Buried Cables Research Articles

Analysis of the earth resistivity response of buried cables

Current flow in homogeneous and layered conducting media is analyzed for the case where an axial conductor or cable is present. The cable is characterized by a specified axial impedance and is assumed to be infinite in length. Various configurations are chosen such as a current point source in an infinite, semi-infinite, and layered region where the cable is taken parallel to the interface(s). The resulting formulas for the potentials reduce to known cases in the absence of the cable. Using these formulations, we present some concrete calculated examples that are relevant to resistivity probing of perturbed homogeneous and layered structures. Only the two-electrode array is treated, but various cable orientations are considered. In general, it is found that a long axial conductor such as a bare cable will distort the potential distribution of the current in a major way. This leads to profound departures from the apparent resistivity curves calculated for idealized homogeneous and layered structures.

Lightning Protection for an Oilfield Automation And Instrumentation System

A description of lightning protection devices in a computer-monitored oilfield automation project located on the south high plains of West Texas near Levelland. Lightning damage was substantially reduced with the present protection system. Introduction For many years, the problem of protecting electrical systems from lightning discharges has plagued power and communications engineers. Only within the past few decades has lightning been a problem to engineers dealing with electrical power systems in oil fields. The problem of lightning protection in West Texas oil fields is unique because of the high concentration of elevated high-voltage lines above flat plains that attract lightning discharges. Most modern, electrically operated oil fields have power distribution systems that are well protected from power distribution systems that are well protected from lightning discharges; and, in many cases, these systems are isolated by sectionalizers and other devices. Even if a portion of a field is disabled by lightning damage, the remainder of the field continues to function normally. This paper concentrates on protecting the low-voltage electronic instrument systems that are very susceptible to even minor voltage surges caused by lightning. In the last several years, the tremendous expansion of oilfield automation and electronic surveillance equipment has required increased emphasis on protecting low-voltage instrument systems from lightning discharges. These systems use DC voltages of near zero to 50 v with 120-v AC-powersources. This paper deals with methods used in a major oilfield automation project to protect various parts of the system from lightning damage. The lightning protection devices discussed are used to protect two of Amoco Production Co.'s computer-monitored oilfield automation projects located on the south high plains of West Texas near Levelland, Tex, plains of West Texas near Levelland, Tex, Fig. 1 shows schematically the automation project operating in one of the locations. This is a secondary waterflood unit with almost 700 wells. The unit is under the surveillance of a computer-monitored oilfield production automation system that utilizes about 1,500 status, production automation system that utilizes about 1,500 status, control, and measurement devices. It has about 150 miles of buried multiconductor cables ranging from 6- t50-pair cables. In this project, "pump-off" controllers are used to control the producing wells, have dry contacts, and indicate satisfactory controller operation and motor status. Water injection wells are controlled by mechanical pressure-regulating valves. The injection rates and pressures are monitored by the computer "reading" amplified signals from turbine meters and pressure transducers. pressure transducers. At satellite tank batteries, the status of tank levels, fire indicators, temperature, and other conditions is monitored with each end device having a set of dry contacts. Producing well tests are obtained by automatically Producing well tests are obtained by automatically switching individual flowlines to fired heater-treaters or separators using electrically operated ball valves. Dry contact meter pulses from these vessels are accumulated by the computer system to determine producing volumes and rates. The telemetry system consists of master and remote telemetry units. Communication between telemetry units is accomplished by frequency modulation using buried copper conductors. JPT P. 1405

Assessment of the influence of soil thermal resistivity on the ratings of distribution cables

The paper summarises the findings of an extensive field investigation into the behaviour of soils affecting operation of buried power cables. Statistical treatment of the large volume of results available leads to the establishment of thermal-resistivity values for defined groups of soils and the main types of load encountered in practice.

Silicone‐polyethylene blends

AbstractPolyethylene and reactive, high molecular weight polydimethylsiloxanes have been blended under conditions of shear and elevated temperature in order to form uniform, thermoplastic blends. The materials can be extruded, coated on wire, injection molded, or compression molded. Despite the thermoplastic nature, a high gel fraction is present. A structure is proposed consisting of microgelled and grafted particles of silicone dispersed in polyethylene. When compared to pure polyethylene, the blends show lowered modulus over a temperature range of −150 to +65°C increased gas permeability, and lowered mixing energies. Electrical properties include corona resistance superior to polyethylene and a resistance to degradation under conditions simulating those experienced by buried cables superior to typical silicone rubber. Other electrical properties are intermediate between the values observed in the component polymers.

