[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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3.1.2.3.4 Arcing With arcing, acetylene is produced along with the other fault gases. Acetylene is characteristic of arcing. Because arcing can generally lead to failure over a much shorter time interval than faults of other types, even trace levels of acetylene (a few parts per million) must be taken seriously as a cause for concern. 3.1.2.3.5 Acid High levels of acid (generally acid levels greater than 0.6 mg KOH/g of oil) cause sludge formation in the oil. Sludge is a solid product of complex chemical composition that can deposit throughout the transformer. The deposition of sludge can seriously and adversely affect heat dissipation and ultimately result in equipment failure. 3.1.3 Sources of Contamination 3.1.3.1 External External sources of contamination can generally be minimized by maintaining a sealed system, but on some types of equipment (e.g., free-breathing devices) this is not possible. Examples of external sources of contamination are moisture, oxygen, and solid debris introduced during maintenance of the equipment or during oil processing. 3.1.3.2 Internal Internal sources of contamination can be controlled only to a limited extent because these sources of contamination are generally chemical reactions (like the oxidation of cellulose and the oxidation of oil) that are constantly ongoing. They cannot be stopped, but their rates are determined by factors that are well understood and often controllable. Examples of these factors are temperature and the oxygen content of the system. Internal sources of contamination are: • Nonmetallic particles such as cellulose particles from the paper and pressboard • Metal particles from mechanical or electrical wear • Moisture from the chemical degradation of cellulose (paper insulation and pressboard) • Chemical degradation products of the oil that result from its oxidation (e.g., acids, aldehydes, ketones) 3.2 <strong>Electric</strong>al Bushings Loren B. Wagenaar 3.2.1 Purpose of <strong>Electric</strong>al Bushings ANSI/IEEE Std. C57.19.00 [1] defines an electrical bushing as “an insulating structure, including a through conductor or providing a central passage for such a conductor, with provision for mounting a barrier, conducting or otherwise, for the purpose of insulating the conductor from the barrier and conducting current from one side of the barrier to the other.” As a less formal explanation, the purpose of an electrical bushing is simply to transmit electrical power in or out of enclosures, i.e., barriers, of an electrical apparatus such as transformers, circuit breakers, shunt reactors, and power capacitors. The bushing conductor may take the form of a conductor built directly as a part of the bushing or, alternatively, as a separate conductor that is drawn through, usually through the center of, the bushing. Since electrical power is the product of voltage and current, insulation in a bushing must be capable of withstanding the voltage at which it is applied, and its current-carrying conductor must be capable of carrying rated current without overheating the adjacent insulation. For practical reasons, bushings are not rated by the power transmitted through them; rather, they are rated by the maximum voltage and current for which they are designed. 3.2.2 Types of Bushings There are many methods to classify the types of bushings. These classifications are based on practical reasons, which will become apparent in the following discussion in three broad areas. Bushings can be classified: 1. According to insulating media on ends 2. According to construction 3. According to insulation inside bushing 3.2.2.1 According to Insulating Media on Ends One method is to designate the types of insulating media at the ends of the bushing. This classification depends primarily on the final application of the bushing. An air-to-oil bushing has air insulation at one end of the bushing and oil insulation at the other. Since oil is more than twice as strong dielectrically as air at atmospheric pressure, the oil end is approximately half as long (or less) than the air end. This type of bushing is commonly used between atmospheric air and any oil-filled apparatus. An air-to-air bushing has air insulation on both ends and is normally used in building applications where one end is exposed to outdoor atmospheric conditions and the other end is exposed to indoor conditions. The outer end may have higher creep distances to withstand higher-pollution environments, and it may also have higher strike distances to withstand transient voltages during adverse weather conditions such as rainstorms. Special application bushings have limited usage and include: air-to-SF 6 bushings, usually used in SF 6 - insulated circuit breakers; SF 6 -to-oil bushings, used as transitions between SF 6 bus ducts and oil-filled apparatus; and oil-to-oil bushings, used between oil bus ducts and oil-filled apparatus. 3.2.2.2 According to Construction There are basically two types of construction, the solid or bulk type and the capacitance-graded or condenser type. 3.2.2.2.1 Solid Bushing The solid-type bushing, depicted in Figure 3.2.1, is typically made with a central conductor and porcelain or epoxy insulators at either end and is used primarily at the lower voltages through 25 kV. Generally, this construction is relatively simple compared with the capacitance-graded type. This was the construction method used for the original bushings, and its current usage is quite versatile with respect to size. Solid bushings are commonly used in applications ranging from small distribution transformers and circuit switchers to large generator step-up transformers and hydrogen-cooled power generators. At the lower end of the applicable voltage range, the central conductor can be a small-diameter lead connected directly to the transformer