Research On Magnetic Equivalent Circuit Model And Short Circuit Of Stator Transposition Bars In A Turbo-Generator

The high efficiency and safety of the turbo-generators is the key to power supply and safe operation of power grid.The stator bars are the important energy conversion components in the turbo-generators.The additional circulating current loss is determined by the transposed method of stator bars,and electromagnetic force density of the transposed strand is directly related to the normal operation of stator bar and its service life.This paper takes a 1240 MW turbo-generator as the research object,magnetic equivalent circuit(MEC)method is calculated on the no-load condition and rated load condition.The rated current is obtained by MEC as the known quantity of circulating current loss calculation of different transposed stator bars and internal short circuit fault calculation of stator bars,the circulating current loss of different transposed stator bars,the modeling method of internal short circuit fault in stator bars,and the short circuit current and electromagnetic force density of strands in different situations are studied.The nonlinear transient MEC model of the turbo-generator is established,and MEC model of air gap which could meet different saturation conditions is proposed.The nonlinear calculation of MEC model based on node magnetic potential is analyzed in detail,and it is proposed that the convergence problem of the nonlinear iterative process is easier solved by using continuous piecewise fitting function between magnetic flux density and relative permeability of stator core and rotor core and relaxation iterative method together.The radial magnetic flux density and circumferential magnetic flux density of air gap,radial magnetic flux density of stator teeth and induced electromotive force are calculated,and compared with the results of the finite element method to verify the correctness of the proposed MEC model.The no-load characteristics are calculated and compared with the finite element results and experimental results to further verify the rationality of the proposed MEC model.The radial flux density of air gap on the rated load condition is calculated and compared with the finite element results.It is pointed out that the rated current of the turbo-generator can be obtained quickly by using the MEC method.The mechanism of circulating current loss of stator bars is analyzed,and 360°transposition methods of stator bars in the slot and non-360° transposition methods of stator bars in the slot are analyzed.The four-row integral transposition method of the stator bars for turbo-generators with the four-row strands is proposed.The threedimensional stator model without end windings is calculated.The double Roebel 540°transposition method,double Roebel 360° transposition method and four-row integral360° transposition method of stator bars are modeled respectively in one slot,and solid windings are modeled in the other slots.The three-dimensional stator model with end windings is calculated.In one slot,double Roebel 540° transposition method,four-row integral 360° transposition method,four-row integral 451° transposition method and the four-row integral 360° transposition with two segments of 78 mm void transposition method of stator bars are modeled.The other slots are modeled by solid windings.An equivalent circuit network model is proposed to consider the short circuit fault between different transposition strands of the stator bar occurring at different positions.The calculation method of all the parameters in the equivalent circuit network model is given,and the calculation of the inductance in the slot and the inductance at the end of the transposed strands is analyzed in detail.The equivalent circuit network method is used to calculate the short circuit currents of two adjacent strands located in the notch and two strands located in different rows in the radial middle of the bars.The proposed equivalent circuit network model is verified by finite element method.The finite element model is established to solve the normal condition of stator bars and the short-circuit fault at two different axial positions.The short circuit currents of stator bars are calculated when the short circuit fault occurs at the same position in the same phase slot and the different phase slots.The short circuit currents are calculated when the short circuit fault occurs at different axial positions in the same phase slot.The size and rule of short circuit currents are analyzed when the short circuit fault occur in the strands in different positions.The electromagnetic force density distributions of strands in the slot and at the end under normal condition and different short circuit fault conditions are discussed.The correctness of the short circuit fault model is verified by calculating the three solve models with all the strands as normal.