[Trade Journal]
Publication: Electric Light & Power
Chicago, IL, United States
vol. 18, no. 3, p. 40-44, col. 1-3
Primary Cables
OFFER ADVANTAGES FOR DISTRIBUTION EXTENSIONS
By G. L. LILLIE, Assistant Engineer of Distribution, Toronto, Hydro-Electric System
OVERHEAD power distribution, using weatherproof braid covered conductors, supported by insulators on crossarms or brackets, is universally acknowledged as the most practical method of power distribution, per capital invested. In appearance, however, it is neither attractive, nor inconspicuous as is the underground system.
In residential districts, where primary has to be extended on small poles unsuitable for open high voltage lines, such as concrete, steel or wood, the whole line has to be rebuilt to add, in most cases, only one conductor. Where trees are prevalent, (as in most residential sections) they must be kept trimmed, and often mutilated, in order to give clearance. Otherwise still higher poles are required.
Underground construction makes the ideal distribution system, but the cost is often prohibitive. A compromise may, therefore, be welcome, and possibly, insulated aerial cable, which can be strung on existing poles, and thus avoid expensive reconstruction, is the solution.
For these reasons the Toronto Hydro-Electric System has for some time been experimenting with different forms of overhead insulated primary cable, especially where necessary to feed small single phase transformer banks of from 50 to 100 kw. in residential districts.
Grounded shielding over the insulation is provided for greater safety, so that if a fault in the insulation should occur, its presence will immediately be detected and the conductor isolated and cleared.
From 1913 to 1929 inclusive, many installations were made of various types of cable, both paper and rubber insulated and shielded either with lead or some form of iron or bronze alloy wire.
Only partial success can be reported from these installations. Paper-insulated lead covered cable suspended upon messenger wire has given years of trouble-free service but, for short primary extensions on residential streets, a more flexible and less expensive in¬tallation is considered advisable. Early experience with self-supporting cables was not particularly successful, due partially to lack of knowledge concerning bends and dead ends. All this experimental work with both paper and rubber insulated cables provided the basis for future developments of shielded primary cables which since have proven quite satisfactory. Several trends developed during the course of the investigation. While properly installed, rubber insulated and braid armored cables apparently would give satisfactory service, the decision was made in 1924 to forego attempts to erect primary cable self-supporting. By 1929, only the P. I. L. C. cables supported by messengers were used and hopes for a light-weight, non-leaded, low-cost insulated primary seemed remote.
The good points of an ariel cable with a grounded sheath were still appreciated but its cost, together with the messenger was almost equal to that of changing poles and installing an open primary. What was needed was a self-supporting insulated conductor that could be strung like tree-wire, dead-ended, and joined readily without excessive costs. Rubber insulation with some form of wire armour appeared logical but the deterioration of rubber and the rusting or mechanical breakdown of the armor was discouraging. The problem was placed before the manufacturers, two of which submitted designs and quotations of interest. In 1930 trial orders were placed and another seven-year period of experimental work, largely upon rubber and varnish-cambric insulated cables with both wire and metallic tape shielding began. This work developed to a point where the Toronto Hydro-Electric System deemed it advisable to prepare its own specifications for a shielded primary based upon its experience and requirements.
Specification
In preparing specifications for the purchase of shielded primary cable, it was desirable to obtain some common ground for competitive purchasing. Where there are several interested manufacturers, each must be given an equal basis for estimating comparable costs.
With this in mind, a definite choice had to be made in the type of insulation, shielding, etc. Varnished cambric insulation had proved quite satisfactory, but with the improvements in rubber, it was decided to concentrate on this for the time at least. Reference was made to the A. S. T. M. requirements and a compromise in the best designs of the various manufacturers was selected.
The specifications call for a 7-wire, stranded conductor, although solid wire has been used with success. Individual wires are to be medium hard drawn, and tinned. The insulation shall contain not less than 40 per cent of best quality Hevea Rubber, which has not been previously used, and shall also conform to exacting physical properties.
Thickness of insulation shall be 8/64 in., minimum thickness to be not less than 90 per cent of the average. High voltage test is not less than 10,000 volts for 5 minutes on each reel length of cable, after 12-hr. immersion in water. Insulation resistance must be not less than 7,900 meghoms per 1,000 feet at 60° F.
Over the insulation a closely woven cotton tape, rubber faced on one side and of minimum thickness of .010 in. shall be applied with at least 1/8 in. overlap. The shielding shall consist of two soft copper tapes of 3 in. maximum width and of minimum thickness of .005 in. each, helically applied in the same direction with the outer tape approximately centred over the spaces between the convolutions of the inner tape. Some makes were previously supplied with a fabric tape over the shielding but we have eliminated the necessity of this. An overall braid is called for, of closely woven cotton hawser cord of minimum thickness .050 in. saturated with a preservative coating and finished smooth and free from tackiness. These specifications are considered to be a minimum requirement, which will no doubt be improved from time to time.
