[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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in the 1960s, it was found that a transformer with a delta-connected primary was more prone to ferroresonance problems because of higher capacitance between buried primary cables and ground. An acceptable preventive was to go to grounded-wye-grounded-wye transformers on all but the heaviest industrial applications. 2.2.5 Operational Concerns Even with the best engineering practices, abnormal situations can arise that may produce damage to equipment and compromise the continuity of the delivery of quality power from the utility. 2.2.5.1 Ferroresonance Ferroresonance is an overvoltage phenomenon that occurs when charging current for a long underground cable or other capacitive reactance saturates the core of a transformer. Such a resonance can result in voltages as high as five times the rated system voltage, damaging lightning arresters and other equipment and possibly even the transformer itself. When ferroresonance is occurring, the transformer is likely to produce loud squeals and groans, and the noise has been likened to the sound of steel roofing being dragged across a concrete surface. A typical ferroresonance situation is shown in Figure 2.2.10 and consists of long underground cables feeding a transformer with a delta-connected primary. The transformer is unloaded or very lightly loaded and switching or fusing for the circuit operates one phase at a time. Ferroresonance can occur when energizing the transformer as the first switch is closed, or it can occur if one or more distant fuses open and the load is very light. Ferroresonance is more likely to occur on systems with higher primary voltage and has been observed even when there is no cable present. All of the contributing factors — delta or wye connection, cable length, voltage, load, single-phase switching — must be considered together. Attempts to set precise limits for prevention of the phenomenon have been frustrating. 2.2.5.2 Tank Heating Another phenomenon that can occur to three-phase transformers because of the common core structure between phases is tank heating. Wye–wye-connected transformers that are built on four- or five-legged cores are likely to saturate the return legs when zero-sequence voltage exceeds about 33% of the normal line-to-neutral voltage. This can happen, for example, if two phases of an overhead line wrap together and are energized by a single electrical phase. When the return legs are saturated, magnetic flux is then forced out of the core and finds a return path through the tank walls. Eddy currents produced by magnetic flux in the ferromagnetic tank steel will produce tremendous localized heating, occasionally burning the tank paint and boiling the oil inside. For most utilities, the probability of this happening is so low that it is not economically feasible to take steps to prevent it, other than keeping trees trimmed. A few, with a higher level of concern, purchase only triplex transformers, having three separate core-coil assemblies in one tank. 2.2.5.3 Polarity and Angular Displacement The phase relationship of single-phase transformer voltages is described as “polarity.” The term for voltage phasing on three-phase transformers is “angular displacement.” 2.2.5.3.1 Single-Phase Polarity The polarity of a transformer can either be additive or subtractive. These terms describe the voltage that may appear on adjacent terminals if the remaining terminals are jumpered together. The origin of the polarity concept is obscure, but apparently, early transformers having lower primary voltages and smaller kVA sizes were first built with additive polarity. When the range of kVAs and voltages was extended, a decision was made to switch to subtractive polarity so that voltages between adjacent bushings could never be higher than the primary voltage already present. Thus the transformers built to ANSI standards today are additive if the voltage is 8660 or below and the kVA is 200 or less; otherwise they are subtractive. This differentiation is strictly a U.S. phenomenon. Distribution transformers built to Canadian standards are all additive, and those built to Mexican standards are all subtractive. Although the technical definition of polarity involves the relative position of primary and secondary bushings, the position of primary bushings is always the same according to standards. Therefore, when facing the secondary bushings of an additive transformer, the X1 bushing is located to the right (of X3), while for a subtractive transformer, X1 is farthest to the left. To complicate this definition, a single-phase pad-mounted transformer built to ANSI standard Type 2 will always have the X2 mid-tap bushing on the lowest right-hand side of the lowvoltage slant pattern. Polarity has nothing to do with the internal construction of the transformer windings but only with the routing of leads to the bushings. Polarity only becomes important when transformers are being paralleled or banked. Single-phase polarity is illustrated in Figure 2.2.11. 2.2.5.3.2 Three-Phase Angular Displacement The phase relation of voltage between H1 and X1 bushings on a three-phase distribution transformer is referred to as angular displacement. ANSI standards require that wye–wye and delta–delta transformers have 0˚ displacement. Wye–delta and delta–wye transformers will have X1 lagging H1 by 30˚. This difference in angular displacement means that care must be taken when three-phase transformers are paralleled to serve large loads. Sometimes the phase difference is used to advantage, such as when supplying power to 12-pulse rectifiers or other specialized loads. European standards permit a wide variety of displacements, the most common being Dy11. This IEC designation is interpreted as Delta primary–wye secondary, with X1 lagging H1 by 11 30˚ = 330˚, or leading by 30˚. The angular displacement of Dy11 differs from the ANSI angular displacement by 60˚. Three-phase angular displacement is illustrated in Figure 2.2.12. FIGURE 2.2.10 Typical ferroresonance situation. (From IEEE C57.105-1978, IEEE Guide for Application of Transformer Connections in Three-Phase Distribution Systems, copyright 1978 by the Institute of Electrical and Electronics Engineers, Inc. The IEEE disclaims any responsibility or liability resulting from the placement and use in the described manner. Information is reprinted with the permission of the IEEE.) FIGURE 2.2.11 Single-phase polarity. (Adapted from IEEE C57.12.90-1999. The IEEE disclaims any responsibility or liability resulting from the placement and use in the described manner. Information is reprinted with the permission of the IEEE.) © 2004 by CRC Press LLC © 2004 by CRC Press LLC
