| Ultra high voltage valve halls,responsible for mission of AC/DC conversion,play the most important role in DC transmission engineering,whose internal equipment is connected to each other by various metal fittings.Shielding ball is the most widely used form of metal fittings in the valve hall.It forms a sphere-plane gap with the surrounding wall or ground.Compared with rod-plane and rod-rod gaps,the sphere-plane gap features larger surface area of corona and greater randomness in initial position of discharge,which affects the initiation of streamer and discharge process characteristics The construction of high-altitude DC projects such as Yazhong-Nanchang±800kV,which are being implemented in China,also puts forward the research demand of air gap discharge characteristics in valve hall of high-altitude converter station.In addition,the surface defects caused during processing,transporting and installing are prone to change the gap structure,to some extent,affect the operating condition of fittings and connected devices even the whole valve hall.Therefore,it is of great academic value and application significance to study the influence of electrode surface state on the discharge characteristics of valve hall sphere-plane gap and its mechanism for the design and optimization of valve hall shielding ball in converter station.This paper,with the funds of National Key Research and Development Program"Research on the key technology of insulators and external insulation of ±1100kV DC converter station",Science and Technology Project of State Grid Corporation of China "Surface defect influence and altitude correction for discharge characteristics of large size electrode gap in valve hall of converter station" and so on,focuses on the gap discharge characteristics under surface defects on shielding ball at high/low altitude areas,and the optical and electrical characteristics of the streamer-to-leader conversion process and law of temperature distribution of leader channel is analyzed.Main research works are as follows:(1)The influence of surface defects on the impact discharge characteristics of shielding ball at low altitude was investigated.The main types and common problems of metal fittings in valve hall of converter station were investigated.7 kinds of electrode surface defects are divided into 3 categories and quantitative indexes are built.Firstly,a 3D electric field simulation model of the shielding ball-plane gap test system with defects was established.The influence of defect locations and sizes on the field intensity distribution on the surface of the shielding ball was analyzed.At SGCC UHV DC test site,by setting the same type of physical defects as in the simulation model,the long air gap discharge tests of sphere-plane was carried out to study the positive switching impulse discharge characteristics of the long air gap under surface defects at low altitudes.Based on the simulation and test results.the degree of influence of each defect type on the gap discharge characteristics was analyzed and determined.(2)Research on the influence of surface defects on switching impulse discharge characteristics of shielding ball at high altitude.At Tibet Yangbajing test base with an altitude of 4300m,aiming at bolts and burrs which have the most significant impact on the surface field intensity and the gap switching impulse discharge characteristics at low altitude area,the shielding ball switching impulse discharge experiments under surface defects and low air pressure were carried out.Compared with test data with the same condition at low altitude,the effect of altitude on discharge characteristic are analyzed.Meanwhile the discharge voltage saturation characteristic curves at different altitudes are obtained.By calculating the gap coefficient under the defects,the influence of the partial change of gap structure on the discharge characteristics of the sphere-plane gap was analyzed.(3)Measurement and analysis of optical and electrical characteristic parameters of long air gap streamer-to-leader transition process.In order to analyze the physical mechanism of the sharply decline of discharge voltage resulting from electrode surface defects,taking electrical and optical signals during discharge process as main information carrier,putting photomultiplier tube(PMT)and integrated optical electric field sensor's merit,high response speed,into full use,optic-electric combined measurement system was established,which was able to realize nanosecond measurement for instantaneous optical power and space electric field strength.After analyzing the feature of space electric field intensity and optical power of sphere-plane gap,burr sphere-plane gap and rod-plane gap,the evident differences among these three gaps were presented especially in initial streamer and dark period.In the stage of initial streamer,the injected charge in sphere electrode was much more than that in another two kinds,while space charge is important factor to affect discharge development.In consideration of the apparent enhancement of Poisson field in dark period,the criteria that the moment when space electric field strength increases sharply in dark period is defined as the accomplishment of conversion from streamer-to-leader of sphere-plane gap.According to this criterion.the theoretical calculation conversion duration of is near to measurement duration.(4)Leader channel temperature field measurement method and thermal characteristics analysis based on background schlieren method and emission spectroscopy.The Background oriented Schlieren(BOS)and optical emission spectroscopy(OES)are extended to the temperature measurement of long air gap leader channel,and the analysis models covering acquisition of light offset,calculation of refractive index and reconstruction of temperature field in BOS are established.The background oriented schlieren characteristics of air gap discharge under switching voltage were observed through the measurement system,and the distribution law of the temperature field of the discharge channel was obtained.The element spectrums in the discharge channel were collected by emission spectrum diagnosis technique.With diagnose of spectrums and temperature calculation under the condition of thermodynamic equilibrium,the value of temperature at the test point was obtained.These two methods can display large-scale overall temperature in the form of "plane" or analyze micro information in the form of high-speed "point",which can be selected and used according to actual test requirements and conditions. |
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