Overhead Line Conductors Research Articles

Thermodynamic models of wet-snow accretion: axial growth and liquid water content on a fixed conductor

For wet-snow accretion on overhead line conductors, occurring at positive air temperatures, the liquid water content of the snow matrix controls the strength of the capillary forces, promoting contact between ice granules, which leads to ice bonding. During this process of metamorphosis, the liquid water content of the snow matrix increases with time and, on reaching a level of 20–40 percent, the internal cohesive forces are greatly weakened, causing shedding of the accreted snow by aerodynamic and gravitational forces. The purpose of this paper is to construct thermodynamic models of wet-snow accretion, by axial growth on a fixed conductor, which estimate the liquid water content during the accretion process. The models depend upon assumptions concerning the wet-snow accretion factor, namely the proportion of mass of a snowflake that adheres on impact with the snow/conductor surface. Models are formulated assuming either that the accretion factor is constant or that it obeys a cosine law (adhesion proportional to the cosine of the angle between the impacting snowflake trajectory and the normal to the surface). A differential equation governing the variation of the liquid water content with deposit time is derived and solved numerically. Solutions of this equation are also obtained using analytical solutions of the evolution equation based on the assumption that the trajectory paths of snowflakes are rectilinear. For a range of meteorological conditions, critical cohesive “precipitation—air temperature” criteria are established for wet-snow shedding, leading to quantitative information on the sawtooth transient wet-snow loading of an overhead line conductor.

Improved overhead line conductors using aluminium alloy 1120

Power conductors utilising the new high conductivity medium strength aluminium alloy type 1120 have been used for transmission and distribution lines in Australia since 1984. Unlike traditional aluminium alloys such as 6201 and 5005 the 50% increase in strength of alloy 1120 over standard EC grade aluminium is accompanied by only a slight reduction in conductivity. Overhead line conductors using this alloy provide lower I/sup 2/R losses without appreciable capital cost penalty. Introduction of these new alloy conductors provides a significant development in transmission line technology. This paper traces the introduction of this alloy in Australia, the development and testing of such conductors, together with details of its application by one major utility. >

Further aspects of dynamical models for rime‐ice and snow accretion on an overhead line conductor

AbstractCurrent three‐dimensional, time‐dependent mathematical models for (dry) rime‐ice and snow accretion on Overhead Line Conductors (OHLC), of finite span and finite torsional stiffness, assume that the airflow past the iced OHLC is given by Attached Potential Flow (APF) and that the effect of aerodynamic moment on the rotation of the OHLC during ice evolution can be neglected. In the present numerical study a CFD code is employed to simulate the turbulent airflow past an iced OHLC and used to validate APF predictions for icing particle impactions, ice evolution and rotation of the OHLC.Comparisons are made for the following: (a) icing particle impaction velocities determined using the CFD code and APF when, for example, the iced surface is fixed at an attitude experiencing lift; (b) the aerodynamic moment, for a chosen ice shape at a range of attitudes, predicted using the CFD code and AFT; (c) the aerodynamic moment, for natural ice shapes, given by APF and measured in wind‐tunnel tests; (d) the effect of aerodynamic moment, predicted using the CFD code and APF, on ice evolution during a short period of icing. Finally, on employing aerodynamic moments calculated using APF modified values, the sensitivity of the ice‐accretion process, across the span of the OHLC, to conductor rotation and various meteorological and physical data for the icing particles is discussed.

The effect of density variations during rime growth on overhead transmission line conductors

Known density relationships, which are functions of the variables and parameters controlling rime-ice accretion on circular cylinders, are incorporated in a heat and mass transfer rime-icing model, to give predictions for the iceload and equivalent ice thickness during the evolution of the ice surface. Whereas predictions for ice thickness and ice shape are found to depend on the choice of density relationship, predictions for the transient iceload are in satisfactory agreement. A recommendation is made for adopting a density relationship which can be readily used to predict iceloads on overhead line conductors, having finite span and finite torsional stiffness.

