Effects Of Negative-sequence Component Changes On Turbo-generator Magnetic Field

For the high rotating speed and the solid rotor of turbo-generator, the negative-sequence current components in the air gap of the generator will generate the rotating magnetic potentials that have synchronous rotating speed and different rotating direction with the rotor. The magnetic potentials will induce eddy currents of second harmonic generation in the large teeth, small teeth and slot wedges on the surface of the solid rotor as well as in the retaining rings and damper windings,leading to the temperature rise of the rotor and the decline of operating efficiency.The negative-sequence currents will also induce various harmonics in the excitation windings, which will further intensify the temperature rise of the rotor. The asymmetric running of the generator caused by the asymmetric loads and other factors will lead to the distortion of the magnetic field in the rotor. Moreover, the damage of the torque ripple generated by the negative-sequence currents to the mechanical structure of the generator rotor shall not be ignored. Therefore, it is crucial to study the effects of negative-sequence current components on turbo-generator magnetic field.In this dissertation, the 1266 MW large half-speed turbo-generator as an example was researched. The 2D electromagnetic field model of the generator by using the finite element software was established, the generator at rated current was simulated,and the result was compared with the electrical quantities from the simulation calculation with the design parameters of the generator to verify the accuracy of the model.The theoretical derivation of the electromagnetic torque of the generator at rated current and at negative-sequence current respectively was conducted by researching a large number of references, to analyze the effects of negative-sequence components on the electromagnetic torque of the generator through formula and find out theregular changes of electromagnetic torque. The model was simulated with the generator at different negative-sequence current components and the torque was conducted a spectrum analysis by using Matlab software to verify the correctness of the theoretical analysis.The effects of negative-sequence components on the magnetic field distribution and the electromagnetic force was analyzed. The simulation results was summarized and analyzed with the change rule of generator field distribution at different negative-sequence current components through; the excitation winding of generator rotor and the electromagnetic force of rotor slot was respectively calculated when the generator running at negative-sequence components to find out the change rule of the electromagnetic force along with the changes of the negative sequence components;the air-gap magnetic density waveform was decomposed by Fourier transform to find out the amplitude of each harmonic and the change rule of each harmonic component;and the relation between the equivalent air gap length and the negative-sequence component was analyzed to summarize the change rule of the equivalent air gap length along with the change of the negative-sequence components.