[Trade Journal]
Publication: American Institute Of Electrical Engineers
New York, NY, United States
p. 1447-1464, col. 1
ENGINEERING DATA RELATING TO HIGH-TENSION TRANSMISSION SYSTEMS
SUB-COMMITTEE REPORT PREPARED BY THE CHAIRMAN
THE present report is an analysis of the engineering data received from 25 power companies, operating high-tension transmission systems, in answer to a printed list of questions prepared by the High-Tension Transmission Committee of the year 1912-1913, and sent by that committee to 105 power companies operating at 25,000 volts and over.
The present Engineering Data Sub-committee has received the replies to these questions and has analyzed the data therein contained as hereinafter presented. In making this analysis, the main purpose has been to present, as far as feasible, the data having general interest in a form which shall at the same time be compact and easy of assimilation. A considerable part of the data which is statistical has been consolidated in tables which show in parallel columns for the several reporting companies various items of interest returned. A second portion of the data received could be presented more intelligibly under the various topical headings to which it related and has been so offered. As a matter of convenience the reporting companies have been listed with a brief statement as to the character of each plant and an abbreviation of the title assigned to it for the purpose of identification, to avoid the reprinting of the full names of companies a large number of times in the body of the report. In addition to the above-listed data, the companies have furnished a large number of blue prints, drawings and maps, many of which contain valuable designs and constructional details. A considerable number of these has been reproduced in the form of cuts at the end of the report. To facilitate examination and to economize space most of this matter has been redrawn to present the salient information free from non-significant detail. Where useful, dimensions and similar data have been given in the reproduced drawings.
The original reports returned from the companies will be on file at Institute headquarters and open to the inspection of any Institute member, should further details be desired. Furthermore, in the list of companies below is given the name and address of the person by whom the report was submitted, and it is very possible that further information with regard to the apparatus or practise of that company referred to in this report, could be obtained by direct application to such person.
Neither the Engineering Data Sub-Committee nor the Institute assumes any responsibility for the correctness of the information here reported. The sub-committee has endeavored, however, to give a clear and fair report of the information offered.
The sub-committee wishes to express its thanks and appreciation for the freedom with which companies have reported and to acknowledge its obligation for the large amount of personal attention and thought which was required on the part of reporting engineers to answer the very comprehensive list of questions that was submitted. Certain reports in particular should receive especial mention as showing an unexpected amount of painstaking description and discussion of some of the most important and delicate problems confronting high-tension transmission engineers. The sub-committee believes that the comparison of experiences and practise herein set forth cannot fail to be of much value to the profession and will certainly be of great assistance to those engineers having high-tension plants to establish who have not the time and opportunity for the extended research that is necessary in the case of plants of the first magnitude.
LIST OF REPORTING COMPANIES
100,000-VOLT GROUP
MISSISSIPPI RIVER POWER CO. , (Miss. P.) Keokuk, Ia.—St. Louis, Mo. Stone & Webster, Eng. Corporation.
The data returned cover a double trunk line electrification. It represents the very heaviest type of large capacity transmission line and is recently completed. The report is a most valuable one.
GREAT WESTERN POWER CO. , (Gt. W'n. P.) California
P. T. Hanscom, Asst. to the President, 233 Post St., San Francisco, Cal.
The data returned cover a large capacity trunk line electrification across the state of California from the Sierra Nevada Mountains to the neighborhood of San Francisco. A very valuable report. This plant has been operating several years.
YADKIN RIVER POWER Co. , (Yad. R. P.) North Carolina
R. J. McClelland, Chief Engineer, Electric Bond & Share Co., 71 Broadway, New York.
The data returned cover a trunk line electrification of medium capacity, connecting a large hydroelectric development supplementary to a previously existing system, and is but recently finished. A valuable report.
PACIFIC GAS & ELECTRIC COMPANY, (Pac. G. &. E.) Central California. San Francisco.
This is one of the largest, oldest and most diversified companies in the country.
CHILE EXPLORATION Co. , (Ch. Ex.) Chile.
Percy H. Thomas, Consulting Electrical Engineer, 2 Rector St., New York.
This line, which is not yet in operation, is a large capacity trunk line for supplying power for operating a very large copper mining development, including the electrical refining of the metal at the mine. It is notable as transmitting power from a power-house at the ocean to a mine at an elevation of 9,000 feet.
NOTE. One company ("X") did not wish its name used. It is a large system in a bad lightning district. It has a very large, widely distributed power load.
85,000-VOLT GROUP
MEXICAN LIGHT & POWER CO. , (Mex. L. & P.) Necaxa to the City of Mexico.
