Design of maximum power point energy storage and inverter for photovoltaic power generation

With the increasing tension of fossil energy in the world and the damage to the environment caused by the use of fossil energy, new energy led by solar energy has been paid much more attention by countries around the world. If the efficiency of solar energy is improved, it will bring breakthrough changes to the world’s energy structure. Based on the related applications of solar photovoltaic power generation, this paper designs an independent photovoltaic power generation system with energy storage device. In the form of DC/DC conversion, the system uses the maximum power point tracking technology of photovoltaic cells to realize the efficient use of solar energy during the charging process. The system inverts the DC voltage after the storage, realizing the transformation of DC voltage to 220 V AC voltage, to realize the utilization of energy in various aspects.


Introduction
With China's slogan of "carbon neutral, carbon peak" forward, solar energy will surely set off a new wave of use in the future national and even global energy market.One way of solar power generation is solar luminescence, which refers to a power generation method that converts light energy into electrical energy without a thermal process.It includes photovoltaic power generation, photochemical power generation, photoinduction power generation, and photobiological power generation.Photovoltaic power generation is a direct power generation method that uses solar-level semiconductor electronic devices to effectively absorb solar radiation energy and convert it into electrical energy and is the mainstream of solar power generation today.In photochemical power generation, there are electrochemical photovoltaic cells, photoelectrolysis cells, photocatalytic cells, and photovoltaic cells are used at present.At the end of June 2023, China's installed photovoltaic power generation capacity of about 470 million kilowatts, an increase of 39.8%; New PV installed capacity increased by 154% year-on-year.However, there are still many problems in photovoltaic power generation, among which the most urgent problem is the inefficiency of energy use.In the case of different temperatures and different light intensities, there is no shadow occlusion and a significant difference in the utilization of solar energy in photovoltaic systems.Such a difference makes the output power of the photovoltaic power generation system can not be stabilized at a high power point, which is also an important external reason for the inefficiency of the power generation system.
Therefore, this paper studies and designs a device to improve the efficiency of solar energy utilization (hereinafter referred to as the "system"), and the design principle is as follows: When the IOP Publishing doi:10.1088/1742-6596/2771/1/012018 2 photovoltaic panel outputs current and voltage (about 20 V) to charge the battery (12 V) and the use of Maximum Power Point Tracking (MPPT) technology, so that the charging device work at the maximum power point, to improve the system of solar energy utilization efficiency.Photovoltaic power generation uses MPPT technology so that the generated energy is stored in the energy storage device.After that, the system uses the full-bridge inverter technology to convert the output DC voltage of the energy storage device into AC voltage, which increases the AC power consumption of the system from the simple DC power consumption, and plays a good role in promoting the development and utilization of the photovoltaic system [1].

MPPT technology principle
MPPT is an electrical technology that helps the photovoltaic panel output more power by adjusting the working state of the electrical module, and the direct current output of the photovoltaic panel is effectively stored in the battery.
As shown in Figure 1, the disturbance resistance R and the metal oxide semiconductor type field effect tube MOSFET (hereinafter referred to as "MOS") are connected.Under the condition that the output voltage is stable, the duty cycle of the MOSFET can be changed to change the average current through the resistance, resulting in a current disturbance.At the same time, the output current and output voltage of photovoltaic cells are affected.At this time, the measurement results of the current change determine the direction of the disturbance in the next cycle.By calculating the product of current and voltage (i.e.power) after the disturbance, if the power increases, it indicates that the direction of the disturbance is correct, and the disturbance continues in this direction in the next cycle.Otherwise, the disturbance is in the opposite direction.After this repetition, the output power of the photovoltaic panel eventually reaches its maximum [2][3][4].

Structure of system
The system is composed of two parts, one is the charging module, and the other is the inverter module.

Charging module
The main function of the charging module is to realize the photovoltaic cells using MPPT technology charge the battery.As shown in Figure 2, the charging module is composed of photovoltaic cells, a DC/DC conversion circuit, a battery, a MOSFET driver circuit, a sampling circuit, and a main controller (STM32 series chips), among which the DC/DC conversion circuit adopts BUCK topology.
The working process of the charging module is as follows: After the photovoltaic cell generates voltage and current and is depressurized by the BUCK circuit, the 12 V battery is charged.In the meantime, the sampling circuit collects and inputs the voltage and current generated by the photovoltaic cell and the voltage and current charged by the battery to the main controller, which controls the photovoltaic cell to always work at the maximum power point according to the compiled MPPT program.

