Conductor Screen Research Articles

Inception of vented water trees in high voltage XLPE cable insulation: Effect of inorganic contaminations inside the semiconductive material

AbstractWet‐design AC subsea power cables are currently rated up to a maximum voltage of 72.5 kV. These high voltage (HV) cables are cost‐effective alternatives to subsea cables kept dry by a protective lead sheath and exhibit superior properties for dynamic applications. However, several vented water trees, degradation phenomena reducing the cable lifetime, are observed in a HV XLPE insulated cable subjected to wet aging in the laboratory. The water trees initiate from a smooth and clean XLPE insulation/semiconductive screen interface. In this paper, we show that voids in the range 15–150 μm are observed in the conductor screen, 50–320 μm from the water tree inception sites. Degradation of the semiconductive material is observed, in the form of porous regions that connect the voids to the water tree inception sites. NaCl impurities are detected on the void surfaces and in the porous regions. These features are proposed to be the pre‐requisites for the formation of the vented water trees observed in this type of cable. The mechanism responsible for the formation of the void and porous regions in the semiconductive cable screen is discussed based on the observations.

Loop-to-Loop Magnetic Coupling Through a Circular Aperture in a Planar PEC Screen of Finite Thickness

An analytical formulation for magnetic field penetration through a circular aperture in a thin perfect electric conductor (PEC) screen with finite-thickness is developed. Especially, we propose an analytical formula for the magnetic coupling between two circular loops separated by the screen and placed coaxially with the aperture. The formula is verified by finite element simulations and experimental data. The thickness effect is explained by the cut off attenuation of TE01 mode in the aperture. The influences of loop-screen-loop distance and loop radius on the coupling intensity are also investigated.

Impact of Cable Configuration on the Voltage Induced in Cable Screen during Work with One-Sidedly Ungrounded Cable Screen

In the latest research, it has been proven that from the point of view of losses in a cable distribution line, the most advantageous operation is to work with two or one phase of metallic cable screen ungrounded. However, such an operation may cause changes in the network characteristics and thus the occurrence of undesirable phenomena. One of those characteristics is the overvoltages in those cable screens, which can lead to cable line damage. The simulation tests presented in this article are closely related to the unusual method of operation of the MV cable screens and their performance, and they address the question of whether in a given system ground fault overvoltages may be a significant threat to the operation of the cable. The research methods used to verify these risks are related to the simulation of the cable line operating states using the DIgSILENT PowerFactory program (DIgSILENT GmbH, Gomaringen, Germany). Overvoltage simulations were performed, taking into account changes in the network configuration, such as the method of cable screens grounding, the length of cable lines, the cross-section of the conductor and cable screen, or the method of operation of the neutral point. The results for the cable line modeled as a part of the MV network with the variables considered during the tests indicate the possible impact of the one-sidedly cable screen ungrounding on overvoltages in this cable screen. The obtained results at the level of a few kV in one-sidedly ungrounded cable screens show that the change of the configuration of the operation of these cable screens may affect the safety of the network operation.

The Effect of Protrusions on the Initiation of Partial Discharges in XLPE High Voltage Cables

This paper is focusing on 3D Finite Elements Analysis (FEA) based modelling of protrusions as defects or imperfections in the XLPE high voltage cable. This study is aiming to examine the impact protrusions have on the initiation of partial discharges. Spherical and ellipsoidal protrusions with different sizes at the conductor screen of the high voltage cable is an essential content of this paper. In addition, a spherical gas-filled void is placed inside and outside the protrusions, and a water tree produced from protrusions is under consideration. The partial discharge influence taking place at the protrusions and the stress enhancement factor is determined for all the variations mentioned to quantify the rise in the inception of partial discharges due to the protrusions.

