Research On Control Strategy Of Step-up Solid-state Transformer For DFIG-based Wind Power Generation System

With large scale of wind farm integration, the impact of wind power on the gridwill increase continuously. Many issues exist in the present modes of grid integrationof wind farm. As mainstream model, doubly-fed induction generator (DFIG) hasshortcomings in overcoming aggravated operational performance under unbalancedgrid voltage condition and achievements of low voltage ride through (LVRT). Toaddress this issue, people are seeking a better mode of grid integration of wind farm.VSC based High Voltage DC (VSC-HVDC) transmission has the advantage of easyconnection of weak AC system. The grid integration of wind farm based on VSC-HVDC has been comprehensively studied. However, when the voltage value of DClink keeps constant, the amplitudes of AC voltages on both sides will be restricted.Solid-state transformer (SST) not only has the function of fault isolation, but also hasthe function of voltage boost. It is meaningful to research the operation and controlstrategy of SST for grid-integration of wind farm.Firstly, based on the topology and operation principles of SST, the equivalentmathematic models of grid side voltage source converter (GSVSC), dual active bridge(DAB) and wind farm side voltage source converter (WFVSC) are established. Underbalanced grid, how to stabilize AC voltages and DC voltages as well as how toregulate the active and reactive power decoupled are researched. This lays afoundation to the study of the improved control strategies under unbalanced gridvoltage and three-phase short circuit fault.For unbalanced grid voltage, the cause of negative sequence current component inthe stator of DFIG using traditional transformer for grid intrgration is analyzed. Tosuppress this component, adapting SST to connect wind farm to the grid is proposed.However, SST became connected to the grid directly. GSVSC will be subject to thegrid negative sequence voltage component. To suppress the second harmonicoscillation in active power and DC bus voltage, the mathematic model of SST undernegative sequence axis is established. The unbalanced voltage control strategyconsidering GSVSC access reactors and equivalent impedance of transformer ispresented. Corresponding commands of positive and negative sequence currents ofGSVSC in positive and negative sequence synchronously rotating axes are separatelyderived. For grid three-phase short circuit fault, the causes of transient DC component inDFIG stator and overcurrent in rotor when using a conventional transformer for gridintegration are analyzed. To suppress the impact of voltages, currents and torque onDFIG, a method of controlling the change slop of WFVSC’s output voltage isproposed. Through analyzing the relationship among the slope of wind farm sidevoltage, transferred energy and the value of DC-link’s equivalent capacitance in SSTand the relationship between wind farm side voltage and grid voltage sag, the valueand slope of wind farm side voltage’s change can be determined. So that the outputpower of the wind farm will be ensured to go down to the balancing power after thefault within buffer time and the voltage of DC link will not exceed the upper limit.The Matlab/Simulink based models of SST and DFIG system are established.Simulation studies of SST control strategies under balanced and unbalanced gridvoltage condition as well as three-phase short circuit are carried out. Simulation resultsdemonstrate the feasibility and effectiveness of the proposed design as well as theimproved control strategies.