Numerical Simulation Of Electric Field Distribution And Structural Optimization For Stator Insulation Of Large Hydrogenerator

With the continuous development of the single-unit capacity and rated voltage level for large hydro-generator,the more and more progressive performance for structure and materials of stator bars'insulation is demanded.Usually,the insulation materials at corner and end regions of stator bars would be destructed by corona discharge which is generated by the concentration of electric field,and the term of service life would be shrinked greatly.An internal screening component and an external corona-protective coating commonly are applied in large hydrogenerator stators.The“smart insulation”which is provided with non-linear conductivity is employed as well.The reasonability of arrangement on structure and materials is depended on the precisely calculation of electric field and losses on stator bars in varies internal screening and corona protection.Taking 1000MW hydrogenator as example,studies into parametric modelling and simulating on stator bars,optimizing the slot and end parts and preparing a non-linear conductive component are conducted.The parametric model of stator bars and ends is constructed by utilizing PTC Creo software,and then comsol and PTC Creo are connected through the Livelink for PTC Creo module.Finally,an automatic simulation system is created for the electric field distribution in slot and the end of stator bar by adopting APP developer in the comsol software.The system greatly improves the research convenient an accessible method for the structural design of the stator bar.When using the simulation system,automatic modeling and simulation calculation can be realized by inputting the relevant parameters such as the structural size,material properties and electromotive force of the generator bar.In case of 0°/360°/0°full transposition structure,since comprehensive effects of the rated electromotive force and the leakage inductance of each strand,a numerical analysis of the stator bar model in the slot is conducted and some results can be got as following.The maximum electric stress would be declined with an increasing chamfer radius in all 4 kinds of inner shields,among them,the fully shielded structure has the best effect of homogenizing the electric field,and the laminate structure has the worst.The value of resistivity of inner shields should between 0.1 and 100?·m.The optimal performance occurs when the resistivity of the inner shield layer is 1?·m and the contact point distance is between 6¹8 times of the transposition pitch?the number of contacts is 4¹0?.Comparing the calculation results among gradient anti-corona structure,nonanti-corona structure and linear anti-corona structure,it can be found that the gradient anti-corona structure induces the minimum surface tangential electric field strength.Furthermore,correlated to same length,the gradient anti-corona structure reduces the electric field stress by 90%and 65%respectively.That indicates a more significant improvement of electric field in the end region caused by gradient structure.Moreover,a training sample space and a testing sample space are created by setting maximum tangential electric field strength as a functional value,in which,the length and resistance of anti-corona layer are setting as independent variables,and then the training sample space is utilized to provide exercises to the neural network model.More superior fitting and estimating accuracy curves can be established through verified neural network model,which is trained by testing sample space,thereupon,combined the trained neural network model with genetic algorithm,6 group of anti-corona structural optimization could be procured.All the proposals would be checked through the loss density and the electric field strength under 3 times rated voltage.Finally,the optimum solution can be determined as segmenting the anti-corona layer into3 sections,of which the resistance respectively is 10⁶,10⁸ and 10¹⁰?·m,and corresponding to the resistance,the length of each layer is 200,100 and 90mm.Under this condition,the maximum tangential field strength at the rated voltage is 2.3kV/cm,the maximum loss density is 0.097W/cm³,and the maximum tangential field strength at 3 times of the rated voltage is 3.56kV/cm.Eventually,the operating reliability of giant hydro-generator is certified by virtue of replacing stator model with 3 slots model,which is created in accordance with optimum structure.To explore the function of micro-nano composite in promoting electric field distribution in the bars,SiC granules in nanoscale and ZnO particles in microscale or nanoscale are fill in epoxy resin?EP?matrix to prepare a series of nonlinear-conductance composite materials which is dissimilar in filler contents and denoted by nano-SiC/EP,nano-ZnO/EP,micro-ZnO/EP,nano-SiC/ZnO/EP and nano-micro-SiC/ZnO/EP.The stress dependency of conductivity in various composites is studied via experiment.Some conclusion could be drawed as following.The composite conductivity increases with increased inorganic filler content.Meanwhile,if the proportion of micro ZnO and nano SiC comes to 2:3?5wt%in total?,the nonlinear coefficient of the composite reaches 0.07 cm/kV.That is individually increasing by 1.06,2.48 and 4.83 times compared with SiC/EP,micro-ZnO/EP,and nano-ZnO/EP in the same quantity ratio of inorganic fillings.A double layer shield is constructed by applying the ZnO/SiC/EP composite as a shield between inner shield and main insulation,it could be proved by simulating calculation and measurement that the maximum electric field stress under 3 times of rated voltage at the stator bar corner is reduced by 17%versus the single layer insulation,meanwhile,the dielectric losses under rated voltage will not change obviously.