High‐altitude tunnel‐catenary gap impulse discharge characteristics and breakdown voltage correction

Air gap impulse discharge characteristics of railway tunnel‐catenary at high‐altitude are of great significance to the electrical insulation design.The existing altitude correction methods for non‐uniform air gaps are not applicable to tunnel‐catenary.To investigate the impulse discharge characteristics of the tunnel‐catenary,a typical structural model was built in an artificial climate chamber.Different altitudes were simulated by adjusting the atmospheric pressure.The 50%breakdown voltages of the 300–700 mm tunnel‐catenary gap were tested at 243–4000 m.The test found that the altitude/atmospheric pressure and gap length have an evident effect on the 50%impulse breakdown voltage,and the influence of humidity is relatively small.The polarity effect was analysed by streamer theory.The{d,P,h}parameter method was employed to obtain the fitting formulas for the impulse breakdown voltage.A high‐altitude correction method for the tunnel‐catenary breakdown voltage based on 1 km altitude was proposed.Field natural tests were conducted to verify the accuracy of the proposed method.The results show that the breakdown voltage obtained by the correction method has an average error of 3.66%in the results of the field tests,indicating that the method has engineering application prospects.

Influence of positive DC electric field on rime icing on the insulator and its experimental verification in the natural environment

The electric field imposes a significant effect on ice accretion on the insulator.The impact of an AC electric field on insulator icing in an artificial climate chamber has been addressed.Compared with AC,DC electric field exerts an attraction on supercooled water droplets,which is more detrimental to the power system.Moreover,the icing on energised insulators is rarely studied in the natural environment.In the present research,an electrical,fluid coupled‐field simulation was carried out on the insulator.The force characteristics and motion deviation features of water droplets at different positive DC electric field strengths were studied.On this basis,rime icing on the insulator energised with different positive DC voltages was investigated in the field.Results show that the charged droplet shifts along the electric field line under a positive DC voltage.The deviation degree of the droplet increases with the increment of the voltage and droplet size and decreases with an increase in wind speed.As the voltage rises,the surface roughness,icing amount and icing length of rime ice‐covered insulator increase.Consequently,compared with that without energisation,the local collision coefficient,icing amount and icing length increase by 17.6%,46.4%and 63.9%,respectively,under a 40 kV DC voltage.

Effect of solid particle type,concentration and size on DC breakdown voltage of short needle-plate air gap:An experiment and simulation study

Unclean air discharge refers to the discharge phenomenon of a large number of polluting gases,droplets or solid particles floating in the air,which will cause insulation breakdown and safety accidents for the power transmission equipment that have been left outdoors for a long time.In order to investigate the effects of particle concentration,particle size and dielectric constant on DC breakdown characteristics of unclean air gap,a self-made discharge chamber was designed in this study.The air was filtered and passed into the discharge chamber to remove the influence of impurities contained in the air on the experimental results.Then,solid particles with different concentration,particle size and particle dielectric constant were added to the discharge chamber to carry out the DC positive polarity breakdown characteristic experiment.The experimental results show that compared with dry clean air,the presence of solid particle increases the breakdown voltage of the gap,and the increase proportion of breakdown voltage is related to the particle concentration,particle size and particle dielectric constant.With the same particle type and particle size,the increase proportion of breakdown voltage increases with the higher concentration,which up to 10.49%.For further investigating,the fluid mechanics model was selected to simulate the process of streamer discharge in unclean air.The results show that compared with dry clean air,the presence of particles enhances the gap discharge ionisation process,but also enhances the adsorption of particles,and the latter has stronger effect than the former,thus increases the gap breakdown voltage.At the same time,the enhancement of adsorption effect increases the electric field intensity of the gap and speeds up the development process of streamer discharge.