above which persons might be burned is about 150˚C (Hayman, 1973). See Section 2.2.7 for a detailed description of underground transformers. FIGURE 2.2.12 Three-phase angular displacement. (Adapted from IEEE C57.105-1978. The IEEE disclaims any responsibility or liability resulting from the placement and use in the described manner. Information is reprinted with the permission of the IEEE.) 2.2.6 Transformer Locations 2.2.6.1 Overhead With electric wires being strung at the tops of poles to keep them out of the reach of the general public, it is obvious that transformers would be hung on the same poles, as close as possible to the high-voltage source conductors. Larger units are often placed on overhead platforms in alleyways, or alongside buildings, or on ground-level pads protected by fencing. Overhead construction is still the most economical choice in rural areas, but it has the disadvantage of susceptibility to ice and wind storms. The public no longer perceives overhead wiring as a sign of progress, instead considering it an eyesore that should be eliminated from view. 2.2.6.2 Underground Larger cities with concentrated commercial loads and tall buildings have had underground primary cables and transformers installed in below-grade ventilated vaults since the early part of the 20th century. By connecting many transformers into a secondary network, service to highly concentrated loads can be maintained even though a single transformer may fail. In a network, temporary overloads can be shared among all the connected transformers. The use of underground distribution for light industrial and commercial and residential service became popular in the 1960s, with the emphasis on beautification that promoted fences around scrap yards and the elimination of overhead electric and telephone lines. The most common construction method for residential electric services is underground primary cables feeding a transformer placed on a pad at ground level. The problems of heat dissipation and corrosion are only slightly more severe than overheads, but they are substantially reduced compared with transformers confined in below-grade ventilated vaults. Since pad mounts are intended to be placed in locations that are frequented by the general public, the operating utility has to be concerned about security of the locked cabinet covering the primary and secondary connections to the transformer. The industry has established standards for security against unauthorized entry and vandalism of the cabinet and for locking provisions (ANSI/NEMA, 1999). Another concern is the minimization of sharp corners or edges that may be hazardous to children at play, and that also has been addressed by standards. The fact that pad-mounted transformers can operate with surface temperatures near the boiling point of water is a further concern that is voiced from time to time. One argument used to minimize the danger of burns is to point out that it is no more hazardous to touch a hot transformer than it is to touch the hood of an automobile on a sunny day. From a scientific standpoint, research has shown that people will pull away after touching a hot object in a much shorter time than it takes to sustain a burn injury. The point 2.2.6.3 Directly Buried Through the years, attempts have been made to place distribution transformers directly in the ground without a means of ventilation. A directly buried installation may be desirable because it is completely out of sight and cannot be damaged by windstorms, trucks and automobiles, or lawn mowers. There are three major challenges when directly buried installations are considered: the limited operational accessibility, a corrosive environment, and the challenge of dissipating heat from the transformer. The overall experience has been that heat from a buried transformer tends to dry out earth that surrounds it, causing the earth to shrink and create gaps in the heat-conduction paths to the ambient soil. If a site is found that is always moist, then heat conduction may be assured, but corrosion of the tank or of cable shields is still a major concern. Within the last several years, advances in encapsulation materials and techniques have fostered development of a solid-insulation distribution transformer that can be installed in a ventilated vault or directly buried using thermal backfill materials while maintaining loadability comparable with overhead or pad-mounted transformers. For further information, see Section 2.2.7.4, Emerging Issues. 2.2.6.4 Interior Installations Building codes generally prohibit the installation of a distribution transformer containing mineral oil inside or immediately adjacent to an occupied building. The options available include use of a dry-type transformer or the replacement of mineral oil with a less-flammable coolant. See Section 2.2.2.6, Coolants. 2.2.7 Underground Distribution Transformers Underground transformers are self-cooled, liquid-filled, sealed units designed for step-down operation from an underground primary-cable supply. They are available in both single- and three-phase designs. Underground transformers can be separated into three subgroups: those designed for installation in roomlike vaults, those designed for installation in surface-operable enclosures, and those designed for installation on a pad at ground level. 2.2.7.1 Vault Installations The vault provides the required ventilation, access for operation, maintenance, and replacement, while at the same time providing protection against unauthorized entry. Vaults used for transformer installations are large enough to allow personnel to enter the enclosure, typically through a manhole and down a ladder. Vaults have been used for many decades, and it is not uncommon to find installations that date back to the days when only paper-and-lead-insulated primary cable was available. Transformers for vault installations are typically designed for radial application and have a separate fuse installation on the source side. Vaults can incorporate many features: • Removable top sections for transformer replacement • Automatic sump pumps to keep water levels down • Chimneys to increase natural air flow • Forced-air circulation Transformers designed for vault installation are sometimes installed in a room inside a building. This, of course, requires a specially designed room to limit exposure to fire and access by unauthorized personnel and to provide sufficient ventilation. Both mineral-oil-filled units and units with one of the less-flammable insulating oils are used in these installations. These installations are also made using drytype or pad-mounted transformers. Transformers for vault installation are manufactured as either subway transformers or as vault-type transformers, which, according to C57.12.40 (ANSI, 2000a), are defined as follows: © 2004 by CRC Press LLC © 2004 by CRC Press LLC