Theoretical aspects of iceload measurements on fixed rods and overhead line conductors: A transfer function for dry rime-ice accretion

This paper is concerned with theoretical aspects of the methodology of iceload measurements on an overhead line conductor (OHLC) of finite span and finite torsional stiffness. The purpose is to establish the relationships between the iceload on a fixed rod or sampler and the spatial average iceload on an OHLC. A theoretical database, generated using dynamical models for rime-ice accretion on conductors, is carefully analysed to express iceload data as a function of meteorological conditions and the mechanical properties of the OHLC. These results provide the required ratios of the iceload on a sampler to that on OHLCs as a function of time during an icing incident. A transfer function, relating theoretically predicted iceloads on an OHLC to that for a sampler, is then constructed and should prove useful in the development of the corresponding methodology for relating sets of field measurements in this way. For design purposes progress in the latter is desirable.

Rime- and glaze-ice accretion due to freezing rain falling vertically on a horizontal thermally insulated overhead line conductor

A theory of atmospheric icing due to freezing rain on an overhead line conductor (OHLC) is developed. The rain falls vertically on a horizontal OHLC that is thermally insulated. It is assumed that the collection efficiency of the accretion surface is unity and that this surface is in thermodynamic equilibrium with the environment. For air temperature T A ⩽ 0°C and raindrop temperature T D ⩽ 0°C, the freezing rain accretes as rime ice, provided that the temperature of the ice surface T l < 0°C. The evolution equation governing the mass transfer at the accretion surface is solved analytically, yielding the shape of the rime-ice surface. Equations governing the thermal state of the rime-ice deposit are also given. These determine the onset of wet growth or glaze accretion at the upper stagnation line during suitable environmental conditions. For environmental conditions producing an ice surface at temperature T l = 0°gC, the freezing accretes as glaze. Equations governing the heat and mass transfer at the surface determine the shape of the glaze surface and the downward viscous motion of the unfrozen water. For T D < 0°C, glaze evolution equations are developed for T A ⩽ 0°C and T A 0°C. Analytical solutions of these equations are obtained. In particular, when T D < − T A < 0°C, the evolution equation predicts a novel limiting growth that is triangular in shape. Further study of the mass and heat transfer conditions, in the neighborhood of this final stage of glaze accretion, shows that it is maintained in thermodynamic equilibrium with its warm air environment.

Statistical distribution of the temperature rise of an overhead-line conductor carrying constant current

The statistical distribution of the temperature, or temperature rise, of an overhead-line conductor carrying a constant current depends on the stochastic variation of the atmospheric parameters, particularly the wind speed and direction, the air temperature and the intensity of solar radiation. The paper presents probability density functions for the temperature, temperature rise, radial temperature difference, and thermal inertia of the 54/7/3.5 mm ACSR conductors in three spans of an outdoor test line erected in the form of a horizontal equilateral triangle at a height of 10 m. The experiment was conducted for a period of 32 months, with a constant current of 1500 A and a constant tension of 30 kN. An unenergized similar conductor was also studied. The results are classified by day and night, and by seasons. Bimodal probability density functions were found for conductor temperature, temperature rise and radial temperature difference.

Snow accretion on overhead line conductors of finite torsional stiffness

A theoretical study of snow loading on the windward side of an overhead line conductor (OHL), of finite span and finite torsional stiffness, is made on employing known meteorological data for dendritic snowflakes to determine their winddriven migration in the vicinity of the conductor and their impaction on the conductor. Two different three-dimensional, time-dependent mathematical/computer models are developed which are capable of simulating a wide variety of environmental conditions known to exist in a snow storm. The numerical results of the complete spectrum of snowflakes model (CSM) are used to check the accuracy of those results obtained from an average snowflake size model (ASM). It is shown that predictions of the considerably less computationally demanding ASM are in almost exact agreement with those of CSM.

Dynamic current rating of overhead lines

Abstract The radial temperatures in three types of overhead line conductor are computed for various wind velocities, wind angles and solar insolation. It is seen that for the higher wind speeds and lower ambient temperatures significant radial temperature gradients exist, especially for the larger conductors. Hence it is concluded that for dynamic rating procedures based on conductor surface temperature this gradient should be taken into account.