Inverter module
As shown in Figure 3, the main circuit structure of the inverter module is composed of three parts, which are a push-pull boost structure, a bridge rectifier structure, and a full bridge inverter structure.The main function is to convert the 12 V DC voltage output by the battery into 220 DC voltage after the push structure circuit and full bridge rectifier circuit, and then convert it into power frequency sine AC voltage after the full bridge inverter structure.1) Front booster circuit.The booster circuit designed in this paper adopts a push-pull circuit topology to boost 12 V DC voltage to 220 V AC voltage and then realizes AC/DC conversion with a full wave rectifier circuit to convert 220 V AC voltage to 220 V DC voltage.That is, 12 V DC voltage is boosted into AC voltage by a push-pull structure and then rectified into 220 V DC voltage by a full wave rectifier circuit.
2) Rear inverter circuit.The input voltage of the rear inverter module is 220 V DC voltage, and the input voltage is high.Therefore, the push-pull topology and half-bridge inverter structure is not adopted, and the full-bridge topology is adopted, that is, the voltage is eventually changed to 220 V AC voltage.
3) The KA7500 pulse width modulation chip is used for both front and rear circuit main control chips.

Charging module circuit
The circuit design of the charging module mainly involves the calculation of inductance and capacitor of BUCK topology, the selection of diode and main control switch, and the design of voltage and current collecting circuit and driving circuit.(1) Short circuit current Isc = 2.85 A, considering that the circuit has a maximum 20% ripple, calculate the ripple current ǻi according to Formula (2).

Capacitor calculation of BUCK topology circuit.
To stabilize the output voltage, it is necessary to increase the capacitance, but the increase in the capacitance value will be accompanied by the increase in the volume, so this paper calculates the required capacitance according to the allowable output voltage ripple.The ripple voltage ǻu is calculated as (4).

Continuation diode and main switch tube selection.
Due to the operating frequency of the BUCK circuit f = 100 kHz, it is necessary to choose a fast recovery diode that can withstand the maximum voltage.According to the calculation, the FGA25N120ANTDTU power diode manufactured by Ammason manufacturer can meet the requirements.
According to the basic characteristic parameters of the system, the open circuit voltage of the photovoltaic cell UOC = 21 V; As a main circuit switching tube, the MOSFET can withstand at least 2.5 times the open circuit voltage.Considering the instantaneous peak of the BUCK topology circuit, it is necessary to increase its withstand voltage value.After selection, the switch tube model NP100P06PLG made by NEC manufacturer can meet the requirements.

Current acquisition circuit design.
Two currents need to be collected, namely, the current output by the photovoltaic array and the current charging the energy storage device.Current acquisition is calculated by collecting the voltage value of a specific resistor and combining Ohm's law I=U/R.The current sampling circuit design of photovoltaic array output is taken as an example to introduce the current sampling circuit, and the rest is similar.As shown in Figure 4, Upv represents the voltage to be collected, and the Upv value is small.Because the controller precision is sometimes difficult to collect, the Upv is amplified by the operational amplifier and collected by voltage amplification.The amplified voltage is U_pv in the figure, and the specific magnification can be set according to needs.The specific calculation process is as follows: The result is: R97 is to prevent damage to the temporary resistance used during the test after power-on.After the test is correct, it is necessary to remove the R97 in the circuit board and short-circuit both ends of the R97 resistance with the solder.As can be seen from the figure, there are two resistors, R58 and R59, that have no resistance value.In the actual design circuit, the PCB does not weld these two resistors, and the positions of these two resistors are reserved to reduce the amplification by welding resistors in parallel with the original resistors, R95 and R96 when the UPV amplification exceeds the sampling range of the ADC after 100 times amplification.
where U is the voltage input to the operational amplifier and is an intermediate variable, which is used to obtain the relationship between the voltage U_PV and UPV of the input STM32 [2].

Voltage acquisition circuit design.
The maximum voltage collected by ADC of the STM32 main control chip is 3.3 V, while the voltages of photovoltaic cells and batteries are both greater than this value, so voltage collection requires sampling in the form of resistance partial voltage, as shown in  4.1.6.Driver circuit design.The driver circuit adopts the optocoupler chip of the TLP250 model as the main body, and checking the characteristic parameters of the chip can meet the use requirements of the system.The main structure of the drive circuit is shown in Figure 6.The driving principle is that chip V0 inputs PWM waveform voltage to the controller and outputs the voltage that can drive the MOSFET on the ANODE through optocoupler isolation.12 V_3 is connected to the positive voltage terminal of the energy storage circuit.

