Reserch On The Recurring Low Voltage Ride Through Of DFIG Wind Power System

The grid codes for Wind Turbine System (WTS) on the Fault Ride Through (FRT) operation become stricter with the increase of the wind power installation into the grid. The recent grid codes in many countries have required the WTS to stay connected with the grid, and provide reactive power support. Moreover, the new grid code in Denmark demands that the WTS should ride through not only one individual grid fault, but also when multiple grid faults happened within a short time (recurring grid faults). The ability to ride through two independent faults within 0.5-3s, and six independent faults in 5 minutes is required for the WTS [1]. Which may be a new challenge to the FRT operation of the WTS.The Doubly Fed Induction Generator (DFIG) WTS uses a back-to-back converter to connect its rotor side to the grid, while its stator side is directly connected to the grid. It can achieve variable speed operation and the cost on the converter is relatively lower compared to the full-scale wind turbine concept. It is presently dominant in the variable speed WTS market,. For DFIG WTS, the voltage dips produced by the grid faults will introduce a transient stator natural flux, which does not rotate with the grid voltage, resulting in a large transient current and voltage in the stator and rotor windings, and this natural flux decays slowly with the normal vector control. On the other hand, the grid code requires the DFIG to stay connected with the grid during voltage dips, and provide reactive power within about 100ms after voltage dips. As a result, The FRT of the DFIG has been an important topic in recent years. However, most of the published works focused on the single fault ride through, the reseach on the recurring faults ride through is not often seen in the existed literature. In this paper, following aspects was done on the recurring faults ride through of DFIG WTS.The dynamic model of DFIG WTS under recurring symmetrical grid faults is established. This dynamic model is based on the FRT strategy using rotor side crowbar-which is a very common FRT strategy for DFIG WTS. The stator flux, rotor current, stator current and electromagnetic torque of the DFIG from the start of the first fault till the end of the second grid fault is caclualted. The influence of the voltage dips level, the grid fault angle and the duration between two faults on the performance of DFIG under the second grid fault is analyzed. By this model, the trainsent current and voltage under the recurring grid fault can be analyzed, the challenge of the DFIG WTS to ride through recurring grid faults can be found out. It can be regarded as the basis for following-up work.Based on the dynamic model of DFIG WTS under recurring symmetrical grid faults, A FRT strategy for DFIG WTS to ride through recurring symmetrical grid faults is investigated. An improved demagnetizing control strategy is introduced and it is applied during the voltage recovery after the voltage dips. A rotor natural current with rotor rotation frequency is generated in the opposite direction of the stator natural flux, so the damping of the stator natural flux can be accelerated. At the same time, the rotor forced current with slip frequency is generated corresponding with the rotor natural current, in order to ensure that the angle between rotor current and stator flux vector is always 1800. So the electromagnetic torque fluctuations can be suppressed. The crowbar is triggered under voltage dips, just the same strategies as that under the single grid fault. With this improved control, the transient rotor current and voltage under recurring faults will be basically the same as that under single grid fault, so the DFIG can ride through the recurring faults as long as it can ride through the single grid fault. The electromagnetic torque fluctuations can be suppressed after the voltage recovery as well, which will further enhance the reliability of the mechanical system.The dynamic model of DFIG WTS under recurring asymmetrical grid faults is aslo investigated. The stator flux, rotor current, stator current and electromagnetic torque of the DFIG from the start of the first fault till the end of the second grid fault is caclualted. The influence of the voltage dips level, the grid fault angle and the duration between two faults on the performance of DFIG under the second grid fault is analyzed. It is compared with the results during the recurring symmetrical grid faults, and the FRT staregy for recurring asymmetrical grid faults is analyzed.The emulation of recurring grid fault in the laboratory is introduced. The demands of the recurring grid fault emulation is analyzed, and a grid emulator based on back-to-back power converter is built. The topology and control scheme of the grid emulator is introduced. Furthermore, the influence of the voltage dip and recovery speed on the performance of DFIG under symmetrical recurring grid fault is analyzed. A revised dynamic model considering the influence of the voltage dip and recovery speed is established. The mathematical model of the DFIG under the recurring grid faults is improved.