Simulation analysis on transient power characteristics of wind power grid-connected system under grid voltage sag

The transient power characteristics of grid connected wind power which composed by many paralleled wind turbines, transformer and collection line, will significantly affect the safety of the power grid and the wind farm itself. In this paper is presented a model of Doubly-fed Wind Power Generation Unit(DFWPGU), which includes two control measures: one is the outer voltage loop and inner current loop model for grid-side converter, the other is the external reactive loop and inner current loop model for rotor side converter. In order to analyze the transient power characteristics, the topology of Doubly Fed Induction Generator (DFIG) wind power integration is built, and a circuit for the two-port network formed by DFWPGU side and grid side is derived by network transformation. Additionally, the mathematical relationship among the power consumption of the two-port network, the grid voltage, line parameters and wind power output is deduced based on the equivalent circuit. The simulation of wind power grid-connected system is conducted in PSCAD/EMTDC. The proposed control scheme of DFWPGU is validated. What's more, it is pointed out that collection line has transient reactive load characteristics during grid voltage dropping and the reactive power loss of collection line changes with line length and DFWPGU's active output.