地中ケーブル周囲土じょうの熱特性と水分移動現象

地中ケーブル周囲土じょうの熱特性と水分移動現象

Computer Program Analysis Heat Flow from Cables Buried in Regions of Discontinuous Thermal Resistivity

The ampacity, or current-carrying capacity, of a buried power cable is determined to a large degree by the effective thermal resistance of the earth to the heat flowing from the cable to the ambient earth surface. Often the heat passes through sharp discontinuities in the earth, where the thermal resistivity changes. Analysis of the thermal circuit in such cases can be performed easily by means of the computer program presented in this paper. Calculations are based on a resistive network model of the thermal path between cable and ambient earth surface.

Discussion on “Calculation of the external thermal resistance of buried cables through conformal transformation” and “Moisture control of cable environment with particular reference to surface troughs”

Discussion on “Calculation of the external thermal resistance of buried cables through conformal transformation” and “Moisture control of cable environment with particular reference to surface troughs”

Electromagnetic surface fields of an inclined buried cable of finite length

The magnetic fields on the surface produced by a buried current-carrying cable of finite length are considered. A homogeneous half-space model of the earth and quasistatic conditions are assumed. The cable is grounded at its end points and oriented at an arbitrary angle to the interface. While, in general, numerical integration is required, the low-frequency limit may be treated analytically. Small cable tilts are shown to modify the direction of the surface field but not the magnitude. The results have possible application to uplink communication and radio location of trapped miners.

Moisture control of cable environment with particular reference to surface troughs

The rate of dissipation of heat from buried cables is dependent on the thermal resistivity of the surrounding backfill. It is well known that, for granular materials, e.g. sands, the thermal resistivities vary markedly with the moisture contents, and high resistivities can be reached because of drying. The paper discusses the factors that lead to moisture migration, and gives practical information on methods of controlling the moisture content in cable backfills. Information on mean wind speeds, hours of daylight and average rainfall are collated to support the theory of establishing a `moisture-control season? that is suitable for UK installations. Results of field trials are analysed, and indicate the major effect of rainfall and the smaller effect of solar radiation on the performances of cables laid at different depths. Difficulties associated with calculating water losses are outlined, and the results of experiments to establish a rate of loss from surface troughs are recorded. Recent cable circuits with automatic-moisture-control facilities are described. These have been confined to surface-trough in stallations employing cement-bound-sand backfills, but the possibility of extending the technique to cable reserves and other types of installation is suggested. The economic advantages of moisture control and future trends resulting from savings in cost and space are also discussed.

A Positive Approach to Corrosion Control of Submersible Vault Type Urd Systems

This presentation establishes corrosion controls for submersible vault type electric systems and includes a computerized analysis of galvanic and electrolytic conditions. Separate tests of anode configuration and of cathodic protection interference with the copper concentric neutral of a direct buried cable are discussed.

Calculation of the external thermal resistance of buried cables through conformal transformation

The calculation of the outside thermal resistance of a buried cable system related to uniform, nonuniform and temperature-dependent resistivity of soil can be made, if a suitable transformation of the thermal field is automatically drawn and the resistance is computed on the transformed field. A suitable conformal transformation is analysed giving rise to maps of the transformed field. On the maps, the calculation of resistances involves simple operations even when the soil is nonuniform and of temperature-dependent resistivity, provided some approximations are introduced. The errors incurred by such approximations are slight and can be checked, when necessary, by direct computation of the thermal field. The calculation of resistances on the maps can also be carried out by means of automatic design programs.

A Model for Currents and Voltages Induced Within Long Transmission Cables by an Electromagnetic Wave

A model is presented for determining the transient currents and voltages induced within a long coaxial cable by a uniform plane traveling wave whose variation with time may be specified. The cable may be located above or within a half-space lossy dielectric with the wave incident from the half-space lossless dielectric. This results in a model for either aerial or buried cables with the source of the wave located in the atmosphere. A method is also presented for generalizing the approach to more complex cables encountered in communication systems. A demonstration of the applicability of this model is presented by analyzing the response of a 0.375-in coaxial cable for both aerial and buried situations. An incident electromagnetic wave with a rectangular time domain waveform is used to demonstrate the transient response. This waveshape is adequate to clearly indicate the transient phenomena. It is found that voltage enhancement results in both aerial and buried cables when the incident wave approaches grazing incidence. This voltage enhancement can be significant especially for aerial cables. Cable shield current enhancement also results for an aerial but not for a buried cable.

