The Research Of Reactive Powerr And Voltage Control For Distributed Grid-Connected Photovoltaic Systems

Since the 21st century, with the shortage of traditional fossil energy and the seriousness of environmental pollution, how to rationally develop and effectively use the green energy becomes the imminent problem all over the world. Solar energy, as a kind of abundant, clean energy, receives more and more attention around the world. PV as an effective way to use solar energy, with the development of photovoltaic grid technology and the growing of grid-connected PV scale, it has brought a lot of adverse effects to the grid. This thesis focuses on control strategies in two-stage three-phase grid- connected PV systems and deeply study reactive power and voltage control strategies under normal power grid voltage and drop operating status of power grid voltage.Firstly, according to the mathematical expression of photovoltaic cells, the simulation model was established in MTALAB/Simulink and its output characteristic was analyzed and simulated. Then the maximum power point tracking methods are introduced. Perturbation and observation method is carried out by MATLAB simulation.The principle of three-phase PV grid-connected inverter is analyzed and the mathematical model of the system is established under dq rotating coordinate system. The decoupling control of the inverter outputting active and reactive power is realized through dq transformation. The inverter output reactive power theory and constraints of photovoltaic system reactive power is realized. The voltage variation by the active power change of PV system and the support to network voltage by PV system outputting a certain amount of reactive power are analyzed.The voltage and current double closed-loop system with PI control is designed and a specific parameter calculation method is given in this paper. The simulation model of the PV system is established, realizing the decoupling control of PV inverter outputting both active and reactive power.The voltage and reactive power control strategy of PV grid-connected system is designed in this paper. The photovoltaic system operates under max power point control when the grid voltage is normal, The grid voltage fluctuations caused by PV output f active power change and load disturbance can be solved by reactive power control of inverter. During the grid voltage sags, a strategy for reactive power and voltage control based on inverter’s providing active power ability is propose. This control strategy can take full advantage of the characteristics of the inverter system, make PV power system connected to the grid with no additional hardware and send reactive power to support voltage of PCC, resulting in the voltage stability. During the grid voltage sags and when the inverter current is limited, the DC side capacitor voltage rising problems caused by DC side power imbalance can be solved by change the control strategy of PV grid-connected system, from max power point control to the constant voltage control, resulting the voltage stability of DC side.