Research On Stator Current And Rotor Eddy-Current Losses Of Nuclear Power Half-speed Generator Under Open-phase Condition

When the generator is running under open-phase condition,it belongs to a kind of special asymmetric operation.The study on the generator under open-phase operation condition is a dynamic synthetic problem,specifically which involves the machine and the electric power system field.When half-speed turbo-generator in nuclear power station is running at grid-connected rated condition,the capacity of machine-network is larger and the system is more complex.Under the considerations on the interaction effect of machine-network,the changes of the current after the fault and the increase of the eddy-current losses in the generator induced by the negative-sequence current,the investigation on the related problems of the generator under open-phase condition plays an important role on theory and engineering application,such as prediction of open-phase condition and the safe operation of the integrated machine-network.In this dissertation the documents about the dynamic problem,asymmetric operation and eddy-current losses domestic and abroad were analyzed at details.Then domestic research status on the asymmetric operation was also presented.Moreover,the existing problems about the open-phase operation are analyzed and the shortcomings of published dissertation study were pointed out.Finally,the main contents of this dissertation were confirmed.In order to more accurately study the dynamic problem of the interaction effect on the machine-network under disturbance and fault condition in the whole system,a field-circuit-network coupled time-stepping finite element model containing excitation system and speed stability control system was presented.In the whole model the finite-element model is adopted for the generator model,taking into account the nonlinear factors containing the magnetic field saturation,the magnetic field distortion,skin effect and so on.Further,according to establish the interface equation among the generator-transformer-power transmission systeminfinite bus system,the machine-network equation coupling was realized.The nonlinear feature,including the magnetic saturation,distortion of electromagnetic field and eddy current,were not only considered,but the dynamic change of speed and parameters of transformer and power transmission system were also considered.The implement measures of excitation system and speed stability control system were given.The entire coupling of the generator 2D magnetic fieldtransformer-transmission-power grid,including the excitation system and speed stability control system,was achieved.The model of field-circuit-network coupled time-stepping finite element for one machine infinite bus system based on excitation and speed stability control was obtained.At the same time the no-load and short circuit characteristic for a 1407MVA huge turbo-generator are experimented.The no-load and short circuit characteristic of this generator are simulated by the model in this dissertation,and then comparing with the experimental data,the validity of this model is proved in this dissertation.The physical processes of the YNd and Yd core transformer high voltage one phase or two phases open are studied by the annlytical method,and the mathematical model of fault currents is established.Based on this model,the phasor diagrams of the stator current under open-phase operation condition are given.After the core transformer,with the two wiring ways,high voltage side one phase or two phases are open,the variation laws of the generator stator current and the universal laws of the stator three phases current are revealed.According to the model of field-circuit-network coupled time-stepping finite element based on excitation and speed control,the stator current is calculated under the two kinds of open-phase conditions,which the one phase or two phases of the YNd11 transformations high voltage(HV)side are off,when the generator is running at rated grid connection connected condition.The relationship between the stator currents under different phase-open conditions are revealed,and the steady negative-sequence current is also calculated under two kinds of fault conditions.Because of adding the speed stability control system to the model,the dynamic influence of the speed control to the fault current was analyzed.Also the influence of nonlinear factors to the simulation results is considered under open-phase condition.At last comparing with the result of analytical method,the validity of the method is proved in this dissertation.According to the field-circuit-network coupled time-stepping finite element model,the eddy-current and eddy-current losses of the wedge and the damper bar,the eddy-current losses density of the large teeth and small teeth were calculated when the generator was running at rated load condition.At the same time,the eddycurrent and eddy-current losses induced by the negative-sequence current were calculated under the grid-connected open-phase condition.And the eddy-current losses distribution laws on each part of rotor were revealed.In order to explain the eddy-current losses distribution on each part of the rotor and magnetic density rate proportioning on corresponding part,the magnetic density rates of the wedge and damper bar were given.To observe the change of the eddy-current and eddycurrent losses in the wedge and damper bar after the fault,the changing curve of the eddy-current and eddy-current losses in the wedges and damper bars were given at A phase open.Considering the skin depth,each wedge is divided into three parts that are the top,the middle and the bottom.Finally,the eddy-current losses density of each part was calculated and the eddy-current distribution laws were given.To analyze the influence of wedge materials on the rotor eddy-current losses,the eddy-current losses of wedges,damper bars and rotor teeth were calculated when different materials were used for the wedges.Comparing simulated results of wedge with different materials the influence rules of wedge material on eddycurrent and eddy-current losses of the rotor were revealed.At the same time the influence of wedge end region connection types on eddy-current and eddy-current losses of the rotor was analyzed.The eddy-current and eddy-current losses of wedges and damper bars and the eddy-current loss density of the rotor teeth were calculated under different conditions which contain the wedge end region break or short and no wedge existing.Comparing with simulated results,the influence of the wedge end region connection types and wedge existing on the distribution of the rotor was analyzed,and this dissertation provided theory base for the engineering practice.Investigations in this dissertation are able to provide some theory references for the prediction of the turbo-generator operating under open-phase condition,and there is important practical significance for the turbo-generator safety and stable operation.