Research On Crowbar Circuit Optimization Control And Fault Characteristics Of Doubly Fed Induction Generator

Under the background of China’s pursuit of "peak carbon dioxide emissions,carbon neutralization" and "constructing new electric power system with new energy as the main part",the transformation of energy industry has become the general trend.Wind power generation is considered to be the most mature technology,the largest scale,and the best form of new energy power generation with the best commercial development momentum.Doubly-fed wind turbine has become the mainstream model due to its cost and performance advantages.However,due to its stator directly connected to the power grid,its ability to resist grid voltage disturbances is poor,so its fault ride-through technology has received extensive attention.Crowbar protection circuit is the most widely used fault ride-through scheme because of its low cost and simple control.However,after crowbar is put into operation,the unit will lose controllability and absorb reactive power from the power grid.It can be considered to improve its protection strategy to improve the transient characteristics of the unit.In this paper,the fault characteristics of doubly-fed wind turbines and the optimal control of crowbar circuit are studied.The main contents are as follows:The research status of low voltage ride through technology of doubly-fed wind turbine at home and abroad is summarized.The mathematical model of doubly-fed wind power generation system is established,and the mathematical equation,equivalent circuit and converter control strategy of doubly-fed induction motor are introduced.Based on the vector equation of doubly-fed machine,the steady-state and fault transient circuit responses are derived.Considering the input delay of the crowbar circuit during the transient process,the fault is divided into two stages before and after the crowbar is put into operation.The superposition principle is used to solve the circuit,and a more accurate expression of the stator and rotor current is obtained.The gridconnected model of the doubly-fed fan is built in PSCAD/EMTDC,and the theoretical derivation results are verified.Then,the key parameters of the crowbar circuit and its influence on the fault transient characteristics of the doubly-fed wind turbine are studied by combining simulation comparison and theoretical analysis.It is found that due to the existence of freewheeling diodes in the rotor-side converter,it is impossible to limit the impulse current flowing through the rotor winding and the rotor-side converter to a lower level at the same time by relying solely on the crowbar circuit.An improved circuit with series protection resistance added to the crowbar branch near the converter side is proposed.The purpose is to make the impedance of the rotor side converter branch controllable,so as to limit the impulse current flowing through the branch.The resistance values of crowbar resistance and protection resistance are set by simulation induction.Based on the improved circuit,aiming at the shortcomings of the traditional crowbar circuit switching strategy,a set of adaptive switching strategies is proposed.The PCC voltage vector is added as the input criterion,and the effective value of the PCC voltage and the effective value of the rotor current are used as the basic removal criterion.Finally,the effectiveness of the proposed protection strategy is verified by PSCAD simulation.