Theory And Method Research On Ultrasonic Stress Measurement For GIS Basin Insulators Based On Acoustoelastic Effect

Basin insulators are insulators of gas-insulated metal-enclosed switchgear（GIS）,and,besides of the function of electrical insulation,they also play the roles of mechanical support,supporting conductor and isolation of gas chamber.Stress is propobaly generated in basin insulators,during their manufacturing,transportation,installation and operation processes.The stress in the basin insulators is accumulated due to external load,and until the stress is greater than the mechanical strength the insulators can withstand,the cracks occur.The cracks can lead to the insulator cracking,and even more serious problems related to safety operation.At present,the overall mechanical strength of the basin insulators is usually evaluated by the hydraulic tests,but there are no researches related to their internal stress measurements.Therefore,the research on the internal stress measurements of the basin insulators can be used to prevent the cracking of the insulators and avoid the occurrence of insulation failure,which is of great significance to the safe and stable operation of GIS.In this paper,the ultrasonic stress measurement technology based on the acoustoelastic effect is used to explore the feasibility of subsurface stress and internal stress measurement for the epoxy composite specimen of the basin insulators,and the ultrasonic method for measuring the internal stress of 126 k V GIS three-phase in-one-tank basin insulators under the hydrolic test is proposed.The main contents of this paper are as follows:1)The feasibility of measuring the subsurface stress of GIS epoxy composite specimen under the uniaxial compression using the ultrasonic longitudinal critically refracted wave(L_CR)method is explored.In this paper,a subsurface stress measurement system for GIS epoxy specimen using the L_CR wave is set up.The L_CR wave is applied,which is very sensitive to the parallel stress,and the relationship between its propagation time and the subsurface stress is investigated.It is found that the propagation time of L_CRwave has a linear relationship with the subsurface stress,and then the acoustoelastic effect is verified.The acoustoelastic coefficient is calculated and the subsurface stress is measured.2)The feasibility of measuring the internal stress of GIS epoxy specimen by using the ultrasonic through-transmission method is explored.The ultrasonic longitudinal wave system based on the through-transmission method for measuring the internal stress of GIS epoxy specimen under the compressive stress is established.The relationship between the velocity of the longitudinal waves propagating perpendicularly to the compressive stress,and the compressive stress is explored.It is found that the ultrasonic longitudinal wave velocity has a good linear relationship with the compressive internal stress,which verifies the acoustoelastic effect.The acoustoelastic coefficient is calculated and the compressive internal stress is measured.The feasibility of internal stress measurements by using the ultrasonic method is verified.The influences of system delay and strain on ultrasonic internal stress measurements are analyzed,and it is found that there are great errors in ultrasonic stress measurements when the strain and deformation are ignored.3)An ultrasonic method is proposed to measure the three orthogonal stresss components（normal,radial and circumferential）in basin insulators.In this paper,the acoustoelastic equations（the relationship between the ultrasonic wave velocity and stress）of longitudinal and transverse waves under the three orthogonal stress components are derived.Based on these equations,a combined method of ultrasonic longitudinal wave and transverse wave is proposed to measure the three orthogonal stress components in basin insulators.This method can illuminate the effects from deformation along the depth direction.This method can be applied in measuring the stress in three-phase in-one-tank basin insulators in the hydraulic test.Besides,a method for measuring the ultrasonic propagation time is investigated.This chapter provides a theoretical basis for the subsequent research of the internal stress measurements in the 126k V GIS three-phase in-one-tank basin insulators in the hydraulic test.4)The required acoustoelastic coefficients of the longitudinal waves or the transverse waves under the uniaxial perpendicular and parallel stress in the above method are also measured.The systems for measuring the acoustoelastic coefficients under the uniaxial perpendicular compressive stress,perpendicular stretching stress and parallel compressive stress are set up.Based on these three systems,the relationships between the ultrasonic wave velocities and stress are investigated,and the acoustoelastic coefficients of longitudinal waves and transverse waves,propagating perpendicularly or parallelly to stress are calculated.A probe placement device for investigating the acoustoelastic effects of ultrasonic waves propagating parallelly to the compressive stress is also developed.5)The ultrasonic measurements for the three orthogonal stress components of 126 k V GIS three-phase in-one-tank basin insulators in the hydraulic test are investigated,and the results are compared with the simulation results.By using the combination method of longitudinal and transverse waves proposed above,combined with the required acoustoelastic effect under the uniaxial stress,the values of normal,radial and circumferential internal stress orthogonal components in the insulator are calculated and the distributions of stress components are evalutated.The feasibility of the proposed methods is verified by the simulation results.