| Large turbo-generator plays an important role in China's power system and national economic development,and the operation stability of the turbo-generator is directly related to the safety and stability of power grid.Asynmmetric load is an common running state of the large turbo-generator,and the resulting negative sequence component will cause magnetic field distortion,rotor temperature rise and torque ripple in the turbo-generator,which will endanger the service life of the turbo-generator and affect the operation safety of the unit.The existing references present the limited negative operation ability only,but the current,magnetic field and force,magnetic torque and rotor temperature caused by the negative operation state are not be research in-depth.The paper foucs on the negative sequence mechanism,magnetic field distribution with negative operation,magnetic torque,rotor temperature with negative operation,etc.According to the operating characteristics and system requirements of large turbo-generator,the mechanism of negative sequence current's damage on large turbo-generator is clarified based on the study of the basic theory of negative sequence operation and the calculation method of feature quantity,and the expressions of the stator single-phase ground short-circuit and the two-phase short-circuit transient negative sequence current are deduced based on the topological structure of the asymmetrical operation of the turbo-generator.Based on the theory of steady-state motion analysis and the expression of the magnetomotive force vector trajectory,the combined magnetomotive force trajectories of the turbo-generator under various negative-sequence components are given,which reveals the influence rules of negative sequence component on the trajectory of the magneto-motive force of the turbo-generator.Taking a large-capacity nuclear power turbo-generator as an example,the steady-state negative sequence and transient negative sequence operation ability are determined by combining the requirement of engineering operation.Based on the analysis of the negative sequence running process and the magnetomotive force of the large turbo-generator,the two-dimensional electromagnetic field field-circuit-motion coupling mathematical model of the turbo-generator is established by combining the magnetic potential differential equation of Maxwell's electromagnetic field.Based on the structural data of a large turbo-generator,the physical model of the negative-sequence operation of the turbo-generator is established by rationalizing the structure of the motor.And the rationality and validity of the model is validated by the experiments of no-load and short-circuit of the real machine.On this basis,the time-space variation law of the large tooth surface of the rotor and the influence law of the negative sequence component on the magnetic field distribution are determined through calculating the magnetic field of the turbine generator under the condition of rated condition and various negative sequence components,analyzing and comparing the distribution characteristics of flux density in the same negative sequence component under different conditions,and analyzing the characteristics of air gap magnetic field under the condition of no load,rated and limited negative sequence component at the same time.The expressions of electromagnetic torque of the turbo-generator with negative sequence components are deduced and the mechanism of the generation of double frequency electromagnetic torque under the condition of negative sequence component operation is clarified based on the expression of the magnetomotive force traveling wave and the virtual displacement method of the synchronous motor.The law of frequency conversion of the electromagnetic torque under the different states when the positive sequence components are the same and the negative sequence components are different as well as when the negative sequence components are the same and the positive sequence components are different is revealed through analyzing the numerical calculation and torque spectrum characteristics of electromagnetic torque under the influence of rated condition and different negative sequence components.And the influence law of negative sequence component on its distribution is revealed through calculating the force distribution of the field windings and the both sides of rotor slots under the negative sequence operation and analyzing the stress distribution characteristics of field winding and rotor groove wall under rated condition and various negative sequence components.Considering the different situations that the rotor teeth of the turbo-generator rotor may be damped or not damped and the rotor slot wedge may adopt magnetic or two non-magnetic materials,the electromagnetic field distribution,air gap flux density waveform and various harmonics of turbo-generator are calculated when the negative-sequence component exists in the armature current of the turbo-generator.The mathematical model of two-dimensional steady-state temperature field analysis of turbo-generator is established by selecting the two-dimensional cross-section of the axial position of the rotor of the turbo-generator as the temperature field.Based on the results of electromagnetic field analysis,the characteristics of the temperature field of the rotor of the turbo-generator are compared and analyzed when the turbo-generator rotor employs different wedges and the armature current has a negative sequence component.The research of this dissertation can present a foundation for structural design,operation performance analysis and negative sequence problem research of large turbo-generator. |
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