Study On Damping Winding Loss And Heating Of Large Tubular Hydrogenerator Under Short Circuit Condition

For large hydro-generators,due to the increasing capacity of the single unit,the design of the fixed rotor structure is becoming more and more complex,and the electromagnetic load and thermal load are relatively high,which makes the internal loss of the generator heating and cooling problems particularly prominent.As an important part of ensuring the safe and stable operation of the generator,the damping winding can not only improve the transient stability of the system,but also reduce the impact of the negative sequence magnetic field on the rotor when the generator is operating asymmetrically.However,when the hydroelectric generator is under extreme operating conditions such as short circuit,the laminated structure of the rotor core greatly limits the generation of eddy currents,and the damping winding becomes the most severely affected part of the magnetic pole area by the negative sequence magnetic field.The additional losses generated are also mainly concentrated on the damping bars,resulting in serious local heating of the damping windings,and even irreversible damage to the generator rotor structure.Compared with other hydro-generators,due to the narrow air gap and slow speed,the cross-flow hydro-generators will face more serious loss of heat and cause a threat to their own safety when a short-circuit fault occurs.For example,in recent years,a number of bulb tubular turbogenerators at home and abroad have successively experienced faults such as electric corrosion of the damping strip and overheating and fusing of the damping strip.Therefore,in order to ensure the safe and stable operation of the generator,this paper conducted a comprehensive and in-depth analysis of the transient changes of the loss and temperature of the damping winding when the generator has different short-circuit faults.First,starting from the real structure and material characteristics of the generator,using the basic theory of electromagnetic field and circuit theory,a three-dimensional layered nonlinear time-varying electromagnetic field-circuit coupling model of the hydroelectric generator is established to solve the magnetic pole under transient conditions System losses.On this basis,considering the ventilation and heat dissipation characteristics of the generator and the thermal conductivity of the material,the heat dissipation coefficient and the thermal conductivity coefficient are determined,combined with the basic principles of heat transfer and Newtonian mechanics,a three-dimensional transient electromagnetic field-temperature field coupling model of the magnetic pole is established to accurately solve the damping for subsequent The winding loss and temperature distribution provide a theoretical basis.Secondly,using the above electromagnetic field-temperature field coupling model,by analyzing the electromagnetic field and temperature field of the hydroelectric generator under rated operating conditions,the damping winding loss under this operating condition is calculated to obtain the electromagnetic field and temperature of the rotor magnetic pole during steady state operation Field distribution.Subsequently,on the basis of steady-state operation,three typical short-circuit conditions of generator three-phase symmetrical short circuit,single relative ground asymmetric short circuit,and phase-to-phase asymmetric short circuit were realized by changing the coupling circuit respectively.The electromagnetic field and temperature field are calculated by transient time-step finite element calculation.The effects of different short-circuit conditions on the electromagnetic field and current of the hydroelectric generator,the distribution of heat loss of the damping winding and the law of transient change are summarized.The damage degree of the machine's damping winding reveals the main cause of the electric corrosion failure of the damping bar of this unit.Finally,on the basis of the above research work and relevant conclusions,further analyze the impact of damping strips with different fault degrees on the damping winding loss heating,and discuss the damping strips at 45 °,90 ° electrical erosion and broken strips The law of damping winding loss and temperature change has laid a foundation for the study of the negative sequence operation capability of the hydroelectric generator,and also provided a theoretical basis for the improvement and optimization design of the damping winding.