C. B. Graves, General Manager, Apartado 124, bis., Mexico, D.F.
The data returned cover a number of transmission lines in a very large system which collects power from a number of power stations and distributes it over a wide range of territory. This plant has been operating for a number of years. A very full and useful report.
APPALACHIAN POWER CO. , (Ap. P.) Bluefields, W.Va.
H. W. Buck, Viele, Blackwell & Buck, 49 Wall St., New York. This plant, which is very modern, illustrates the problem of the distribution of power in moderate quantities to a distributed load. It has been operating but a short time. A valuable report.
SOUTHERN SIERRAS POWER CO. , (SOU. S. P.) Southern part of California.
R. G. Manifold, Engineer, Riverside, Cal.
The data returned cover a high-voltage construction for an extremely long-distance transmission and operates in conjunction with a previously existing system of large extent. It operates at present at a lower voltage than ultimately contemplated. A valuable report.
PENNSYLVANIA WATER & POWER Co. , (Pa. W. &. P.) Holtwood to City of Baltimore.
J. A. Walls, Chief Engineer, Baltimore, Md.
The data returned cover a heavy trunk line electrification in a very difficult lightning district, receiving power from the well-known Susquehanna River hydroelectric development. The engineering of this line has been particularly carefully worked out and the report is especially full and valuable.
60,000-VOLT GROUP
WASHINGTON WATER POWER CO., (Wash. W.P.) Washington.
O. F. Uhden, Chief Engineer, Spokane, Wash.
The data returned cover a new trunk tower transmission line and also standard 60,000-volt wood pole construction. This is part .of a very large and widely distributed system. A valuable report.
TORONTO POWER CO., (Tor. P.) Ontario. Canada.
F. G. Clark, Chief Engineer, Toronto, Ont., Canada. This is a large-capacity trunk line through a bad lightning district, from Niagara Falls to Toronto. A good report.
SAN JOAQUIN LIGHT & POWER CORP. , (San. J. L. & P.) California.
Lloyd N. Pearl, General Superintendent, Fresno, Cal.
This is a widely spread distribution system using wooden poles in the Southern Central district of California. A particularly dry climate.
NIAGARA, LOCKPORT & ONTARIO POWER CO. , (Niag. L. & 0.P.) Western New York.
L. C. Nicholson, Marine Bank Building, Buffalo, N.Y.
This is a double trunk line through the western part of New York State in a very bad lightning district. This plant has been in operation a number of years and its engineering has been very carefully studied. A valuable report.
PORTLAND RAILWAY, LIGHT & POWER CO. , (Port. R.L. & P.) Oregon.
O. B. Coldwell, General Superintendent, Broadway, Portland, Ore. The data returned cover some of the more modern construction of an extensive generating and distribution system of large capacity. A valuable report.
SOUTHERN CALIFORNIA EDISON CO. , (SOU. C.E.) California.
J. A. Lighthipe, Los Angeles, Cal.
The data returned cover a portion of a large and well-known system feeding into Los Angeles, Cal.
CHIPPEWA VALLEY RAILWAY, LIGHT & POWER CO. , (Ch'a. V.R.L. & P.) Wis. Eau Claire, Wis.
WESTERN STATES GAS & ELECTRIC CO. , (W. St's. G. & E.) California. Stockton, Cal.
The data returned cover an extensive distribution system in Central California. Wood-pole type of construction.
PUGET SOUND TRACTION, Light & Power Co. (P. S. T. L. & P.) Wash.
John Harisberger, M. T. Crawford, S. C. Lindsay, Seattle, Wash.
The data returned cover a large generating distribution system around Seattle, with modern transmission lines. A portion of the plant has been in operation for a number of years.
CITY OF SEATTLE LTG. DEPT. (City S. L. Dpt.) J. D. Ross., Seattle, Wash.
25,000-50,000-VOLT GROUP
UTAH LIGHT & RAILWAY CO. , (Ut. L.R.) Utah.
O. A. Honnold, Electrical Engineer, Salt Lake City, Utah.
The data returned cover a portion of a very extensive system centering around Salt Lake City, Utah.
CANADIAN NIAGARA POWER CO. , (C.N.P.) Niagara Falls, Canada, and Buffalo.
L. E. Imlay, Superintendent, Niagara Falls, N.Y.
The data returned cover a trunk line from Niagara Falls, Canada, to Buffalo. It represents four heavy capacity lines, parts of which have been in operation a number of years.
MT. WHITNEY POWER & ELECTRIC CO. , (Mt. Wh'y. P. & E.) California.
Fred G. Hamilton, Superintendent, W D., Visalia, Cal.
A transmission distributing company located in irrigating district of Central Southern California.
UNION TRACTION CO. (Un. T.) Indiana.
