The Design of a High Tension Testing Transformer Citation Wells, Albert William (1914) The Design of a High Tension Testing Transformer. Bachelor's thesis, California Institute of Technology. doi:10.7907/85ez-zh38. https://resolver.caltech.edu/CaltechTHESIS:07112024-202203199 Abstract No abstract. Item Type: Thesis (Bachelor's thesis) Subject Keywords: (Electrical Engineering) Degree Grantor: California Institute of Technology Division: Engineering and Applied Science Major Option: Electrical Engineering Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: Unknown, Unknown Defense Date: 1 January 1914 Record Number: CaltechTHESIS:07112024-202203199 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:07112024-202203199 DOI: 10.7907/85ez-zh38 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16550 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 16 Jul 2024 23:42 Last Modified: 16 Jul 2024 23:42 Thesis Files PDF - Final Version See Usage Policy. 11MB Repository Staff Only: item control page

Thesis The Design of a High Tension Testing Transformer by Albert W. Wells Class of Nineteen Hundred and Fourteen Department of Electrical Engineering Throop College of Technology Pasadena, Cal - 1914 - The Design of a 250,000 Volt Testing Transformer. The transformer, the design of which is given in this thesis, is a transformer for experimental work and high voltage testing such as would naturally come under the work and direction of the Electrical Engineering Laboratories of a College of Technology. The design calls for a transformer of one hundred kilowatts capacity and a maximum potential of two hundred fifty thousand volts with one end of the high voltage winding grounded. Two such trans- formers, operated in series by connecting their grounded sides togethe~c would give a maximum potential of half a million volts between their ungrounded terminals, with only 250,000 volt potential strains to ground or neutral. Voltage control will be obtained by varying the low tension voltage, except for the lower voltages around onehalf and one-fourth of the normal. These reduced voltages will be obtained by changing the connections of the low tension winding, which will be in four parts, from multiple, to series-multiple, and series. Following the general tendencies of design in transfor mers of this character, it is of the core type, with half of the windings on each leg of the core, and the condenser principle is utilized in all the high tension insulation. The. d esign given was taken from three comparative designs 2 which were started to determine the ratio of voltage to turns of wire which would give a transformer of lowest first cost for materials. Using .090 by .110 inch rec- tangular copper wire for , the low tension winding, No. 26 B andS gage for the high tension winding, and high grade transformer steel for the core, the most economical design called for about six volts per turn of wire. Si::x: and one- quarter was chosen as this allows the use of simple constants in changing the readings of commercial meters to the actual values of voltage and kilowatts developed by the transformer, the meters being connected to special terminals which will be brought out for that purpose. These terminals are such that the maximum voltage on the meters and to ground will be limited to two hundred, thus allowing for the protection of the instruments, and the operator from the high voltages when measurements are being taken. The Core The volts per turn having been determined., and the maxinn.im flux density limited arbitrarily , to 70 .,000 lines per square inch, which is known to be a safe value., the cross section of irom required was calculated., using the formula e = J.d.1:-i-f- in \7hich e is the vol ts per /cJ turn, p the total flux, and f the frequency in cycles per second. Transposing .L )( /() 8 -f.~-1 I Dividing 2,820,000 by 70,000 gives the necessary cross section of iron to be 40 square inches. area. This is the net Allowing a space factor for stacking of the larnin- ationa of 0.9, the gross area required is 44.5 sq. in. Making the core square, with each side measuring 6.75 inches, the cross section becomes 45.56 sq. in., which is on the safe side as it allows a slight reduction in the flux density. The window opening of the core will have a vertical height of 3 ft. 8.75 in. and a horizontal width of 3 ft. 1.75 inches. Expressed in inches these dimensions are re- spectively 44.75 and 37.75 inches. The mean length of mag- netic circuit will, therefore, be 187.2 inches, or 475 cm. With the joints in the core arranged as shown in drawing No.3, the lengths of the individual punchings will be 4 ft. 3.5 in. and 3 ft. 8.5 inches. The sum of these two lengths is which is the length of stock sheets of this steel. 