The design of transmission systems requires electric field distribution estimation, which, in case of HVDC application is strongly sensitive to thermal and electrical configuration as well as to the nature of dielectric materials being used owing to the resistive field distribution. In this paper, the field distribution in a dielectric bilayer of XLPE and rubber materials, as representative of cable junctions, is estimated based on experimental data on field and temperature dependencies of conductivity. Through space charge measurements on bi-layer dielectrics, it is shown that the space charge density and electric field distributions are to a first order estimation consistent with data issued from conductivity measurements. Most notably, the interface charge building up between the two dielectrics changes sign, depending on field and temperature. However, in the high field range (order of 20 kV/mm), charge build-up in the bulk of dielectric materials introduces further distortion to field distribution.

Correlating conductivity and space charge measurements in multi-dielectrics under various electrical and thermal stresses

A number of silicone rubber composite insulators have been examined following 15 years of service on a coastal 400 kV transmission line in the UK. Extensive measurements of their hydrophobicity, as determined by contact angle, are given along with a description of their appearance. The hydrophobicity change varied from the low voltage end to the high voltage end with the lowest contact angles being found in the middle of the string. The sheds also aged differently around their circumference and this was reflected in discoloration differences on different sides of the insulator, in addition to hydrophobicity changes. The greatest change to properties was witnessed on the core of the insulators on which contact angles of less than 70 degrees were recorded

/pdf/aging-of-silicone-rubber-composite-insulators-on-400-kv-21zvj7z17j.pdf

Aging of silicone rubber composite insulators on 400 kV transmission lines

Surface analysis of 400 kV silicone rubber composite insulators is presented. These insulators have experienced 15 years of service on a coastal 400 kV transmission line. Inspection of the insulators shows low levels of degradation through cracking and oxidation of title surface. This has been quantified by microscopy, energy dispersive X- ray analysis (EDX), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The analysis shows non-uniform ageing over each shed and ageing on the south side more advanced than on the north side. It is believed that non-uniform ageing was due to environmental factors including natural UV radiation and prevailing wind direction, and the resultant pattern of growth of organic species. EDX and FTIR were found to be the most useful and effective tools for analysis of these polymeric insulators.

/pdf/surface-analysis-of-asymmetrically-aged-400-kv-silicone-2fdh822d95.pdf

Surface analysis of asymmetrically aged 400 kV silicone rubber composite insulators

According to hydrophobicity and hydrophobicity transfer studies on silicone rubber surfaces, although the silicone rubber can transfer its hydrophobicity to various contaminants deposited on its surface, the hydrophobic states of water droplets on clean silicone rubber surfaces and on various contaminated silicone rubber surfaces were found to be different based on evaluation by contact angle measurement. Furthermore, surface discharge and aging of materials decreases the hydrophobicity of the surface. Focused on the difference of hydrophobicity and the effect of water droplets on the hydrophobic contaminated surfaces, some contrastive computations and tests of water droplets on various surfaces were carried out in this study. The results showed that the water droplets existing on the surface of silicone rubber materials might change the electric field and voltage distribution along the surface, and even reduced the initial voltage of the corona discharge, which was verified with a comparison test on a high voltage side of polymer insulator monitored by a photomultiplier tube (PMT) system. Since the leakage current provides much useful information to diagnose the state of outdoor insulation, this study investigates the time and frequency distributions of surface discharges on ceramic insulators, new room temperature vulcanized (RTV) silicone rubber coated insulators and aging RTV insulators with various hydrophobic states with the short time Fourier transform and Gabor transform methods. Then a correlation was found between the distribution of the spectrum of surface discharges and hydrophobicity via the joint time and frequency analysis of discharges on various hydrophobic surfaces.

Effect of Hydrophobicity on Electric Field Distribution and Discharges Along Various Wetted Hydrophobic Surfaces

Discharges between water drops are important in the ageing of hydrophobic outdoor insulators. They may also be important in the processes leading up to flashover of these insulators in high pollution conditions. This paper considers discharges between drops when a limited alternating current is available, as experienced by an ageing insulator in service. A phenomenon is identified in which the length of a discharge between two drops is reduced through a particular type of distortion of the drops. This is visually characterized as a liquid protrusion from each of a pair of water drops along the insulator surface. This process is distinct from vibration of the drops, general distortion of their shape and the very fast emission of jet streams seen in very high fields. The process depends upon the discharge current, the resistivity of the moisture and the hydrophobicity of the insulation surface.

