Study On Low Voltage Traversal Of Doubly - Fed Machine With Improved Crowbar Protection

With the growing demand of energy and continuous failure of conventional energy, wind energy, an emerging green energy, gains more and more attention. With the rapid development of wind power technology, the proportion of wind power capacity in the grid increases year by year. The impact on the grid is also growing. In order to safeguard the security and stability of the system, all the countries have formulated the LVRT requirements that the wind turbine will not work out of the grid.Currently, in order to ensure that the DFIG will not work out of the grid and at the same time, it can meet the requirements of the LVRT when the grid voltage is broken-down, the study can be roughly divided into the following two aspects:First, improving the excitation control strategy of the converter which is only applicable to the grid voltage drop under mild conditions. While if the drop is sharp, it is difficult to meet the requirements. Second, increasing hardware protection circuit. It requires install Crowbar circuit beside the DFIG rotor when the voltage is drop, but this method makes DFIG rotor shorted, then the DFIG will turn into induction motor running and absorb large amounts of reactive power from the grid. In the end, it aggravates the instability of the grid voltage near the wind farm. In the view of the above situation, this paper presents an improved Crowbar circuit. On the one hand, it can protect the DFIG under the condition of three-phase short circuit, on the other hand, it can achieve low voltage ride through when the voltage drops to the limit.In order to facilitate the study of the features of DFIG when the three-phase circuit is shorted and the voltage drops to the limit, this paper establishes a dynamic mathematical models of DFIG on the basis of operating characteristics of the wind turbine and DFIG. Achieving the state space model according to the change of the current which is on the basis of mathematical equations. Taking instant-conditional process of the three-phase short circuit and the voltage drop as the composition of the stable operation and reverse voltage operation according to the superposition principle. According to the requirements of the three-phase short circuit and LVRT, this paper analyzes the operating characteristics of the three-phase short circuit and the voltage drop, it achieves the main factors of the three-phase short circuit and the voltage drops to the limit. Putting forward the improved Crowbar circuit under the condition of the three-phase short circuit. Achieving parameter setting of the Crowbar circuit on the basis of the study of the Crowbar circuit. Establishing the mathematical models on the analysis of the operating characteristics when the voltage drops to the limit. Then it studies the control strategy which consists of the Crowbar circuit and the rotor excitation voltage. At last, it analyzes the operating characteristics of the LVRT.Based on the theoretical study and according to Matlab/simulation platform and the mathematical model of the DFIG three-phase short circuit and the voltage drops to limit LVRT, it builds a simulation model of the DFIG three-phase short circuit and the voltage drops to limit LVRT. Theoretical analysis and simulation results show that the improved Crowbar circuit not only can realize the protection of the DFIG during the three-phase short circuit, but also can achieve LVRT when the voltage drops to the limit.