Research On Control Strategy For Two-stage Photovoltaic Grid-connected Inverter System

In recent years, the photovoltaic grid-connected power generation, which has been sustaining an exponential growth rate, will inevitably play an important role in the new energy power generation for the future. The rapid growth of photovoltaic(PV) power systems both at home and abroad is driven by the trend of global environmental degradation and the gradually reducing primary energy sources. As the grid-connected inverter systems are the key component of the entire photovoltaic power systems, it is very significant to ensure their grid-connected reliability and safety. In this thesis, the three-phase grid-connected inverter system is focused on and studied. The thesis will conduct research on the modeling of three-phase photovoltaic power system, the control strategy of the system connected to grid under balanced gird voltage or unbalanced grid voltage, and the related low-voltage ride-through of the photovoltaic grid system.The thesis will take the two-stage photovoltaic grid-connected power system with three phases and three lines as research object. The PV array and the Boost DC-DC converter circuit are used in the first stage. The engineering model of the solar cell is established to facilitate research applications and the MPPT control structure with voltage closed-loop for pre-stage Boost converter is used, so the first-stage circuit can implement the boost function and maximum power point tracking(MPPT) function. In the second stage, the general math model of the three phase inverter with LCL filter is established. The characteristics of LCL filter are analyzed and the parameter design methods of LCL filter are introduced. Due to the resonant peak of the LCL filter, damping control is necessary to ensure system stable. When the grid voltage is balanced, the voltage and current double closed-loop control structure based on proportional integral is employed to control the grid-connected current to guarantee current balance and unity power factor. Simulation results can verify the stable and safe operation of the photovoltaic power generation system with balanced grid voltage.In actual operation, voltage imbalance may occur. According to output characteristics of photovoltaic array and the requirement of grid-connected inverter, the PV grid-connected system is studied, and the mathematical model of unbalanced gird is established to control the Boost chopper and the inverter system independently. In order to restrain the negative sequence component, orthogonal signal generator based on second-order generalized integrator(SOGI) is used to separate the positive-negative sequence component and realize the phase locking of gird voltage. At the same time, both the control strategy of suppressing the negative sequence current and the control strategy of restraining the DC voltage harmonic in dq synchronous rotation reference frame are proposed. In the end, simulation model of PV grid-connected system is established to verify the validity of proposed strategy under the unbalanced grid voltage conditions.According to output characteristics of photovoltaic array and the requirement of grid-connected inverter, this paper has a study on two-stage PV grid-connected system to realize low-voltage ride-through. The requirements on the low-voltage ride-through of the photovoltaic power station are introduced and a brief analysis of the operation status of the inverter under unbalanced voltage sags is also offered. During the period of the gird faults, constant power control strategy is used in the first-end stage to make the PV array output constant power and rear-end invert control can make sure of injecting reactive power into the gird. With these methods, DC voltage and gird-connected current can be well controlled. Besides, a control strategy which could suppress the negative sequence current under unbalanced voltage sags is also proposed. Finally, simulation model of PV grid-connected system is established to confirm the validity of proposed LVRT control strategy under different grid voltage sags.