Calculations Of The Fault Current And The Unbalanced Magnetic Pull Of Turbo Generator With Inter-turn Short Ciruit In Field Windings

Inter-turn short circuit in field windings is a typical fault threatening the safety operation of large synchronous generator. The on-line monitoring of the fault is always a difficult problem in the field of the generator fault diagnosis. Electromechanical information fusion intergrates the fault electrical characteristics and mechanical characteristics. It can narrow the dead zone of fault monitoring to the utmost extent, timely detect early fault on small number of turns, and avoid serious accidents caused by the fault in the long-running.With the support of National Natural Science Foundation of China "On-line monitoring of inter-turn short circuit in field windings of the generator based on electromechanical information fusion", the emphasis of this paper is to calculate the fault current and unbalanced magnetic pull (UMP). The two basic problems are the foundation of on-line monitoring based on mechanical and electrical information fusion, the main research content and conclusion are as follows:(1) Under different level of saturation and magnetic distortion on the air-gap magnetic field, calculation and analysis of the inductance parameter are carried out, by using the multi loop method and finite element method (FEM). The influence of nonlinear variation of the magnetic field in the motor on the inductance parameters is explored. This law is further verified by the comparison of the inductance parameter under the network load and no-load operating conditions. Calculation and analysis show that multi-loop method can only ensure the accuracy of the fundamental component of inductance parameters between the excitation winding and the stator branch. However, the finite element method calculates the inductance parameters more correctly, by taking full account of the true distribution of the magnetic field(2) The mathematical model of field-circuit coupling in internal fault of the generator is established by simultaneous equations of the multi-loop equation of generator windings and the finite element equations of magnetic field. The simulation calculation is carried out by using ANSOFT software. The failure experiments on two electrical machines with different pole pairs are carried out under no-load, resistance load and network load operating conditions. The experiment results coincide well with the simulation results, which verifies the correctness of the field-circuit coupling model.(3) The early fault is generally small number of turns shorted, and the air gap magnetic field changes little caused by the short circuit fault circuit. So, the change of magnetic field induced by this fault is not considered, the inductance parameters calculated by FEM can be substituted into the multi loop mathematical model. The model of field-circuit combining model is set up. The circuit equation and magnetic field equation are decoupled to avoid iteration in solving the coupling. The comparison of fault simulation results and experimental results of electrical machines verifies the correctness and the rapidity of the field-circuit combining model. This method provides more effective tool for the research on fault.(4) Based on the harmonic magnetic motive force (MMF) caused by the harmonic current, this article derives expression of the unbalanced magnetic pull(UMP) in coordinate axis direction of stator cartesian coordinates. According to the characteristics of the stator and rotor MMF, the expression of frequency of UMP and methods of analysis of UMP are discovered. UMP can be calculated based on field-circuit coupling model. UMP of electrical machines calculation results coincide well with the simulation results, when the turn of the field winding is short circuit, which forms the foundation for the fault monitoring by use of electromechanical information fusion.