Study On Excitation Winding Inter-turn Short Circuit Fault Of Hydro-generator

Excitation winding inter-turn short circuit is one of the common internal faults of hydro-generator.When the fault occurs,the excitation magnetic potential will be locally lost,the excitation current will increase,and the output reactive power will be reduced,which will affect the generator’s operating performance.Internal currents such as eddy currents on the rotor side and circulating currents in the stator windings due to faults can also increase the heating of the generator.However,as the fault increases,it may cause excessive vibration and burnout of the rotor winding,and even threaten the secure and stable operation of the power network.Therefore,the investigation of excitation winding inter-turn short circuit fault characteristics is of great significance for fault diagnosis.To study the change of air-gap magnetic field in the excitation winding of hydro-generator with inter-turn short circuit,the trouble mechanism of inter-turn short circuit is analyzed.The formulas for the excitation magnetic potential and air gap magnetic density of the hydro-generator during normal operation and under trouble conditions are derived,the influence of fractional harmonics on excitation magnetic potential and air gap magnetic density is emphasized,and the phase change of air gap magnetic density before and after the fault of hydro-generator is disclosed.Finally,a hydro-generator with multiple pairs of poles is taken as the research object,and the field-path coupling method is used to establish a two-dimensional finite element simulation model of the hydro-generator.By simulation of the excitation winding fault with one turn of coil short circuit,the change characteristics of the air-gap magnetic field during the fault are obtained,and the correctness of the above theoretical analysis is validated.To study the stator winding branch loop current characteristics when the rotor winding of a hydro-generator is short-circuited between turns,the distribution law and phase relationship of the parallel branches of the generator stator winding are analyzed with the conductor electric potential in the stator slot as the basic analysis unit,and the electric potential difference and harmonic loop currents between parallel branches with different branch spacing in the same phase are calculated.In addition,the phase relationship between different branches of the stator winding in the same phase is studied to reveal the influence of the branch spacing of the hydro-generator on the harmonic loop current,and the harmonic characteristics of the branch loop current manifested between different branches of the stator winding are not exactly the same.Through the harmonic characteristics characterized by the circulating current in the stator winding after the fault,the on-line diagnosis of the inter-turn short circuit fault of the excitation winding of the multi-pole and multi-branch hydro-generator can be carried out.Finally,a hydro-generator is simulated and the simulation results show that the harmonic components contained in the loop current of the stator winding branches after the fault are consistent with the theoretical derivation results in the paper,and the results of this study can be widely applied to hydro-generators of different structure types.In order to study the frequency characteristics of rotor eddy currents in the event of inter-turn short circuit fault of excitation winding of hydro-generator,the expressions of magnetomotive force of each branch of rotor winding after short circuit fault and the expressions of three-phase synthetic magnetomotive force are derived,the rotational speed of synthetic magnetic field relative to stator and rotor is analyzed in detail,and the frequency characteristics of induced currents of synthetic magnetic field on rotor side are further analyzed to reveal the frequency characteristics of eddy currents generated on rotor surface by stator branch loop current when the fault occurs.The relationship between the frequency characteristics of rotor eddy currents and the structural parameters of the motor after the occurrence of a fault is obtained by performing fault simulation calculations on a hydro-generator.This paper thoroughly investigates the short circuit fault between turns of excitation winding of hydro-generator,analyzes the effect of short circuit fault on air-gap magnetic field and the characteristics of harmonic loop current between stator winding branches during the fault,and further studies the frequency characteristics of current induced by stator winding branches on rotor side.The relevant research results provide theoretical references for the diagnosis of inter-turn short circuit faults in the excitation winding of hydro-generator,which is helpful to improve the operation efficiency and reliability of hydro-generator.