With the rapid development of the global economy, the massive consumption of traditional fossil fuels has led to a global energy crisis, prompting countries worldwide to dedicate themselves to the development and utilization of new energy sources.
Solar energy, with its abundant resources, renewable nature, and lack of environmental pollution, has become one of the most promising new energy sources. Continuous advancements in solar cell and power electronics technologies have enabled solar photovoltaic (PV) power generation to achieve significant development and become the mainstream of solar energy utilization. Among various solar PV power generation system structures, solar AC modules combine individual PV modules and low-power inverters and connect them to a single-phase low-voltage grid. This structure overcomes the power losses caused by partial shading and series/parallel connections of PV modules in PV arrays, and facilitates optimal coordination between PV modules and inverters.
AC modules are considered an important development direction for current and future grid-connected inverters. The challenge for these modules lies in optimizing cost, efficiency, and lifespan. Based on this, this paper conducts an in-depth study of AC module inverters and their control technologies, addressing the characteristics and problems of current solar AC modules. The grid-connected inverter is the core of the entire solar AC module power generation system. This paper introduces capacitor idle technology into traditional flyback inverters and proposes an improved AC module inverter structure.
The novel inverter employs only four switches. Two switches operate at high frequency to achieve maximum power point tracking and adjust the grid-connected current waveform; the other two operate at low frequency to control the polarity of the grid-connected current. This simplified circuit structure and operating mode reduce system costs. The paper analyzes the working principle of the proposed inverter, presents its PWM control mode, and elucidates its operating mode and energy exchange process through the analysis of six operating states within one switching cycle.
Since the intermediate decoupling capacitor operates in an idle mode, the inverter is decomposed into relatively independent front-end DC circuits and rear-end inverter circuits. Furthermore, the front-end and rear-end circuits are simplified and equivalent to a unified circuit form, which essentially operates in Buck-Boost mode. Detailed analysis of the inverter’s operation reveals that discontinuous conduction mode, capacitor idler technology, and the application of absorption capacitors enable the high-frequency switch to operate in a zero-current turn-on and zero-voltage turn-off state. Soft-switching operation improves system efficiency. Capacitor idler technology also transfers power ripple in the single-phase inverter circuit to the voltage fluctuations of the intermediate decoupling capacitor, eliminating the need for large-capacity electrolytic capacitors, which is significant for extending system lifespan. Furthermore, the absorption capacitor provides a path for energy feedback from the leakage inductance of the flyback transformer, reducing voltage overshoot on the switch during turn-off and improving circuit reliability.
Besides the soft-switching operation of the high-frequency switch, improving overall system efficiency requires real-time adjustment of the photovoltaic module’s operating point based on its operating status, ensuring it always operates near the maximum power point. This paper proposes a new perturbation-matching maximum power point tracking (MPPT) technique based on a combination of the perturbation observation method and the incremental conductance method. By adjusting the duty cycle of the main switch, the inverter’s input impedance equals the equivalent output impedance of the photovoltaic module, thus pinpointing the maximum power point. This method only requires adding a certain perturbation to the duty cycle. By comparing the fluctuation value and average value of the photovoltaic module voltage, maximum power point tracking can be achieved. Therefore, the proposed method does not require detecting the output current of the photovoltaic cell, saving measurement and control system costs and complex calculations, while ensuring the accuracy of maximum power point tracking. Analysis shows that the input impedance of the inverter can be adjusted by perturbing the switch duty cycle or switching frequency. Therefore, this paper proposes two perturbation matching tracking methods: the duty cycle perturbation matching method and the equivalent switching frequency perturbation matching method, and provides detailed derivation and discussion of the proposed methods.
AC module inverters need to capture the maximum power of the solar cells on the one hand, and provide energy to the grid and load in the form of grid-connected current on the other. Addressing the steady-state second harmonic fluctuations in the intermediate decoupling capacitor voltage, this paper proposes a feedforward SPWM correction control based on the peak current control method and utilizing the energy equivalence principle of the flyback transformer to ensure the waveform quality of the grid-connected current. Furthermore, considering that the average value of the intermediate decoupling capacitor voltage changes with the power transmitted by the circuit, this paper proposes to use closed-loop control of the average capacitor voltage to control the amplitude of the grid-connected current. The proposed joint control scheme for grid-connected current amplitude and waveform only requires detecting the voltage of the intermediate decoupling capacitor to control the grid-connected current, eliminating the need to measure the primary current of the flyback transformer and saving control system costs.
As AC modular inverters are connected to the grid, in addition to power quality control for photovoltaic grid connection, anti-islanding effect detection and protection are also required. Based on an analysis of the advantages and disadvantages of the active interference method and the active frequency shift method, this paper proposes a comprehensive islanding effect detection method combining the active interference method and the positive feedback frequency shift method. Under normal circumstances, only the active frequency shift method is used for detection. When the frequency deviation reaches a certain value, the active interference method is activated, and the amplitude and frequency of the output voltage are detected and judged simultaneously. The combination of the two methods leverages their respective strengths, avoiding the deterioration of grid-connected current quality and periodic power loss under normal conditions, while also accelerating the detection time and reducing the detection blind zone. Based on the proposed novel AC modular inverter topology, maximum power point tracking scheme, grid-connected current control method, and islanding detection technology, a 100W solar AC modular photovoltaic grid-connected power generation system experimental platform was built. This paper designs the main circuit and control system, and conducts experiments including inverter grid connection, maximum power point tracking, and islanding detection. Finally, analysis and evaluation are presented.
Simulation, experimental, and evaluation results show that the proposed novel AC module grid-connected power generation system demonstrates correct theoretical analysis, feasible practical scheme, and superior performance, laying an important theoretical and practical foundation for further research.