Supergrid condition assessment

The overhead-line conductors on the national Grid are aging, and will eventually require replacement. The selection process involves measuring the deterioration of individual routes

The thermal rating of overhead-line conductors part II. A sensitivity analysis of the parameters in the steady-state thermal model

The thermal rating of an overhead-line conductor is the current required to produce a certain maximum permissible temperature under specified atmospheric and other conditions. The thermal rating is found by solving the heat equation, which contains a large number of parameters. The sensitivity of the thermal rating to a variation of each parameter over a practical range is examined, and the relative importance of each parameter is given.

A Non-contact Technique for Determining Harmonic Currents Present in Individual Conductors of Overhead Lines

A Non-contact Technique for Determining Harmonic Currents Present in Individual Conductors of Overhead Lines

A Non-Contact Technique for Determining Harmonic Currents Present in Individual Conductors of Overhead Lines

This work derives the relations between harmonic or fundamental currents in overhead three-phase circuits and the magnetic fields measured under the lines. These relations are used to calculate harmonic currents in each conductor from the magnetic field data. Calculated and experimental field patterns are shown, and a specific technique for field measurements and current calculations is outlined. Laboratory results indicate that a good level of accuracy is attainable with this non-contact technique.

Overhead line conductors — the prospect for aluminium alloy

The present dominance of steel-reinforced aluminium conductors is being challenged for certain applications by all-aluminium-alloy conductors. In the UK, the requirements for future overhead-line construction and reconductoring, and the development of the use of overhead lines to incorporate communication channels, appear to be advantageous for the use of this type of conductor

架空線の劣化におよぼす腐食の影響

架空線の劣化におよぼす腐食の影響

Discussion on “Long-term-creep assessment for overhead-line conductors”

Discussion on “Long-term-creep assessment for overhead-line conductors”

An elementary appraisal of structural problems arising from the provision of twist in overhead line conductor bundles

Overhead transmission line bundles suffer from a variety of wind-induced vibration phenomena, and the most troublesome of these is probably “subconductor oscillation”—a low frequency flutter condition experienced by the leeward of two bodies exposed to an airstream and ascribable to wake-generated aerodynamic forces. Since this flutter condition is found to be critically dependent upon the position of the leeward body in the wake generated by its neighbour, it has been suggested that by twisting the entire bundle of conductors through a sufficiently large angle, the flutter-producing aerodynamic forces will be decorrelated in the spanwise direction and flutter will thereby be eliminated. An angle of 30° per subspan (typically 160–180 ft between bundle spacers) has, in fact, been suggested in the literature. It is shown herein, by using simple theory, that bundle twisting inevitably leads to a reduction in bundle torsional stiffness and ultimately to torsional instability. Of particular interest is the conclusion that the failure of a single spacer on a twisted line can lead to large angles of twist in the failed subspan and, perhaps, to complete static instability. These, and other structural factors encompassed herein, must be taken into account in assessing the efficacy of bundle twisting as a flutter ameliorative.

Long-term-creep assessment for overhead-line conductors

Based on the strain-hardening-material law, a method of long-term-creep prediction is proposed which takes into account all practical line-design and operating conditions and estimates the reduction in conductor tension and the increase in its sag due to creep with acceptable accuracy. Correlation between theory and practice is illustrated by both laboratory and field measurements. Selected examples are given on the application of the method for line design and for the selection of optimum sagging procedure of a line. Areas of further research and investigations on overhead-line creep are indicated.

Corrigendum: Magnetic properties in axial 50Hz fields of steel core wire for overhead-line conductors

Corrigendum: Magnetic properties in axial 50Hz fields of steel core wire for overhead-line conductors

Magnetic properties in axial 50 Hz fields of steel core wire for overhead-line conductors

As a result of the spiralling of the power-frequency current around the core of a steel-cored overhead-line conductor, a longitudinal alternating magnetic flux is set up in the steel core. This flux produces active and reactive power losses, which give rise to increases in the effective resistance and inductance of the conductor. Results are given of experimental work to determine the effects of conditioning, galvanisation, preforming, stranding, temperature and tensile stress on the magnetic characteristics of three sizes of 85ton grade 0.55%-carbon-steel wire. It is shown how the increase of resistance and inductive reactance of the conductor can be calculated for conditions in service.