V.T. Morgan

The Overall Convective Heat Transfer from Smooth Circular Cylinders

Publisher Summary Accurate knowledge of the overall convective heat transfer from circular cylinders is of importance in a number of fields, such as boiler design, hotwire anemometry, and the rating of electrical conductors. The wide dispersion in the published experimental data for the heat transfer from smooth circular cylinders by natural and forced convection is attributed to various factors associated with the experiments. The error due to heat conduction to the supports is particularly important with natural convection, especially where the heat loss and the temperature rise of the cylinder are calculated from the voltage drop across it. A common cause of error is the use of too small a space ratio, so that the temperature and velocity fields are distorted. To reduce this error to less than l%, the space ratio D c /D for natural convection or D T /D for forced convection should exceed 100. The error caused by blockage with wind tunnel measurements can be calculated depending on the type of tunnel. One of the greatest sources of error with forced convection is the failure to allow for the effect of stream turbulence.

The thermal rating of overhead-line conductors Part I. The steady-state thermal model

Abstract The stead-state current rating of short transmission and distribution lines is often determined by the maximum permissible temperature of the conductors to avoid excessive sag or long-term annealing of the conductors. A thermal model to calculate the relationship between the current and the conductor temperature is proposed, and the parameters in the model are examined in detail.

Effect of elevated temperature operation on the tensile strength of overhead conductors

The loss of tensile strength of wires and stranded conductors operating at elevated temperatures depends on the cold work experienced in their manufacture, and the temperature and the time duration at that temperature. It is shown that existing equations for calculating the loss of strength for aluminum, 6201 aluminum-alloy and copper conductors are unsatisfactory, and a new equation is proposed. Published experimental data are examined, and possible relationships between the rate of loss of strength and the various stages of recovery and recrystallization are identified.

The Loss of Tensile Strength of Hard-Drawn Conductors by Annealing in Servicec

Although the literature contains many results of tests on the loss of tensile strength of wires held at various temperatures for various time durations, the data are very disperse, and it is difficult to apply them to determine the loss of strength of a particular conductor as a result of long-term annealing in service. This paper examines the effect on the loss of strength of the reduction in cross-sectional area during drawing, the operating temperature, and the time duration of annealing. The effect of the heat capacity of wires and conductors on the loss of strength following a short-circuit or a lightning impulse current is shown. The need for more work to determine the effects of tension and stranding on the loss is stressed.

Statistical distributions of wind parameters at Sydney, Australia

The characteristics of the wind at 10 m height were studied over a period of 32 months. The sampling interval was 20 ms and the averaging time was 10 min. Probability density functions are given for the speed, direction, inclination and intensity of turbulence of the wind. Frequency contour plots are given for wind speed vs solar time, wind speed vs wind direction, wind speed vs global solar irradiance and wind speed vs the intensity of turbulence of the wind. Differences between the results for day and night and between various seasons are examined.

The radial temperature distribution and effective radial thermal conductivity in bare solid and stranded conductors

The radial temperature distribution in monometallic and bimetallic, solid and stranded circular cylindrical conductors is calculated for uniform circumferential temperature, uniform current density, and uniform radial thermal conductivity. An analysis of the radial heat flow in concentric-lay stranded conductors is presented. Parallel heat paths are formed by the asperities and the air gaps at the contacts between crossing wires, and the air voids between wires in adjacent layers. Most of the heat is transferred by conduction through the air gaps and the air voids; there is negligible heat transfer by radiation and convection. It is shown that the temperature difference between layers depends on the number of contacts per unit length and the area of each contact. The latter depends on the radial force, and hence on the tension. The calculated effective radial thermal conductivity for a 61/3.5 mm AAC conductor falls within the range of experimental values. Tests were performed on pairs of crossing, 3.5 mm, hard-drawn aluminum wires to determine the relationships between the radial force per contact, the crossing angle, the apparent area of contact, and the loading duration. >

The Current Distribution, Resistance and Internal Inductance of Linear Power System Conductors—A Review of Explicit Equations

This paper reviews the present state of knowledge of explicit equations for calculating the dc and ac current distributions and the resistances and internal inductances per-unit length of linear electrical conductors used in power transmission and distribution systems. These conductors may be homogeneous wire or rod, tubular, triangular, elliptical or rectangular busbars, helically stranded nonmagnetic conductors (AAC or AAAC), or bimetallic stranded conductors, such as the commonly used aluminum conductor steel reinforced (ACSR). In general, the current density in an isothermal homogeneous conductor is uniform with direct current (dc), but with alternating current (ac), skin effect, and proximity effect, can cause nonuniform distribution of current, hence, increased resistance and decreased internal inductance. With stranded steel-cored conductors, the dc density within each section is inversely proportional to its resistivity. However, with ac at power frequency, the spiraling of the currents in the nonferrous layers causes a longitudinal magnetic flux in the steel core, which results in hysteresis and eddy current power loss in the core, and a circular magnetic flux in the nonferrous wires, which results in a nonuniform distribution of current density between the layers of nonferrous wires. These effects give rise to increased resistance and reduced internal inductance. The effects of current amplitude, frequency, temperature, and tensile stress on conductor properties are discussed.

Effects of alternating and direct current, power frequency, temperature, and tension on the electrical parameters of ACSR conductors

Many high-voltage transmission lines are constructed with aluminum conductors, steel reinforced (ACSR). The stranded steel core supports much of the tension and the aluminum strands conduct most of the current. The current flowing in the strands spiraling around the core causes a longitudinal magnetic flux in the core, which increases its permeability and causes a redistribution of the currents in the layers of aluminum strands. This redistribution and skin effect, hysteresis, and eddy current losses in the core cause changes to the resistance and inductance of the conductor. This paper describes measurements, in a specially constructed nonmetallic hut, of the DC and 50-Hz resistances and the internal inductance of a 54/2.26-mm aluminum + 19/3.74-mm steel (Pawpaw) ACSR conductor, having three layers of aluminum wires. The effects of current, temperature, and tension were determined, and the effective radial thermal conductivity was derived. Measured values are compared with those calculated from the electromagnetic model, and the model is used to study the effect of frequency, in the range from 25 to 60 Hz, on the resistance and internal inductance.

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

Abstract 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.

Effect of magnetic induction in a steel-cored conductor on current distribution, resistance and power loss

The presence of a steel core in the commonly used aluminum conductor, steel reinforced (ACSR) on overhead transmission lines causes an increase in the AC resistance of the conductor, as a result of the magnetic induction in the core. This induction, which is higher with an odd number of aluminum layers, causes hysteresis and eddy current power losses in the steel, and a redistribution of current in the layers of aluminum wires. The effects of the total current in the conductor and the temperature of the steel core on the current distribution, the AC/DC resistance ratio and the power loss are determined for a Grackle conductor with three layers of aluminum wires. It is shown that the resistance ratio and the power loss can be significantly reduced at higher currents by careful design of the lay length (pitch) of each layer of aluminum wires.