Numerical Analysis On Circulating Current Loss And Electromagnetic Force Of Stator Transposition Bar In Large AC Electrical Machine

Large AC electrical machine is the main power equipment in electric power production, with the economic development and social requirement, the safe, clean and efficient energy is required increasingly by higher demands. With the growing capacity of AC electrical machine, the losses of various parts in electrical machine increase, especially the increasing additional circulating current losses of stator bars lead to more serious excessive temperature rise of local bars, local electromagnetic force of strands is growing, which greatly threaten the safe operation and service life of the electrical machine. Transposition ways of stator bars are diverse, the electromagnetic field environment of bars in operation is complex, additional circulating current loss of bars and electromagnetic force density between transposition strands are difficult to be measured, and accurate calculations of circulating current loss and electromagnetic force density for stator transposition bar become the key issues in the design of large AC electrical machine.In view of the limitation that traditional analytical algorithm in calculating circulating current losses of stator bars could not reflect local electromagnetic field distribution, the physical model of inadequate 540° transposition stator bars with void transposition in three-dimensional stator global domain of a water-cooled turbo-generator is set up. The equivalent modeling method of the hollow strands is introduced, and numerical calculation of magnetic field for transposition bars in stator global domain is made, the distribution regularities of strand current and circulating current loss in left and right transposition sets are calculated. According to the current distribution characteristics of stator bar strands in four rows, a stator hybrid transposition bar with two circles and its transposition method are put forward, not only the amount of copper of stator bar with equal current-carrying capacity of strand is reduced, but also the strand current distribution in the whole stator bar is effectively balanced, and the maximum current and circulating current loss of bars are both reduced. The strand current of stator transposition bar is measured by experiment, and the correctness of numerical calculation is validated.According to transposition structure of stator bar and distribution characteristics of strand sections along axial direction, a multi-section method coupled field-circuit is put forward for calculating circulating current losses of stator transposition bars in large AC electrical machine. By the method of linear space approximation,transposition structure of stator bar is simulated, and the complex numerical modeling of transposition bar is simplified. By combining the method coupled field-circuit and setting up corresponding boundary conditions, multi-section coupling calculation of electromagnetic field and circuit for stator bar is realized, the size of numerical calculation is decreased, and the leakage magnetic field distribution and strand current distribution of transposition bars are solved fast. By comparing calculation results of magnetic flux density, current density, strand current and induced voltage with three-dimensional finite element method, the effectiveness and high efficiency of multi-section method coupled field-circuit for calculating circulating current losses of stator transposition bar are verified.By analyzing the transposition structure of strands, a subdivision strategy and corresponding calculation method of circulating current loss of stator bar is proposed,which satisfies the requirement of similar elements in three-dimensional scaled boundary finite element method, and the existing difficulties in the element subdivision of air domain surrounding transposition bending with three-dimensional scaled boundary finite element method are solved. By comparing the calculation results with finite element method, the element subdivision method with scaled boundary finite element method not only effectively overcomes the difficulties of physical modeling for transposition strands, but also substantially reduces the size of the element subdivision, the calculation results are anastomotic.According to the Lorentz force method, electromagnetic force density distribution of transposition strands in the stator bar of water-cooled turbo-generator is calculated,based on same-phase current, out-phase current and different transposition factors,electromagnetic force density distributions and influence regularities of strands and transposition bendings along different axial positions are researched. The influence of different transposition ways on radical electromagnetic resultant force of a single strand of stator bar is calculated, and the relationship between different transposition ways and radial electromagnetic force distributions is definite,which provides a theoretical basis for the optimization design of stator bar transposition ways.