8 ft. The standard widths of the stock are 24 and 30 in. and unless special widths may be obtained at a reasonable p:-cice at the time when this transformer is built, the more economical method would be to use the 30 in. material and cut 4 short and 4 long punchings from each sheet, wasting only the three inch strip. Basing calculations on data furnished by The American Sheet and Tin Place Co~any fox their Apollo Special Electrical Steel, the weight of the laminated core will be 4 1725 lbs., and with a flux density of 70,000 lines per sq. in. the core loss will be 0.9 watts per pound or a total of 1550 watts. Counting all four sides of the core as being radiating surfaces, the radiation of heat from core to oil will be about 0.31 watts per sq. in. of surface. No. 29 gage or 14 mil iron will be used. The core will be rigidly clamped togeth er by means of bolts and clamps. Three-quarter inch bolts will be us- ed for this, and they will be spaced as shown on drawings No. 2 and 3. All bolts on the core structure are to be insulated from the core by horn fibre tubes. The bolts which pass thru the legs of the core will have plates made of 4 in. lengths of ¼in. by 2 in. strap iron under their heads and nuts. These plates will be insulated from the core by means of horn fibre. The yokes will be held be- tween 6 in. channels, with insulated bolts passing through the channels and yokes. The lower yoke clamp will also serve as a footing for the entire internal structure, as support for the external. barrier, and as guides when the transformer is placed in, or removed from, its tank. The upper yoke clamp will carry the upper barrier braces, the coil steadying clamps, and the lifting eyes. The details of the footing are shovr.n in drawing No. 3, as are also the lifting eyes. ing No. 4. The coil clamps are shown in detail in draw- 5 Low Tension Winding The low tension or 2300 volt windings will be wound directly on the legs of the core. The winding is to be in four separate sections or coils, two wound on ea ch le g, and all eight leads will be brought out so that the fom~ coils may be operated in lli"'l.1.ltiple, series-multiple, or seri es. The normal full voltage condition is to op e rate the four coils in multiple on 2300 volt mains. The other connect ions, with the same low tension voltage, give high t ension voltages of 125,000 and 62,500 respectively. Al t ho the core on which the low tension coils are wound is rectangular, the coils themselves are to be cylindrical in form, with an internal diameter one-half inch greater than the diagonal of the core. meter will be ten inche s. This internal dia- Wooden strips spaced around the core, with oil ducts between the m will form an approxi mately cylindrical surface on which to form the low tension coils. Each of the four coils will con s ist of 368 tums of 0.95 by .lJ.O in. 1·ectangular double cotton covered wire wot1ncl on edge, and should occupy not more than 41 inches of the ava ilable window opening of 44.75 inches. Beginning at the wooden forming strips which for m a cylindrical sm.•face ten inches in dia me ter, the low tension winding will be built up of the followin g materials in the 01·der mentioned: 2 layers of 1/16 inch pressboard, 2 layers of 10 mil 6 varnished cambric, the first low tension coil, 1 layer of cambric, 2 layers of 1/16 inch pressboard, 1 layer of cambric, the second low tension coil, 1 layer of cambric, and finished with 2 layers of 1/2 lap cotton tape. The entire low ten- sion windings and its insulation should not exceed 0.7 in. in thickness, and the insulation cylinders as wound in place will be 43 inches long. With this construction the mean length of turn of the innel' coils will be 29. 87 in. and of the outer coils 31. 57 inches. The average of these is 30.72 in. 1!7hich gives a total length of wire in the 4 x 368 turns of 45,220 inches or 3770 feet. At 40.3 lb. per 1000 feet the copper will weigh 136 lb. The resistance, at 0.7964 ohms per 1000 ft. will be 3.002 ohms with the coils connected in series. The effective resistance for the series-nmltiple connection will be 0.75 ohms, and for the multiple connection 0.19 ohms. With a low tension cur-rent of 50 amperes, which is about normal full load current, the current in each of the four coils will be 12.5 amperes; 12.5 The RI drop per coil will be 0.75 x = 9.38 volts, which is 0.408% of 2300, and the RI£ 1os s will be 0.75 x 12.sf.= 117.5 watts per roil, and fo1· the four coils 470 watts. For the radiation of this loss there will be available 5780 sq. in. o f surface. value of 0.0815 This gives the l ow watts pe1· s ·q uare inch of radiating surface. 7 Hi gh Tenoion Winding The high tension winding will be much more complex in its winding and insulation than the low tension coils. The entire winding will be of No. 26 B andS gage double cotton covered wire. required. 