[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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The test procedure given in ANSI/IEEE Std. C57.19.00-1991 [1] is to first immerse the lower end of the bushing in oil at a temperature of 95C and then pass rated current through the bushing. When the bushing comes to thermal equilibrium, a test voltage equal to 1.2 times the maximum line-ground voltage is applied, and tan is measured at regular, normally hourly, intervals. These conditions are maintained until tan rises no more than 0.02% (0.0002 p.u.) over a period of five hours. The bushing is considered to have passed the test if it has reached thermal stability at this time and it withstands all of the routine dielectric tests without significant change from the previous results. 3.2.9 Maintenance and Troubleshooting Once a bushing has been successfully installed in the apparatus, without breakage or other damage, it will normally continue to operate without problems for as long as its parent apparatus is in service unless it is thermally, mechanically, or electrically overloaded. However, such overloads do occur, plus some designs have design flaws, and bushings can sustain incipient damage during shipment or installation. All of these potential problems must be watched by periodic inspections and maintenance. The type and frequency of inspection and maintenance performed on bushings depend on the type of bushing plus the relative importance and cost of the apparatus in which the bushing is installed. The following gives a brief description of some common problems and maintenance that should be performed [8, 17, 18]: 3.2.9.1 Oil Level Proper oil level is very important in the operation of a bushing, and abnormal change in oil level can indicate problems within the bushing. Loss of oil can indicate that the bushing has developed a leak, possibly through a gasket, a soldered or welded seal, or an insulator that has been cracked or broken. A leak on the air-end side may be an indication that water has entered the bushing, and the water content and dielectric properties of the bushing oil should be checked as soon as possible to determine whether this has occurred. Excessive water can cause deterioration of the dielectric integrity along the inside of the insulators, particularly the oil-end insulator and, if used, the bushing core. Furthermore, excessive loss of oil in a capacitance-graded bushing can cause oil level to drop below the top of the core. Over time, this will cause the insulating paper to become unimpregnated, and the bushing can suffer dielectric failure, possibly an explosive one. An abnormal increase in oil level on a bushing installed in a transformer with a conservator oilpreservation system may indicate a leak in an oil-end gasket or seal, or the oil-end insulator may be cracked or broken. 3.2.9.2 Power-Factor/Capacitance Measurements Two methods are used to make power (dissipation) factor and capacitance measurements [17, 18]. The first is the grounded specimen test (GST), where current, watts, and capacitance of all leakage paths between the energized central conductor and all grounded parts are measured. Measurements include the internal core insulation and oil as well as leakage paths over the insulator surfaces. The use of a guardcircuit connection can be used to minimize the effects of the latter. The second method is the ungrounded specimen test (UST), where the above quantities are measured between the energized center conductor and a designated ungrounded test electrode, usually the voltage or test tap. The two advantages of the UST method are that the effects of unwanted leakage paths, for instance across the insulators, are minimized, and separate tests are possible while bushings are mounted in apparatus. Standards [8] recommend that power factor and capacitance measurements be made at the time of installation, a year after installation, and every three to five years thereafter. A significant increase in a bushing’s power factor indicates deterioration of some part of the insulating system. It may mean that one of the insulators, most likely the air-end insulator, is dirty or wet, and excessive leakage currents are flowing along the insulator. A proper reading can be obtained by cleaning the insulator. On the other hand, a significant increase of the power factor may also indicate deterioration within the bushing. An increase in the power factor across the C 1 portion, i.e., from conductor to tap, typically indicates deterioration within the core. An increase across the C 2 portion of a bushing using a core, i.e., from tap to flange, typically indicates deterioration of that part of the core or the bushing oil. If power factor doubles from the reading immediately after initial installation, the rate of change of the increase should be monitored at more frequent intervals. If it triples, then the bushing should be removed from service [8]. An increase of bushing capacitance is also a very important indicator that something is wrong inside the bushing. An excessive change, on the order of 2 to 5%, depending on the voltage class of the bushing, over its initial reading probably indicates that insulation between two or more grading elements has shorted out. Such a change in capacitance is indication that the bushing should be removed from service as soon as possible. 