[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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than one of these insulating materials can be used in conjunction. The following text gives a brief description of these types: 3.2.2.3.1 Air-Insulated Bushings Air-insulated bushings generally are used only with air-insulated apparatus and are of the solid construction that employs air at atmospheric pressure between the conductor and the insulators. 3.2.2.3.2 Oil-Insulated or Oil-Filled Bushings Oil-insulated or oil-filled bushings have electrical-grade mineral oil between the conductor and the insulators in solid-type bushings. This oil can be contained within the bushing, or it can be shared with the apparatus in which the bushing is used. Capacitance-graded bushings also use mineral oil, usually contained within the bushing, between the insulating material and the insulators for the purposes of impregnating the kraft paper and transferring heat from the conducting lead. 3.2.2.3.3 Oil-Impregnated Paper-Insulated Bushings Oil-impregnated paper-insulated bushings use the dielectric synergy of mineral oil and electric grades of kraft paper to produce a composite material with superior dielectric-withstand characteristics. This material has been used extensively as the insulating material in capacitance-graded cores for approximately the last 50 years. 3.2.2.3.4 Resin-Bonded or -Impregnated Paper-Insulated Bushings Resin-bonded paper-insulated bushings use a resin-coated kraft paper to fabricate the capacitance-graded core, whereas resin-impregnated paper-insulated bushings use papers impregnated with resin, which are then used to fabricate the capacitance-graded core. The latter type of bushing has superior dielectric characteristics, comparable with oil-impregnated paper-insulated bushings. 3.2.2.3.5 Cast-Insulation Bushings Cast-insulation bushings are constructed of a solid-cast material with or without an inorganic filler. These bushings can be either of the solid or capacitance-graded types, although the former type is more representative of present technology. FIGURE 3.2.3 Exponential plot of oil-filled, capacitance-graded bushing. E(d) = 2V/[d ln (D 2 /D 1 )] (3.2.2) will be the greatest at d = D 1 . The lengths of grading elements and the diameters at which they are positioned are such as to create a more uniform radial-voltage distribution than found in a solid-type bushing. As seen from Figure 3.2.3, the axial voltage distribution along the inner and outer insulators is almost linear when the proper capacitance grading is employed. Thus both insulators on capacitance-graded bushings can be shorter than their solid-bushing counterparts. Capacitance-graded bushings involve many more technical and manufacturing details than solid bushings and are therefore more expensive. These details include the insulation/conducting layer system, equipment to wind the capacitor core, and the oil to impregnate the paper insulation. However, it should be noted that the radial dimension required for the capacitance-graded bushing is much less than the solid construction, and this saves on material within the bushing as well in the apparatus in which the bushing is used. Also, from a practical standpoint, higher-voltage bushings could not possibly be manufactured with a solid construction. 3.2.2.3 According to Insulation inside Bushing Still another classification relates to the insulating material used inside the bushing. In general, these materials can be used in either the solid- or capacitance-graded construction, and in several types, more 3.2.2.3.6 Gas-Insulated Bushings Gas-insulated bushings [5] use pressurized gas, such as SF 6 gas, to insulate between the central conductor and the flange. The bushing shown in Figure 3.2.4 is one of the simpler designs and is typically used with circuit breakers. It uses the same pressurized gas as the circuit breaker, has no capacitance grading, and uses the dimensions and placement of the ground shield to control the electric fields. Other designs use a lower insulator to enclose the bushing, which permits the gas pressure to be different than within the circuit breaker. Still other designs use capacitance-graded cores made of plastic-film material that is compatible with SF 6 gas. 3.2.3 Bushing Standards Several bushing standards exist in the various countries around the world. The major standards have been established by the Transformers Committee within the IEEE Power Engineering Society and by IEC Committee 37. Five important standards established by these committees include the following: 1. ANSI/IEEE Std. C57.19.00, Standard Performance Characteristics and Test Procedure for Outdoor Power Apparatus Bushings [1]. This is the general standard that is widely used by countries in the Western Hemisphere and contains definitions, service conditions, ratings, general electrical and mechanical requirements, and detailed descriptions of routine and design test procedures for outdoor-power-apparatus bushings. 2. IEEE Std. C57.19.01, Standard Performance Characteristics and Dimensions for Outdoor Power Apparatus Bushings [6]. This standard lists the electrical-insulation and test-voltage requirements for power-apparatus bushings rated from 15 through 800-kV maximum system voltages. It also lists dimensions for standard-dimensioned bushings, cantilever-test requirements for bushings © 2004 by CRC Press LLC © 2004 by CRC Press LLC
