Modelling on thermal-fluid field in SF_(6)insulated high voltage direct current bushing with its influence on electric field distribution and metal particle motion

The insulation performance of SF_(6)insulated high voltage direct current apparatus are often analysed in a pure electric field or electro-thermal coupled field while the fluid behaviour of SF_(6)gas flow is often ignored.However,frequent flashover along the support insulators inside the SF_(6)insulated wall bushing indicates that the fluid characteristics of SF_(6)may have considerable influence on the interior insulation performance of the bushing.This paper has simulated the thermo-fluid coupled field in the SF_(6)insulated wall bushing based on the finite element method model.The result shows that buoyant flow of SF_(6)has evident influence on the temperature distribution in the bushing.On this basis,the effect of the thermo-fluid field on the electric field distribution along the insulator as well as on the motion behaviour of the metallic particles is analysed.Furthermore,the effect of different thermal boundary conditions on analysis of the multiphysics field in the bushing is discussed and the most suitable one for simulation and experiment is suggested.Comparisons are made between different supporting structures and modification advices are proposed.This paper can serve as a good reference for subsequent research studies on SF_(6)insulated apparatus in which the temperature gradient or the motion of the micro-sized metallic particles shall be considered.

Influence of frequency on the surface discharge characteristics of PEEK under positive repetitive square voltage

In this study, the surface discharge current pulses of polyetheretherketone (PEEK) material under positive repetitive square voltage in a nitrogen atmosphere are measured. The influences of different voltage amplitudes and frequencies on the detail parameters of forward discharge and backward discharge current pulses are statistically analysed. The results show that as the square voltage amplitude increases, the current pulse amplitude, fall time and pulse width of both forward and backward discharge current increase, and the rise time does not change significantly. As the voltage frequency increases, current pulse amplitude, fall time and pulse width of both forward and backward discharge current decrease, and the rise time does not change significantly. Due to the independence of the discharge at different repetitive cycles, the specific discharge process in one cycle is analysed in detail to explain the influence mechanism of the voltage amplitude and frequency on the discharge current. By mean of the Richardson-Schottky and the Cavallini relaxation model, the relationship between discharge voltage ratio and surface charge, and the decay process of surface discharge are analysed. Furthermore, the influences of amplitude and frequency of the positive repetitive square voltage on the PEEK surface current pulses are explained qualitatively.

A simple method considering the synergetic effect of space charge and inhomogeneous material conductivity for DC electric field computation

The current continuity equation is usually applied to solve DC electric field distribution when there is inhomogeneous material conductivity due to temperature gradient or multi-layer dielectrics.However,in the presence of charge accumulation,which may result from the process of electrode injection,impurity ionisation or charge trapping,the material conductivity is in tangled relationship with local field strength,and the current continuity equation can be hardly applied due to the ignorance of conductivity distribution.Thus,quantitative analysis on the effect of space(surface)charge for DC apparatus with inhomogeneous material conductivity has been a difficult problem,which requires a complicated physical model to solve.This paper has proposed a simple method to compute the synergetic influence of inhomogeneous conductivity and the space charge on the DC field distribution;through assigning bulk(interface)charge as functions of local material conductivity and field strength,the effect of inhomogeneous conductivity can be incorporated in Gauss'law and the space charge accumulation can be further included in the equation.The effect of space(surface)charge on electric field distribution in DC apparatus with temperature gradient as well as multiple-layer dielectrics is simulated through the proposed method,which shows that the method has offered a convenient approach for investigating the effect of charge accumulation on DC field distribution under various insulation structure and working conditions.