Research On Technologys Of Low Voltage Ride Through For Doubly-fed Wind Power System Based On Energy Shaping Control

The doubly-fed induction generator based on wind turbine(DFIG-WT) is sensitive to grid faults. When the grid voltage sags, disconnecting the DFIGs with grids will not only lead to wind power not being fully utilized but also seriously affect the safe operation of power grid. According to the new grid codes, DFIGs should keep operating during voltage sag, in other words, DFIGs should have the capability of low voltage ride through(LVRT). So improving the LVRT capability of DFIGs is a challenging job and it is extremely meaningful to the development of wind power industry. Based on energy-shaping control strategy, this paper makes a research on how to enhance the LVRT capability of DFIG.First, latest progress and research status of wind power generation technology, energy-shaping control scheme and the LVRT of DFIG are introduced. The advantages of application energy-shaping control scheme in DFIG is analyzed and it is pointed out that the research on the control scheme is only limited to the power grid voltage steady state. So a further study of LVRT technology based on energy-shaping control strategy is needed. Then the application of hard propection circuit and improved control LVRT strategies is analyzed. A contrastive analysis of the advantages and the disadvantages of the two kinds stratages is made.Second, a LVRT control strategy on the basis of the theor y of Port-Controlled Hamilton(PCH) and 2L gain disturbance attenuation is designed. Considering the sag of grid voltage as a bounded external disturbance, γ-dissipative inequality is satisfied by adding an additional control which is introduced to mitigate the adverse effects resulted from external disturbances. Simulation results demonstrate that the proposed strategy in this paper makes contributions to the stability of power system under slight grid voltage sag.Then, the protection scheme of DFIG rotor side connected with crowbar circuit under grid voltage abrupt drop are studied. Through analysis of transient response of rotor currents after rotor winding shorted by crowbar circuit, the rotor transient current mathematical expression is derived which is used to guide the selection of the optimal value of crowbar resistance. Additionally, influences of different crowbar cut-off time on DFIG are simulation analysized. During the crowbar actived, DC bus capacitor and grid-side converters can be considered as static synchronous compensator(STATCOM) and reactive power is offered to grid by revising the q-axis current reference value.Eventually, logic switch rules among different control strategies during voltage sag are researched and crowbar cut-off time is further optimized according to the extent of voltage dip. Based on the switch rules, swich control Stateflow model of DIFG wind power system is established, realizing the automatic switchover among control schemes under different working conditions.