Lightning Shielding of Plastic Telephone Cable

Data are presented on the surge characteristics of various types and combinations of metals when applied as a shield for buried telephone cables. Surge currents were applied to the shield of 500-foot lengths of cable and the potentials developed between the shield and conductors were measured at selected points along the cable length in order to determine relative susceptibility to lightning influence. The shield metals included copper, aluminum, stainless steel, and low-carbon steel, and combinations of these metals. This program was begun and completed in the calendar year 1968. This program may have a substantial impact in the future designs of cable used in the Rural Electrification Administration systems.

External thermal resistance of two buried cables. Restricted application of superposition

New formulas are derived for the external thermal resistance of one of two equally loaded identical cables laid direct in the ground at the same depth. The formulas are based on a restricted application of the principle of superposition, and remain valid when the cables are close or touching. Two assumptions made are that the depth of the buried cables is much greater than the cable radius, and that the cable surfaces are isothermal at a common temperature. The former assumption is justified as the cable depth is generally equal to at least ten times the cable radius. The latter assumption is recognised to be an approximation, but the present approach may prove to be more acceptable than alternative techniques. Comparison is made with previously published formulas.

External thermal resistance of two buried cables

External thermal resistance of two buried cables

External thermal resistance of three buried cables in trefoil-touching formation. Restricted application of superposition

An approximate formula, with error bounds, is derived for the thermal resistance between three touching circular cylinders and a plane. The formula is based on a restricted application of the principle of superposition. The distance between the cylinder configuration and the plane is assumed to be much greater than the cylinder radius, which is the case for the evaluation of the external thermal resistance of three equally loaded identical cables laid direct in the ground in trefoil-touching formation. Numerical calculation indicates that formulas now in use for this application may give values that are noticeably on the high side. The method may be used to obtain approximations to the solution of other thermal resistance problems involving a plane as one of the boundaries.

External thermal resistance of buried cables and troughs

The paper describes how an integral-equation method of potential theory may be used to compute the external thermal resistance of an arbitrary configuration of buried cables or of a buried trough. It is assumed, following Goldenberg, that the outer surface of each cable or cable group is isothermal, this being approximately true when the layer of serving is very thin around the metallic sheath of each cable. With this assumption, numerical results have been obtained for a number of examples. Results for pairs of cables and for three cables in trefoil-touching formation confirm Goldenberg's theoretical estimates for large depths of burial. Results for three touching cables at the same depth and for a rectangular trough appear to be new. The method adopted here is valid for any depth of burial and is also applicable to cables whose shapes are markedly noncircular.

Calculation of steady-state temperature rise of water-cooled buried cables using a new iterative method

The paper describes a new iterative method for calculating the steady-state temperature rise of water-cooled cables buried in a homogeneous soil. Cables and water pipes are represented by line heat sources and sinks of variable magnitude and position, and a computer program is used to calculate the necessary magnitudes and positions of the heat sources and sinks needed to represent the temperature distribution accurately. Open boundaries are used, and this avoids possible errors resulting from the usual practice of imposing isothermal surfaces at finite distances from the cables. It allows a check to be made on boundary conditions used for relaxation calculations or electrolytic-tank measurements. However, the method of analysis described here is considered superior to relaxation for calculating cable temperature rise in a homogeneous soil. A detailed example is given of a calculation carried out at one point in a cable run, including the complete temperature distribution in the soil. A technique is then outlined for applying the method at various points along a cable run, and consequently for calculating the rate of water flow required to maintain an acceptable conductor temperature.

Discussion on “Transient heating of buried power cables”

Discussion on “Transient heating of buried power cables”

Transient heating of buried power cables

An account is given of past work in the field of thermal transients in buried cables. From the publications of Morello and Van Wormer, it follows that the earliest simplified theoretical formulas for the conductor transient, produced for application to small-diameter cables, can be correct only for times sufficiently large for the effect of the thermal capacity of the cable to be negligible. For such large times, the simple exponential integral formula is shown to give a cable-surface attainment factor that is closer to the solution of the proposed model of Whitehead and Hutchings than is the more complicated Whitehead-Hutchings formula itself, although the difference is generally negligible. A comparison is given between the cable-surface attainment factor based on a constant rate of heat flow from the cable to the soil, and the corresponding exponential-integral formula.