3.2.9.3 Damage or Contamination of Air-End Insulator Insulators, particularly those that are porcelain, are susceptible to damage during handling, shipping, or from flying parts of other failed equipment. Chips can be broken out of the sheds, and these can generally be repaired by grinding off the rough edges with a file and painting over the break with a suitable paint. In some cases, a composite material can be used to fill the void caused by the break [17]. On the other hand, some damage may extend into the body of the insulator. These areas should be watched closely for oil leakage for a period after the damage is noticed, since the impact may have cause the insulator to crack. Bushings used in highly polluted environments should be washed periodically. Washing involves either de-energizing the apparatus and cleaning the insulators by hand with a cleaning agent or using a suitable jet of low-conductivity water while energized [17]. 3.2.9.4 Improper Installation of Terminals Improper installation of either the top or bottom terminals can cause thermal problems. A common problem with threaded terminals is that force of some kind must be placed on the mating threads so that a good current path is formed; in the absence of such force on the joint, the terminal is not suitable for carrying large currents. These problems can be found by using thermography methods [17]. Some manufacturers require that threaded terminals be torqued to a certain minimum value. Another method commonly used is to cut the member with the female threads in half. Then, it is first threaded onto the bushing top terminal and finally clamped such that it compresses onto the male threads of the top terminal. 3.2.9.5 Misaligned or Broken Voltage Tap Connections In some cases, usually during rough shipment of bushings, the core will rotate within the insulators. This can cause the connection to the tap layer to break. In some older bushings, a spring connection was used to make the tap connection, and this connection sometimes shifted during handling or shipment. Both of these problems cause the tap to become inoperative, and the power-factor/capacitance measurements involving the tap cannot be made. 3.2.9.6 Dissolved-Gas-in-Oil Analysis It is not recommended that dissolved-gas-in-oil analysis (DGA) samples be made on a routine basis because bushings have only a limited supply of oil, and the oil will have to be replenished after several such oil samples have been taken. However, if power-factor/capacitance measurements indicate that something is wrong with the bushing, DGA samples are indicated. Large amounts of CO and CO 2 gases indicate deterioration of paper insulation within the bushing, whereas other DGA gases indicate byproducts of arcing or thermal overheating, just as they do in transformer insulation. References 1. ANSI/IEEE, Standard Performance Characteristics and Test Procedure for Outdoor Power Apparatus Bushings, IEEE Std. C57.19.00-1991 (R1997), Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1997. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
2. Easley, J.K. and Stockum, F.R., Bushings, IEEE Tutorial on Transformers, IEEE EH0209-7/84/0000- 0032, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1983. 3. Nagel, R., Uber Eine Neuerung An Hochspannungstransformer Der Siemens- Schuckertwerke, Elektrische Bahnen Betriebe, 4, 275–278, May 23, 1906. 4. Reynders, A.B., Condenser Type of Insulation for High-Tension Terminals, AIEE Trans., 23, Part I, 209, 1909. 5. Spindle, H.E., Evaluation, Design and Development of a 1200 kV Prototype Termination, USDOE Report DOE/ET/29068-T8 (DE86005473), U.S. Department of Energy, Washington, DC. 6. IEEE, Standard Performance Characteristics and Dimensions for Outdoor Power Apparatus Bushings, IEEE Std. C57.19.01-2000, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 2000. 7. IEEE, Standard Requirements, Terminology and Test Procedures for Bushings for DC Applications, IEEE Std. C57.19.03-1996, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1996. 8. IEEE, Guide for Application of Power Apparatus Bushings, IEEE Std. C57.19.100-1995 (R1997), Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1997. 9. IEC, Bushings for Alternating Voltages above 1000 V, IEC 137, International Electrotechnical Commission, Geneva, 1995. 10. Ueda, M., Honda, M., Hosokawa, M., Takahashi, K., and Naito, K., Performance of Contaminated Bushing of UHV Transmission Systems, IEEE Trans., PAS-104, 891–899, 1985. 11. IEEE, Recommended Practice for Seismic Design of Substations, IEEE Std. 693-1997, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1997. 12. McNutt, W.J. and Easley, J.K., Mathematical Modeling — A Basis for Bushing Loading Guides, IEEE Trans., PAS-97, 2395–2404, 1978. 13. ABB, Inc., Instructions for Bushing Potential Device Type PBA2, instruction leaflet IB 33-357-1F, ABB, Inc., Alamo, TN, September 1993. 14. Lapp Insulator Co., Extend Substation Equipment Life with Lapp-Doble Test Terminal, Bulletin 600, Lapp Insulator Co., LeRoy, NY, 2000. 15. IEEE, Standard Techniques for High Voltage Testing, IEEE Std. 4-1995, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995. 16. Wagenaar, L.B., The Significance of Thermal Stability Tests in EHV Bushings and Current Transformers, Paper 4-6, presented at 1994 Doble Conference, Boston, MA. 17. EPRI, Guidelines for the Life Extension of Substations, EPRI TR-105070-R1CD, Electric Power Research Institute, Palo Alto, CA, November 2002. 18. Doble Engineering Co., A-C Dielectric-Loss, Power Factor and Capacitance Measurements as Applied to Insulation Systems of High Voltage Power Apparatus in the Field (A Review), Part I, Dielectric Theory, and Part II, Practical Application, Report ACDL-I and II-291, Doble Engineering Co., Boston, MA, Feb. 1991. This section refers to LTCs immersed in transformer mineral oil. The use of other insulating fluids or gas insulation requires the approval of the LTC’s manufacturer and may lead to a different LTC design. 