Research About The Problems Of Loss, Heat And Ventilation In Large Hydro-Generators

With the technologic development of electric machine design and manufacture, the power and the power density of hydro-generators are becoming larger and larger. At the same time, the problems due to loss, temperature rise and ventilation become more and more serious. To ensure the steady operation and lifespan of large hydro-generators, it is necessary to study the ways of decreasing loss and heat and improving ventilation effect thoroughly. Focus on the key technologies of design and manufacture for large hydro-generators, the problems of stator bar transposition, no-load voltage waveform, loss and temperature rise of rotor damper system, mixed ventilation system, temperature rise of stator are researched by the methods of electric circuit, wind circuit, thermal path, electromagnetic field and temperature field in this thesis. The main studies and achievements are as follows.Firstly, various transposition methods of stator bars and their effects on loss reduction are researched according to the structure feature of large hydro-generators. The advantages and disadvantages of 4 different transposition methods, including 360o full transposition, 360o void transposition, deficient 360o transposition and 360o extended transposition, are analyzed respectively. To calculate the strand circulating currents and their additional losses, the simplified formulae of leakage induction e.m.f. in the strands of 4 different transposition methods are derived and presented completely. And then, an optimal design software for stator bar transposition of large hydro-generators is developed on the platform of Microsoft WINDOWS. The effects on loss reduction of 4 different transposition methods are compared and discussed for 4 hydro-generators. Furthermore, a new idea of mixed transposition and 3 methods, including deficient 360o extended transposition, deficient 360o void transposition and 360o void extended transposition, are proposed respectively. The results show that all of the improved methods can reduce the additional losses by more than 95%, among them the 360o extended transposition method is the best, comparing with the traditional Roebel bar which uses 360o full transposition method. The 3 new mixed transposition methods provide more flexible options for the stator bar transposition optimization of large hydro-generators.Secondly, 3 different models for the computation of electromagnetic field and rotor losses, including 2D nonlinear steady-state electromagnetic field finite element coupling with the analytic formulae of losses, 2D nonlinear time-varying moving electromagnetic field finite element and 2D nonlinear time-varying moving electromagnetic field finite element coupling with circuits, are established respectively. At the same time, a model of 3D anisotropic steady-state temperature field finite element for the computation of rotor temperature is built up. These models provide the foundations for the comprehensive analysis of electromagnetic fields and temperature field, and the calculation of rotor eddy current, losses and temperature rise. The results show that the model of 2D nonlinear time-varying moving electromagnetic field finite element coupling with circuits is more accurate by comparison with the test data.Thirdly, by applying the model of 2D nonlinear time-varying moving electromagnetic field finite element coupling with circuits, the effects on improving no-load voltage waveform are researched for the first time according to the asymmetric magnetic pole structure design of large hydro-generators. The results show that eccentricities of pole shoe and damper bars have obvious effects on weakening tooth harmonic, improving generator no-load voltage waveform and power quality. The asymmetric magnetic pole structure design is a good alteration of traditional methods such as skew slots of stator core, and has advantages of the simpler structure and cheaper manufacture.Fourthly, by combining the finite element models of 2D nonlinear time-varying moving electromagnetic field and 3D anisotropic steady-state temperature field, a series of computation and analysis are carried out for different operation conditions, different rotor structures and materials of large bulb hydro-generators. The results show that the eddy current and loss of each damp bar are different, the loss and temperature rise of the damp bar at the windward are larger than that at the lee side. The highest temperature of the rotor occurs on the damper winding at the conditions of rated operation and 12% negative sequence operation.Finally, axis- and radial mixed ventilation system and the second recurrent cooling system are researched according to the structure feature of large bulb hydro-generators. The models of equivalent wind circuit and stator equivalent thermal path are established respectively, and a general iterative algorithm with good convergence is deduced and proposed for the calculation of complex wind network. Furthermore, a software for ventilation and heat calculation of large hydro-generators is developed on the platform of Microsoft WINDOWS. The mixed ventilation system and the stator temperature rise are analyzed and calculated based on the new structure design of the uneven section of the generator stator core lamination. The results show that the uneven section structure design of stator core lamination has obvious effects on improving the distribution of branch wind quantity, decreasing the temperature rise and heat of stator core and winding. The temperature rise of the generator is quite low and the generator can operate continually and safely even though overloaded 10%.