Page 1 and 2: ELECTRIC POWER TRANSFORMER ENGINEER
Page 3 and 4: I wish to recognize the interest of
Page 5 and 6: Contents 1 Theory and Principles Ch
Page 7 and 8: 1.3 Equivalent Circuit of an Iron-C
Page 9 and 10: designer starts to make a design fo
Page 11 and 12: • Transient voltages generated du
Page 13 and 14: the transformer’s bushings and, m
Page 15 and 16: % regulation = [(V NL - V FL )/V FL
Page 17 and 18: In core-form transformers, the wind
Page 19 and 20: FIGURE 2.1.14 Layer windings (singl
Page 21 and 22: the transformer design, which may b
Page 23 and 24: consumer’s service circuit is a d
Page 25 and 26: FIGURE 2.2.3 Single-phase transform
Page 27: is installed so that the tank never
Page 31 and 32: FIGURE 2.2.16 Two-bushing subway. (
Page 33 and 34: carbon steel or the very expensive
Page 35 and 36: FIGURE 2.2.31 Radial-style dead fro
Page 37 and 38: FIGURE 2.2.36 Complete transformer
Page 39 and 40: voltage in each winding simultaneou
Page 41 and 42: mounted transformers can have arres
Page 43 and 44: V S + V S I V L Z=R+jX a) V L V S V
Page 45 and 46: Z = jX (2.3.19) 0.7 Then the curren
Page 47 and 48: X1 L* X1 S =X1 L X2 L* X3 L* a) S L
Page 49 and 50: 150 V(%) 100 50 0 -50 40 80 T(s) 12
Page 51 and 52: 2.3.8.1.2 Series Connection • The
Page 53 and 54: A six-pulse single-way transformer
Page 55 and 56: The degree of coupling also influen
Page 57 and 58: where I h = current for the hth har
Page 59 and 60: In the case of liquid-filled transf
Page 61 and 62: FIGURE 2.4.21 A 36-pulse system mad
Page 63 and 64: Vacuum-pressure encaps ulated — T
Page 65 and 66: FIGURE 2.6.1 Typical wiring and sin
Page 67 and 68: a) H1 b) Ip X2 FLUX H2 LEAKAGE FLUX
Page 69 and 70: and for CTs is TCF = RCF - (PA/2600
Page 71 and 72: RCF x Bx Bt RCF t - RCF 0 co
Page 73 and 74: 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
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2.7 Step-Voltage Regulators Craig A
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Source Bypass Switch Source Bypass
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2.7.4 Theory A step-voltage regulat
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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
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FIGURE 2.8.20A Performance of a rid
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• Input frequency or frequency ra
Page 103 and 104:
References 1. Sola/Hevi-Duty Corp.,
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FIGURE 2.9.5 Typical phase-reactor
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FIGURE 2.9.10 Two generator arrange
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• Overloading of lines • Increa
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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 = (
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ate of rise of transient-recovery v
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the LLCR is provided in IEEE Std. C
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aluminum shielding, i.e., an oil-im
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Leo J. Savio ADAPT Corporation Ted
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3.1.2.2.2 Heat Dissipation A second
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FIGURE 3.2.1 Solid-type bushing. ma
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1.6 cm. This means that most of the
Page 131 and 132:
where HS = steady-state bushing ho
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the conductivity of the water-solub
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of 178 ohm-m, and a test duration o
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2. Easley, J.K. and Stockum, F.R.,
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FIGURE 3.3.5 Reactance-type LTC, in
Page 141 and 142:
FIGURE 3.3.10 LTC with delta connec
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3.3.5 Selection of Load Tap Changer
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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
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3.5.4 Three-Phase Transformer Conne
Page 155 and 156:
FIGURE 3.5.4 Interconnected star-gr
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3.6.1.1 Standards ANSI standards fo
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ANSI standards. LTC control setting
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FIGURE 3.6.7 Discharging the capaci
Page 163 and 164:
FIGURE 3.6.10 Standard switching-im
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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
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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
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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
Page 191 and 192:
29. Concordia, C. and Rothe, F.S.,
Page 193 and 194:
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
Page 203 and 204:
where C = capacitance between the t
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L ijo = N i N j ijo (3.10.31) 1.4
Page 207 and 208:
% 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
Page 213 and 214:
6. Costs to set up and use a mobile
Page 215 and 216:
Records of relay operation must be
Page 217 and 218:
• Has heating occurred as the res
Page 219 and 220:
a reference value) of the currents
Page 221 and 222:
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
Page 235 and 236:
TABLE 3.14.2 Relevant Documents for
Page 237 and 238:
Page 239 and 240:
Page 241:
TABLE 3.14.13 Relevant Documents fo
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[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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