Economies of Primary Cable
At the present date, the amount of shielded primary cable purchased since June, 1930, is 137,540 ft., and the total now in service amounts to 143,119 ft. (27.1 miles). The System's 267 active installations supply 544 overhead distribution transformers of 13,100 kw. capacity, which is 7.9 per cent of the total in this class of service.
The cost of the above extensions, up to the end of August, 1939, was ap-proximately $105,076 (not including transformers). It has been estimated, that, if cable had not been used, it would have cost $195,562 to replace concrete poles with 35 ft. cedars, transfer lines and street lighting fixtures and string one No. 2 open primary. This represents a saving of $90,486 or $338 per extension or approximately $6.90 per kw. supplied. These figures are very conservative, as in many cases actual costs include items, such as re-construction and replacing of secondaries, which are not properly a part of primary cable extensions. The average length installed to date is 537 ft. (See Fig. 2). Improved methods have lowered prices so that a 500-ft. extension now costs approximately $219.31 or $0.44 per ft. This includes equipping of runoff pole with hardware and deadends, replacing one
24-ft. concrete pole with new type 28-ft. for transformers, transferring lines and street lighting lantern, raising one street lighting secondary one arm, installing cutout at line end, and stringing shielded primary cable. On the basis of cost of $393 for average length of 537 ft., or $368 per 500 ft., we have an improvement of $149 on each new installation over previous average costs. Compared with this, present estimated cost of replacing concrete poles with 35-ft. cedars and stringing one No. 2 primary wire for 500 ft. is $527.36, or 140 per cent greater. This reconstruction may be necessary where heavy feeders are required, but this is seldom the case on residential streets at right angles to main thoroughfares.
Cable Saves In New Construction
For original construction in new residential districts, the use of shielded primary on concrete poles can, also, be more than justified. Its cost per 500 feet is only 11 per cent greater than for an open primary wire on 30-ft. cedar poles, and 16 per cent less than for a 35-ft. lead with cross-arms.
The improved appearance and less likelihood of complaints cannot be valuated in dollars and cents, but instructions from the executive department in 1934, that, "where primaries are necessary on streets having only concrete poles, they be made of insulated cable and on existing poles", speaks for itself.
In addition to the normal requirements for shielded primary on pole lines, several new applications have been developed. In one instance, 300 ft. have been run along the side of a brick building to supply a transformer on a pole on private property. This was installed on consumer's order and obviated entirely, the necessity for a number of poles. In several cases, shielded cable is used for primary drop leads of line transformer located in close proximity to accessible parts of buildings, such as windows, fire escapes, or on small wooden poles where transformers are mounted on brackets, etc. This has also led to the Toronto Hydro-Electric System's recommendation that shielded primary cable be required to supply transformers on joint pole construction with other Utilities, where transformers are located below their cable.
In February of this year an experimental 4,100-volt, 3-phase, und e r-ground lateral service was installed from overhead mains on opposite side of street to supply transformers inside building. Three separate single conductor cables were individually suspended across the street between high cedar pole and concrete pole, and continued down pipe lateral on this pole into underground vault, without joint or pothead, at less than 50 per cent of the cost of using standard P. I. L. C. cable, and with greatly improved appearance. (See Fig. 4).
Reduces Tree Trimming Expense
The shielded primary cable offers some attractive possibilities as an alternative to extensive tree trimming or re-building. The Toronto System's tree trimming expense of $13,500 is increasing annually. Trees grow but poles do not.
Taking the capital cost of building a 45 ft. pole line with 3 No. 2 primaries, No. 10 house lighting and No. 6 street lighting secondaries, as approximately $1,400 per 1,000 ft., it can be argued that the same size lines may be installed on 30 ft. poles using shielded primary cable at approximately the same or slightly less cost, but without the need for constant tree cutting (see Fig. 7).
Where tree growth requires line replacement, a saving of $300 per 1,000 ft. may be made in cutting pole line down to 30 ft. size and using shielded primary with secondaries on brackets. This saving would be increased to $500.00 if it were otherwise necessary to increase height of poles 10 feet.
With an existing 30-ft. cedar line, carrying secondaries only, it is possible to install three No. 2 shielded primaries for approximately half the cost of replacing with 45 ft. poles and stringing open primary, and at the same time avoid all primary tree interference.
Installation Details Are Important
Eliminating messenger wire installations, the Toronto System has 212 installations of self-supported shielded primary cable. The self-supported cables are secured at each end by suitable dead-ending devices and held on intermediate poles by insulators or clamps. Insulators are not necessary electrically but provide a convenient, low-cost support.
With cable installations, there is always present the possibility of breakdown between conductor and the grounded shield, if the cable is not properly installed. Care must be taken to guard against two possible sources of trouble. The first is mechanical pressure on the cable surface such as at an insulator or dead end; the second is the lack of sufficient clearance between the conductor and shield when making a joint or dead end.
