Theoretical Analysis And Online Monitoring Of The Interturn Short Circuit Of Field Windings In Multiphase Star Brushless Exciter At Nuclear Power Plant

As an important part of large-capacity nuclear power plants,multi-phase star brushless excitation can provide high-quality excitation currents for generators.It is significant for the safe and stable operation of nuclear power units to realize on-line monitoring of turn-to-turn short circuit fault of excitation winding.According to the pivotal structure of multiphase star brushless exciter,the fault can be effectively monitored through the harmonic characteristics of stator excitation current.In this paper,mathematical modeling,simulation analysis,fault characteristic mechanism analysis and fault online monitoring device are carried out for the inter-turn short circuit fault of the excitation winding of the nuclear power multiphase star brushless exciter.Firstly,the mathematical model of multiphase star brushless exciter under normal operation and short circuit fault of excitation winding is established,including the voltage and flux linkage equations of excitation winding,rotating armature winding and DC load.Considering the actual connection between stator and rotor of motor,the voltage equation of multiphase star brushless excitation system is established with a single coil as the basic unit,and finally solved iteratively,which provides a theoretical basis for studying fault characteristics.In order to verify the mathematical model,the theoretical analysis and simulation verification of the multiphase star brushless exciter under normal operation and short-circuit fault of excitation winding are proposed.In this paper,the 3n-phase P-pole-pair brushless exciter is taken as the analysis object,and the armature current induced by the excitation magnetomotive force through the air gap is analyzed from the change of excitation magnetomotive force before fault and after.Then,according to the change of armature current and armature reaction magnetic field,the existence condition of armature reaction magnetic field is analyzed,and finally the harmonic characteristics of excitation current before and after the fault are obtained.The multi-phase star brushless exciter is simulated and analyzed by Ansys Maxwell simulation analysis software.The simulation results and theoretical analysis show that the stator excitation current mainly contains 6 times harmonics except DC component in normal operation.When the turn-to-turn short circuit fault of the excitation winding occurs,different harmonic characteristics will appear in the stator side excitation current under different armature winding forms.The combination modes of multiphase star-shaped armature windings are complex and diverse.In order to explore the excitation current harmonic characteristics of multiphase star-shaped brushless exciter under different armature winding forms,this paper discusses the armature winding forms of star-shaped brushless exciter in two aspects: the relative spatial position of each phase n branch winding and the relative spatial position of three-phase winding.And gives a unified analysis method of fault harmonic characteristics of star-shaped exciter under corresponding winding forms.Taking two armature winding forms as examples,the corresponding simulation models are constructed,and the correctness of theoretical analysis is verified.Finally,according to the unique harmonic characteristics of excitation current of exciter stator obtained by simulation calculation and theoretical analysis,this paper puts forward the monitoring principle of inter-turn short circuit fault of excitation winding of multiphase star brushless exciter and its setting method,and laid a theoretical foundation for online fault monitoring.