| For large hydro generator, it is essential to be operated in certain asymmetrical conditions, in which the negative sequence component would be emerged in armature current. It is proved that such negative sequence current could do damage not only to the generator, but also to the power system. As the unit capacity of the hydro generator keeps increase continuously in recent years, the damage by such negative sequence current could do, will be more severe. The damper windings, which are always equipped on hydro generators nowadays, could increase the capacity of the hydro generator to operate asymmetrically, and decrease the damage that such negative sequence current could do to the rotor. So, to study the current of the damper windings is necessary in process of analyzing the asymmetrical operation characteristics of the hydro generator.The dissertation starts with researching on the method of calculating the damper winding current while hydro generator operates asymmetrical. Aiming at a tested synchronous generator with the structure of salient pole, the field-circuit coupling method is applied. Such method could take the influence of the stator slots, the shape of the pole shoes and the saturation of the magnetic field into account, and therefore it could get more accurate calculation results. While calculating the damper winding current, the ends of the damper windings are usually simplified analytically. Such simplification could be suitable for the generator with the conventional structure. However, if the structure of the end of the rotor were changed like the tested generator in this paper, such simplification would bring errors to the calculated results. For purpose of calculating the damper winding currents of the hydro generator with unconventional structure at the end of the rotor, a modified method by means of solving the electromagnetic field at the end of the rotor by numerical way is brought forward in this dissertation. The modified method in the paper could calculate the damper winding current in hydro generators not only with the conventional structure, but also with the unusual structures at the end of the rotor as the tested machine in the paper.The method presented in the paper is proved to be reliable by the measured data acquired from the tested machine which is equipped the wireless current sensors on the damper bars for the sake of acquiring the instantaneous value of the damper winding current. And then, the harmonics of the damper winding current are studied while the generator operates asymmetrically. In the paper, the magnetic field in the machine is decoupled into the excited field, the positive sequence field and the negative sequence field. The influences of each field on the harmonics of the damper winding current are analyzed separately. And the differences of the results under the linear and nonlinear magnetic field are compared and discussed.Finally, the method mentioned above is applied on a350MW hydro generator. The currents, the losses and the heat of the damper bars in this generator are calculated while it operates asymmetrically. The asymmetrical operations include the static negative sequence condition and the transient negative sequence condition. In the former condition, the static negative sequence current is ruled within the national standard of hydro generator in China. In the latter condition, the currents, losses and heats of the damper windings are analyzed in two sudden short circuit accidents which are the sudden short circuit between two phases and the sudden short circuit between one phase and the ground. To get the vector form of the negative sequence current, the instantaneous values of the armature currents acquired from the method mentioned above are transformed. After the negative sequence currents are acquired, the permitted time of short circuit ruled by the national standard could be determined. And then the heats of the damper windings within the permitted time are calculated. |
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