Strain insulators were used for the first deadends of self-supporting cable. The cable was doubled back on itself without removing the armor. This is now considered very severe treatment because it squeezes the insulation between the conductor and shield, reducing its thickness and inviting puncture. However, deadends made thus have given trouble-free service. The progressive development of deadends, based upon experience, is shown in Fig. 5.
Turns, bends, and angles are potential trouble spots unless care is taken to eliminate excessive pressure on the cable surface. Line insulators are not suitable for making changes in directions. Developments in this direction included the use of two spool-type insulators and reenforced cable insulation, the use of a large diameter wooden spool which led to the development of a bronze clamp, and finally to the now standard grip.
In 1936, the flexible wire grip, commonly used for cable pulling and secondary cable supports, was adopted as the standard method of deadending cables and of making changes in direction (See Fig. 6). It is light in weight, flexible, low in cost, inconspicuous, easy to install, and fulfills all requirements extremely well.
While the location of primary cable upon concrete poles has been a debatable question, most installations are made on the upper secondary street lighting position, it being necessary to lower the secondary to do this. Alternatives are the use of a top extension (for clearance) or the position on the lower street lighting secondary which is advantageous where transformers are located on concrete poles. One installation was made without insulators using a metallic clip. This support was practical but more expensive than the conventional insulator.
Splices or joints in armoured cable constitute a weak point and have, therefore, been kept to a minimum. Linemen ordinarily have not the experience or training necessary in the making of high potential joints involving grounded armors. All paper insulated lead covered cables on messengers were finished off by skilled underground jointers. This procedure is expensive both as to labor and material and was one of the reasons for adopting nonleaded shielded cable. The first steel wire armored cable was jointed by weatherproof leads to cutouts at each end and simply taped over, and it still works. On the next oldest installations porcelain tube joints were first used. A very fine rubber splicing tape is now used for sealing joints.
Until 1932 there was no need for any other jointing than at each dead-end, and the occasional line splice, but with transformers erected individually on concrete poles, a tap had to be made in the cable for the first transformer. In making this tap, the cable was prepared by removing weatherproof braid and armor for about 8 inches. The insulation was then, also, removed from centre for 3 inches tapering it 1/2 inch at each side. The end of the tap was then prepared in a similar manner, and after binding with No. 14 bare copper, and soldering, the joint was built up with splicing compound tape half lap¬ped to 50 per cent greater thickness than original insulation. This was then covered with two layers of varnished cambric tape, and one layer of friction tape. Copper braid tape was then wound over the joint, beginning with two strands at tap armour and winding one strand each way to meet cut ends of line armor. Braid was soldered to armor and the joint was completed with friction tape and compound.
In 1936, any further need for either "T" or "Y" joints was eliminated by the use of a transformer cutout (Fig. 8), having two connections on the line side, thus permitting of an entirely separate piece of cable to be used for the second transformer, either on the same pole or the adjacent one.
The shielding of all aerial cables is connected electrically and mechanically to the common neutral wire at every alternate pole.
Until 1932, transformers on cable extensions were hung on crossarms on 35 ft. cedar poles. (See Fig. 9.) As this transformer pole in each case replaced a concrete and had similar poles on either side of it, we received many complaints about its appearance. It, therefore, became advisable to improve methods of transformer installation. A modified 28 ft. concrete pole was first used. Since then 196 25-kw. transformers have been so erected, a standard pole now being used with clamp for holding transformer bracket and the ground wire dispensed with and cases grounded to common neutral only. Until 1937, these transformers were installed singly on adjacent poles. They are now hung on one pole, back to back (Fig. 9), thus lowering costs and eliminating losses in secondary neutral connection between them.
On cedar poles, used for transformer erection in concrete leads, improved methods were also tried. As the wire arrangement on concrete poles is essentially vertical, the horizontal line crossarms were discarded and cable deadended on peak of pole. Shielded primary lead was then run down face of the pole as previously described, or more directly, using wire cable grips, from deadend to cutout on end of trans-former crossarms. With this arrangement, a 30-ft. pole can now be used in place of the 35 ft. previously required, and a much less conspicuous installation results.
As the first types of cable installed were of an experimental nature, it was quite to be expected that some troubles would develop. By their occurrence, we have benefited, and eliminated weak points in design, installation, and operation.
Operating Results
Since 1931, there have been 55 breakdowns necessitating minor repairs. Only one of these occurred in installations which have been installed since 1935. In analyzing causes for breakdown, a majority (22 out of 55) were due to pressure on the cable surface at angles. All but three of these took place in a type of cable with particularly soft insulation. Eleven cases were traced to breakdown between conductor and shield in splices or joints. Five were due to defect in material or injury during installation. One breakdown was attributed to lightning and the remainder to miscellaneous causes such as falling trees, etc. With the present standard construction where deadends are eliminated, stress removed by grips at angles, and splices and joints minimized, the primary cable should provide an excellent operating record.
Our experience has been, that nonleaded shielded primary cable, though far from a perfect product, has great possibilities and that for saving, safety and sightliness it serves a very useful purpose in the modern electrical distribution system.