/pdf/stability-of-alternating-current-discharges-between-water-4jaf6vvl0b.pdf

Stability of alternating current discharges between water drops on insulation surfaces

Dielectric barrier discharge (DBD) excitated by pulsed power is a promising method for producing nonthermal plasma at atmospheric pressure. Discharge characteristic in a DBD with salt water as electrodes by a home-made unipolar nanosecond-pulse power source is presented in this paper. The generator is capable of providing repetitive pulses with the voltage up to 30 kV and duration of 70 ns at a 300 Ω resistive load. Applied voltage and discharge current are measured under various experimental conditions. The DBD created between two liquid electrodes shows that the discharge is homogeneous and diffuse in the whole discharge regime. Spectra diagnosis is conducted by an optical emission spectroscopy. The air plasma has strong emission from nitrogen species below 400 nm, notably the nitrogen second positive system.

Generation of Atmospheric Pressure Plasma by Repetitive Nanosecond Pulses in Air Using Water Electrodes

By use of self-built natural pollution test station, the experimental investigation of natural pollution deposit (NPD) features of polymeric insulators operated under DC voltage in northern China’s inland areas for two contaminated seasons is carried out. In this paper, the surface pollution deposit law of DC insulator shed, the influence of rainfall on pollution deposit (equivalent salt deposit density, ESDD) and non-soluble deposit density (NSDD), unevenness of pollution between top and bottom surface, dust to salt ratio of insulator surface, the orthogonal ratio of ESDD to NSDD and so on are analyzed. Research results show that not all rainfall achieves obvious purging efficiency on pollution deposited on insulator surface; both the amount of precipitation and rainfall intensity are important factors influencing purging effect; the maximum pollution degree of polymeric insulator occurs before rainy season in the summer, but not before spring rain. It is recommended that these features of pollution deposit should be adequately considered in the DC outdoor insulation design.

Natural Pollution Deposit Test of Polymeric Insulators Operated under DC Voltage

Oil-paper is used as the main insulation materials in the convert transformer, one of the vital equipment in HVDC power system. The characteristics of space charge in oil-paper insulation are complex, so the research on space charge phenomenon in converter transformers is instructive on insulation design. This paper reports an investigation into the space charge accumulation and dissipation in a two-layer oilpaper insulation system under negative and positive voltages. Planar samples have been subjected to DC electric stresses for extended periods of time and the space charge distribution has been measured by the pulsed electroacoustic (PEA) technique. The influence of the interface between oil-paper layers on space charge dynamics has been analyzed, so does the influence of the polarities of the applied voltage. The test results showed that both the interface and the polarity affected the behavior of space charge. The interface between two oil-paper layers acted as a barrier to both negative and positive charge, as a result charge accumulated adjacent to the interface. The polarity of applied voltage played an important role in determining the interface charge accumulation, since the charge accumulated at the interface was in accordance with applied voltage, namely negative voltage for negative charge and positive voltage for positive charge. Because of the interface, the space charge dynamics in two-layer samples were slower than single layer paper and the interface trapped charge made charge around it dissipated slowly.

Effect of interface on space charge behavior in multi-layer oil-paper insulation

A needle-plane electrode system was used to study the hydrophobicity of silicone rubber(SR)under direct current(DC)coronaIt is found that the influence of DC corona on hydrophobicity of SR is slightHumidity,transfer time and voltage also slightly affect the hydrophobicity of SRIn particular,this influence is slighter when the surface of SR is contaminatedThe phenomenon under DC corona is evidently different with that under alternating current coronaSo,after analyzing the surface discharge,surface charge accumulation and change of surface structural under DC corona,this paper considers that the chemical reaction in SR surface is too weak to change the hydrophobicity of SR under DC corona

Influences of AC Corona on Hydrophobicity of Silicone Rubber

In this paper, model of composite insulators were designed to study distortion, movement of the water droplets when discharging occurs on the shed surface of composite insulators applied with uniformity electric field. The distortion and movements of single and multiple water droplets on the surface of insulator in ac and dc electric fields were studied. The results show that the water droplets gradually increase in length in the action of electric field, and the distortion considerably increases with the increase of the electric fields. From these results, the effect of water droplets on flashover voltage is analyzed.

https://ieeexplore.ieee.org/iel5/8642/27389/01218545.pdf

Zhou Yuanxiang

Papers

Generation of Atmospheric Pressure Plasma by Repetitive Nanosecond Pulses in Air Using Water Electrodes

Natural Pollution Deposit Test of Polymeric Insulators Operated under DC Voltage

Effect of interface on space charge behavior in multi-layer oil-paper insulation

Influences of AC Corona on Hydrophobicity of Silicone Rubber

Study of water droplet discharge by electric field computation and high-speed video