1.6 cm. This means that most of the current will flow in the region from the outer portion of the conductor and radially inward to a depth of the skin depth . Second, the losses generated within a conductor will be: P loss = I 2 R = I 2 L/A = 4I 2 L/(D 12 – D 02 ) (3.2.3) where I = rated current = resistivity of the conductor material, ohmm = 1.7241 10 –3 ohmm for copper with 100% IACS (international annealed copper standard) = 3.1347 10 –3 ohmm for electrical aluminum alloy with 55% IACS L = length of conductor, m A = cross section of conductor = (D 12 – D 02 )/4 D 1 = outside diameter of conductor, m D 0 = D 1 – , m skin depth of the conductor in the case that a tubular conductor is used 0.0127 m for copper, 0.0159 m for aluminum, both at 60-Hz frequency It can be seen from Equation 3.2.3 that P loss decreases as D 1 increases. Hence, design practice is to increase the outside diameter of the conductor for higher current ratings and to limit the wall thickness to near the skin depth. There are other technical advantages to increasing the outside conductor diameter: First, from Equation 3.2.2, observe that electric-field stress reduces as d = D 1 increases. Therefore, a larger diameter conductor will have higher partial-discharge inception and withstand voltages. Second, the mechanical strength of the conductor is dependent on the total cross-sectional area of the conductor, so that a larger diameter is sometimes used to achieve higher withstand forces in the conductor. FIGURE 3.2.4 Pressurized SF 6 gas bushing. rated through 345-kV system voltage, and partial-discharge limits as well as limits for power factor and capacitance change from before to after the standard electrical tests. 3. IEEE Std. C57.19.03, Standard Requirements, Terminology and Test Procedures for Bushings for DC Applications [7]. This standard gives the same type of information as ANSI/IEEE Std. C57.19.00 for bushings for direct-current equipment, including oil-filled converter transformers and smoothing reactors. It also covers air-to-air dc bushings. 4. IEEE Std. C57.19.100, Guide for Application of Power Apparatus Bushings [8]. This guide recommends practices to be used (1) for thermal loading above nameplate rating for bushings applied on power transformers and circuit breakers and (2) for bushings connected to isolated-phase bus. It also recommends practices for allowable cantilever loading caused by the pull of the line connected to the bushing, applications for contaminated environments and high altitudes, and maintenance practices. 5. IEC Publication 137 [9], Bushings for Alternating Voltages above 1000 V. This standard is the IEC equivalent to the first standard listed above and is used widely in European and Asian countries. 3.2.4 Important Design Parameters 3.2.4.1 Conductor Size and Material The conductor diameter is determined primarily by the current rating. There are two factors at work here. First, the skin depth of copper material at 60 Hz is about 1.3 cm and that of aluminum is about 3.2.4.2 Insulators Insulators must have sufficient length to withstand the steady-state and transient voltages that the bushing will experience. Adequate lengths depend on the insulating media in which the insulator is used and on whether the bushing is capacitively graded. In cases where there are two different insulating media on either side of an insulator, the medium with the inferior dielectric characteristics determines the length of the insulator. 3.2.4.2.1 Air Insulators Primary factors that determine the required length of insulators used in air at atmospheric pressure are lightning-impulse voltage under dry conditions and power-frequency and switching-impulse voltages under wet conditions. Standard dry conditions are based on 760-mm (Hg) atmospheric pressure and 20C, and wet conditions are discussed in Section 3.2.6.1, High-Altitude Applications. Bushings are normally designed to be adequate for altitudes up to 1000 m. Beyond 1000 m, longer insulators must be used to accommodate the lower air density at higher altitudes. Under clean conditions, air insulators for capacitively graded bushings are normally shorter in length than insulator housings without grading elements within them. However, as the insulator becomes more contaminated, the effects of grading elements disappear, and the withstand characteristics of graded and nongraded insulators become the same over the long term (15 to 30 min) [10]. Further guidance on this subject is given in Section 3.2.6.2, Highly Contaminated Environments. 3.2.4.2.2 Oil Insulators Since mineral oil is dielectrically stronger than air, the length of insulators immersed in oil is typically 30 to 40% the length of air insulators. In equipment having oil with low contamination levels, no sheds are required on oil-immersed insulators. In situations where some contamination exists in the oil, such as carbon particles in oil-insulated circuit breakers, small ripples are generally cast on the outer insulator surface exposed to the oil. © 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
Page 77 and 78: also some uncertainty in repeatabil
Page 79 and 80: FIGURE 2.6.25 Standard two-element
Page 81 and 82: 2.7 Step-Voltage Regulators Craig A
Page 83 and 84: 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: FIGURE 3.2.1 Solid-type bushing. ma
Page 131 and 132: where HS = steady-state bushing ho
Page 133 and 134: the conductivity of the water-solub
Page 135 and 136: of 178 ohm-m, and a test duration o
Page 137 and 138: 2. Easley, J.K. and Stockum, F.R.,
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
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Primary Current (Amps) 60 40 20 0 -
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current transformer having two prim
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87R1 87BL1 (A) Independent Harmonic
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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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