Studies on Fault Characterstics and Analytical Models of Integrated Variable-speed Constant-frequency Wind Powe Generator
|Keywords||fault analysis of power system wind generation short-circuit current relayprotection equivalent model|
Wind generation is a strategic choice to solve the global energy crisis andenvironmental problems. The operation characteristics of power grid during fault maychange against the background of rapid growth of wind power. The problems ofambiguous transient characteristics, indistinct fault mechanism and incompleteanalytical method still exist for the new power system formed by the interconnection ofwind generations. These limitations make it difficult to set and calculate the relayprotection and threaten the safety of electrical equipments. Therefore, the keytheoretical problems about the fault mechanism and fault calculation of windturbine-integrated system are systematically studied in this paper. The fault analysis ofpower system contained wind generation is extended from the existing simulation studyon transient characteristics to the analytical analysis of fault mechanism and the faultcalculation. The research is also been expanded from the domain of symmetric fault toasymmetric fault. This research provides the necessary bases and references for thestudy and implement of power system planning, protection and control. It has importanttheoretical meanings and practical value in deepening the study of wind generation andguaranteeing the safety operation of power system.The impacts of large-scale wind generation on power grid are summarized on thebasis of overview about development history, achievement and prospect of wind powerin China. The connotation of power system fault analysis is expounded. The effects ofwind generation on the fault analysis methods are analyzed. The researches progressesof fault analysis about power grid contained wind generation are comprehensivelyreviewed in this paper. The characteristics and deficiency of existing researches are thenanalyzed.From the view of physical process and construction features of doubly fedinduction generator (DFIG), the transient models represented by instantaneous valueand space vector are completely built through the methods of coordinate transformationand space vector. The theoretical basis and transformational relation of models are thenelaborated. On the basis, the transient processes of DFIG under symmetrical andasymmetric fault are analyzed in detail by using mathematical analysis. The equivalentconversion of vector is adopted and introduced during the analysis. The characteristicsand changing regularity of electric quantities in stator and rotor winding under impact action of voltage drop are proposed.In order to solve and explicitly express the rotor current, the elaborate derivationsand theoretical analyses are conducted with innovation on the dynamic-couple processof rotor winding of DFIG-based wind turbine under grid fault. Consequently, the staticand dynamic impacts of converter controls on rotor current are explored and obtained.According to the existing problem about rotor current calculation, the dynamicalequations of rotor circuit in time fields which embody the transient dynamic of statorflux and control action of rotor-side converter are built. Consequently, the time-domainexpressions of rotor currents under symmetric and asymmetric fault are proposed.Considering the terminal transient voltage of DFIG presents common features ofsynchronous generator and induction generator, the characteristics of stator voltage ofDFIG-based wind turbine under grid fault are analyzed. The integrated processing ofgrid and generation are adopted. The stator voltage expression of DFIG understeady-state and fault-state are deduced using the mathematical model of DFIG and thegrid constraint of terminal voltage and current. In this foundation, the changecharacteristics of terminal voltage are further analyzed.The expressions of short-circuit current contributed by DFIG-based wind turbineare deduced against different fault types and operation conditions of wind generation.Then the generating mechanism, composition and changing patterns of short-circuitcurrent of DFIG-based wind turbine are presented. In order to solve the fault calculationof power grid, an equivalent model of DFIG-based wind turbine is proposed. Besides,the polytropic effect of DFIG-based wind turbine on short-circuit current of distributionnetwork are analyzed by building an equivalent circuit of distribution network. Thecharacteristics of short-circuit current in distribution network contained DFIG-basedwind turbine are then expounded through simulation.Considering the classical model of power system cannot fully reflect the operationcharacteristics of direct-driven wind turbine, the dynamic model of power systemcontained FPD-PS is built to grasp the strongly coupling between interfaced converterand power grid. The controller dynamics of FPD-PS are modeling in detail, and theprinciple of simultaneous power balance is adopted to build the system model accordingto the method of structure-reserved model. This dynamic model reserves the integrity ofsystem structure, perfectly shows the dynamic characteristics and control constraint ofFPD-PS. The method is further used to model the multi-machine power system withhybrid power sources of FPD-PS and synchronous generators. It provides a model basis for the transient analysis for new power system formed by the access of wind power andother renewable energy.