Electric field calculation and optimization for stress cone of DC cable joint based on the coaxial double-layer insulation model

This paper presents a DC electric field calculation and optimization algorithm for a coaxial double-layer insulation. The stress cone curve of a 320 kV DC cable joint was optimized utilizing the algorithm under different conditions. The obtained results show that the stress cone curve becomes steeper with an increase of the x-axis electric field. The curve also becomes steeper with a decrease of the temperature difference between the stress cone and the conductor screen. The interface charge density along the interface between the XLPE and SiR decreases continually with the reduction of the temperature difference. The thermo-electric coupling simulations were performed using finite element simulation software. The obtained results illustrate that the temperatures, electric fields and interface charge density of the stress cone obtained by the algorithm and the simulation are consistent. Moreover, the optimized stress cone curve achieves the intended x-axis electric field under the given temperature difference. It is appropriate to use the algorithm to optimize the electric field in the stress cone area due to the evident efficiency.

Bistate Frequency Selective Surfaces Made of Intertwined Slot Arrays

Novel arrangements of active frequency selective surfaces (AFSSs) with integrated voltage control wiring are proposed for bistate (transparency/reflectance) operation at specified frequencies. The AFSSs are comprised of passive arrays of intertwined patterns of slots in a conductor screen and an active dipole array with p-i-n diodes placed on either the same or opposite sides of a thin dielectric substrate. Simulation and measurement results show that such AFSSs exhibit good isolation (~15 dB) between the translucency and reflection states at normal incidence that slightly decreases at oblique incidence. The proposed AFSSs maintain the high angular and polarization stability over broad fractional bandwidths inherent to the constituent periodic arrays of intertwined conductor patterns with substantially subwavelength unit cells. The merits of these AFSS arrangements also include resilience to parasitic effects of real switches, whose insertion loss in the ON state only enhances the AFSS ON/OFF isolation. Such AFSSs are essential elements for reconfiguring and controlling the electromagnetic architecture of buildings.

Modeling the Effect of Human Body on ToA Ranging Using Ray Theory

In line-of-sight (LoS) conditions, the accuracy of time-of-arrival (ToA) based localization has proven to be superior to the received-signal-strength (RSS) and the angle-of-arrival (AoA) localization techniques. The accuracy of TOA ranging is limited to unexpected ranging errors caused by the human body obstructing the direct path (DP) between a transmitter and a receiver. Validated analysis of this effect is a challenging problem, and this paper proposes an analytical Uniform Geometrical Theory of Diffraction (UTD) approach, based on a hybrid applicability of conductor cylinder, wedge, and screen models, to analyze behavior of first path in proximity of human body that is in angular motion. The model introduced can cover for a variety of antenna positioning in respect to human body by addressing creeping waves for on-body, far-field and near-field waves for off-body propagations. Result of analytically calculated ToA and path-loss for shortest diffracted path is validated with UWB measurements conducted by a vector network analyzer sweeping 3–5 GHz of bandwidth. The analytical approach proposes an enhancement to existing RT algorithm to include the effect of human body.

Enhanced nonresonant light transmission through subwavelength slits in metal.

We analytically describe light transmission through a single subwavelength slit in a thin perfect electric conductor screen for the incident polarization being perpendicular to the slit, and derive simple, yet accurate, expressions for the average electric field in the slit and the transmission efficiency. The analytic results are consistent with full-wave numerical calculations and demonstrate that slits of widths ∼100 nm in real metals may feature nonresonant (i.e., broadband) field enhancements of ∼100 and transmission efficiency of ∼10 at infrared or terahertz frequencies, with the associated metasurface-like array of slits becoming transparent to the incident light.