G. H. Kelsay, Anderson, Ind.
The data returned cover construction of an interurban electric railway system of wide extent.
TABLES
Tables I to V, inclusive, herewith, give statistical information as to the several companies and are self-explanatory. Where the information submitted has been too extended to incorporate in the table, it is given in the following notes.
NOTES REFERRED TO IN TABLES
NOTE 1. Angles are turned by shortening standard spans and taking only part of angles exceeding 10 deg. on any one pole. Below 10 deg. the following table is used:
On angles over 50 deg. the last tangent span is made 300 ft.—the first and last angle spans are 50 ft. with an angle of 5 deg., and subsequent spans are 50 ft. with 10 deg. angle each.
Vertical angles are similarly handled.
NOTE 2. Right angles and heavy angle corners are turned by specially guyed dead end poles, Fig. 6, —angles under 10 deg. turned by standard poles guyed, and angles from 10 to 30 deg. are made on two or more poles. Vertical angles are not specially constructed except as to guying.
NOTE 3. Mex. L. & P. reports that the altitude is important. Gaps are set as follows:—Necaxa, altitude 2950 ft., voltage 85,000, gap 6 in.; El Oro, altitude 8000 ft., voltage 81,000, gap 6 in.; Pachuca, altitude 8000 ft., voltage 84,000, gap 7 in.
NOTE 4. Lumping together total interruptions and partial interruptions, and in the latter including even minor losses of load, such for instance, as a direct current breaker on a synchronous motor generator set opening up, although the synchronous apparatus holds in, we have the following percentages:—
It is almost impossible to tell whether these insulators were punctured resulting in their breaking, or whether they cracked due to a spillover and then punctured. With the new insulator or Hirt type very little damage can be done by spillover.
b. From 1907-1912, about 150 insulators lost.
1912—about 75 insulators lost, and about 200 others changed on account of cracks.
1913, about 200 insulators changed on account of cracks and probably 50 insulators lost.
NOTE 8. The telephone system is protected by:—
a. A No. 8 steel ground wire carried on the same crossarm as the telephone wires, and on the side toward the transmission line.
b. Drain coils at terminals and at the two substations.
c. Vacuum arresters and spark gap at all telephone stations permanently connected to line. Aluminum cell arresters at terminals and at Hulls substation.
d. Insulating transformers at power house and substations.
NOTE 9. Telephone wires transposed according to "A", "B"-"C"-"B" system. "A" transposition between pairs; "B" and "C" transposition between wires of each pair.
NOTE 10. No standard distance for transposition. Our 28 mile two circuit tower line has three complete cycles in each circuit. The majority of pole lines have a transposition every mile (one cycle every three miles) Where our high-tension lines parallel foreign telephone or telegraph lines, transpositions are made every 1/3 mile.
NOTE 11. Long fuses (30 inches long) with spark gap to ground. At all stations "bleeder " coils with middle point grounded are installed. Operator must stand on insulated platform to use telephone.
NOTE 12. Reactance coils are used in main low-tension buses in the power house between groups of generators.
NOTE 13. Between the low-tension bus and the transformers at the power house, and between certain sections of the bus at the power house and at the substation.
Short-circuit current on transmission line at full voltage in power house, right outside the power house, is about 5 to 6 times normal full load current at start.
Without these reactances the starting wave of the short-circuit current at the same point would be about 8 to 9 times full load current.
No objectionable features. These coils have been in service all year 1912 and 1913.
NOTE 14. The coils will be located between sections of 12,000-volt generator busbars and will limit the flow of current to the capacity of the bus section which it protects.
NOTE 15. Coils will have 6 per cent reactance, and will be placed close to terminals of the 12,000- and 15,000-kw. steam turbines.
NOTE 16. The factor of safety of new poles is entirely up to the judgment of the designing engineer, and depends greatly on the climate, kind of soil and economic conditions. Our towers were built to stand if all conductors were cut on one side, and no allowance was made for the tendency of the ground wires to support the tower.
Specifications for Towers.
Each crossarm will be sufficiently strong to withstand a horizontal strain, in any direction applied at the end of the crossarm of 5000 pounds; and a vertical strain applied at the end of the crossarm of 1000 pounds.
Each ground wire support will be sufficiently strong to withstand a horizontal strain in the direction of the line of 8000 pounds and a vertical strain of 1000 pounds.
