Control of Thermal Environment of Buried Cable

L. H, F I N K A S S O C I A T E M E M B E R A I E E

Τ ΉΕ OPERATING TEMPERATURE of under-ground current-carrying conductors is affected directly

by the thermal resistance and capacitance of their imme­diate environment. With pipe-type or directly buried cable, where soil alone comprises that environment, the thermal characteristics of that soil are of major importance. Soils vary considerably in respect to these characteristics, even in a given locality, and along the route of a cable line several miles in length, the range in values usually will be appreciable.

Economic utilization of such a line is restricted if its available capacity is limited by the presence of extremely unfavorable thermal conditions along a small portion of its length. It is desirable, therefore, to take corrective action wherever there exist restrictive conditions of excep­tional severity.

A limited investigation has been conducted to determine the most practical solution to this problem. The necessary corrective action could take the form of improving the thermal characteristics of the soil immediately surrounding the cable or of replacing that unsatisfactory soil by backfill of superior characteristics in the amount necessary to effect the desired improvement. Investigation showed that the former approach is not practical, and that the latter is generally the most satisfactory despite the costs involved in the extensive excavation and backfilling which it entails.

The characteristics of an ideal backfill can be postulated. Primarily, it should be composed of a low-cost material with a high intrinsic thermal conductivity, have a high solids content, and be characterized by retention of moisture under adverse conditions. The material which most nearly meets these conditions is quartz, but because properly graded pure quartz sand is unavailable at low cost in the Philadelphia (Pa.) area, the backfill adopted for field use as a result of this investigation was a silicic sand, designated concrete sand, locally available in quantity at low cost. Its thermal conductivity was found to be satisfactory not only at normal values of moisture content, but also at a value (0.7 per cent by volume) lower than any ever likely to be encountered in the field. Its water retention characteristics were evaluated and found satis­factory. Its specification limits of grain size distribution are such as to assure a uniformly high solids content. This high solids content is an important factor in producing the foregoing attributes, and inspection of the grain size distribution provides a ready means for control of this characteristic.

Several sands and a sandy loam were subjected to a temperature of 180 F over a period of from 20 to 70 hours. A comparison of the samples showed that their final moisture contents, while apparently independent of their initial moisture, varied directly as their solids contents.

Even with the most economical backfill material, it is

desirable to minimize the amount of excavation and back­filling to be done. The work of investigators such as J. H. Neher^ has provided a means for determining the mini­mum amount of such work necessary to achieve a given degree of improvement. The principle of superposition is utilized, in that the heat flow from a cable system is treated independently of other thermal gradients. On

Fig. 1. Trenching for corrective backfilling; size of trench to receive backfill having a thermal resistivity of 70 cm C/watt, in order to reduce the over-all eflfec­tive thermal re­sistivity to 120 cm C/watt in areas where the soU has the resistivity shown on the abscissa. These dimensions assume a 6-inch pipe with 36 inches of cover

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this basis, it is possible to determine, for a given installation, the minimum trench dimensions which fulfill the following simple condition: If the soil from the trench is replaced by a backfill having a given thermal conductivity superior to that of the original soil, the thermal resistance of the backfill within the trench, in series with that of the sur­rounding soil, is equal to the total thermal resistance which would be encountered if the pipe were buried in a homo­geneous soil having the maximum acceptable thermal resistivity. Given a backfill having a resistivity of 70 cm C/watt, Fig. 1 gives the minimum trench dimensions necessary to achieve an over-all effective resistivity of 120 cm C/watt in soils with resistivities varying from 140 to 200 cm C/watt. The 6-inch pipe and 36 inches of cover on which Fig. 1 is based have been used in Philadelphia Electric Company 69-kv pipe-type installations. Shnilar curves can be constructed, of course, for other pipe sizes and depths of burial.

R E F E R E N C E

1. The Temperature Rise of Biiried Cables and Pipes, J . H. Neher . AIEE Trans-actions, voL 68, pt. I, 1949, pp. 9-21.

Digest of paper 54-75, "Control of the Thermal Environment of Buried Cable Systems,** recommended by the AIEE Committee on Insulated Conductors and approved by the AIEE Committee on Technical Operations for presentation at the AIEE Winter General Meeting, New York, Ν . Y., January 18-22, 1954. Scheduled for pubUcation in AIEE Power Apparatus and SysUms, 1954.

L. H. Fink is with the PhUadelphia Electric Company, Philadelphia, Pa.

J U N E 1954 Fink—Thermal Environment of Buried Cable 505