Search

Measurement of partial discharge (PD) is a vital method to assess the health of the electrical insulation in high-voltage power equipment. As diagnostic testing at very low frequency (VLF) is increasingly being used, it is important to investigate PD behaviours under such a low-frequency voltage excitation. This study presents the PD characteristics at VLF excitation under different applied voltage waveforms, including sinusoidal and trapezoid-based waves. Also, the effects of cavity size on PD behaviours at VLF are investigated. Experiments were performed to measure PDs in a cylindrical void bounded by solid insulation. The results show that discharge activities at VLF, in general, exhibit lower magnitude and repetition rate when compared with power frequency. Also revealed is the strong dependency of discharge parameters on the rate of voltage rise. Physical mechanisms to explain discharge behaviours are given.

There are many factors, which influence the behaviours of partial discharge (PD) within a void located in a solid dielectric material. One of the important factors is the waveform of the applied voltage on the electrode of a system, such as its amplitude and frequency. In this study, a two‐dimensional axial‐symmetric model of a cylindrical void in polyethylene layers has been developed using finite element analysis software. The model was used to simulate PD activity in the void under square waveform applied voltage under different amplitudes of the voltage. The model has considered the effect of charge propagation along the void surface during a PD occurrence. The obtained simulation results were compared with the measurement results reported from previous literature to validate the PD model that has been developed in this work. It was found that the simulation and measurement results are within reasonable agreement with only slight disagreement. From the comparison, critical parameters from the model that affect PD behaviours under square waveform applied voltage were found to be the electron generation rate and the inception, extinction and critical charge propagation fields. These findings may increase an understanding of PD behaviours within a void in a dielectric material under square waveform applied voltage, which is important for insulation diagnosis. [ABSTRACT FROM AUTHOR]

In high voltage (HV) electrical power system, variety of solid, liquid and gaseous materials are used for insulation purpose to protect the incipient failure inside the HV power equipment. Among these the solid insulation is widely used for high voltage power equipment HV electrical power system. Most of insulating materials are not perfect in all respect and contains always some impurities. The presence of air bubble is one of such impurities in insulating materials and highly undesirable for such type of insulation which causes a local weak zone inside the insulator. Insulation of the HV power equipment gradually degrades inside the insulator due to cumulative effect of electrical, chemical and thermal stress. Due to the high voltage stress the weak zone inside the insulator causes the partial discharge (PD) which is known as local electrical breakdown. As a result the insulation properties of such materials are enormously degrades its quality due to the PD. In this work, the simulation of PD activity due to presence of a small cylindrical void inside the solid insulation material of high voltage power equipment is studied with the MATLAB Simulink platform. [ABSTRACT FROM AUTHOR]

• Under shock compression lower than HEL of copper, plastic deformation occurs due to the existence of a cylindrical void. • The mechanism of plastic deformation caused by low-speed shock compressions is that the shock wave is reflected from on the free surface of a cylindrical void and then results in change of Schmid factor. • The plastic deformation caused by shock compression is significantly different from that caused by quasi-static uniaxial compression. The main reason is that the activated slip system under shock compression is totally different from that under uniaxial compression. • The significant difference of stacking fault structures caused by shock and quasi-static uniaxial compressions is mainly attributed to the large resolved shear stress gradient formed on the surface of void by shock compression. • The size of void has a slight influence on the velocity of shock wave. The plastic deformation decreases with the decrease of void diameter. This phenomenon can be well explained using the concept of geometrically necessary dislocations. Molecular dynamics simulations are performed to investigate the response to low-speed shock compression of single-crystal copper with cylindrical voids. The results show that plastic deformation occurs under shock compression that is lower than the Hugoniot elastic limit for copper due to the existence of cylindrical voids. An analysis of the interaction between shock waves and the free surface of voids is conducted to determine the mechanism of plastic deformation. Quasi-static uniaxial compression is also investigated. It is found that the resulting plastic deformation is significantly different from that caused by shock compression. Possible reasons for these differences are systematically analyzed. In addition, the effects of the shock velocity, shock direction and axial direction of cylindrical voids on the plastic deformation response are investigated and the relevant mechanisms are analyzed. Finally, the size effect of cylindrical voids is investigated and the underlying mechanism is analyzed based on the concept of geometrically necessary dislocations. [ABSTRACT FROM AUTHOR]

Abstract: Assuming the voids in a pearlitic steel are cylindrical, a cylindrical void-cell model is presented. From the analysis of the void-cell model a void evolution equation is obtained. Defining a new intrinsic time and the softening function related to the void evolution, and introducing them into an endochronic constitutive equation, the constitutive equation involving void evolution is derived. The corresponding numerical algorithm and the finite element approach are offered, which are applied to analyse the stress and porosity distributions of the unnotched and notched cylindrical specimens of a pearlitic steel. The analytical result of the unnotched specimen shows that the porosity of the specimen increases with the increase of its plastic deformation, which is consistent with the experimental data. The analytical result of the notched specimen reveals that the stress of considering void evolution is larger than that of no considering void evolution. Both the stress and porosity reach their maximum at the notch root. The latter agrees with the experimental result. [ABSTRACT FROM AUTHOR]