Each tower will be sufficiently strong to successfully resist the turning moment due to the application of a horizontal strain in the direction of the line of 5500 pounds applied at the point of support of either ground wire; also of a horizontal strain in any direction of 3000 pounds applied to the points of support of any three of the conducting wires. It will also be sufficiently strong to successfully resist an overturning due to a strain parallel with the line of 5500 pounds applied at the points of support of the two ground wires or a total 11,000 pounds; also to a strain at right angles to the line of 5000 pounds applied at the points of support of the two ground wires or a total of 10,000 pounds; also to a strain of 9000 pounds in the direction of the line, which load will be considered as concentrated at the intersection of the middle crossarm and the center line of the tower, and simultaneously a horizontal strain of 500 pounds at the points of support of each of the six conducting wires and two ground wires or a total of 4000 pounds which strain will be in a direction transverse to that of the line; also of a stress of 12,000 pounds applied horizontally at center of gravity of wires either at right angles or parallel to the line. Accompanying the above mentioned horizontal stresses will be a vertical downward strain of 500 pounds applied to each of the supports of the six conducting wires and two ground wires.
In addition to the above specified stresses each tower will be subjected to stresses due to its own weight and to a wind pressure in a horizontal direction of twenty pounds per square foot on the superficial area of the tower. The strength of each tower will be sufficient to resist the combined action of all of the above stresses without permanent distortion.
NOTE 17. Old insulators assembled test 120,000 volts for 1 min. New insulators, each disk tested with a low capacity transformer having steep wave front for 2 min. of continuous static discharge over insulator. Strain insulators same test.
NOTE 18. A special test tower was constructed and tested with following tests and all towers were built as duplicates of test tower:—
Test No. 1. A horizontal pull of 12,500 lb. in a direction parallel or at right angles to the lines will be applied at the intersection of the middle crossarm and the center line of the tower.
Test No. 2. A horizontal pull of 4,000 lb. in a direction parallel to the line and applied at the ends of any two crossarms (aggregate pull 8,000 lb).
Test No. 3. A horizontal pull of 6,000 lb. in a direction of the line applied at the end of any crossarm.
Test No. 4. A vertical load of 1,500 lb. applied at the end of any crossarm.
The tower must withstand the above force without permanent distortion in any member, and if such distortion should take place the contractor must replace such members with others until the tower successfully meets the requirements of the tests.
All of the standard towers covered by this contact must then be constructed strictly in accordance with the design and sizes of material contained in the successful test tower.
NOTE 19. On three of the present four 22,000-volt circuits from Niagara Falls to Buffalo on the Canadian side are in use Electrose insulators which have been in service a considerable number of years. The fourth line which was installed in 1912 also uses Electrose insulators but of a radically different design planned to be puncture-proof in severe impact tests. Test and forms of these insulators are described in the Trans. A. I. E. E., p. 2121, Vol. XXXI, meeting December 1912.
NOTE 21. The foundations of towers consist of 7-ft. legs with foot piece, the whole generally loaded with stone and well backfilled.
NOTE 22. Material of insulators porcelain with slate glaze. (Slate glaze shows up arc smoke marks better than brown glaze and attracts the attention of gunners less, but does not show up broken petticoats as well.
NOTE 23. The towers were built for two ground wires, but only one ground wire located on apex of tower was erected. This makes the horizontal spacing from ground wire to conductor 7} ft. and the vertical spacing 71 ft.
NOTE 24. All standard towers set on earth stubs while the heavy towers are set in concrete.
TOPICAL TREATMENT
A large number of comments made by the various reporting companies on a number of topics of especial interest to engineers are here grouped together under their appropriate subjects as follows:
LONG SPANS (QUESTION A-19)
The following notes of interest were returned:
Miss. P. Longest span with standard tower and conductor 1425 ft.
The maximum span used on this line is 3200 feet, and occurs at the crossing of the Missouri River. This and other long spans are shown in plan and profile on the accompanying drawings (Figs. 30, 31 and 33.) The conductor cable in these spans consists of a Fin. high-strength galvanized 19 strand steel core overlaid with 20 strands of No. 10 B. & S. gauge hard drawn copper wire. The cable is filled with a compound for the exclusion of air and moisture. Each circuit is carried on a single tower line, conductors in a horizontal plane, spaced 20 ft. apart, with two ground wires 10 ft. above at point of support. These river crossing towers were especially designed and vary in height from 60 to 230 ft. above foundations. See drawings.
Gt. Wn. P. One span 2300 ft. on special towers; one 2740 ft. with No. 000 B. & S. "Minot" stranded wire; conductor balanced by counterweights to give uniform tension—Figs. 3 and 4.
Mex.. L. & P. One 1400 ft. with a difference in elevation of 350 ft.; cable size and towers standard.
Pa. W. & P. Longest span with standard conductors and towers 1280 ft. Longest span 1800 ft. with No. 0000 B. & S., 7 strand hard drawn copper and towers 115 ft. high over all above foundations. Span sag 120 ft. (6.7%). Distance between conductors, vertically 10 ft., horizontally 15 ft.—Ground wires above conductors—no trouble.
