According to market research in recent years and statistics from the National Energy Administration as of 2013, in the field of solar photovoltaic power generation systems in China, the main focus is on DC power generation systems, where solar cells, batteries, and DC loads are connected in parallel. When there is insufficient sunlight, the output voltage of solar cells is low, and they are powered by batteries to directly load. Due to its simple design, easy implementation, and controllable cost, this type of structure has been widely used. But its drawbacks are also obvious, often due to differences in load DC voltage, which makes it difficult to unify standards, leading to poor compatibility of the system. Especially in the civilian field, due to the fact that the load is mostly AC, the response of photovoltaic power supply products for DC systems in the market is mediocre. In addition, in recent years, the rapid development of solar photovoltaic grid connected power generation worldwide has made the application of solar photovoltaic inverter technology in photovoltaic power generation systems increasingly important.
As a power conversion device, solar photovoltaic inverters convert direct current into alternating current, and their inverter technology has become increasingly mature in the field of electrical and electronic technology. Many of its products have been successfully launched into the market and have been widely recognized by society. As the core functional component of solar photovoltaic power generation systems, the performance of inverters directly affects the operational quality of photovoltaic power generation systems. With the development and increasing variety of solar photovoltaic inverter technology, solar photovoltaic power generation systems can be applied to various fields related to daily life and production.
The design of solar photovoltaic inverters adopts pulse width modulation analog control method, using the digital output of microprocessors to control the analog circuit. Due to its simple control, flexibility, and good dynamic response, it can keep the output voltage of the power supply constant when working conditions change, generating corresponding control waveforms, It is a very effective technique for controlling analog circuits using digital signals from microprocessors. It is very advantageous for various aspects such as harmonic suppression, dead zone control, and regulating output voltage.
1. Overall structural design
The working principle of solar photovoltaic inverters is that the direct current generated by solar cells is boosted through the front stage
The circuit (this design uses a push-pull circuit) is raised to around 310V to provide the input voltage to the solar photovoltaic inverter. Then, the high-frequency AC output from the solar photovoltaic inverter is rectified, high-frequency filtered, and polarity converted to 220V 50Hz AC. The system mainly consists of two parts, the front-end DC-DC converter and the back-end DC-AC inverter. In this system, the rated DC voltage output by the solar panel is about 48V. The function of the DC-AC inverter is to convert the converted DC power into 220V 50Hz sinusoidal AC power. The DC-AC part adopts a full bridge inverter, and the core chip of the control circuit is AT89C2051.
2. Hardware design
After collecting relevant information according to the design requirements and conducting comprehensive analysis and research, a preliminary plan and design concept were obtained:
The solar photovoltaic power generation system consists of solar cell arrays, controllers, inverters, battery packs, and other components. The function of the inverter is to convert the direct current generated by the solar battery pack or the direct current released by the battery when the voltage and power of the battery pack decrease into alternating current for the load’s needs if an AC load is connected to the solar photovoltaic power generation system.
Due to the variation of the terminal voltage of the photovoltaic array with load and sunlight intensity, the solar photovoltaic inverter must be able to operate normally within a wide range of DC input voltage and ensure the stability of AC output voltage. This design is mainly aimed at a solar inverter controller with a small microcontroller as the core. By utilizing the internal resources of a small microcontroller and eliminating harmonics offline, a satisfactory sinusoidal voltage output can be obtained.
Digital signal input and analog signal input refer to various feedback quantities, as well as protection and detection signals. After the microcontroller detects various signals, it calculates them according to pre-set programs or control strategies, and then outputs control signals through digital and analog output. After amplification, the high-frequency electric power circuit is controlled to achieve transformation. The AC output is transformed by a transformer to obtain the required AC power.
3. Software design
The inverter power control software consists of three parts: the main program, timer interrupt service program, and external interrupt service program. The working mode of initializing the microcontroller is completed by the main program, during which the frequency of the inverter power output is sampled. When the output frequency changes, its encoding value will change accordingly, and the frequency change flag will be set to facilitate the timer interrupt service program to timing according to the new switch angle data and achieve the switching of the driving signal. The timer interrupt service program mainly completes the timing of switch switching angle data and the output of corresponding driving signals to achieve harmonic elimination pulse width modulation control. The external interrupt service program is mainly responsible for handling the fault protection of the inverter power supply. When a fault interrupt request occurs, the microcontroller responds to the interrupt and checks again to see if a fault has occurred. If a fault is confirmed, the driver signal is blocked and a fault code is output.
This design adopts the company’s features, including program memory, two bit timers, and is a low-cost and highly integrated chip. Sample the output frequency of the solar photovoltaic inverter, input it in encoding mode, select control data based on the input encoding, and switch the switching device according to the control data through internal timing control, and then output the signal from the working end to achieve control.
4. Conclusion
From the perspective of environmental protection and energy conservation, this article provides a design scheme for solar photovoltaic inverters. Pulse width modulation control technology uses the digital output of a microprocessor to control the voltage pulse width to achieve voltage transformation. This article effectively reduces the harmonic elimination of the output voltage of the solar photovoltaic inverter through offline harmonic elimination control, thereby obtaining a relatively satisfactory sine wave voltage output.