多源卫星高度计新融合产品的研发与评估

卫星高度计在海洋环流观测、海平面变化分析和海洋环境预报等多个研究和业务领域得到广泛应用.当前在轨运行的高度计卫星已达5颗(Sentinel-3A、Jason-3、Cryosat-2、Saral/Alti Ka以及HY-2A),观测精度不断提高,但卫星高度计融合产品的有效分辨率(可分辨海面涡旋的最小尺度)仍需进一步提升.本研究以变分原理为基础,改进背景场、背景误差协方差矩阵以及观测误差,建立一种新的多源卫星高度计资料融合方法.以南海及其附近海域为目标区域,在格距为0.08°的规则网格上,融合所有在轨运行的5颗卫星高度计的沿轨数据,得到逐日融合产品.评估分析表明:新方法得到的融合产品的有效分辨率达到125 km左右,与国际上广泛应用的AVISO产品相比(格距0.25°,有效分辨率240 km),有效分辨率提高近一倍,融合产品的精度进一步提高,可用于描述更小尺度的海洋环流结构.

Effect of inverted T arrangement on AC pollution flashover characteristics of insulator strings

The arrangement of insulator strings has an effect on its pollution flashover characteristics.A new arrangement of insulator strings,inverted T-type arrangement,is proposed in this study.The electric field distribution is simulated and the pollution flashover characteristics are tested in an artificial climate chamber.The test results indicate that the inverted T-type arrangement can greatly improve the flashover voltage of polluted insulator strings,the‘7+2’inverted T-type arrangement can increase the flashover voltage of each insulator by 8.5%on average,and the‘3+3’inverted T-type arrangement can increase the flashover voltage of each insulator by 13.4%.The discharge process is different from that of the normal suspension string when the insulator string is inverted T-shaped,i.e.a local arc is produced at the low voltage end of the insulator string and develops downward along the suspension part.While as for the horizontal tension string parts,it is difficult to generate arc during discharge development.

Progress in research on ice accretions on overhead transmission lines and its influence on mechanical and insulating performance

Atmospheric ice accretion on transmission lines is of great danger to the security of service of electrical power system. This paper reviews the progress in research dealing with the formation of ice accretions on transmission lines and the effects of ice on the mechanical and electrical performance of transmission lines. The results show that ice accretions on transmission lines can be categorized into five types： glaze, hard rime, soft rime, hoar frost, and snow and sleet. In all types of ice accretions, glaze grown in a wet regime is of the greatest danger to the transmission lines. Meteorological conditions, terrain and geographic conditions, and some other factors significantly influence the ice accumulation speed and the ice amount. Drastic decrease of mechanical property and electric property as a result of severe icing is the main reason for ice accidents. The amount of ice, the asymmetrical ice accretion, and the asynchronous ice shedding can considerably change the conductor strain, conductor sag, variation amount of the span, displacement of the insulator string, and the tension difference. The amount and type of ice, the uniformity of ice accumulation, and the con- ductivity of freezing water have significant influence on the flashover voltage of ice-covered insulators.

Three-dimensional electric field simulation and flashover path analysis of ice-covered suspension insulators

Flashover of ice-covered insulators seriously affects the safe operation of transmission lines.It is necessary to study the electric field distribution for flashover analysis of ice-covered insulators.The electric field of ice-covered insulators was mostly calculated by a two-dimensional(2D)axisymmetric model that cannot be well corresponded to actual icing situations.In this study,a 3D electric field simulation model of ice-covered suspension insulators under moderate icing condition with applied DC voltage was established.Two characteristic parameters,E_(av)and E_(max)were proposed to measure electric field distortion degree.Based on the simulation results,the effects of water film conductivity,icicle length,icicle deviation angle,and icicle distribution on Eav and Emax were studied.The results showed that icicles have a significant influence on the electric field distribution of insulators.E_(av)increases with the increase of icicle length and decreases with the increase of icicle deviation angle.E_(max)increases with the increase of the icicle length,icicle spacing and adjacent icicle length difference.When the icicle deviation angle is 45°,E_(max)is the largest.Lastly,the possible flashover paths were analysed based on the simulation results,which would provide a theoretical basis for building a flashover model of ice-covered insulators.

