Research On Three-Phase Grid-Connected Photovoltaic System

Research on the photovoltaic(PV) system has already become the focus of the social concerns, the goal is to pursue the high efficiency of the PV system, and the PV system's efficiency is affected by many factors.The PV array's output voltage and current are nonlinear, they change along with the illumination and temperature's changes. It is necessary to track the maximum power point by the MPPT control algorithm to ensure that the PV array outputs its maximum power. Based on the traditional perturbation and observation(P&O) algorithm, adopt the dP-P&O with constant voltage start, and simulate how the length of step impact on the tracking speed and the static precision of the maximum power point tracking. This paper choose the dP-P&O algorithm with constant voltage start and a variable step to realize the maximum power point tracking, Simulation results reveal this algorithm can track fast and simultaneously avoid the fluctuation around the maximum power point, deeply improve the PV array's output efficiency.The total harmonic distortion(THD) of the grid-connected current and the power factor are two important performance indexes to evaluate the PV system's performance. In order to reduce the pollution which is brought by the high switching frequency to the grid, it is necessary to add a filter sector between the inverter and the grid. After analyzing and comparing the performance of the L filter, the LCL filter and the LCL filter with the damping resistance, the optimized LCL filter with the damping resistance was chosen as the filter sector, and then simulate the filter's impact on the system's Static performance. Calculate and analyze the invertor's Dynamic mathematical model, and give the inverter's average signal model. It is necessary to get the phase and frequency's information to realize the unity power factor control when the grid is unbalance. This paper adopts PI controllor to let ud/uq approximately equal to a given reference 0 to realize the phase locked, and then simulate in the situation of asymmetrical grid voltage to comfirm whether tracing results are accurate. Based on invertor's mathematical model, control the current in dq coordinate by PI control to realize the active power and reactive power's decoupling control. Then regulate the PI parameter of the voltage outer loop and the current inner loop according the pole-zero placement principle, the simulation result shows that the system's dynamic response has been improved and static error has been reduced.Stability of the grid-connected PV system was researched and the stability criterion was comfirmed by simulation. Do the simulation in luminous with defference change ratio to observe the system's performances, which include the PV's output performance and the grid-connected current's chages.