Application of UPFC Based on Improved Double-loop Decoupling PI Control in Photovoltaic Systems

The mutation of solar irradiance can cause the output power of photovoltaic power plant to mutate, and it takes great changes to active power and reactive power which feed into the power grid. When the system security constraint is exceeded, the photovoltaic power plant will stop running. In this paper, the structure and the mathematical model of large grid-connected photovoltaic power plant are introduced, then an improved double loop PI decoupling control system is proposed based on the d-q model of UPFC parallel converter. On the basis of traditional PI control, DC load current is directly used as feed-forward control in the control system of capacitor voltage. It is not only easy to get feedback, but also conducive to design and operate the series controller and the parallel controller independently. Finally, this paper sets up a Simulink model of UPFC in the environment of Matlab/Simulink after analyzing its principle. And then, this model was applied into a three-phase system to observe its influences to power quality. The simulation results show that this control system based on the UPFC can effectively control the voltage and the power flow, maintain bus voltage and reduce reactive exchange. It can also improve the active photovoltaic power transmission, as well as maintaining the stability of the system.