Abstract: This paper describes a high-efficiency, high-performance frequency converter specifically designed for photovoltaic water pumps, using the Intel 80C196MC chip as its control core. The system employs a successive approximation method to achieve maximum power point tracking (MPPT) of the solar cells, while simultaneously utilizing a constant V/f method to achieve efficient energy conversion of the AC photovoltaic water pump system.
Keywords: 80C196MC chip; photovoltaic water pump; maximum power point tracking
Today, with the massive increase in the consumption of conventional energy sources such as oil and coal, the increasingly deteriorating ecological environment is forcing countries around the world to actively seek a new path to sustainable energy development. Clean energy sources such as solar, wind, and geothermal energy have gradually gained human attention, with solar energy undoubtedly holding the most prominent position. Currently, in remote areas far from the power grid, such as Northwest my country, Tibet, and Inner Mongolia, many people lack access to clean drinking water. These areas are also rich in solar energy resources; therefore, developing photovoltaic water pump technology in these regions has significant social and economic benefits. However, most inverters currently available on the market are ordinary frequency converters, which, when used in this system, cannot effectively implement various protection functions and lack maximum power point tracking (MPPT) functionality for solar cells, resulting in a huge waste of solar cell capacity. This article introduces a high-efficiency, high-performance frequency converter specifically designed for photovoltaic water pumps.
1. Composition of a photovoltaic water pump system
The composition of the photovoltaic water pump system is shown in Figure 1.
Figure 1. Composition of the photovoltaic water pump system
The main input parameter of this system is solar radiation intensity (Φ), and the output parameter is water flow rate (θ). The overall system efficiency is η=Hθ/Φ, where H is the head. The MPPT and voltage converter are the objects of this study.
As shown in Figure 1, the photovoltaic array is the energy input terminal of the system. When the solar irradiance Φ is constant, its maximum output power is also constant. One of the main functions of the frequency converter developed in this system is to ensure that the photovoltaic array always operates at this maximum power point, i.e., the MPPT problem; the second function is to optimize the matching between the system output voltage and load characteristics, i.e., constant V/f control of the motor. From the above two aspects, it can be seen that when the solar irradiance Φ and the solar cell capacity are constant, the system efficiency reaches its maximum, that is, the water flow rate can reach its maximum while keeping the pump head constant.
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) Power devices of the system
This system uses power MOSFETs, which are voltage-controlled unipolar devices. Power MOSFETs do not have minority carrier storage effects, have high input impedance, fast response, high operating frequency, no secondary breakdown, low drive power, and a simple drive circuit. Furthermore, due to their positive temperature coefficient, they can automatically balance current and prevent hot spots. Therefore, the system uses two power MOSFETs in parallel to increase current capacity and reduce system cost. Simultaneously, the main circuit can detect solar cell voltage, DC bus current, and the U-phase and V-phase current values of the motor to achieve various system protection functions.
3. System control circuit
3.1 System control circuit function
The control circuit of this system is a fully digital intelligent control circuit based on the new generation 16-bit microcontroller 80C196MC manufactured by Intel. Its main function is to complete the following functions of the system under software control and based on necessary external information:
1) Based on the established V/f curve and the working principle of the 80C196MC on-chip peripheral waveform generator (WG) unit, an SPWM signal is sent to keep the V/f value constant, thereby achieving 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 can be achieved while completing the frequency conversion speed regulation.
3) Based on various fault signals, take corresponding handling measures and provide alarm displays for various faults.
3.2 System Control Chip and Peripheral Block Diagram
The 8XC196MC is a true 16-bit embedded microcontroller launched by Intel in 1992, following the MCS51 and MCS96 series. Utilizing CHMOS technology, its processing speed is significantly improved. Furthermore, it integrates many commonly used functional modules onto the chip, making the user system more compact, more resistant to interference, and more reliable. The control chip used in this system, the 80C196MC, is one of the 8XC196MC series microcontrollers. Its most distinctive feature is the integrated waveform generator (WG), which greatly simplifies the method and steps of generating SPWM. Different frequencies and pulse widths of SPWM can be obtained simply by calculating the values of the registers WG-RELOAD and WG-COMPX online. Its peripheral circuit block diagram is shown in Figure 3.
Figure 3 Block diagram of CPU peripheral circuitry
3.3 Input Interface Circuit
This system has six 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. The interface circuits for DC-side voltage detection and DC-side overcurrent and short-circuit protection are shown in Figures 4 and 5, respectively. The interface circuits convert the input signal to the 0-5V level required by the chip. This part of the circuit should have a relatively high input impedance to minimize the impact on the device signals, and the output impedance should match the input impedance of the chip's internal A/D port.
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 utilizes the power characteristic curve of the solar cell and employs a successive approximation method to achieve maximum power point tracking (MPPT). The MPPT module compares the solar cell's operating voltage and the load's operating frequency in two consecutive tests, outputting a voltage value that gradually approaches the maximum power point. Simultaneously, to eliminate system oscillations and improve dynamic response speed, a control method combining PI regulation and soft starting is designed. By continuously changing the load's operating frequency, the solar cell's operating voltage eventually equals the MPPT's output voltage value. Figures 7 and 8 are the flowcharts for the MPPT and PI programs, respectively; the soft starting program flowchart is omitted.
Figure 7 MPPT Program Flowchart
Figure 8 PI Program Flowchart
The software of this system adopts a modular design, mainly including the main program, WG module, MPPT module, PI module, and soft starter module. This modular approach simplifies the complex software, making it easier to understand and modify, as well as expand its functionality. The flowchart of the main program is shown in Figure 9.
Figure 9 Main Program Flowchart
5 Conclusion
The frequency converter designed based on the above ideas basically achieves maximum power point tracking of solar cells, while the voltage closed loop ensures constant V/f, thus greatly improving the system's working efficiency. At the same time, the system has a variety of comprehensive protection functions such as short circuit, undervoltage, stall, dry running, and overload, and can work in various harsh environments, making it a promising candidate for application in some remote western regions.
