Research On Voltage Control Of Grid-connected DFIG Based Wind Farm

With the expansion of wind power capacity, and the increasing proportion of wind power in grid, people pay more and more attention to the control problem of reactive power and voltage. Doubly-fed induction generator with its flexible capacity of reactive power regulation has become the mainstream choice type for the wind farm. However, when the grid is severely disturbed, DFIG is not sufficient to undertake the reactive power required by the system, so it is necessary to install additional reactive power compensation equipments to ensure the security and stability of grid. Therefore, by researching the structure of DFIG, a coordinated control strategy between DFIG and reactive power compensation equipments is proposed, which can maintain the voltage level at the point of common coupling.At first, the paper introduces the working principle of the variable speed constant frequency wind power generator, then constructs the mathematical model of DFIG. Based on the theoretical analysis above, the paper analyzes the DFIG reactive power limit caused by stator winding limit, rotor-side converter limit and grid-side converter limit, etc. It provides a basic for further investigation of reactive power and voltage regulation of DFIG based wind farm.According to the reactive power compensation ability of DFIG, a rapid voltage control strategy is proposed. With a grid-connected wind farm as an example, this paper simulates the strategy from two aspects of steady state and transient based on MATLAB software. The result shows that DFIG can guarantee voltage level under steady state, but it is difficult for DFIG to supply enough reactive power for gird when a fault occurs in the system.Finally, a multi-stage coordinated control strategy with DFIG, capacitor bank and SVC is proposed. The scheme of switching of capacitor bank is pre-performed based on the predicted wind power first of all; then adopts different strategies within different conditions between DFIG and SVC. Simulating this strategy by combining a grid-connected wind farm. The simulation of a practical example shows that the control strategy can accurately track the voltage at the point of common coupling, improve the ability to withstand voltage drop and ensure the stable operation of wind farm.