City S. L. Dpt. Longest span 780 ft. standard double-pole construction.
Wash. W. P. One 1500 ft., 1/4-in. "Siemens-Martin" steel as conductor.
San. J. L. & P. Span across Kings River at Piedra, six 3/0 aluminum cables, carried about 1700 ft. across river and anchored on hillsides to cedar poles. Two sets of three wires each are attached to two poles, wires in a vertical plane six ft. apart and attached to poles with two Locke No. 273 strain insulators. Guys are placed for each wire and run to anchorage in rocks. About 200 ft. sag is obtained with wires clearing river about 150 ft. All wires swing in unison in a high wind and no trouble has been experienced.
Niag. L. & 0. P. See drawing, Fig. 34.
C. N. P. See drawing, Fig. 8. The transmission line crosses the Niagara River at Buffalo where there is a span of 2192 ft., from a 150-ft. tower on the American side to a 202-ft. tower on the Canadian shore. The tops of these towers are at the same elevation. The line is then carried over the village of Fort Erie with a span of 1667 ft. to a 61-ft. tower on Bertie Hill. The top of this tower is 107 ft. below that of the High tower. The minimum clearance of the cables above the river is 130 ft. On the high towers the cables are arranged on 15-ft. triangles and on the Bertie Hill tower on 10-ft. triangles.
The twelve conductor-cables are made up of 19 strands of No. 10 B. & S. gage bi-metallic wire and are stressed up to 5400 lb. This tension is kept constant by counterweights on the Buffalo and Bertie Hill towers. The counterweights are supported by steel cables which run over sheaves at the top of the towers and are- connected to each bi-metallic cable through two pairs of spool insulators. Drop cables pass down and through the tower to the Buffalo terminal station and on the Bertie Hill tower to the bus-bars. The busbars and switches are so arranged that any circuit on the pole lines can be connected to any circuit on the long spans. At the high tower, the cables are connected to galvanized iron chains which rest on insulated saddles and extend about 13 ft. on each side of the tower. Jumper cables are carried over the saddles.
In addition, spans of 800 and 1435 ft. were reported by other companies and no cases of trouble.
SPECIAL FEATURES OF CONSTRUCTION (QUESTION A-20)
The following notes relate to special features of interest in construction:
Ap. P. All suspension insulators are ballasted with 30-1b. cast iron weights. See PROCEEDINGS A. I. E. E., February 1914, page 227.
Port. R. L. & P. Experience has shown that it is cheaper and quicker to erect steel towers in position from the ground up.
ANCHOR TOWERS ON TANGENTS (QUESTION B-13-15).
The following reports were made on the use of anchor towers on tangents:
Miss. P. Approximately every mile.
Gt. Wn. P. Average every two miles—designed to stand with all wires cut.
Ap. P. Two per mile—designed to stand with all wires cut. Sou. S. P. Every five miles, designed for 24,000 lb.
Pa. W. P. At least every fifth tower—on average five to mile.
San J. L. & P. Poles guyed both ways every half mile, will stand with three conductors cut.
Niag. L. & 0. P. Every mile on steel towers; every half mile on " A " frames; all to stand with all three conductors cut.
Sou. C. E. No, use line guys.
Ut. L. & P. Every 1-1/2 to 3 miles, according to wind conditions; designed to stand 7000 lbs. at center crossarm in addition to stress on regular line towers.
C. N. P. Only at two ends of line and two intermediate curves; designed to stand all conductors cut.
DETERIORATION (QUESTION A-25)
The following interesting notes on deterioration were received: Gt. Wn. P. Slight rusting where towers were not properly galvanized. Wires corrode.
Yad. R. P. Line two years old—no deterioration noticed.
Pa. W. & P. No deterioration observed upon examination of buried portions of galvanized towers. One particular set of gusset plates near top of tower showing signs of rust during 1913 ; no rust or deterioration elsewhere. No signs of deterioration in conductors. Insulators both on transmission line and in stores showing deterioration, due possibly to temperature expansion effects. About 4 per cent of insulators examined to show such deterioration, not due to electrical causes.
Wash. W. P. We have noted no deterioration in conductors. Some insulators placed in service in 1904-1906 indicate that they may have deteriorated, but as the manufacture of porcelain at that time was far less efficient than now, no results of long time tests on those would indicate what will obtain on the ones of later manufacture. Towers were placed in 1910, and no deterioration has been noticed.
Tor. P. Except for some deterioration of ground wire and hemp core of conductor, no deterioration noticed.
San J. L. & P. No deterioration noticed as yet-60,000-volt system in use only three years.