Inverter module circuit
Both the front and rear stages of the inverter module use KA7500B as the main control chip, and its pin function is shown in Figure 7.The following description requires reference to the chip pins in Figure 7.The pin functions are as follows: VCC: working voltage input; C1, C2: final output triode collector; E1, E2: final output triode emitter; EA1+, EA1-: in-phase and inverse-phase outputs of error amplifier 1; EA2+, EA2-: in-phase and anti-phase outputs of error amplifier 2; VREF: reference voltage output; OUTPUT CONTROL: output control, grounding for parallel single-terminal output, high level for push-pull output; COMP: The specific working process is as follows: In the push-pull mode of KA7500B chip, if Pin 10 is high level, Pin 9 is low level, then Q1 gate is high level, and Q1 is switched on according to the working principle of MOSFET; If Pin 10 outputs a low level (0 V), the junction capacitance of the MOSFET Q1 is discharged, the emitter voltage of Q7 is raised and switched on, and the gate potential of Q1 is pulled to a low potential and cut off.The operating process of the triode Q8 and MOSFET Q2 is the same as the above process [6].According to [6], in Figure 9(a), the control signal PWM5 and control signal PWM6 are connected to the MOSFET gate of S5 and S6 of the full-bridge topology of the inverter circuit in Figure 3, respectively.Pins 9 and 10 of the rear driver chip KA7500B are connected to the "ground".The input power end of the KA7500B chip (Figure 7 Pin 12) is connected to the VCC_2 end of the drive circuit to ensure the ongoing of S5 and S6.
The MOS in the full-bridge inverter circuit is N-type.When there is a pressure difference between the G pole and the S pole, the MOSFET will be on.In Figure 9(b), PWM3 and PWM4 are connected to the S3 and S4 grids of the full-bridge topology of the inverter circuit after Figure 3; Q3S and Q4S are connected to the S3 and S4 sources of the MOSFET, respectively.In Figure 9 (b), the rear-drive circuit is connected to the control chip KA7500B Pin 12, namely the power input pin, to ensure the conduction of S3 and S4.The Q13 and Q14 emitters are connected to the "ground" to ensure the shutdown of Q3 and Q4.4.2.3.High-frequency transformer design.The high-frequency transformer plays the role of electrical isolation in the front booster circuit.According to the device parameters and circuit structure of the system, the transformer has 1 primary winding, and 2 secondary windings are used according to the needs of the project.Among them: one outputs about 12 V AC voltage, filtered to provide voltage for other power conversion chips on the circuit board; The other is for the post-inverter.Transformer design needs to go through the selection and calculation of core, skeleton, and winding methods.The process is not to be described here.After calculation and analysis, soft magnetic ferrite material is selected as the core material, and the core of the EI33 structure is selected.According to [7], after calculation: The magnetic core parameters of the EI33 are Ap=1.5854cm 4 >Ap1=1.49cm 4 , which meets the requirements.
where Ap is the product of the core area, Ae is the effective cross-sectional area of the core, and Aw is the window area of the core.
The ratio of the number of turns at the input end and the number of turns at the output end of the transformer needs to be determined according to the voltage ratio of the input and output, according to the formula of the number of turns on the original side: where: V is the input voltage of the primary side winding of the transformer, Ae is the cross-sectional area of the core center column, f is the working frequency of the transformer, and Bm is the magnetic induction intensity of the core.The final transformer pin figure is shown in Figure 10.

Design of data acquisition system
The data acquisition system uses LabView as the upper computer platform and transmits data to the circuit board by USB communication. Figure 11 shows the main interface of the data acquisition part.The input voltage, output voltage, real-time duty ratio, real-time power, and other parameters can be seen through the upper computer platform, and the baud rate of the system can be set by manually entering the baud rate position as shown in Figure 11, and the start and end of the system can be controlled by the start and stop buttons.

Conclusion
As a pollution-free secondary energy source, solar energy is bound to play an increasingly important role in the future energy system.Photovoltaic power generation, as a way to effectively use solar energy, will also attract more and more attention.Starting from the practical application of photovoltaic power generation, this paper designs an independent photovoltaic power generation system with an energy storage device and inverter device, which effectively solves the problem of low efficiency of photovoltaic cells, and realizes the multi-purpose function of photovoltaic power generation with energy storage device and inverter device.This paper mainly designs the hardware system, and for the software algorithm design, I hope more readers can provide better solutions.

3 Figure 2 .
Figure 2. Structure block diagram of system charging module.

Figure 3 .
Figure 3.The main circuit structure of the inverter module.

6 Figure 5 .
Figure 5. Capacitor C plays a filtering role to ensure the stability of the collected voltage signal.Adjusting the proportional relationship between the sampling resistance and the partial voltage resistance can satisfy the voltage collection range.
amplifiers 1 and 2 to compensate for voltage feedback signals; D.T: Dead zone time control; CT, RT: CT external oscillation capacitor, RT external oscillation resistance, for the oscillation frequency control end; GND: Ground.4.2.1.Front drive circuit design.According to the pin function of the KA7500B chip introduced in [5], the front drive circuit shown in Figure 8 is designed.Pin 9 and Pin 10 in the figure are respectively connected to Pin 9 and Pin 10 of the chip in Figure 7.

Figure 8 .
Figure 8. Front drive circuit.4.2.2.Rear drive circuit design.Figure 9 is the driving circuit diagram of the rear stage, Figure 9(a) is the driving circuit of the rear stage inverter S5 and S6 in FIG. 3, and Figure 9(b) is the driving circuit of the rear stage inverter S3 and S4.

Figure 11 .
Figure 11.The main interface of the data acquisition system.