Influence of pollution chemical components on AC flashover performance of various types of insulators

The pollution on insulators is composed of various chemical components,which threaten the stability of external insulation.However,the existing insulation designs have not taken this issue into consideration.This work aimed to study the influence of pollution chemical components on flashover performance of insulators with various materials.Flashover tests of porcelain,glass and silicone rubber insulators polluted by eight types of common contaminants were conducted.Based on the test data,the influence of chemical components and their solubility on equivalent salt deposit density(ESDD)and insulator flashover performance were analysed;and the electrical properties of different types of insulators were compared.Results show that the ESDD contribution rate changes with chemical components.The dissolution characteristic has a significant influence on the measurement of ESDD;slightly soluble contaminants need to be considered separately.The corresponding insulator flashover gradient for ions can be summarised as Cl^(−)<NO_(3)^(−)<SO_(4)^(2−),K^(+)<Na^(+),NH_(4)^(+)<Ca^(2+).For a glass insulator,the flashover gradient is 4.9-17.0%higher than that of a porcelain insulator.For a silicone rubber insulator,this gap reaches 14.6-24.4 and 83.3-90%for freely soluble pollution and CaSO_(4),respectively,this phenomenon can be explained by the combined action of the slightly soluble character of CaSO_(4)and hydrophobicity transference of silicone rubber.

Calculation of conductive ions distribution in icicle and melting water conductivity

The flashover problem of ice-covered insulators has seriously affected the safe operation of transmission lines.Numerous researches have shown that the high conductivity water film on the surface is determined by the conductive ions distribution in the icicle which is the key factor affecting the flashover voltage of ice-covered insulators.In our previous work,the conductive ions distribution in the icicle has been obtained through experi-ments and simulation analysis.In order to calculate the conductivity of the water film,the conductive ion-specific content at different positions in the icicle has been calculated mathematically.In addition,based on the simulation of icicle melting process,the icicle melting process is analysed,and the melting water conductivity of the icicle is calculated.Moreover,melting ice test is carried out in artificial climate chamber to verify the validity of the calculation method.The calculated results are in good agreement with the test results.The conclusion obtained here would provide theoretical basis for establishing a flashover model of ice-covered insulators.

Novel dc flashover model for predicting flashover voltage of the iced insulators

This work first proposes a novel flashover model based on the deduced residual resis-tance of the ice layer. The most important finding of the field icing tests is that the double‐layer ice‐melting water film exists on the inner and outer surfaces of the ice layer. The conductivity of melting water on the inner and outer surface of the ice layer has been measured, and the corresponding surface conductivity has been calculated. According to the proposed flashover model, the ratio η of the resistance of the ice layer to that of the pollution layer gradually increases with the increase of the ratio k of the resistance of the outer surfaces to that of inner surfaces, and eventually approaches saturation. Further-more, the icing flashover criterion has been modified, and the voltage gradient of the inner surface is less than that of the outer surface. Test results show that the ice flashover voltage is lower than the pollution flashover voltage between 2.18-9.69% and 14.75- 20.32% under light ice and semi‐cylindrical ice, respectively. The simulation results based on the proposed model and the classic model are compared with the field test data. Results show that the proposed model could attain a high accuracy of dc flashover voltage and demonstrate the effectiveness of the proposed model.

Calculation of 3D transient Eddy current by the face-smoothed finite element-boundary element coupling method

To calculate the distribution of the magnetic field and eddy current density on the surface of an aluminium plate, a method that couples the face-smoothed finite element method (FS-FEM) to the boundary element method (BEM) is proposed in this study. This method combines the advantages of the FS-FEM and BEM, which can rapidly and accurately calculate the distributions of vertical magnetic field and eddy current field on the surface of an aluminium plate. The structural parameters and material properties of the coil and aluminium plate are considered. An accurate three-dimensional calculation model is established. Then, the vertical magnetic field and eddy current field distributions are calculated in this study. In the case of the same grid density, compared with the finite element-boundary element coupling algorithm, the simulation results show that the FS-FEM and the boundary element coupling method have obvious advantage in improving the calculation accuracy. The maximum relative error between the calculated results and measured results is only 4%. The proposed method in this study is available for reference to the transient open-domain eddy current field analysis.

