Design of the hottest photovoltaic inverter system

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With the growth of energy consumption, the deteriorating ecological environment and the improvement of human awareness of environmental protection, countries all over the world are actively looking for a sustainable and pollution-free new energy. Solar energy, as an efficient and pollution-free green new energy, and a substitute for conventional energy in the future, is particularly valued by people. The direct application of solar energy mainly includes photothermal conversion, photoelectric conversion and photochemical conversion. Photoelectric conversion (photovoltaic technology) is the most promising one

1. Working principle and circuit design of the system

the overall block diagram of the photovoltaic system is shown in Figure 1. After loading to a fixed load,

Figure 1 overall block diagram of the system

it can be seen from Figure 1 that the whole system includes two main links: charging and inverter. Solar cell is the basis of this system, and its efficiency directly determines the efficiency of the system

1.1 charging control part

(1) working characteristics of solar cells

as the basis of photovoltaic system, the working characteristics of solar cells, including working voltage and current, are closely related to sunshine and solar cell temperature. Figure 2 and figure 3 respectively show the relationship curve between working voltage, current and sunshine when the solar cell temperature is 25 ℃, and the curve between output power of solar cells and sunshine (s) and U

as can be seen from Figure 2, the power at any point on the curve is p=ui, which is not only related to u and I, but also related to sunshine (s), solar cell temperature, etc. It can be further seen from Figure 3 that since the working efficiency of the solar cell is equal to the ratio of the output power to the power projected on the area of the solar cell, in order to improve the working efficiency of the system, the solar cell must work at the maximum stable and reliable power point as far as possible, so that the solar cell with the lowest power can obtain the most power output. In Figure 2 and figure 3, points a, B, C, D and e respectively correspond to the maximum power points in different sunshine hours

Figure 2 working voltage, current and sunshine relationship curve

Figure 3 output power and sunshine relationship curve

(2) maximum power point tracking (MPPT) of solar cell

it can be seen from Figure 1 that the system first uses solar cell array to charge the battery, and stores solar energy in the battery in the form of chemical energy. In this process, the self seeking optimal control method is usually used to make the solar cell work at the maximum power point. The whole control process can be divided into two stages:

1) determine the output voltage UreF when the solar cell works at the maximum power point

2) change the charging current of the solar cell to the battery to stabilize the output voltage of the solar cell at UreF

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

1.2 inverter design

(1) inverter circuit design

sine wave inverter adopts single-phase full bridge circuit, and IGBT is used as the power device of the inverter circuit. IGBT is a voltage controlled device, which integrates the advantages of power MOSFET, bipolar crystal and more body tubes on the edge of forgings. It has the advantages of simple driving circuit, large voltage and current capacity, high working frequency, low switching loss, large safe working area and high working reliability. The inverter converts the DC voltage output by the battery into a SPWM wave with a frequency of 50Hz, and then converts it into a standard sine wave voltage of 220V through the filter inductor and power frequency transformer. In this way, the system structure is simple, and the high-order harmonic components in the waveform can be effectively suppressed

the working mode of the inverter adopts SPWM control mode, and the sinusoidal value of 0 ~ 360 ° is pre tabulated and pre stored in EPROM. The switching mode signal is generated by comparing the sine wave reference signal with a triangular carrier signal. It mainly has two types: unipolar and bipolar. When the switching frequency is the same, because the sinusoidal wave generated by bipolar SPWM control has more harmonic content and switching loss than unipolar, the unipolar SPWM control is adopted in this system

(2) control core

Figure 4 is the control block diagram of the system. The control chip 80C196MC is a real 16 bit single chip microcomputer launched by Intel in 1992 after MCS96. Its data processing ability is stronger and the execution speed of instructions is faster. In particular, the most distinctive three-phase waveform generator (WG) unit is integrated inside, which greatly simplifies the software and external hardware used for SPWM waveform generation, thus making the whole system structure simpler. In order to make the output signal and its complementary signal not effective at the same time, a dead band generator circuit is set inside the chip, so as to avoid the IGBT up and down on the same bridge arm, and protect the IGBT

Figure 4 control block diagram

(3) system voltage stabilization control

in order to provide voltage that meets the accuracy requirements, corresponding system voltage stabilization control methods must be adopted, and its control block diagram is shown in Figure 5

Figure 5 system voltage stabilization control block diagram

voltage stabilization control is realized by generating interruption in 80C196MC's on-chip and off-chip device - waveform generator (WG), in which the measurement of feedback voltage is completed at the time of interruption. Its control mode adopts a compound control mode combining feedback control and feedforward control. Moreover, based on the conventional digital PI regulator, this system proposes a piecewise variable coefficient PI regulator, that is, when the deviation of the system is large, the integral coefficient (KI) and proportional coefficient (KP) are large; When the deviation of the system is small, the integral coefficient and 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 also meets the performance requirements of scratch resistance, tackiness, stress whitening and shrinkage, as shown in Figure 6

Figure 6 topology structure of main circuit

2. Software design of the system

the system software adopts modular design, including main program module, WG module, PI regulation module and MPPT module

the main program module completes the initialization of the system, assigns initial values to each unit, judges whether there are running signals and judges various faults. At the same time, in order to avoid excessive peak current during startup, the system adopts soft start mode to make the output voltage rise to the given value in a slope

wg interrupt module mainly takes out the corresponding sinusoidal value from the sinusoidal table, and then sends it to wg-compx register, so as to obtain SPWM waves with different pulse widths

pi regulation module is mainly used to quickly stabilize the system output voltage to 220V when sudden load is applied

mppt module is mainly used to track the maximum power point of solar cells

3. Test results

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

Figure 7 voltage waveform under no load

figure 8 voltage waveform under load

4. Conclusion

experiments show that this design method is feasible. (end)

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