| As the world energy crisis and the increasingly serious environmental pollution problems, development and application of new energy get more and more attention all over the world. Solar energy is a kind of clean green energy source. Photovoltaic grid-connected power generation is an important way using solar energy currently. The power control and power quality problem of photovoltaic grid-connected power generation has always been the hot issues. Photovoltaic grid-connected power conditioner (PVPC) is a multifunctional converter system which contains photovoltaic grid-connected generation, reactive power compensation and harmonic attenuation. It expands photovoltaic inverter function, improves the equipment utilization efficiency and power quality. It has a broad development prospects. This paper is dedicated to the key issues in grid-connected system.This paper introduces the present research situation of Maximum Power Point Tracking method, grid-connected topological structure and control methods in detail. It analyses the topological structure and working principle of PVPC system and builds the mathematical model in abc stationary coordinate system.Incremental conductance method (INC) is the most commonly used algorithm in MPPT. It points out that the fixed step size is too small to track the maximum power point (MPP), but too large to cause system oscillation. A variable step size INC method has the drawbacks of slow tracking and big power loss. In view of the disadvantages above methods, this paper proposes an improved INC method. When the external conditions change rapidly, it calculates the approximate MPP voltage firstly, after that it conducts the maximum power point tracking based on the approximate value. It makes up the tracking defects when environmental conditions change rapidly. To verify the feasibility of this method, this paper gives the MPPT HIL simulation experiment based on RT-LAB real-time simulation platform. The results show that this method has high efficiency, fast response and small voltage fluctuations.Finally, the PVPC system has been modeled and simulated, the former boost realizes MPPT function and backward stage implements grid-connected and harmonic compensation. Closed-loop system transfer function of inner current loop has the right half plane zero and close from the imaginary axis, so that the control system Q value is larger and the setting time is longer. It lags the system harmonic compensation time. According to the shortages this paper proposes an improved optimization control strategy, namely, the inner current loop gives a low pass pre-filter. It can eliminate the zero in the closed-loop transfer function and simplifies the function into a standard second-order system. Compared with the traditional control strategy, the proposed control method can shorted the adjustment time, reduce the system oscillations and improve system dynamic performance. Finally, the system simulation has been carried out using the MATLAB/Simulink software. The results show that the effect of maximum power point tracking and harmonic compensation is good. |