winding, and such a lead typically passes through an arbitrarily shaped bore of an outer and inner porcelain or epoxy insulator(s). Between the two insulators there is typically a mounting flange for mounting the bushing to the transformer or other apparatus. In one unique design, only one porcelain insulator was used, and the flange was assembled onto the porcelain after the porcelain had been fired. At higher voltages, particularly at 25 kV, more care is taken to make certain that the lead and bore of the insulator(s) are circular and concentric, thus ensuring that the electric stresses in the gap between these two items are more predictable and uniform. For higher-current bushings, typically up to 20 kA, large-diameter circular copper leads or several copper bars arranged in a circle and brazed to copper end plates can be used. The space between the lead and the insulator may consist of only air on lower-voltage solid-type bushings, or this space may be filled with electric-grade mineral oil or some other special compound on higher-voltage bushings. The oil may be self-contained within the bushing, or it may be oil from the apparatus in which the bushing is installed. Special compounds are typically self-contained. Oil and compounds are used for three reasons: First, they enable better cooling of the conductor than does air. Second, they have higher dielectric constants (about 2.2 for oil) than air, and therefore, when used with © 2004 by CRC Press LLC © 2004 by CRC Press LLC
FIGURE 3.2.1 Solid-type bushing. materials with higher dielectric constants, such as porcelain or epoxy, they endure a smaller share of the voltage than an equally sized gap occupied by air. The result is that oil and compounds withstand higher voltages than air alone. Third, oil and other compounds display higher breakdown strengths than air. The primary limitation of the solid bushing is its ability to withstand 60-Hz voltages above 90 kV. Hence, its applications are limited to 25-kV equipment ratings, which have test voltages of 70 kV. Recent applications require low partial-discharge limits on the 25-kV terminals during transformer test and have caused further restrictions on the use of this type of bushing. In these cases, either a specially designed solid bushing, with unique grading shielding that enables low inherent partial-discharge levels, or a more expensive capacitance-graded bushing must be used. 3.2.2.2.2 Capacitance-Graded Bushings Technical literature dating back to the early twentieth century describes the principles of the capacitancegraded bushing [2]. R. Nagel of Siemens published a German paper [3] in 1906 describing an analysis and general principles of condenser bushings, and A.B. Reynders of Westinghouse published a U.S. paper [4] that described the principles of the capacitance-graded bushing and compared the characteristics of these bushings with those of solid-type construction. Thereafter, several additional papers were published, including those by individuals from Micafil of Switzerland and ASEA of Sweden. The value of the capacitance-graded bushing was quickly demonstrated, and this bushing type was produced extensively by those companies possessing the required patents. Currently, this construction is used for virtually all voltage ratings above 25-kV system voltage and has been used for bushings through 1500-kV system voltage. This construction uses conducting layers at predetermined radial intervals within oil-impregnated paper or some other insulation material that is located in the space between the central conductor and the insulator. Different manufacturers have used a variety of materials and methods for making capacitance-graded bushings. Early methods were to insert concentric porcelain cylinders with metallized surfaces or laminated pressboard tubes with embedded conductive layers. Later designs used conductive foils, typically aluminum or copper, in oil-impregnated kraft paper. An alternative method is to print semiconductive ink (different manufacturers have used different conductivities) on all or some of the oil-impregnated kraft-paper wraps. FIGURE 3.2.2 Oil-filled, capacitance-graded bushing. Figure 3.2.2 shows the general construction of an oil-filled, capacitance-graded bushing. The principal elements are the central circular conductor, onto which the capacitance-graded core is wound; the top and lower insulators; the mounting flange; the oil and an oil-expansion cap; and the top and bottom terminals. Figure 3.2.3 is a representation of the equipotential lines in a simplified capacitance-graded bushing in which neither the expansion cap nor the sheds on either insulator are shown. The bold lines within the capacitance-graded core depict the voltage-grading elements. The contours of the equipotential lines show the influence of the grading elements, both radially within the core and axially along the length of the insulators. The mathematical equation for the radial voltage distribution as a function of diameter between two concentric conducting cylinders is: V(d) = V [In (D 2 /d)]/[ln (D 2 /D 1 )] (3.2.1) where V = voltage between the two cylinders d = position (diameter) at which the voltage is to be calculated D 1 and D 2 = diameters of the inner and outer cylinders, respectively Since this is a logarithmic function, the voltage is nonlinear, concentrating around the central conductor and decreasing near the outer cylinder. Likewise, the associated radial electric stress, calculated by © 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
Page 75 and 76: 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: 3.1.2.2.2 Heat Dissipation A second
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 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
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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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