Research On The Fault Characteristics Anyalysis And Protection Strategy Of Grid Connected Wind Farm With DFIGs

Because of the economic and technical advantages, Doubly-fed induction generator (DFIG) has become one of the most widely used wind turbine generators in the world. High penetration of wind farm integration will bring challenges to the safety and stabilization of power grid. Considering the complex structure and converter control strategy of DFIG, the short-circuit current provided by the DFIG is obviously different from the conventional synchronous generator. Therefore, it is necessary to study the fault transient characteristics of DFIG and the influence to relay protection. Based on the research of mathematical model of DFIG, this article has studied the transient characteristics of the stator flux linkage, and the fault current characteristics considering the rotor-side converter control and Crowbar protection circuit, and the stator and rotor short-circuit current mathematical expressions are deduced, finally,it is verified by PSCAD/EMTDC. Based on the analysis above, the wind generator protection and collector line protection strategy are studied.According to the operationg principle and structure of DFIG, applying the common coordinate transformation theory, the mathematical models of DFIG are established under the three-phase static coordinate system and synchronous rotating coordinate system. The grid-side and rotor-side converter control models of DFIG are derived, and based on the analysis above, the simulation model of DFIG are established in PSCAD/EMTDC.Based on the space vector model of DFIG in synchronization reference frame, the fault transient process of DFIG are analyzed considering the rotor-side converter control, and the stator and rotor short-circuit current mathematical expressions are deduced. The stator flux linkage,the rotor and stator current transient characteristics under the grid fault are simulated by using simulation software PSCAD/EMTDC.The basic principle of Crowbar protection circuit of DFIG is analyzed, based on the vector space model in the three-phase static coordinate system, the stator and rotor short-circuit current mathematical expressions are deduced considering the Crowbar action of DFIG. The fault characteristics of DFIG are simulated under the Crowbar protection operating strategy, and the influences of the Crowbar resistance and rotor speed on fault characteristics before the grid fault are simulated by using simulation software PSCAD/EMTDC.Based on fault characteristics of DFIG under grid fault, the protection strategy within the wind farm has been studied. First of all, from the protection configuration of the generator protection, considering the grid demand for wind power integration, we need to coordinate generator protection with the low voltage ride through characteristics. Aiming at the problems of existing protection configuration of the collector line, the relay protection configuration and setting of the collector line with the fuse of step-up transformer are studied.