Study On Asymmetric Short-circuit And Negative Sequence For Large Half-speed Turbo Generator

The external asymmetric short-circuit faults of synchronous machine includesingle phase short-circuit fault, interphase short-circuit fault, two phase to groundshort-circuit fault. When the asymmetric short-circuit fault happened for synchronousmachine, the negative sequence and zero sequence current will be produce in themachine except the positive sequence current, and in addition to the fundamentalwave, there are a series of high order harmonic. The reverse rotating magnetic fieldgenerated by the negative sequence current, can induce the current of twicefrequency in the field winding, damping winding and the solid rotor of steam turbinegenerator, and cause the copper loss and iron loss, cause the rotor overheated, and thelow operation efficiency. Therefore, the research to the synchronous generatorasymmetric short-circuit fault is importance.In this paper, as an example, the1250WM steam turbine generator at half speedwas researched, firstly, the2D finite element model was established, and the ratedoperating conditions was simulated, after the simulation, the result was comparedwith the designed value, in order to verify the accuracy of the modeling.Secondly, the external asymmetric short-circuit fault was simulated on the basisof the model. Through the build of the external circuit, study the changing of theelectromagnetic field when the single phase short-circuit fault, two phase to groundshort-circuit fault and the interphase short-circuit fault were happened under no-loadcondition for generator, and obtain the changing law of the excitation current, thearmature current and electromagnetic torque and so on; And using the method ofFourier transform, the problem of the air-gap magnetic field harmonic that wascaused by the negative sequence components was analyzed under all kinds of short- circuit fault. In addition, using the analytic method, the three kinds of asymmetricshort-circuit fault were analyzed, and then compared with the result of the finiteelement method.Then making use of the method of voltage-variable, the transient negativesequence component of the asymmetrical short-circuit current was calculated. Thevoltage-variable method can determine positive and negative sequence componentthrough the real number, when the sequence component was determined, thesequence component of induced current in the rotor can be confirmed, and we canget the expression in the ratio of the each sequence current components when it hasthe maximum induced current in the rotor, it has a certain practical significance tostudy the effect of negative sequence of motor.In the end, influence that the negative sequence component to the rotor body,wedge and damping winding was analyzes. Using the finite element model, the eddycurrent and loss distribution of rotor tooth and damping bar that under the singlephase short-circuit, two phases to ground short-circuit and interphase short-circuitwhich can be calculated. And then, according to the different materials of slot wedge,the influence to rotor and eddy current loss of damping bar is analyzed, which basedon the same kind of asymmetric operation fault and different slot wedge material,and finding out the changing rule.