40 .,ooo turns of wire will be The winding will be subdivided into 56 separate, flat, form-wound coils, which wiJ.l be wound separately and assembled later. There will be three classes of co ils., all the coils of each class being similar as regards insulation, number of turns per layer, and number of layers of wire, but will have different diameters. This variance of diameter is to save material in those coils which are around the smaller parts of the insulation between the high and low tension windings. Drawine; No. 1 shows the differ- ent diameters of the coils and their relative positions., also their sequence of connection. Drawing No. 6 shows by means of a table the different dimensions of the individual coils. The first class of coils will be four in number and will be the first f our of the entire aeries, begin~ing at the high tension terminal. Altha these coils will be 7.15% of the number of coils, they will contain but 1.3% of the total turns, because they will have to stand the highest potential strains and the sur ge s. The next twelve coils in the se:r- ies will be second class coils and will contain 8.7% of the turns, and the re main ing 40 coils will be thi1·d class coils 8 and contain 90% of the turno. The foundation of each coil will be a ring of pressboard one-quarter of an inch thick and of a width equal to the width of the coils before taping. See table on Drawing 6. The internal dia- meter of this ring i• the internal diameter of the coils. The insulation between layers will in all cases be one thickness of 15 and one of 10 mil paper, with whatever else is called for, depending on the class of coil. This 10 mil paper is to be cut three-quarters of an inch wider than required and the edges folded over as shown in drawing No. 6. The edges wii1 thus be 40 mils thick and form between them a channel 30 mils deep in which to lay the wire. The 15 mil paper will not be folded. The first class of coils will be very heavily insulated. They will have two turns of wire per layer and only 65 layers. The total turns will tlrus be 130, which at 6.25 volts p er turn gives 812.5 volts per coil. The wire will be reemlforced on both sides with 2 strands of four co1·d machine th1·ead 1 v1hich, as shown in the Westinghouse catalogue 1 section 804, has a diameter nearly equal to that of the double cotton covered wire used. Varnished cambric will be placed around the turns of the layers as shown on drawing No. 6. The second class coils will not be as heavily insulated as the first class. Altho the voltage per coil is more the strain to ground and the surges will be less. These coils 9 will have four turns of wire per layer, a.nd 73 layers, which gives a total voltage per coil of 1825. Each turn of wire is reenforced with one strand of the same kind of thread used in the first class coils. Varnished cambric is placed ove1· and tmder adjacent tuxns of wire and thread, as shown in drawing No. 6. The third class coils will be vmund entirely without cambric or thread. There will be ten turns of wire per layer and ninety layers, thus giving 900 turns per coil. The voltage per coil will be 5625. Aside from the stepped diameters the entire 40 of these coils will be similar, except the last one, No. 56. This is the coil, one terminal of which is grounded, and it differs from the others in that all the taps for the measuring instnune nts will be brought out from it. At a point 8 turns from the terminal lead the winding will be stopped, and tap No. 1 brought out. The winding will commence again at this same point, starting from tap No. 2, and be continued for 8 !JlOre turns where tap No. 3 will be brought out without intern1.pting the con- tinuity of the wire. At a point 16 turns from tap No. 3, the fourth and last tap wi 11 be brouisht out. The scheme of these tap s, and their location relative to the ground terminal is shown on drawing No. 5. Taps 1 and 2 make a break in the winding in which to insert an ammeter 01· the current coils of a wattmeter. The potential coils of a 10 wattmeter would be connected from ground (the tank of the transformer) to taps land 2, 3 or 4. With full voltage on the high tension winding, the voltage from ground, to taps land 2, will be 50; to tap 3, 100; and to tap 4, Aft el' the coils are wound they will be taped with 200. ope layer of cotton tape, with a minimum lap of one half, and thoroughly varnished. When the coils are stacked on the co1·e during the assemblipg process they are to be separated, each by twelve wooden strips one inch wide, and of such thickness as to make the distance between centers of adjacent coils 1.5 inches. The bottom coil will be support- ed on blocks which will raise it 2 inches from the bottom yoke, and level it as well. Blocks will also be placed on top of each stack of coils and the coils held in place by the screws which pass thru the brackets on the upper yoke clamp and bear against these blocks. These supporting brackets are sho,vn in detail in drawing Ho. 4. The amount of wire required for the high tension windings will be about 334,000 feet~ As this weighs 0.77 lbs. per 1000 ft, the total weight will be 257 lbs. The resis- tance at 40.84 ohms per 1000 feet would be 13,640 ohms. Basing all calculations at 0.5 ampere, which is 25% overload, the RI drop will be 6820 volts, which is 2.73% of 250,000. The RI.fl loss will be 3410 watts. With a total radiating surface on the 56 coils of 57,800 sq. in., the radiation per square inch will be, on an average, 0.059 watts. 