Research On Single-Phase Grid-Connected Photovoltaic Inverter

As the main power conversion unit of the Photovoltaic(PV) grid-connected(GC) system, the PV GC inverter has two main tasks. One is ensure that the PV modules is operated at the maximum power point(MPP). The other is to inject a sinusoidal current with unit power factor to the grid. The main circuit topology, modulation identification, main circuit parameter design, control strategy and the overall system software design optimization of the PV GC inverter must be studied in depth to accomplish these two tasks.The main circuit of the PV GC inverter is initially studied in this thesis. The topology of full-bridge inverter with line-transformer is determined after analysis. With the comparing and analyzing of two main SPWM modulations, the unipolar SPWM modulation is determined. After the output filter and the DC-link capacitor is analyzed and studied, the LCL-filter with damping resistor is determined as the GC filter.Secondly, the dual-loop control strategy of the PV GC inverter is analyzed and studied in detail. With the modeling of the GC inverter, the current instantaneous control strategy with PI control is determined by comparing and analyzing several current control mode and the control parameters of the current loop are optimized with the simulation of the characteristics of open-loop and close-loop. With the modeling of DC-link voltage loop, the parameters of the voltage loop is optimized through analytical and simulation methods.The MPPT control strategy of the PV GC inverter is then analyzed and studied. With the analyzing of PV cell output characteristics, the MPPT control theory is studied based on Boost topology. The perturb and observe(P&O) method is determined, improved and implemented as the MPPT control strategy of this thesis by comparison and analysis.Consequently, the digital control strategy of the PV GC inverter is designed and implemented. The digital phase-lock-loop(PLL) and the software control strategy of the system is researched and studied and then the experimental waveforms are given and analyzed.