Near-field diffraction in a two-dimensional V-groove and its role in SERS

Optical field distribution in micro-nano geometries of miniaturized optical devices is often significantly different from that in identical but macroscopic geometries. Plasmon effects and near-field diffraction can enhance the local field intensity, leading to enhanced cross section for light absorption and scattering, which can be utilized in substrate-enhanced spectroscopies for the detection of trace amounts of adsorbed chemicals. A specific problem is an ingenious but only empirically described way to enhance signal intensity in Raman spectroscopy by the use of a substrate patterned with gold coated micron size pyramidal pits. While Raman enhancement on nanostructured substrates is generally attributed to surface plasmons, here the micron size, and thus the sub-wavelength to near-wavelength dimensions suggest that resonant enhancement emanating from optical near-field diffraction might also play a role. To answer this question, light diffraction in a projection of the pyramidal pit: a V-groove, was modelled with a modified Neerhoff-Mur formalism suitable to calculate electromagnetic field distribution in sub-wavelength structures. Under the boundary conditions a perfect conductor screen was assumed, which excludes plasmon effects. The calculations show that interference in the cavity causes a modest resonant increase in local intensity and that near-field diffraction strongly influences the field distribution, which is explained with the electrodynamic edge effect. The magnitude of the resonant electric field on its own cannot account for the experimentally observed Raman enhancement. However, a resonant enhancement of a similar magnitude is expected for the emitted Stokes frequencies. In this case the geometry implements the conditions for the classical electromagnetic Raman enhancement, ~E(4), in a good agreement with experimental results.

Overvoltage attenuation in power cable lines—A simplified estimation method

Cable lines impart attenuation and distortion to overvoltages coming from overhead lines. In particular the attenuation is an important factor for the insulation coordination of a cable line and consequently for its reliability. The evaluation of such attenuation needs the utilization of numerical programs and their utilization, although it becomes an obliged step, is not an easy task without a specific skill. In this paper a simplified method is proposed which is based on the results of a specific numerical program, in order to evaluate the overvoltage attenuation introduced from cable lines for both the conductor screen (insulation stress) and screen to ground (jacket stress) modes of propagation. Although such method introduces some approximation, it is a useful tool in order to have both a first and conservative evaluation of the overvoltage stressing the cable line insulation when the impinging overvoltage to the cable line is known.

Severe degradation of the conductor screen of service and laboratory aged medium voltage XLPE insulated cables

The main purpose of this paper is to show the strong correlation between corrosion of the metallic aluminum conductor and the formation of interconnected cracks / voids in the conductor screen, creating initiation sites for vented water trees in service aged medium voltage XLPE cables. The results show that porous structures in the conductor screen previously reported for laboratory aged insulation systems, also develop in the conductor screen in service aged medium voltage XLPE cables. These structures can bridge the screen and serve as path for contaminants and corrosion products from the aluminum conductor and initiate water trees. A prerequisite for the formation of such structures is the presence of liquid water at the interface between the conductor and conductor screen causing corrosion. The initiation site of such structures has been identified, and is likely caused by environmental stress cracking (ESC). Initiation sites were determined in all cables, but porous structures in the conductor screen were only observed in the cable suffered from service failure, where liquid water had entered the cable conductor between the strands. Severe degradation of the XLPE insulation was observed at the initiation sites for water trees growing from these structures.

Conditions of discharge-free operation of XLPE insulated power cable systems

A coherent methodology was developed for analysing the risk of partial discharge inception in the insulation of high voltage cables containing spherical voids located at the conductor screen interface and fissures located at the insulation screen interface. These two types of insulation defects present the two most important cases from the point of view of insulation integrity in high voltage cable systems. Our approach was based on the notion of sensitivity of dielectric stress to dimensional change of cable insulation and the notion of sensitivity of the size of discharge free defect to the change of electric field intensity. We investigated how modifications of insulation dimensions can affect the risk of discharge in typical defects. The obtained results revealed that so called slim design of HV cables requires that the size of voids is reduced by approximately a third to ensure discharge free operation. The size of fissures must be reduced approximately tenfold in comparison to the permissible size of similar fissures in a standard design to ensure discharge free operation at the interface with accessories. This poses a very high technological demand on new cable system designs of this type. Increasing the size of cable core creates an increased risk of partial discharge, mainly in fissures. Based on the dimensions of a typical 110 kV cable we showed that a reduction of fissure dimensions by approximately a quarter is required for a discharge free operation if the conductor size increases from 1000 mm2 to 2000 mm2. We have quantified this way the extent to which quality of the insulation extrusion process as well as precision of accessories assembly must improve, in order to assure discharge free operation of cable systems after typical design modifications.

