Design of Photovoltaic Inverter Power System

Keywords: Photovoltaic inverter power supply system, 80C196MC single-chip microcomputer, maximum power point tracking, SPWM control Chinese Library Classification Number: Document Identification Code: Article Number:

introduction

With the growth of energy consumption, the deteriorating ecological environment and the improvement of human environmental awareness, countries around the world are actively looking for a sustainable and pollution-free new energy source. As a highly efficient and pollution-free green new energy source and a future alternative to conventional energy, solar energy is particularly valued by people. The direct application of solar energy mainly includes three forms: photothermal conversion, photoelectric conversion and photochemical conversion. Photoelectric conversion (i.e. photovoltaic technology) is the most promising one.

1 System working principle and circuit design

The overall block diagram of the photovoltaic system is shown in Figure 1.

Figure 1 Overall block diagram of the system

As shown in Figure 1, the whole system consists of two main links: charging and inversion. Solar cells are the basis of the system, and their efficiency directly determines the efficiency of the system.

1.1 Charging control part

1.1.1 Working characteristics of solar cells

As the basis of photovoltaic systems, the working characteristics of solar cells, including working voltage and current, are closely related to sunshine and solar cell temperature. Figures 2 and 3 respectively show the relationship curves between working voltage, current and sunshine when the solar cell temperature is 25°C, and the curves between the output power of the solar cell and sunshine ( S ) and U.

As can be seen from Figure 2, the power at any point on the curve is P = UI , and its value is related to U and I as well as sunshine ( S ), solar cell temperature, etc. As can be further seen from Figure 3, since the working efficiency of the solar cell is equal to the ratio of the output power to the power projected onto the solar cell area, in order to improve the working efficiency of this system, the solar cell must be operated at the maximum power point as much as possible, so that the maximum power output can be obtained with the solar cell with the smallest possible power. In Figures 2 and 3, points A, B, C, D, and E correspond to the maximum power points at different sunshine times.

Figure 2 Relationship curve between working voltage, current and sunshine

Figure 3 Output power and sunshine relationship curve

1.1.2 Maximum Power Point Tracking (MPPT) of Solar Cells

As shown in Figure 1, the system first uses a solar cell array to charge the battery, storing solar energy in the form of chemical energy in the battery. In this process, a self-optimal control method is usually used to make the solar cell work at the maximum power point. The entire control process can be decomposed into two stages:

1 ) Determine the output voltage value U _ref when the solar cell operates at the maximum power point ;

2 ) Change the charging current of the solar cell to the battery so that the output voltage of the solar cell is stable at U _ref .

These two stages are realized by the control circuit through detecting the output voltage and current of the solar cell using the successive comparison method.

1.2 Inverter Design

1.2.1 Inverter circuit design

The sine wave inverter uses a single-phase full-bridge circuit and uses IGBT as the power device of the inverter circuit. IGBT is a voltage-controlled device that combines the advantages of power MOSFET and bipolar transistors. It has the advantages of simple driving circuit, large voltage and current capacity, high operating frequency, low switching loss, large safe working area, and high working reliability. The inverter converts the DC voltage output by the battery into an SPWM wave with a frequency of 50Hz, and then converts it into a standard sine wave voltage of 220V through a filter inductor and an industrial frequency transformer. This method has a simple system structure and can effectively suppress the high-order harmonic components in the waveform.

The inverter works in SPWM control mode, and the sine values ​​of 0 to 360 degrees are pre-tabled and stored in EPROM. The switching mode signal is generated by comparing the sine wave reference signal with a triangular carrier signal. There are two main types: unipolar and bipolar. Under the same switching frequency, the harmonic content and switching loss of the sine wave generated by the bipolar SPWM control are greater than those of the unipolar one, so this system uses unipolar SPWM control.

1.2.2 Control Core

Figure 4 is the control block diagram of the system. The control chip 80C196MC is a true 16-bit single-chip microcomputer launched by INTEL in 1992 after MCS96. It has stronger data processing capabilities and faster instruction execution speed. In particular, it integrates the most distinctive three-phase waveform generator (WG) unit, which greatly simplifies the software and external hardware used for SPWM waveform generation, making the entire system structure simpler. In order to prevent the output signal and its complementary signal from being effective at the same time, a dead zone generator circuit is set inside the chip, thereby avoiding the IGBT on the same bridge arm from being directly connected up and down, protecting the IGBT.

Figure 4 Control block diagram

1.2.3 System voltage control

In order to provide a voltage that meets the accuracy requirements, a corresponding system voltage regulation control method must be adopted. The control block diagram is shown in Figure 5.

Figure 5 System voltage regulation control block diagram

The voltage stabilization control is realized by generating an interrupt in the waveform generator (WG), an on-chip external device of the 80C196MC, in which the feedback voltage is measured during the interrupt. The control method adopts a composite control method combining feedback control and feedforward control. Furthermore, based on the conventional digital PI regulator, this system proposes a segmented variable coefficient PI regulator, that is, when the system deviation is large, the integral coefficient ( KI ₎ and the proportional coefficient ( KP ₎ are large; when the system deviation is small, the integral coefficient and the proportional coefficient are also small. Therefore, this control method can not only ensure the dynamic response speed of the system, but also meet a certain static voltage stabilization accuracy.

The complete main circuit topology is shown in Figure 6.

Figure 6 Main circuit topology

2 System Software Design

The system software adopts modular design, including main program module, WG module, PI adjustment module and MPPT module.

The main program module completes the system initialization, assigns initial values ​​to each unit, determines whether there is an operation signal and judges various faults. At the same time, in order to avoid excessive peak current at startup, the system adopts soft start mode to make the output voltage rise to a given value in a ramp.

The WG interrupt module mainly takes out the corresponding sine value from the sine table and then sends it to the WG-COMPX register to obtain SPWM waves with different pulse widths.

The PI regulation module is mainly used to quickly stabilize the system output voltage to 220V when a sudden load is added.

The MPPT module is mainly used to track the maximum power point of solar cells.

3 Test results

Based on the above control concept, a series of high-power prototypes have been successfully developed. For the 10kW prototype, its efficiency η≥85%, frequency accuracy ≤0.1%, output voltage accuracy ≤0.5%, and its no-load and loaded voltage waveforms are shown in Figures 7 and 8 respectively.

Figure 7 Voltage waveform when no load

Figure 8 Voltage waveform with load

4 Conclusion

Experiments have proved that this design method is feasible.