Research and design of high-efficiency and high-performance inverter for photovoltaic water pumps

Nowadays, with the massive increase in the consumption of conventional energy such as oil and coal, the deteriorating ecological environment has forced countries around the world to actively seek a new sustainable energy path. Clean energy such as solar energy, wind energy, and geothermal energy has gradually attracted human attention, and among them, solar energy is undoubtedly in the most prominent position. Now, in remote areas far away from the power grid such as Northwest my country, Tibet and Inner Mongolia, many people cannot drink clean drinking water, and these areas are also very rich in solar energy resources. Therefore, the development of photovoltaic water pump technology in these areas has obvious social and economic benefits. However, most of the ordinary inverters sold on the market are used in this system. They cannot realize various protection functions well, and they do not have the maximum power point tracking function of solar cells, resulting in a huge waste of solar cell capacity. This article introduces a high-efficiency and high-performance inverter dedicated to photovoltaic water pumps.

1 Composition of photovoltaic water pump system

The composition of the photovoltaic water pump system is shown in Figure 1.

Figure 1 Composition of photovoltaic water pump system

The main input parameter of this system is the sunshine intensity (Φ), the output parameter is the water flow rate (θ), and the total efficiency of the system is η=Hθ/Φ, where H is the head; the MPPT and voltage converter parts are the objects studied in this system.

As shown in Figure 1, the photovoltaic array is the energy input end of the system. When the sunshine intensity Φ is constant, its maximum output power is also a constant value. One of the main functions of the inverter developed by this system is to make the photovoltaic array work at this maximum power point at all times, that is, the MPPT problem; the second function is to make the match between the system output voltage and the load characteristics optimal, that is, the constant V/f control of the motor. From the above two aspects, it can be seen that when the sunshine intensity Φ and the solar cell capacity are constant, the system efficiency reaches the highest, that is, the water flow can reach the maximum under the premise of keeping the pump head unchanged.

2 Main circuit structure of the system

1) Circuit topology

The main circuit topology of the system is shown in Figure 2.

Figure 2 Main circuit topology

2) System power devices

The power device of this system adopts power MOSFET, which is a voltage-controlled unipolar device with no minority carrier storage effect, high input impedance, fast action, high operating frequency, no secondary breakdown, low driving power, and simple driving circuit; at the same time, due to its positive temperature coefficient, it can automatically balance the current and will not produce hot spots. Therefore, the system uses two power MOSFETs in parallel to expand the current capacity, thereby reducing the cost of the system. At the same time, the solar cell voltage, DC bus current, motor U-phase and V-phase current values, etc. can be detected from the main circuit to achieve various protections of the system.

3 System control circuit

3.1 System control circuit function

The control circuit of this system is a fully digital intelligent control circuit with the new generation 16-bit single-chip microcomputer 80C196MC produced by Intel as the control core. Its main function is to complete the following functions of the system according to the necessary external information under the control of software:

1) According to the determined V/f curve and the working principle of the waveform generator (WG) unit on the 80C196MC chip, an SPWM signal is sent to keep the V/f value constant to achieve variable frequency speed regulation.

2) Based on the DC side voltage and current values ​​detected by the detection element, combined with the power characteristic curve of the solar cell and the corresponding software, the maximum power point tracking of the solar cell is achieved while completing the variable frequency speed regulation.

3) Take corresponding treatment measures according to various fault signals and give alarm displays for various faults.

3.2 System control chip and peripheral block diagram

8XC196MC is a real 16-bit embedded microcontroller launched by Intel in 1992 after MCS51 and MCS96 series microcontrollers. Due to the use of CHMOS technology, the chip's computing speed is greatly improved; at the same time, it further integrates many commonly used functional modules into the chip, making the user system more compact, more anti-interference and more reliable. The control chip 80C196MC used in this system is one of the 8XC196MC series microcontrollers. The most distinctive unit, the waveform generator (WG), is integrated inside it. It greatly simplifies the method and steps of generating SPWM. You only need to calculate the values ​​of registers WG-RELOAD and WG-COMPX online to get SPWM with different frequencies and pulse widths. Its peripheral circuit block diagram is shown in Figure 3.

Figure 3 CPU peripheral circuit block diagram

3.3 Input interface circuit

This system has 6 detection channels, which respectively complete the detection and protection of the system's DC side voltage, DC side current, output AC voltage and output AC current. Among them, the interface circuits of DC side voltage detection and DC side overcurrent and short circuit protection are shown in Figure 4 and Figure 5 respectively. The interface circuit converts the input signal into the 0-5V level required by the chip. The input impedance of this part of the circuit should be relatively large to minimize the impact on the device signal, and the output impedance should match the input impedance of the A/D port in the chip.

Figure 4 Solar cell voltage detection

Figure 5 DC side overcurrent and short circuit protection

4 System control principle block diagram

The control principle block diagram of the system is shown in Figure 6.

Figure 6 System control principle block diagram

As shown in Figure 6, this system combines the power characteristic curve of the solar cell and uses the step-by-step approximation method to realize the maximum power point tracking (MPPT) of the solar cell. The function of the MPPT module is to output a voltage value that gradually approaches the maximum power point by comparing the working voltage of the solar cell and the working frequency of the load twice before and after. At the same time, in order to eliminate system oscillation and improve the dynamic response speed, a PI adjustment combined with a soft start control method is designed. By continuously changing the working frequency of the load, the working voltage of the solar cell is finally equal to the output voltage value of the MPPT. Figures 7 and 8 are the MPPT and PI program flow charts respectively, and the soft start program flow chart is omitted.

Figure 7 MPPT program flow chart

Figure 8 PI program flow chart

The software of this system adopts modular design, mainly including main program, WG module, MPPT module, PI module and soft start module. After such modularization, the system software becomes simple and easy to understand, and it is also easy to modify and expand functions. The main program flow chart of the system is shown in Figure 9.

Figure 9 Main program flow chart

5 Conclusion

The inverter designed according to the above idea basically realizes the maximum power point tracking of the solar cell, and the voltage closed loop ensures the constant V/f, thus greatly improving the working efficiency of the system; at the same time, the system has various complete protection functions such as short circuit, undervoltage, stall, dry and overload, and can work in various harsh environments. It has broad application prospects for some remote western regions.