3.3.1 Design Principle The LTC changes the ratio of a transformer by adding turns to or subtracting turns from either the primary or the secondary winding. The main components of an LTC are contact systems for make-andbreak currents as well as carrying currents, transition impedances, gearings, spring energy accumulators, and a drive mechanism. In new LTC-designs, the contacts for make-and-break-currents will be replaced more and more by vacuum interrupters. The transition impedance in the form of a resistor or reactor consists of one or more units that are bridging adjacent taps for the purpose of transferring load from one tap to the other without interruption or appreciable change in the load current. At the same, time they are limiting the circulating current for the period when both taps are used. Normally, reactance-type LTCs use the bridging position as a service position and, therefore, the reactor is designed for continuous loading. The voltage between the mentioned taps is the step voltage. It normally lies between 0.8% and 2.5% of the rated voltage of the transformer. The majority of resistance-type LTCs are installed inside the transformer tank (in-tank LTCs), whereas the reactance-type LTCs are in a separate compartment that is normally welded to the transformer tank. 3.3.1.1 Resistance-Type Load Tap Changer The LTC design that is normally applied to larger powers and higher voltages comprises an arcing switch and a tap selector. For lower ratings, LTC designs are used where the functions of the arcing switch and the tap selector are combined in a so-called arcing tap switch. With an LTC comprising an arcing switch and a tap selector (Figure 3.3.1), the tap change takes place in two steps (Figure 3.3.2). First, the next tap is preselected by the tap selector at no load (Figure 3.3.2, 3.3 Load Tap Changers Dieter Dohnal For many decades power transformers equipped with load tap changers (LTC) have been the main components of electrical networks and industry. The LTC allows voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. From the beginning of LTC development, two switching principles have been used for the load-transfer operation, the high-speed-resistance type and the reactance type. Over the decades, both principles have been developed into reliable transformer components available in a broad range of current and voltage applications to cover the needs of today’s network and industrial-process transformers as well as ensuring optimum system and process control (Goosen, 1996). FIGURE 3.3.1 Design principle — arcing switch with tap selector. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
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ELECTRIC POWER TRANSFORMER ENGINEER
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I wish to recognize the interest of
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Contents 1 Theory and Principles Ch
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1.3 Equivalent Circuit of an Iron-C
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designer starts to make a design fo
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• Transient voltages generated du
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the transformer’s bushings and, m
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% regulation = [(V NL - V FL )/V FL
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In core-form transformers, the wind
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FIGURE 2.1.14 Layer windings (singl
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the transformer design, which may b
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consumer’s service circuit is a d
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FIGURE 2.2.3 Single-phase transform
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is installed so that the tank never
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above which persons might be burned
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FIGURE 2.2.16 Two-bushing subway. (
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carbon steel or the very expensive
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FIGURE 2.2.31 Radial-style dead fro
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FIGURE 2.2.36 Complete transformer
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voltage in each winding simultaneou
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mounted transformers can have arres
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V S + V S I V L Z=R+jX a) V L V S V
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Z = jX (2.3.19) 0.7 Then the curren
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X1 L* X1 S =X1 L X2 L* X3 L* a) S L
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150 V(%) 100 50 0 -50 40 80 T(s) 12
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2.3.8.1.2 Series Connection • The
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A six-pulse single-way transformer
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The degree of coupling also influen
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where I h = current for the hth har
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In the case of liquid-filled transf
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FIGURE 2.4.21 A 36-pulse system mad
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Vacuum-pressure encaps ulated — T
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FIGURE 2.6.1 Typical wiring and sin
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a) H1 b) Ip X2 FLUX H2 LEAKAGE FLUX
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and for CTs is TCF = RCF - (PA/2600
