| With the deepening of energy crisis and the continuous increase of public concern to the environmental problems, wind power reveals a trend of dramatic increase all over the world. The structure of doubly fed induction generator is different from the traditional synchronous and asynchronous motors. The research of DFIG transient characteristics and fault mechanism is not enough, and mature failure analysis method and mathematical calculation model have is needed to be established.Firstly, a modeling method for the RSC transient analysis have been proposed in order to make the DFIG transient analysis suitable for more power grid faults. The waveform characteristics of important transmission nodes of the control signal are studied, and the "trend" of the rotor voltage and current is obtained with Simulink. And strict mathematical logic is used to prove that the "trend" meets RSC inner loop control strategy. Then, a RSC transient modeling method which applies more control strategy is concluded.For the resolution of applying Superposition principle, Laplace transform is complicated; the mathematical model is solved based on the state transition matrix and the flux linkage as the state variable. Positive diagonal solution determines the frequency and decay time constant of the solution of the state variable. And if the regions of two negative angle not equal zero,it will migrate the frequency and attenuation time. Otherwise, there is no impact. Rotor converter transient equivalent model based on the control strategy correspondingly changes two areas below the four regions and the integral solution is suitable for various fault types. Simulation results show that the active power and reactive power have a serious impact on the first cycle, and the effect is weak in the follow cycles.The effect of RSC control strategy in regard to the DFIG transient current is demonstrated. Nowadays, the DFIG’s ability of Low Voltage Ride Through is getting better. And the effect of rotor side converter control strategy is more and more obvious, because the crowbar circuit protection isn’t relied on under a certain degree of voltage dips. Motor model under the control strategy of rotor side converter is resolved by state transformation matrix. The generator using PI control strategy is used as the research object. The transient current amplitude and attenuation time constant are affected by proportionality coefficient which is as well as additional resistance value of the rotor-side winding. What’s more, the frequency, amplitude and decay time of transient current are affected by integral coefficient. However, this value is small. Then, it is the main reason of the change of the transient current frequency that voltage compensation term is used in the decoupling control strategy. Finally, frequency is also affected because the control strategy of active reactive power calculation expression is different. Therefore, changing control strategy in time will help weak the transient component and cooperate with protections.Finally, study of fault current characteristics of DFIG considering impact of crowbar protection is proposed. The transient current of DFIG is more accurate when considering the influence of the control strategy of the converter. Current expression of Motor model is resolved by state transformation matrix and both impact of crowbar protection and the control strategy of RSC are considered during calculation. DFIG physical mechanism of the transient process was expounded from the Angle of the electromagnetic field. It is concluded that the rotor winding may withstand voltage changed three times. What’s more, calculation expression, being applicable to the calculation of engineering is given. The results are validated by the actual short circuit test data of wind farm. |
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