △ILfmax. The function of the filter capacitor is to filter out the high-order harmonics in the output voltage together with the filter inductor, thereby improving the waveform of the output voltage. The larger the filter capacitor, the smaller the THD value of the output voltage. However, from the circuit point of view, when the output voltage remains unchanged, increasing the filter capacitor will increase the current of the filter capacitor, increase the reactive energy of the inverter, increase the loss, and reduce the efficiency. Therefore, the filter capacitor should not be too large. Therefore, the principle of selecting the filter capacitor is to make the value as small as possible while ensuring that the THD value of the output voltage meets the requirements. At the same time, CBB capacitors with good high-frequency characteristics and small losses should be used as much as possible [4]. The power device switching frequency of the inverter designed in this paper is 15kHz, and the design cutoff frequency fC is 2kHz. Considering the system margin, after calculation and comprehensive consideration, the filter inductor is 9mH and the filter capacitor is 3μF.

4 Control circuit structure and control strategy

The control circuit mainly includes four parts: signal detection circuit, drive circuit, protection circuit and communication circuit, as shown in Figure 3.

The control strategy mainly adopts PI control. Among them, the independent inverter adopts a dual closed-loop control scheme of voltage average value outer loop and voltage instantaneous value inner loop to achieve stable voltage output; the grid-connected inverter adopts CVT type maximum power point tracking, and generates the reference current of the current inner loop through real-time voltage tracking. The current inner loop uses instantaneous value feedback to achieve tracking control of the grid-connected current and realize the feeding of solar energy into the grid.

4.1 Independent inverter control

The independent inverter adopts a dual closed-loop control system with voltage average value outer loop and voltage instantaneous value inner loop feedback. The control block diagram is shown in Figure 4. Among them, the voltage average value outer loop regulator is PI regulation, and the voltage instantaneous value inner loop regulator is P regulation. The error between the output voltage average value feedback value Uf and the voltage given signal Ug is formed by the PI regulator to form the amplitude given of the voltage inner loop, and then multiplied by the discrete sine table data to form a discrete sinusoidal voltage signal as the given of the voltage instantaneous value inner loop. The error signal between the voltage instantaneous value given value and the feedback value is then generated by the P regulator to generate a PWM control signal, which is written into the comparison registers CMPR1 and CMPR2 inside the DSP. After comparison with the triangular carrier, 4 PWM1~PWM4 switching signals are generated to control the on and off of the power devices in the main circuit. The generated high-frequency SPWM signal is filtered by the output LC filter to generate a standard sinusoidal output voltage, and then boosted to 220V/50Hz by the step-up transformer to ensure the stability of the output voltage.

[page]4 Independent inverter control block diagram

4.2 Grid-connected inverter control

(1) Solar photovoltaic grid connection

The grid-connected inverter adopts a DC voltage outer loop and a grid-connected current inner loop control strategy. Among them, the DC voltage outer loop uses a PI regulator to achieve maximum power point tracking of the solar photovoltaic module, and its output is the amplitude of the grid-connected current. The system first detects the grid voltage frequency and phase, and makes the grid-connected current and the grid voltage in phase through a phase-locked link. The grid-connected current given value is multiplied by the discrete sine table data as the grid-connected current given value, and the current inner loop regulator uses a P regulator. The output value of the P regulator is written into CMPR1 and CMPR2, and after comparison with the triangular carrier, 4 PWM1~PWM4 signals are generated to control the conduction and shutdown of the power devices in the main circuit. The high-frequency SPWM signal generated is filtered by the inductor L to generate a standard sinusoidal grid-connected current in phase with the grid voltage. After filtering by the inductor L, the grid-connected current with the same frequency and voltage is input to the grid. The control block diagram of the grid-connected inverter is shown in Figure 5.

(2) Battery grid connection

In order to feed back the excess energy in the battery to the grid, the system must be operated in the battery grid-connected state. In this state, the grid-connected current is determined by the battery discharge curve [8]. In order to reasonably protect the battery and prevent excessive discharge current and battery over-discharge, this paper feeds back the battery energy to the grid by real-time sampling of the battery terminal voltage and discharge current. The battery grid-connected control block diagram is shown in Figure 6.

5 Communication part

The communication part is mainly used to complete the system status display and parameter setting. In this system, the host computer uses the 8-bit microcontroller PIC16F877A produced by Microchip. Its serial port communication with TMS320F2812 adopts RS-485 communication protocol, and two MAX485 chips are used to realize data exchange between the two. The communication principle diagram is shown in Figure 7.

6 System Software Design

The system software adopts modular design, mainly including four parts: main program, timer interrupt program, capture interrupt program, power protection interrupt program. Among them, the main program mainly detects whether the operation status of the device is normal and the command sent by the host computer, and waits for the arrival of the interrupt; the timer interrupt is mainly used to complete AD detection and SPWM generation; the capture interrupt is mainly used to complete the phase-locking purpose in the grid-connected inverter to ensure that the grid-connected current is synchronized with the grid voltage.

7 Experimental results and conclusions

When the inverter is in independent inversion, with resistive load, the output power is about 210W, and the inverter output voltage and current waveforms are shown in Figure 8. When the inverter is in grid-connected mode, the grid-connected current and grid voltage waveforms are shown in Figure 9. In the figure, the purple is the grid voltage waveform, and the green is the grid current waveform. The two have the same frequency and phase, achieving the unity power factor of the grid.

As shown in FIG8 , when the inverter works in an independent inverter state, it can output an ideal sinusoidal voltage waveform; as shown in FIG9 , when the inverter is connected to the grid, the output current is basically in the same frequency and phase as the grid voltage, achieving a unity power factor when connected to the grid.

Notes

Anhui Provincial Science and Technology Department's annual key project: Application and development of household solar photovoltaic power supply (06022010);

This work was supported by the National Natural Science Foundation of China (Project No. 50707003).

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