Research On Converter Control Of Photovoltaic Grid-connected System With Composite Energy Storage

A number of factors, such as growing energy demand, the shrinking of the traditional fossil energy and environmental issues, have become a powerful impetus to promote the development of renewable energy. Photovoltaic power generation as one of the main trends in renewable energy fields has stimulated the national competition. This paper studies the model of photovoltaic cells, maximum power point tracking(MPPT), grid-connected control technology and composite energy storage technology.Firstly, this paper establishes the model of photovoltaic cells, and obtains the P-V and P-I characteristics under different light and temperature. This paper also analyzed the influence of various environment variables on photovoltaic power generation and MPPT algorithm and proposes a variable step size P&O algorithm. The algorithm is based on the relationship between the slope of P-V characteristic curve and the maximum power point(MPP).The closer to MPP the slope of the curve, the smaller its absolute value is, and at a value of zero at the MPP. This algorithm use the slope of the curve as the size of the disturbance voltage values, to avoid the oscillation phenomena in the vicinity over the MPP, and the simulation proved that the algorithm is correct and effective.Secondly, this paper utilizes a full-bridge inverter, and compares the output current with instruction current. Then PWM signals drive power switching devices after the formation of the deviation of the PI regulator by the triangular wave modulation. Due to the current tracking characteristic of the method associated with PI parameters, so this paper also has carried on the PI parameters setting. The simulation proved that the current tracking control method based on triangular wave comparison method can track the grid voltage accurately, and achieve the goal of grid-connected.Finally, this paper analyzes the impact on the grid when the PV output power fluctuates and the important role of energy storage system in this process. So this paper construct grid integrated generation system with composite energy storage. The energy storage device is constituted by super capacitors and batteries paralleling on the DC bus via the Bi-DC/DC converters. Super capacitors with fast response speed, high power density, and high charge and discharge times etc, can make up for the disadvantage of battery whose charge and discharge is slow and the power density is not high enough. According to the characteristics of the energy storage system, a new grid-connected strategy to smooth the output power fluctuations of PV system and it is proposed to smooth the high and low frequency fluctuations of the power injected into grid. Simulation results showed that the proposed strategy can effectively suppress the system output power fluctuations, while reducing the current harmonics injected into grid.