Niag. L. & 0. P. Galvanized towers develop rust spots in about seven years. Insulators to some extent deteriorate by puncture of an occasional skirt. No noticeable deterioration of cable except by occasional burning by arcs.
Port R. L. & P. The transmission line has been in service less than two years and we have, therefore, no observations of deterioration except in the matter of insulators, there having been a considerable number of failures in suspension insulators and insulators in a strain position since the line was put in service.
Sou. C. E Insulator shells crack, presumably due to expansion of cement or steel pin.
Ch'a V. R. L. & P. Insulators give more trouble with age. Ut. L. & R. Wood poles with carbonized butts last 10 years in this climate.
Pug. S. T. L. & P. None, if proper factors of safety were observed in original installations. Steel towers have to be painted every two years, if not galvanized. Cedar poles rot off at the ground in from 15 to 20 years.
City S. L. Dpt. Poles rot at ground line.
DEFLECTION OF SUSPENSION INSULATORS (QUESTION B 20-21)
As to how much angular deflection of conductor was assumed under wind conditions and how much was actually observed, the following data were reported:
Miss. P. 26 deg. 45 min., with in. ice, assumed.
Gt. W'n. P. 45 deg. assumed.
Yad. R. P. 45 deg. assumed.
Ap. P. 30 deg. on swinging of strings; held down by 50-lb. weights.
Sou. S. P. 45 deg. assumed-45 deg. observed on swings.
Pa. W. & P. Approximately 60 deg. Probably never more than 30 deg. angular deflection from vertical due to wind observed under either steady wind conditions or swings. No good records on actual angular deflection. Conductors do not swing violently, and angular deflection is not the same at all points in a span for one conductor, but is the same for all conductors. See drawing, Fig. 54.
Wash. W. P. 50 deg. assumed, 36 deg. observed.
Ut. L. & P. 60 deg. from vertical assumed, this value observed in swings.
DESIGN FACTORS OF SAFETY (QUESTION B-22)
As to the factors of safety provided in conductors, towers, against overturning foundations, and overhead ground wires, the following data were reported:
Miss. P. Conductors 2, towers 3, foundations 2, ground wires 2.
Gt. Wn. P. Conductors 2, towers 2, foundations 3, ground wires 3.
Yad. R. P. Conductors 25,000 lb. per sq. in.
Mex. L. & P. Conductors 2 and 3, foundations 1.5.
Ap. P. Conductors 2, 3, towers 2, foundations 5, ground wires 10.
Sou. S. P. Conductors 2.5, towers 1.7, foundations 1.7.
Pa. W. P. For conductors (alum.) up to elastic limit—towers tested for maximum designed strength at factory—foundations practically 4—ground wire just up to elastic limit.
Wash. W. P. For conductors elastic limit, for towers 1, for foundations 1, for ground wire 1. These factors are taken in view of the fact that the maximum load conditions assumed were very severe.
San. J. L. & P. For conductors 6, for poles 3.
Niag. L. & O. P. For conductors 1 (elastic limit), towers 2, foundations, 2.
Sou. C. E. For conductors 22,000 lbs. per sq. in working stress.
City S. L. Dpt. Factor for conductors of 3 over elastic limit.
OVERHEAD GROUND WIRES AS PART OF STRUCTURE (QUESTION B-23)
In answer to the question as to whether overhead ground wires are relied upon as part of the line structure most of the companies replied no, but the following comments were received.
Yad. R. P. Yes.
Pa. W. P. Ground wire gives some stiffness lengthwise of line, damping longitudinal vibrations of towers, but is not relied on as part of the mechanical supporting structure.
Ut. L. & P. No, but it undoubtedly acts as a guy wire.
CUTTING OUT OF LOAD (QUESTION C-1)
A loaded circuit is usually cut off by an oil switch, sometimes on high tension, sometimes on low tension. The following replies are noted:
Gt. Wn. P. Drop generator load and open generator oil switch on low-tension side. Do not switch on high-tension side.
Yad. R. P. (a) Reduce voltage 60 per cent and then open low-tension oil switch (b) Open low-tension oil switch at full voltage (c) Open high-tension oil switch at full voltage.
Mex. L. & P. Cut out sections of line one at a time loaded or unloaded. Experience shows that this method gives less trouble from surges on oil switches and switch bushings.
OPENING SHORT CIRCUIT. (QUESTION C-2)
To open a short circuit that holds on, the following companies reduce the voltage of the generators:
Miss. P., Gt. Wn. P., Sou. C. E., C. N. P.
