The Research Of The Grid’s Fault In Riding Through The Grid

Wind power is one of the clean power sources, and wind power station remotesfrom the load centers, parallel operating with the big power grid through the longdistance transmission. With the increase of the wind generator capacity, the influencebetween the generator and the grid after parallel operation is more and more obvious,especially wind power units operate out of the system automatically when the voltagefalls caused by grid fault. But the power grid requires that wind generator cancontinue parallel operating to realize wind power system security operation in theperiod of the voltage falling, in order to prevent the serious impact to the power gridby wind generator operation out of the system, and adding the recovery difficulty ofthe power system. In the subject of security operation and control of the double-fedwind power system, this dissertation researches on the relevant safety controltechnologies of wind power.Firstly, the mathematical models of DFIG system grid side converter and rotorside converter are established in the frames of the three-phase stator static coordinatesystem and two phase rotation d, q coordinate system. Based of it, when the voltage ofthe big power grid appears fluctuation and fall, grid side converter uses the vectorcontrol strategy of power grid voltage directional dc side voltage and current doubleclosed-loop, and the rotor consideres the DFIG control model of stator voltagechanging and the current dynamic process. But in the traditional model, it is handledas zero, then stator flux orientation and stator voltage orientation are usedrespectively to improve vector control strategy. By the simulation validation, theimproved control strategy is effective for improving the rotor side converter controlrotor current and double-fed type wind power system security operation.Secondly, the dissertation mainly analyzes the influence on double-fed inductiongenerator from power grid three-phase symmetric short-circuit fault. According to thecircuit superposition principle, the expressions of starting motor stator current androtor current are presented when double-fed induction generation occurres thesymmetrical fault. The occurring time of rotor current maximum in the fault period iscalculated, and the expression of the rotor current maximum is gotten. Crowbar circuitis switched on, and the relationship of rotor current maximum and rotor voltagemaximum is determined, and reasonable choice estimate expression of Crowbarresistance is given. The influence on rotor current, machine voltage and reactive power caused by Crowbar resistance is analysed by simulation. The simulation resultsshow that selecting resistance within the reasonable scope of Crowbar resistance cannot only restrain effectively rotor current in the fault period, but also help grid voltagerecovery after grid fault.Then, in order to research the influence on wind power system for the safeoperation from Crowbar protection circuit switch-on and cut-off time, three solutionsare used to analyze the influence on each system reference from Crowbar circuitcut-off time. The simulation result shows that Crowbar should be cut off before thegrid fault is eliminated once Crowbar protection circuit is switched on, and the timebetween Crowbar protection circuit fault cut-off and grid fault elimination is shorter,the system response is better. What’s more, it can make the every system oscillationminimum, and reactive power is injected into the grid at the time of grid fault to helprecovering grid voltage, so that the time of system restore is the shortest.Finally, through the assembly and debugging of1.5MW double-fed inductionwind power inverter, the system hardware and the process of control debugging aremastered, and the contact between theory and practice is realized.