Research On The Phased And Nonlinear Propagation Mechanism Of Water Trees Based On The Microstructures And Viscoelastic Properties Of XLPE

Water treeing is one of the main factors which leads to the degradation and life shortening of XLPE(Crosslinked Polyethylene)cable insulation.During the manufacturing,laying and operation processes of XLPE cables,microscopic defects can inevitably be initiated in the insulation layer of the cables,and water trees can be initiated from the microscopic defects under the long-term effects of electric field and water.When the water trees propagate to a certain length,electrical trees are prone to be initiated at the tips of the water trees and lead to the breakdown of the cables.The cable breakdown and the power blackout caused by the water tree aging has been a problem which cannot be overlooked,and special attention was paid to it by industrial community as well as foreign and domestic researchers.It is generally believed that the water tree is an hydrophilic,permanent,aged structure which is composed of micron-sized voids and nanoscale channels.Existing researches show that electro-mechanical aging is an important process in water tree propagation,and the water tree propagation theory which takes electro-mechanical aging as the fundamental process is also called electro-mechanical damage theory.The electro-mechanical damage theory considers that in the water tree propagation process,the water filled microvoids can exert Maxwell forces on surrounding materials and lead to the fatigue deformation and fracture of the molecular chain segments of the material,and further initiating microcracks and water tree branches.The water tree propagation behavior mainly consists of two aspects,one is the increase in water tree length,and the other is the increase in microvoid density in water tree regions.It is found in the water tree propagation that the water trees have phased propagation phenomenon,namely the propagation behavior of water trees can change significantly in different aging periods.However,the present electro-mechanical damage theory cannot well explain the phased propagation phenomenon of water trees.The water tree propagation behavior is closely related with the microstructure of water trees.The microstructure of water trees largely decides the electric field in water tree regions,and further decides the propagation behavior of water trees.However,the present electro-mechanical damage theory generally believes that the water tree propagation concentrates in the amorphous regions of the materials,while there is a lack of concern about the damage phenomenon and damage mechanism of the crystalline regions in water tree propagation.As a result,the phased propagation phenomenon of water trees cannot be fully understood.Moreover,the present electro-mechanical damage theory considers that the damage of water tree regions(initiation and development of microvoids)develops linearly,namely the damage rate of water tree regions(material damage in unit fatigue cycle)is a constant,while it has not noticed the nonlinear damage phenomenon in water tree propagation,namely the damage rate in water tree regions can increase with the increase of aging time or frequencies,and the influence of the changes of the viscoelastic properties of the water tree regions under different aging conditions on the damage properties of the materials and water tree propagation.Based on the electro-mechanical damage theory of water tree propagation,this thesis starts from the propagation characteristics of water trees in different aging periods,and investigates the microstructural damage characteristics of the amorphous regions in water tree regions and their influence on water tree propagation.The damage characteristics and mechanism of the crystalline regions in water tree regions are investigated.Combining with the damage characteristics of the amorphous regions,the phased propagation mechanism of water trees is illustrated.In addition,the damage characteristics in water tree regions under aging conditions of different times and frequencies are investigated.The changing characteristics of the viscoelastic properties of the material under different times and frequencies,and its influence on the mechanical fatigue damage,the nonlinear damage mechanism in water tree regions under different aging conditions are illustrated.The main research works of the thesis are as follows:The microstructural damage characteristics of the amorphous regions in water tree regions in different aging periods are investigated,and the influences of the amorphous regions damage on electric field distribution and water tree propagation are explored.It is found that the water tree propagation can be divided into three stages,namely initial stage,stagnation stage,and subsequent stage.In the initial stage,the microvoids in amorphous regions are small and isolated,the electric field is high at the water tree tips,and the propagation rate of the water trees is high.In the stagnation stage,the isolated microvoids in amorphous regions interconnect with each other and leads to noticeable decrease of the electric field at the water tree tips and the propagation rate of the water trees.In the subsequent stage,irregular microvoids emerge in the amorphous region and leads to electric field distortion.As a result,the propagation rates of local water tree branches increase again and irregular water tree shapes are formed.The damage characteristics of the crystalline regions in water tree regions in different aging periods are investigated,and the damage mechanism of the crystalline regions in water tree propagation and its influence on water tree propagation are analyzed.It is found that the crystalline regions of XLPE can also be damaged in water tree propagation.When the microvoids in the amorphous regions develop to the crystalline regions,they can squeeze the crystalline regions and further lead to the deformation of the crystalline regions and the initiation of dislocations.Afterwards,the dislocation proliferates to lead to the damage of the crystalline regions.In the initial stage,dislocations are initiated in the three XLPE main crystal faces of(110),(200),(020)and slip for long distances along the crystalline regions,leading to the formation of slender slip bands and dislocation steps.At this time the crystalline regions are not damaged.In the stagnation stage,dislocations in different crystal faces encounter and form jogs,which leads to the stack of dislocations and the initiation of microcracks in the crystals,which further develops into larger microcracks.With the increase in the density of the dislocation stack,different types of dislocations in different crystal faces move and proliferate to form V shaped grooves and L shaped,U shaped dislocation pits et al.Meanwhile,microvoids are initiated at the bottom of the dislocation pits and the crystalline regions begin to be damaged.In addition,the damage of the crystalline region is an important reason for the stagnation of water trees.When microcracks are initiated in the crystalline region,the isolated microvoids in the amorphous regions are interconnected by the microcracks.As a result,the electric field at the water tree tips and the water tree propagation rate decrease remarkably.The damage characteristics in water tree regions under different aging times and frequencies are investigated,and the causative factor for the nonlinear damage of water trees is illustrated.It is found that in water tree propagation,the damage of the material in water tree regions has nonlinear characteristics.On the one hand,under the same aging frequency,the damage extent in water tree regions increases nonlinearly with aging time.On the other,under the same aging time,the damage extent in water tree regions increases nonlinearly with aging frequencies.Moreover,the nonlinear damage of water trees under different aging conditions is caused by the changes of the viscoelastic properties in water tree regions.The viscoelastic properties and the mechanical fatigue damage characteristics of XLPE under different conditions are investigated,and the cause for the nonlinear damage in water tree regions under different aging conditions is analyzed.It is found that in the mechanical fatigue,with the increase in the action time of the fatigue stress,the elastic modulus of the material decreases and the elastic energy decreases,accordingly the energy consumption rate of the material increases and increasing nonlinear damage appears with time.While with the increase in frequencies of the fatigue stress,the molecular chain segments are harder to keep up with the external forces,accordingly the energy consumption rate of the material increases and increasing nonlinear damage appears with the increase in frequency.The nonlinear damages in water tree regions under different times and frequencies are also induced by the changes of the viscoelastic properties of the material.With the increase of the aging time or the aging frequencies,due to the changes of the viscoelastic properties of the material in water tree regions,the energy consumption rate in water tree regions increases and increasing nonlinear damage properties appear with time or frequency.