11 But as 90% of the total loss is in 40 of the coils, the radiation on these coils will be 0.074, which is very low for a transformer for such intermittent use as one of this character has. At normal load the radiation would be only 0.0475 watts per sq. in. at a maximum. About 43,000 feet of the ten and fifteen mil folded strip will be required to build up the high voltage coils. Insulation between high and low tension windings. The high and low tension windings will be separated by two built up bushings, one surrounding each leg of the transformer. These bushings will consist of a series of concentric cylinders, separated by oil spaces. The cylinders will be held in place by spacing strips placed in the oil ducts. Shellac will be the ma-t;e1~ial used to bind the bushings into one solid mass. The first or inner cyl- inder will be of solid bakelite micarta construction, with an internal diameter of twelve inches, a length of 3 ft. 6 in. and a thickness of 0.75 inches. The remaining cylinders will be of three thicknesses of .056 inch fullerboard, securely held together with shellac, and having the joints broken so as to avoid weak spots. On the outside of each cylinder, except the outside one, will be placedtwelve spacing strips one inch wide, with a length equal to the next cylinde1 to 1 be wound, each composed of one thickness of .125 inch and two of .030 inch fullerboard. Se>ccessive rows of these 12 strips will not be staggered, but will extend radially like the spokes of a wheel, so as to form an inflexible bushing support. The arrangement of these strips is shovm in drawing No. 9. This construction will give each cylinder a thickness of 0.168 inch, and each oil duct a radial dimension of 0.185 inch. The bushings will be stepped as shown in drawing No. 8. Each step will have~ radial dimension of 0.706 in., this being the thickness of two fullerboard cylinders and two oil ducts. The other dimension will be equal to the dis- tance between centers of the high tension coils, i.e., one and one-half inches, thus making any pair of cylinders three inches shorter than the next inner pair. At each step will be a band of tin foil one inch wide, extending around the cylinder and placed so as to leave 1.5 inches between the edge of the foil and the end of the cylinder. This will b1·ing the two bands on any cylinder immediately under the edges of the next cylinder. The pair of bands on each cylinder will be connected electrically with a half-inch strip of ten mil copper. Each band will be connected to the inner terminal of . high ten s ion coil in whose plane it lies. The bands will thus serve to dis- tribute the potential strains over the surface of the bushing, and, with their connecting wires, to connect the high tension coils i-n their proper sequence. 13 Each buahine; will be supported on a wooden ring for med of one inch material. The internal and external diameter will be respectively 12 and 16 inches. These rings will be supported on blocks which will rest on the yoke and its clamps. Similar rings at the top, with blocking between them and barrier braces, will st eady the bushings and keep them in place. To wind up the bushi ngs about 7 5 sheets of fuller board, measuri ng 40 x 42 •inches will be required . As these weigh 3.6 lbs. per sheet, the total weight will be 270 lbs. Including the spacing str i ps and micarta foundation, each bushing will weigh about 150 lbs. and cost for material at 17 cents per pound, about twenty-five dollars, exclusive of the inner mica:rta cylinder. The External Barrier. Surroundin g the high tension coils an d between the m and t he tank will be the external barrier. This will con s ist of layers of press-board sep ar a ted by oil spaces . The structure wi 11 be car:ci ed on a wooden fr amework o:r support which will be secured to the core footin g at the bottom and braced to t he upp er core clamps at the top. The details of t hi s supporting fra me are shown in drawing No. 10. The barri e1· wi 11 be stepped similar to t he bushings, each step mea suring one-ha lf by one and one-half inches. Each laye1· of pr e ss-board will be composed of two thicknesses of oneeighth inch pre t.rn-board, and t he laye r s will be separa ted by one-fourth inch oil spaces, wooden strips placed about 14 12 inches apart, 1 x ¼inches, being used as spacers. The na r rowest layer will be twelve inches wide, and each following one three inches wider. There will be twelve layers of press-board in the barrier, thus giving three inches of solid material between the windings and tank. To this will be added seven inches of oil, one inch of which will be outside the barrier, two and three-quarters inches between the press-board layers, ancl the rest ins i de the barrier. As shown on the drawings the barrier will be 3 ft. 9 inches high. To co nstruct this barrier about 160 sq. yds of one-eighth inch press-board will be r equired. The weight of this will b e about 1200 lbs. and at 16 cents a pound will cost, for the material alone, about 190 dollars. The Terminal Bushing . As the