Research Of Fault Ride-through Control For Direct-Drive Wind Turbine

Along with the enlargement of capacity of wind turbine and the scale of wind farm, the influence of wind power integration on the operation of power grid is increasingly outstanding. The wind turbine is required to meet with grid code to achieve fault ride-through under grid fault condition. Compared with doubly-fed system, direct drive wind turbine which integrates to power grid by using full power converter which is generally thought to decouple the grid and generator set is easier to achieve fault ride-through, thus having a good prospect of application. However, The wind turbine will face a series challenge under grid fault condition, especially under unbalanced grid conditions. The extraction of the synchronous signal of grid-connected power conversion is the precondition for a stable operation. Thus, this paper makes research on methods of extracting grid voltage synchronization signal and the control strategy of fault ride-through control for direct-drive wind turbine, is also studied.The control of grid-side converter of the direct-driven wind power turbine needs to extract the grid voltage synchronization signal and to achieve positive and negative sequence separation under asymmetrical grid voltage and harmonics conditions. The existing ways of extracting synchronous signal and positive and negative sequence are subjected to harmonic interference. Due to the limited performance of frequency detection and fundamental component separation of the existing grid voltage synchronization method under harmonic condition, a grid voltage synchronization method based on decoupled multiple synchronous reference frame PLL (DMSRF-PLL) is proposed. This DMSRF-PLL employs the positive, negative sequence and low-order harmonics dq transformation and their decoupling to achieve high speed and high performance output under asymmetric and harmonic condition for frequency, phase and positive, negative sequence dq components picking up. The measured results verified the performance of the proposed DMSRF-PLL method under complex grid condition. When applied to direct-driven wind power system connected to grid, the simulation result also shows that this method can eliminate the influence of the grid voltage harmonic on the output current.The power relationship is analyzed first based on the gird fault characteristics, and the challenges faced by wind turbine (take PMSG as a reference) under fault operation are summarized to two categories, active power imbalance and power fluctuation problems.The first problem is introduced by the dipping of positive-sequence grid voltage, while the second one is contributed by negative-sequence grid voltage. The reported fault ride-through control schemes are reviewed according to the summarized two challenges. The pros and cons of each control methods are identified. Finally, the further researches in direct-drive wind turbine fault ride-through control are discussed.A novel control method of PMSG wind turbine is proposed in this paper according to the problems caused by wind turbine which operated under asymmetric fault. This method can eliminate the grid side negative sequence current component and 2nd order voltage ripple in the dc link at the same time. The proposed control method employs the grid voltage feed forward to reduce the grid side negative sequence current component, while a novel power control concept, output power of generator tracking the grid side power, is applied to control the electro-magnetic power of the PMSG to eliminate the 2nd order voltage ripple in the dc link. Simulation results show that the proposed control method is effective under varies of fault conditions.To verify the accuracy of the proposed DMSRF-PLL, the three-phase grid-connected converter experimental prototype with digital control is developed, and detailed design process of software and hardware is given. Experimental verification of DMSRF-PLL method is test, this prototype is tested, The measured results verified the performance of the proposed DMSRF-PLL method under complex grid condition. The prototype is tested at the situation of steady working and the experimental results...