Research On Stator Side String Impedance Low Voltage Ride-Through Technology Of Doubly-Fed Induction Generator

Under the background of new energy revolution and energy transformation,wind power generation,as a new main power source for the construction of a new power system,shows a rapid development trend.With the continuous improvement of wind power penetration,the power grid will be more seriously affected by wind power,which also puts forward higher requirements for the safe grid-connected operation of wind farms.The doubly-fed induction generator is the main wind turbine type,and it is of great significance to study the low-voltage ride-through technology of the doubly-fed wind turbine and improve the active support ability of the unit during the fault.This paper focuses on the low-voltage ride-through technology of the stator string impedance of doubly-fed fans,and the main contents are as follows:The research background and significance of this paper are expounded,and the research status of low-voltage ride-through technology is sorted out and summarized.The mathematical models of doubly-fed induction motor,rotor-side converter RSC and grid-side converter GSC are established,and the dual closed-loop vector control strategies of RSC and GSC are analyzed.Based on the existing theory,an evaluation model of active support capacity of wind turbines during faults is established.The output of the unit during the fault is divided into two parts:the output of the stator side and the output of the GSC of the mesh side converter.Taking the d and q axis currents of RSC and GSC as variables,the mathematical models of active and reactive output of the unit during the fault period were established,and the active support ability of the unit during the fault period was evaluated and analyzed.Based on the transient characteristic analysis,a resistance optimization method for the series impedance on the stator side is proposed.Firstly,the mathematical relationship between the series impedance value and the rotor transient current,rotor open electromotive force and flux decay time constant is established,and then the feasible domain of the impedance value is determined by taking the time requirements of the rotor winding overcurrent,overvoltage and reactive current response during the fault as the boundary conditions.Using the active support capacity evaluation model of the unit during the fault,the impedance value is optimized with the reactive power optimal as the objective function,which is essentially a multivariate function nonlinear programming problem.Finally,the simulation verifies the validity of the selected impedance value.Based on the existing control ideas,a low-voltage ride-through comprehensive control strategy based on the stator side string impedance is proposed.Firstly,the threshold for the voltage dip depth is determined according to the slip rate of the wind turbine operation before the failure.When the voltage drop is less than the threshold,feedforward compensation is added on the basis of the traditional double closed-loop vector control strategy.When the voltage drop is greater than the threshold,the stator side puts in series impedance,and the command value of the inner loop current is given to ensure that the unit outputs reactive power to the greatest extent.The advantage of this strategy is that the response time requirements of reactive current are considered under the premise of satisfying the transient overcurrent and DC capacitor overvoltage constraints of the rotor,and the reactive output capability of the unit during the fault period is taken into account.Finally,the simulation verifies the effectiveness of the proposed control strategy and the advantages over the traditional pry bar circuit.