Research On Characteristic Analysis And Detection Method Of Rotating Diode Open Circuit Fault In Brushless Exciter

The brushless excitation system has the advantages of large output excitation current,low failure rate,low pollution and simple maintenance.In recent years,it has become the preferred excitation mode for large synchronous generator sets.Rotating rectifier bridge diode is a high fault risk point in brus hless excitation system.Because the rotating rectifier works with generator at high speed and large load for a long time,huge centrifugal force,excessive electric load and heat load will lead to open-circuit fault of diode,resulting in unplanned shutdown of the whole unit,which seriously affects the safety and reliability of unit operation.Based on the structural characteristics of the brushless excitation system,this thesis explores the detection method of the open-circuit fault of the rotating diode by analyzing the change characteristics of the main magnetic field before and after the rotating diode failure.The main works are as follows:1.Research on rotor vibration characteristics of open-circuit fault of rotating diode.According to the space layout and induced electrom otive force characteristics of armature winding of brushless exciter,the conduction law of armature winding is analyzed,and the variation characteristics of armature m agnetic field before and after rotating diode fault are studied.The unbalanced magnetic tension of the rotor after a diode failure is calculated analytically using the air gap permeance method and the Maxwell stress tensor method,and the vibration frequency of the rotor on the X-axis and Y-axis is predicted by combining the vector decomposition principle and the on-off law of each phase winding.Using a finite element simulation,the correlation characteristics between the unbalanced magnetic pull of the rotor and the open-circuit fault of the diode can be obtained,and the prediction that the amplitude of the fundamental frequency of the rotor,(P±1)times fundamental frequency and(2P±1)times fundamental frequency will increase significantly after a diode fault is verified.2.Research on the shaft voltage characteristics of rot ating diode open-circuit fault.Using the Fourier analysis method and the air gap permeance method,the main magnetic field characteristics before and after the open-circuit fault of the rotating diode are analyzed considering the inherent slight dynamic eccentricity and static eccentricity of the exciter.Based on the special shape magnetic flux component,the characteristic harmonics of the shaft voltage are derived,and the theoretical derivation results are verified by finite element simulation.3.An online detection method for rotating diodes open-circuit faults of brushless exciters based on flexible coils is proposed.According to the characteristics of the incremental magnetic potential of the armature after the rotating diode fails,it is proposed to wrap a flexible coil on the stator yoke of the exciter,and use the rotating frequency component of the induced voltage of the flexible coil to diagnose the rotating diodes open-circuit faults.A modified 184 B 6.8k W 3-phase 5-pole brushless excitation synchronous generator set was customized,and a two-dimensional finite element simulation model of the exciter was established according to the actual electrical structure parameters of the set.Through finite element simulation and prototype experiment,the feasibility of the proposed method is proved.