Research On Advanced Control Strategy For Transformerless Single-Phase Grid-connected Inverter

Motivated by the booming installations of grid-connected photovoltaic (PV) systems, several countries have revised their grid codes in pace with the sustained growth both in quantity and scale. PV systems with high efficiency and reliability will be a trend in the future. This thesis deals with the key techniques of transformerless single-phase grid-connected PV inverter.The first part describes an adaptive and variable step maximum power point tracking (MPPT) method based on current predictive controllers, which is based on the modeling of PV array. The experimental results show that the proposed MPPT algorithm has good steady-sate and dynamic performance. Since the power-voltage characteristics of PV arrays are characterized by multiple local peaks and one global peak with the effects of bypass diodes and blocking diodes under partial shading conditions (PSCs), a universal model of PV arrays under PSCs is established in this part, on which base a multi-peak voltage estimation method is proposed. A comparison between the estimated peak voltage and the actual one is conducted under different shading conditions. The proposed multi-peak voltage estimation method has been verified and it is proved to be accurate.The second part presents advanced control strategies for single-phase grid-connected PV inverters. Second order generalized integrator (SOGI) based phase locked loop (PLL) algorithm is introduced and tested under disordered grid at the first beginning. Then a novel control strategy is introduced in this part. In contrast to a common dual loop controller with dc-link voltage outer and inverter current inner, dc-link voltage is flexibly controlled with different stage of two-stage systems to minimize voltage variation during grid-fault period. In the aspect of current injected to the grid, constant peak current strategy is adopted to avoid triggering over-current protection. The amount of reactive power delivered for grid recovery depends on sag depth of grid voltage. A compensator is added in the proposed strategy to meet the target of reducing 2nd-order ripple in reference active current. Simulation results on the whole system shows that the proposed strategy is effective with the capability of low-voltage-ride-through (LVRT).By considering both the advantages and disadvantages of the existing topologies of transformerless single-phase grid-connected PV inverter, an optimized full-bridge structure is proposed in the third part. The principle of the inverter topology is analyzed in detail, and the control strategy of the grid-connected photovoltaic generation systems is studied. The experimental platform of single-phase grid-connected inverters is set up in the last part. The experiments have been carried out to verify the theoretical analysis. Experimental results indicate that the proposed grid-connected photovoltaic inverter has low common-mode leakage current and high quality of grid-connected waveforms.