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RCF x Bx Bt RCF t - RCF 0 co
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2000 1000 5% Vex BANDWITH FROM NOMI
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This may be more useful when operat
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also some uncertainty in repeatabil
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FIGURE 2.6.25 Standard two-element
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2.7 Step-Voltage Regulators Craig A
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Source Bypass Switch Source Bypass
Page 85 and 86: 2.7.4 Theory A step-voltage regulat
Page 87 and 88: FIGURE 2.7.22 Equalizer winding inc
Page 89 and 90: TABLE 2.7.4 Increase in Ampere Load
Page 91 and 92: References IEEE, Standard Requireme
Page 93 and 94: 300% FIGURE 2.8.4 Schematic of a co
Page 95 and 96: TABLE 2.8.1 Typical Sizing Workshee
Page 97 and 98: Input with Interruption Ferro Outpu
Page 99 and 100: FIGURE 2.8.20A Performance of a rid
Page 101 and 102: • Input frequency or frequency ra
Page 103 and 104: References 1. Sola/Hevi-Duty Corp.,
Page 105 and 106: FIGURE 2.9.5 Typical phase-reactor
Page 107 and 108: FIGURE 2.9.10 Two generator arrange
Page 109 and 110: • Overloading of lines • Increa
Page 111 and 112: FIGURE 2.9.23 34-kV, 25-MVAR (per p
Page 113 and 114: V 90 ( X X ) s T (2.9.10a) where,
Page 115 and 116: Therefore, Line reactive losses = (
Page 117 and 118: ate of rise of transient-recovery v
Page 119 and 120: the LLCR is provided in IEEE Std. C
Page 121 and 122: aluminum shielding, i.e., an oil-im
Page 123 and 124: Leo J. Savio ADAPT Corporation Ted
Page 125 and 126: 3.1.2.2.2 Heat Dissipation A second
Page 127 and 128: FIGURE 3.2.1 Solid-type bushing. ma
Page 129 and 130: 1.6 cm. This means that most of the
Page 131 and 132: where HS = steady-state bushing ho
Page 133 and 134: the conductivity of the water-solub
Page 135: of 178 ohm-m, and a test duration o
Page 139 and 140: FIGURE 3.3.5 Reactance-type LTC, in
Page 141 and 142: FIGURE 3.3.10 LTC with delta connec
Page 143 and 144: 3.3.5 Selection of Load Tap Changer
Page 145 and 146: FIGURE 3.3.19 Effect of the leakage
Page 147 and 148: References Goosen, P.V. , Transform
Page 149 and 150: If i 1 is the full-load current rat
Page 151 and 152: TABLE 3.4.1 Loading Recommendations
Page 153 and 154: 3.5.4 Three-Phase Transformer Conne
Page 155 and 156: FIGURE 3.5.4 Interconnected star-gr
Page 157 and 158: 3.6.1.1 Standards ANSI standards fo
Page 159 and 160: ANSI standards. LTC control setting
Page 161 and 162: FIGURE 3.6.7 Discharging the capaci
Page 163 and 164: FIGURE 3.6.10 Standard switching-im
Page 165 and 166: voltage of the excited winding, rea
Page 167 and 168: FIGURE 3.6.18 Current, voltage, and
Page 169 and 170: TABLE 3.6.3 Transformer Short-Circu
Page 171 and 172: FIGURE 3.7.3 Control for voltage re
Page 173 and 174: FIGURE 3.7.6A Feeder with power-fac
Page 175 and 176: 3. Circulating current (current bal
Page 177 and 178: FIGURE 3.7.10 Control block diagram
Page 179 and 180: Primary Current (Amps) 60 40 20 0 -
Page 181 and 182: current transformer having two prim
Page 183 and 184: 87R1 87BL1 (A) Independent Harmonic
Page 185 and 186: Winding 1 Secondary Currents Data A
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Y Y 20 18 3 rd Harmonic CTR1=40 CTR
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230 kV 3 180 MVA 138 kV 5 CTR1 = 2
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29. Concordia, C. and Rothe, F.S.,
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FIGURE 3.9.1 Measurement of pressur
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H 2m 1. Vertical Forced Air 2. Prin
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Gade, S., Sound Intensity Instrumen
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nected. This steady-state voltage d
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where [A] = state matrix [B] = inpu
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where C = capacitance between the t
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L ijo = N i N j ijo (3.10.31) 1.4
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% VOLTAGE % VOLTAGE 100 BIL 90 50 3
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29. Degeneff, R.C., Reducing Storag
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All connections should be cleaned a
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6. Costs to set up and use a mobile
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Records of relay operation must be
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• Has heating occurred as the res
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a reference value) of the currents
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The next data-processing step is to
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temperature. As the transformer coo
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3.13.3.2 Instrument Transformers Th
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FIGURE 3.13.5 Analysis of bushing s
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temperature. Under abnormal conditi
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3.14.1.2 Accredited Standards Commi
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FIGURE 3.14.4 IEC technical-committ
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TABLE 3.14.2 Relevant Documents for
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TABLE 3.14.13 Relevant Documents fo
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[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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