Modeling Of Stator Internal Faults In The Fractional Pole-Path Ratio Synchronous Generator And Research On Transient Electromagnetic Problems

It is very important to ensure the steady and normal operation of large synchronous generators,which concerns with maintaining the security and stability of power stations.The internal faults in stator windings are common and destructive faults of generators.Therefore,analysis and calculation of internal faults and designing protection schemes for large synchronous generators have always been the focus of attentions and researches at worldwide.The fractional pole-path ratio winding,a new type AC winding,can availably handle the mismatch problem of capacity,voltage and speed of large synchronous generators.And this new type of winding has been applied to a pumped storage power station in China.In this dissertation,a large fractional pole-path ratio synchronous generator is taken as the research object,and the new problems such as the modeling of internal faults of this generator,calculating eddy current losses of damper windings and dynamic electromagnetic force of stator core and the main protection configuration,are studied.The calculation method of mutual inductance parameters of two stator coils with arbitrary pitch are derived;moreover,the calculation method of mutual inductance parameters between stator coils and rotor loops,including field and damper loops,are also presented.The general transformation matrix which is applicable to typical internal faults is presented,and then the multi-loop model of fractional pole-path ratio synchronous generators is built,where the influences of space harmonics in the airgap and stator winding actual distribution on the simulations are taken into account.In addition,the effect of core localized saturation is modeled by modifying the airgap permeance coefficient of fault coils.The multi-loop model built in this dissertation is also applicable to the analysis of internal faults in conventional winding synchronous generators.The precise circuit-coupled finite element model(CCFEM)of synchronous generators with internal faults is proposed.Based on the actual structure of stator coils,their localized models are built,in which each coil is divided into six parts and each part is the basic unit of stator windings.Using this model,the internal faults occurring inside coils can be accurately simulated,better approximating the location of the actual fault point;thus,the fault currents can be calculated more accurately.The rotation coupling technique for internal fault models of large synchronous generators is proposed,and the handle method of relative motion between stator and rotor is simplified.Based on the actual structure of fractional pole-path ratio windings,the precise CCFEM of large fractional pole-path ratio synchronous generators is built.No-load,three-phase sudden shorted circuit,and turn to turn short-circuit experiments were carried on a synchronous generator.The current waveforms,obtained from simulation results,are compared with experimental results to verify the validity of the precise CCFEM proposed in this dissertation.The multi-loop model and the precise CCFEM of a large fractional pole-path ratio synchronous generator are built,and three types of internal faults are simulated,respectively.The comparisons of simulation results are made between the multi-loop model and the CCFEM to verify the validity of the multi-loop model proposed in this dissertation.The influence of core localized saturation on fault currents in the turn to turn fault condition is studied,and the general range of local saturation coefficients at different short-circuit turns is given.A program for the short-circuit analysis and calculation and main protection configuration of generators in hydropower stations is developed.Taking a large fractional pole-path ratio synchronous generator as an example,the fault information is discussed for this generator,and the precise CCFEM of this generator is built.The internal faults,including 8856 types,of the large fractional pole-path ratio synchronous generator are simulated.The influences of damper winding models which adopt the actual damper structure and the approximate damper cage structure,respectively,on the action situation of protection schemes are studied.The protection schemes include zero-sequence current transverse differential protections,split-phase transverse differential protections,and incomplete longitudinal differential protections.Moreover,the influences of simulation results obtained from different fault positions on the action situation of protection schemes are also studied.Finally,the main protection scheme is recommended for the large fractional pole-path ratio synchronous generator.The eddy current losses of damper windings and the dynamic electromagnetic force of stator core are studied in detail for the large fractional pole-path ratio synchronous generator under internal faults condition.Aiming at the turn to turn fault which may cause dead region of protection schemes,i.e.,the turn to turn fault with one coil shorted,the amplitude of eddy current loss of damper windings is calculated and the eddy current loss changing with time and space distribution are studied.The location of maximum eddy current loss in damper windings is found under the turn to turn fault condition.In addition,the influences of the d-axis position in case of faults and the number of short-circuit coils on the eddy current losses are also analyzed.The eddy current losses of damper windings are studied comparatively under the branch to branch fault in the same phase and phase to phase fault conditions,respectively.The location of maximum electromagnetic force at stator tooth top and tooth side wall is found under the turn to turn short-circuit fault condition.The local electromagnetic force changing with time and space distribution is studied.The study provides the basis for the optimization design of damper windings and core structure and fault diagnosis technologies of synchronous generators.