transformer is to operate with one si d e grom1ded, only one high tension lead will be required. will be of the condenser type. of bakelite micarta This It will be built up solid with the layers of tin foil to dis- tribute the potential strains worked in according to modern paractice~. The center of the bushing will be a 3/8 inch brass tube through which the t erminal lead wi 11 pass, the connection bein g at the top, so as to avoid making it inaccessible und erneath the oil. Potential strains and corona will be relieved at the ends bf the bushing by means of a three inch diameter brass ball at the lovrer end and a big disk of tin foil covering a light wooden framework at the 15 top. This disk will be fotir feet in diameter and six inches thick. The total length of the terminal will be nine feet. A cast iron ring around the largest diameter section will support it in the transformer cover. outline drawing of the bushine;. Dratiing No. 12 is an The Electric Journal for August, 1913, contains a full description of condenser terminals, and the methods used in their construction. The dimensions are given on drawing No. 12, and the building instructions will be taken from the Electric Journal mentioned. The Tank and Cover. The tank which is shown in outline in drawing No. 13 will be built of 3/8 inch boiler plate, aides and bottom. The inside dimensions of the tank will be: Length - 9 ft. Width - 5 II 3 II Height 6 II 0 It 1 in. A thermomete1 will be fitted, also an oil gage, and a 3 •in . 4 globe valve for the removal of the oil. As t his tank is to be puxchased in the open mark et the details of const 1"tlction and reenforcement vii 11 be left to the manufactu1 4 eri. - An angle iron arouno the top will be desired to carry the cover. The cover of the tank will be of boiler plat e the same as the tank, reenforced on the under side to prevent sa gg ing. Lifting eyes will be in s erted for convenience in handling. 16 Openings wil 1 be cut in the cover as follow·s: a hand hole for reaching the low tension terminal board, an opening for the terminal block for the low tension leads, an opening for the high tension bushing, and a small opening for a terminal block for the meter connection to the high tension winding. All these openines are to be closed tight to keep out dust and moisture. Drawing No. 14 shows the cover in outline. The Terminal Blocks. Terminal blocks of fibre will fill those openings in the cover through which the leads are brought. For the low tension leads No. G stranded, cambric covered cable will be used, and for the high tension meter connections, braided wire, such as is used for pigtails on brushes will be used. This saQe kind of braided wire will be us- ed to connect the high tension coils together and to the ground connection_ on the core, also, to connect the ground t errninal of the core to the tank cover. in cotton sleeving. These will be run Binding posts, numbered from 1 to 4 and connected to the taps of the same numbers in the high tension. coil will be mounted on the little high tension block. Under posts 1 and 2 will be flat springs which will press against each other. These springs, at their point of contact, will be separated by a piece of paper which will puncture at 500 volts or less. The purpose of this is to automatically ground the end of the winding if 17 for any reason the meter circuit should become opened. A cylinder or dr~m switch, the plan of which is shown in drawing No. 5, will serve to change the connections of the low tension coils. Bibliography. The principal technical articles read and used in the preparation of this thesis a~e: Air as an Insul a tor When in the Presence of Insulating Bodies of Higher Specific Inductive Capacity, in the A.I.E.E. proceedings for March, 1913, by C. T. Forte scue ands. W. Farnsworth. The Application of a Theorem of Electrostatics to Insulation Problems, in the A.I.E.E. for March, 1913. Insulation, an article in the May A.I.E.E. proceedings, 1913. High Tension Testing, by A. B. Hendricks, Jr., A.I.E.E. proceedings, February, 1911. Notes on Transformer Desi gn, Elect.ric Journa l, January, 1912. Abnormal Strains in Tra.~sformers, General Elect ric Review, Decembe~, 1912. Magnetic Leakage in Transformers, General Electric Review, Decei:iber, 1912. Corona and Dielectric Strength in Air, General Electric Review, December, 1912. 18 The Use of Artificial Insulating Materials in the Construction of Electrical Apparatus, in the London Electrician, May 5, 1913. The Ratio of Transformation in the General Transformer with Distributed Windings, in the London Electrician, August 15, 1913. Simple Transformez- Formulae, London Electrician, January 3, 1915. Protection of the Internal Insulation of a Static Transformer, by w. s. Moody, in the A.I.E.E. Proceedings for June, 1907. The Condenser Type of Outlet Terminal,Theory of Design, by C. Fortescue. Conotruction and Application, by J. E. Mateer, in the Electric Journal for August, 1913. ffiFA -rr,A I ©: __,.,~- I ,· ~ I I ~,~- - ~ ~ ====== (R) ~ . !© . ·. • I . ., • •