Research On Control And Protection For DFIG During Voltage Dip

With the increasing share of the wind power in the power system, the impact of its integration is becoming more widespread. Random and intermittence of wind has more and more influence on the stability of power system. Since2011, the cascad-ing disconnection failures of power system caused by disturbance frequently occur in large-scale wind power base of China’s North and Northwest. The preliminary analysis shows that the cascading disconnection closely related to low voltage ride through (LVRT) and high voltage ride through (HVRT) capability of the wind pow-er generation system and its reactive power control system. Based on its merit, doubly-fed induction generator (DFIG) has become one of mainstream model of wind-driven generator which connected with grid directly, and also the most com-mon types in all of the cascading disconnection. In order to explore the causes, op-eration and protection technology under the grid voltage dip were proposed in view of the existing problems.The analysis states that improving the control ability of DFIG under the volt-age dip is the key to deal with cascading disconnection. The method of instaneous sequence component to get the positive and negative sequence was improved and expanded to non-power frequency. The simulation result shows that this algorithm is more effective and efficient compared with the traditional method. The imple-mentation is broken down into four phases, and different objectives and strategies are presented during the voltage dip. Base on the rotor power variation, a novel feed forward control strategy is proposed for DIFG. The simulation results show that the proposed strategy can significantly improve control performance and realize unified control of grid side and rotor side converter.The cascading disconnection was revealed in the grid voltage recovering peri-od. The sudden changes of the phase angle and its impact on the vector orientation under grid voltage dip and recover, was deeply analyzed. The phase angle compen-sation theory and improved control strategy is proposed under voltage dip. In nor-mal operation stage (without crowbar), stator flux oriented control is used. In volt-age dip stage (with crowbar), deactive the rotor side converter and active grid-side converter. The phase angle compensation theory is adopted in grid voltage recover-ing period. The simulations and dynamic test shows that the improved control strategy effectively can decrease the rotor over-current and reduce the risk of cas-cading failures. Consequently, the crowbar is the most commonly method for DFIG to realize low voltage ride through. In order to study the reason of cascading disconnection and the influence among the DFIGs with crowbar, the resistance of crowbar was calculated based on the estimation formula of peak current on the stator and rotor side. The simulations show that complexity and tight coupling between DFIGs and the wind farms. Crowbar circuit has significant influence on dynamic feature of system. The resistance value and switching control strategy should account for the needs of electrical constraints and the reactive compensatory requirement for power system. The measurement of field-tests are carried out based on the SCR’s three-phase ac structure and IGBT’s three-phase rectifier structure of crowbar cir-cuit protection scheme. These data indicate that crowbar circuit should be disabled after1~2periods at the fault clearance, because its negative effects of reactive power consuming in the grid voltage recovering period. Active crowbar circuit can effectively strengthen the ability to resist grid failures, for its good controllability and enough stability margins.The real wind power delivery case with detailed power network topology and parameters is modeled, and based on it the transient process of large-scale wind turbines cascading disconnection is recurred. The dynamic reactive power response capabilities of wind turbines and reactive power compensation devices during the transient process are analyzed. A reactive power and voltage emergency control strategy with the consideration of above dynamic reactive power response capabili-ties is proposed. In the proposed emergency control strategy, the grid-side convert-ers of wind turbines are utilized to dynamically provide reactive power to the grid to mitigate voltage drop in real time. In addition, reactive power compensation devices are actively disconnected to the grid before fault removal to restrain the transient overvoltage, and then reconnected to the grid according to voltage level in recover-ing period to provide reactive power support. The simulation validates the feasibil-ity and effectiveness of the proposed control strategy.