Note also the following comments:
Sou. S. P. The hydro-electric plants are tied in by non-automatic switches on the low-tension side while the steam plant has oil breakers with definite time circuit relays on the low-tension side. The high-tension switches in the main tower line are of the Bowie air-break type and are non-automatic. As operated at present, when short circuit occurs on tower line, the steam plant breakers clear the southern end of the system of trouble, leaving the steam station with all load in that territory. The hydroelectric. plants then drop voltage to low value and test for location of trouble. Sou. C. E. Separate main system into sections and cut out step up transformers on high-tension side.
AUTOMATIC OVERLOAD RELAYS (QUESTIONS C 3-8)
Automatic overload relays are generally used, and in many parts of the various systems. The majority are definite time limit or inverse time limit.
The overload settings run from 100 per cent to 300 per cent overload, and the definite time limits from to 10 sec.
A half dozen companies use overload relays of progressively greater time element distributed from the load to the generator.
Miss. P. Use inverse time limit automatic overload breaker to cut apart groups of generators on the 11,000-volt generator busbars.
Niag. L. & 0. P. and Sou. C. E. report success with this selective action; Yad. R. P., Mex. L. & P. , and Wash. W. P. report partial success.
Pa. W. & P. Automatic overload circuit breakers are used in connection with 13,000-volt cable feeders, station auxiliary transformers at both power house and substation, and transmission lines, in the last case, however, not the high-tension circuit breakers, but the low-tension circuit breakers of those transformers connected with the line being opened.
In connection with 13,000-volt cable feeders, we use inverse-time relays; for the transformers and transmission line, definite-time-relays.
(a) The lowest tripping current for the relays connected with our 13,000-volt cable feeders is 100 per cent overload, based on cable rating; with 700-1400 per cent overload these relays will trip in 1 sec. (inverse time).
(b) The relays for the substation transformers are reverse-power relays set to trip at 50 per cent over load in reverse direction, and connected with a three-sec. definite time element.
(c) The power house transformer relays trip at 140 per cent overload 7 sec. definite time.
Time-element relays are normally used with progressive timing of the elements. This refers particularly to the relay system used for the 13,000-volt a-c. underground cable system in Baltimore, of which a part belongs to the Pa. W. & P. Co. and a part to our customers' distributing systems. The larger part of the relays for this system are Type C Westinghouse overload inverse-time relays improved by F. E. Ricketts's compensating coil, which produces a relay curve with less steep characteristic and for heavy overloads can be brought to approach a definite time. Both tests and experience have shown that this type of relay can give good selective action for several relays in series.
Bellows type relays were previously used in this connection but were found to be not sufficiently reliable and were replaced by relays of the type referred to above.
Westinghouse Type C, improved, reverse-power relays with selective element are also used.
These reverse-power relays are used at the sub-station end of two transmission lines working in parallel. When a short circuit, which is not cleared -in any other way, occurs on one line, it will trip the low-tension side of transformers at the substation connected with this line, while overload or time relays will trip the low-tension side of the corresponding transformers at the power house. If the other transmission line is not affected, the reverse-power relays for this line will remain open. In order to give another device (arc extinguishers) time to relieve lightning arcs, these relays for the transmission lines are furnished with definite time-limit relays (W. Type E); these have at present the following setting:
The different time setting for the two circuits is chosen in order to prevent one line from opening at the substation, while the other opened at the power house, in case both lines should be in trouble. As soon as one circuit is cleared, an interlocking device prevents the other from opening by any relay action.
If after the clearing of one of the two paralleled transmission lines, the other still shows the trouble the field will momentarily be taken off all the generators at the power house simultaneously, and restored again.
Should this action not clear the second line, the switches must be opened by hand. Our experience so far shows, however, that permanent line trouble (wires down, etc.) never has taken place on both circuits at the same time.
Pug. S. T. L. & P. Success generally but not always.
Aside from the Pa. W. & P. , the Ut. P. & R. and Pug. S. T. L. & P. are the only companies using reverse-energy relays; the former reported " always " act selectively—the latter does not state the result.
Note also Cty S. L. Dpt. Use Westinghouse Type C, reverse-energy relays which act selectively when the power factor does not drop too low as on a very heavy short circuit.
DROPPING SYNCHRONOUS LOAD (QUESTION C-9)
The following report that they seldom or never succeed in carrying synchronous load through a heavy main-line short circuit:
Gt. Wn. P., Mex. L. & P., Sou. S. P., Tor. P., San J. L. & P., Port. R. L. & P., W. St's G. & E., C. N. P., Un. T., Cty. S. L. Dpt.
Other reports—
Ap. P. " Sometimes." Lightning arcs are frequently cleared by arc suppressers without losing synchronous load.
Pa. W. & P. Lightning arcs are frequently cleared without the least loss of load, by arc suppressers.