Use of a conductor screen to magnetize NdFeB magnets

The magnetizer presented in this paper consists of a magnetic core made with silicon iron sheets with a large airgap in which the permanent magnets to be magnetized are located, This magnetic structure is energized by two symmetrical coils placed on the central leg above and below the airgap. The cross section of the airgap is four times the area of the permanent magnet, so with a copper plate that surrounds the magnet (not completely), effective concentration of the flux density lines is obtained. The copper plate was called electromagnetic conductor screen. The capacitor-discharge-bank was rated 1080 /spl mu/F, 2000 V and 3.5 tesla were obtained in the central airgap. The paper presents the experimental results and theoretical analysis used to evaluate the conductor screen time-constant of the proposed magnetizing system.

The Effect of Service and Test Conditions on Water Tree Growth in XLPE Cables

Experimental studies of tree growth are often performed under one single set of test conditions. The aim of this work has been to systematically vary the test conditions to study their influence on tree growth in one type of XLPE cable. It was found that water in the conductor dramatically increases the number, size and growth rate of trees. In such tests the temperature of the outer screen should be controlled as well as that of the conductor, because the temperature gradient determines the degree of supersaturation in the insulation wall. Stationary temperatures are found to cause more severe tree growth than cycling temperatures. This is contrary to the situation with dry conductor where cycling of both the conductor and outer screen temperature greatly enhances tree growth.

Effects of Conductor Screening and Oil-Types on the Electrical Characteristics of Oil-Filled Model Cables

The effects of conductor screening with various shielding materials and oil types with different gassing tendencies on the electrical characteristics of oil-paper cables have been studied using model cables suitably terminated with stress relief cones. Results indicated that the impulse strength of the models was not significantly influenced by the types of conductor screen or the gassing tendencies of the insulating oils tested. However, metallized carbon black (CB) paper used in conjunction with CB paper exhibited much superior power factor-stress characteristics and therefore would be the preferred conductor screen design for oil-filled power cables, especially EHV cables.

Effect and physical behaviour of carbon-black papers used a conductor screen

Effect and physical behaviour of carbon-black papers used a conductor screen

Effect and physical behaviour of carbon-black papers used as a conductor screen

Conductor screens are now in general use on high-voltage power cables. They take several forms, and are especially effective in bringing about an improvement in a.c. electric strength. Good-quality cable paper loaded with a carbon-black material is one form used for conductor-screening purposes, but it can have adverse effects when applied to oil-filled cables. These include an increase in the ionisation factor and a decrease in the impulse electric strength. The paper records the result of comprehensive investigation into many aspects of carbon-black-paper screening, and claims conclusively to establish the behaviour of carbon loading when used in paper for screening purposes.

60-Cycle AC Breakdown of 345-kV Paper Insulated Cable

High-voltage-time tests at 60 cycles were made with an 800-kV series resonant test set on 21 samples of 345-kV impregnated -paper-insulated high-pressure oil-pipe-type cable. The breakdown level of cables, expressed in equivalent volts per mil for one hour life (V/mil/h) under 200-psig oil pressure decreased from 1130 to 850 V/mil/h when the conductor temperature was increased from 70 to 110°C. At 100 psig the breakdown level decreased from 870 to 670 V/mil/h from 50 to 80°C. One sample at 50 psig with a conductor temperature of 62°C had a breakdown level of 600 V/mil/h. Fault paths terminated in radial holes at the conductor and outer shield with a longitudinal running path between these terminations. There was no regular pattern to the fault paths, relative to the conductor screen or to gaps between turns of the insulating tapes.