Study on the dc flashover performance of standard suspension insulator with ringshaped non-uniform pollution

Under dc electric field,pollution may be more easily deposited around insulator string caps and pins,distributing non-uniformly on its inner and outer parts,and presenting ring-like shape.dc Flashover tests of 7-unit XHP-160 insulator string were carried out.During the tests,the ring-shaped non-uniform pollution was simulated using solid-layer method by changing the ratio I/O of inner-to-outer surface salt deposit density(SDD)and radius(r)of the heavily polluted area(the inner part).On the basis of the test data,the effects of ring-shaped non-uniform pollution on dc flashover characteristics were studied.Then,the relative variation trend of flashover voltage,surface pollution layer resistance,and leakage current were discussed.Research results show that the ratio I/O affects the dc flashover performance of the insulator string.The flashover voltage increases with the increase of I/O.With the increase of the radius r,the flashover voltage will increase at first and then decrease.Under ring-shaped non-uniform pollution,surface pollution layer resistance of insulator will increase,which results in lower leakage current and higher flashover voltage.The results will be of certain value in providing references for outdoor insulation selection.

AC flashover performance of different shed configurations of composite insulators under fan-shaped non-uniform pollution

Currently,it is founded that under unidirectional wind and certain landform,composite insulators are always fan-shaped non-uniformly polluted.In this paper,artificial pollution tests on three types of composite insulators with different shed configurations and under various fan-shaped non-uniform pollution conditions were carried out.Then the flashover performances of composite insulators,porcelain insulators and glass insulators were compared.Results indicate that there is a big difference between ac flashover performance of composite insulators under non-uninform pollution and uninform pollution.The flashover voltage of composite insulator is largely influenced by salt deposit density(SDD),the ratio of SDD of windward side to leeward side(W/L),the occupation ratio of leeward side k and the shed configurations.The relationship between SDD and U50 still meets negative power function when composite insulator is fan-shaped non-uninform polluted.There is a reduction of 17.8-27.6%in the flashover strength when the ratio W/L of SDD decreases from 1/1 to 1/15.The shed configurations of composite insulators have great effects on the flashover performance.Composite insulators always have better withstand property compared to glass and porcelain insulators under either uniform pollution or fan-shaped non-uniform pollution.

Audible noise evaluation for six-phase overhead lines transformed from existing three-phase double circuit infrastructures with uprated voltages

The drive to reduce carbon emissions and meet climate change targets set by governments worldwide has led to the growth of renewable energy sources and electric vehicles in recent years.Resulting intermittent generation and centralised demand require increased transmission capacities when operating associated power systems.Existing transmission lines are not capable of meeting planned needs.In many countries,obtaining a new right of way to build an overhead line(OHL)is increasingly difficult.There is therefore an urgent need to study techniques for transmission line uprating.Simply increasing voltages is not feasible because elevated surface potential gradient at conductor surfaces increases audible noise(AN)levels unacceptably.The feasibility of high phase order,especially six-phase,transmission techniques has been previously demonstrated,and it has been proved to be an effective way to uprate existing three-phase double circuit OHLs with increased voltages.This study analyses two typical overhead line structures,whose conductor surface potential gradient and AN level are calculated.The average and maximum surface potential gradient of the six-phase OHL are 18.5%and 18.6%lower than that of the three-phase ones.The AN of the six-phase single circuit OHL is predicted to be 6.33 and 6.45 dBA lower than that of the three-phase double circuit OHL after the voltage is increased by 45%and 50%for the lines considered.It is shown that six-phase OHLs can increase capacity by allowing elevating phase voltages without increasing acoustic noise levels.

Electrical characteristics of an energised conductor under various weather conditions

When a transmission line is subjected to adverse weather conditions,rain drops or ice can be formed on its surface,leading to discharge phenomena known as corona.The corona discharge increases corona loss,leakage conductance,and capacitance,the latter two effects facilitating harmonic conditions and power quality degradation.This study investigates the electrical characteristics of an energised conductor under rain,rime,and glaze ice conditions.The experiments were carried out in a climate room using a corona cage.It was found that the leakage conductance of a corona-cage configuration was at its highest under glaze ice condition and at its lowest under rime condition for the same background surface electric field.With the presence of icicles,the change in capacitance under glaze ice accretion was complex.From the experimental results,it can be concluded that the corona-cage configuration is mainly characterised by its capacitive nature before the corona onset.When the corona discharge occurs,the corona cage exhibits a resistive–capacitive nature.With the increase of applied voltage,the configuration becomes more resistive.