Wash. W. P. We have automatic switches on all lines feeding out of the different stations and when these act properly we very seldom lose any synchronous load.
Niag. L. & 0. P. Save synchronous load by automatic arc extinguishers, when arcs only are involved.
Ut. L. & P. Yes, when short circuit is cleared in three seconds.
Pug. S. T. L. & P. Sometimes we can and sometimes we cannot. If the duration of short circuit is three or four seconds synchronous apparatus always drops out.
CUTTING OUT ONE OF Two PARALLEL LINES (QUESTION C-10)
In answer to the question as to when two lines parallel at both ends could be cut out without losing the load the following were received:
Miss. P. Two St. Louis lines parallel at both ends and have been separated in a number of cases automatically without losing the load.
Mex. L. & P. Four lines are operated in parallel and as a rule one line can be cut out without losing the load.
Ap. P. Sometimes.
Wash. W. P. Have such lines but cannot cut them out without losing load.
San J. L. & P. Lines are tied together at load end by tiebreaker set light; at supply end lines are separated by operator.
Niag. L. & 0. P. Have tried this but have abandoned the attempt.
Port. R. L. & P. Yes, but cannot be automatically separated Sou. C. E. All main lines, cannot separate.
C N. P. Cannot separate such lines.
Un. T. Cannot separate such lines.
LOCATING TROUBLE (QUESTION C-11)
Practically all plants sectionalize the line, test with generator voltage and patrol to locate line trouble.
Yad. R. P. Use also a Wheatstone bridge method.
Niag. L. & 0. P. Use a special loop test described in the
TRANS. A. I. E. E. ~~, June 1907.
C. N. P. Uses a loop test.
DISTRIBUTION OF POWER BETWEEN POWER HOUSES AND REGULATION OF VOLTAGE (QUESTIONS 20 AND 21)
No points of especial interest appear in answer to questions on how power distribution between generators and voltage regulation are secured.
EFFECT OF HEAVY SHORT CIRCUIT (QUESTION C-23)
As to the effect of a heavy short circuit near one power station on a large system:
Pa. W. & P. When a short circuit occurs near one power house, the effect of this depends entirely on how long a time it lasts.
(1) If it is a lightning arc on the transmission line it will normally be cleared by arc suppressor.
(2) If it is cable trouble on the 13,000-volt distributing system, it will normally be cleared by opening automatically, the proper feeder switches. If the trouble hangs on for more than four seconds the fields of the generators will be destroyed and restored automatically at all three power houses simultaneously.
OPERATION WITH ONE SIDE GROUNDED (QUESTION C-29-31)
In answer to a question as to whether the lines were ever operated with one side grounded, even for a brief period, the following were received:
Pa. W. & P. For a few minutes, no effect; ground was cut off by the time the ground resistance was red hot.
Ut. L. & R. All one night on 28,000-volt circuit; no effect except unbalancing of system.
C. N. P. For about two hours with no effect except a slight unbalancing of current in conductors.
Ap. P. For two hours with no effect.
Wash. W. P. For several minutes causing whole system to be unbalanced.
Gt. Wn. P. For about 1/2 hour; one oil switch bushing and one string of insulators punctured.
Sou. S. P. "No; effect too severe."
Tor. P. On several occasions for five to fifteen minutes, on one occasion four hours. On the occasion when the system was operated for four hours the ends of the cable that were down were 1000 ft. apart, the ground was highly charged and the barbed wire on the right-of-way fence was also highly charged. A man attracted by the display due to this ground walked into the charged area, then tried to climb the barbed wire fence and was killed. A dog approached the barbed wire fence some distance away and after investigation started for remote regions. Claims were made for damages to cattle. These were paid, although it could not be found that any cattle were really injured.
In operating on a ground we have no means of knowing whether or not the wires are down, and as it is possible that there may be two grounds miles apart with an open circuit in the conductor between, we consider it a very risky thing to continue such operation and would only do so as a last resort.
San J. L. & P. For two and a half hours on 60,000 volts; for one and one third hours on 30,000 volts. The effect was unbalanced voltage on the particular feeder having a ground; unbalanced load on nearest generating plant, private telephone line out of commission, troubles reported from Sunset and other telephone systems.
Niag. L. & 0. P. On one occasion when neutral was not grounded, for two hours; effect "violent."
Pug. S. T. L. & P. For 10 minutes,—severe strains, discharging lightning arresters—telephone wires hot.
Presented at the 31st Annual Convention of the American Institute of Electrical Engineers, Detroit, Mich., June 25, 1914, under the auspices of the Engineering Data Sub-Committee. (Subject to final revision for the Transactions.)
