[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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allow convenient removal of heat, sufficient mechanical strength to withstand forces generated during system faults, acceptable noise characteristics, and an electrical insulation system that meets the system steady-state and transient requirements. There are two common transformer structures in use today. When the magnetic circuit is encircled by two or more windings of the primary and secondary, the transformer is referred to as a core-type transformer. When the primary and secondary windings are encircled by the magnetic material, the transformer is referred to as a shell-type transformer. 3.10.8.2 Core Form Characteristics of the core-form transformer are a long magnetic path and a shorter mean length of turn. Commonly used core-form magnetic circuits are single-phase transformers with two-legged magnetic path, with turns wound around each leg; three-legged magnetic path, with the center leg wound with conductor; and a legged magnetic path, with the two interior legs wound with conductors [2,3]. Threephase core-form designs are generally three-legged magnetic cores, with all three legs possessing windings, and a five-legged core arrangement, with the three center legs possessing windings. The simplest winding arrangement has the low-voltage winding nearest the core and the high-voltage winding wound on top of the low. Normally, in the core-form construction the winding system is constructed from helical, layer, or disk-type windings. Often the design requirements call for a winding arrangement that is a more complex arrangement, e.g., interleaving high- and low-voltage windings, interwound taps, or bifurcated windings having entry and exit points other than the top or bottom of the coil. Each of these variations has, to one degree or another, an effect on the transformer’s transient-voltage response. To ensure an adequate insulation structure, during the design stage each possible variation must be explored to evaluate its effect on the transient overvoltages. 3.10.8.3 Shell Form Shell-form transformer construction features a short magnetic path and a longer mean length of electrical turn. Fink and Beaty [43] point out that this results in the shell-form transformer having a larger core area and a smaller number of winding turns than the core-form of the same output and performance. Additionally, the shell form generally has a larger ratio of steel to copper than an equivalently rated core-form transformer. The most common winding structure for shell-form windings is the primary-secondary-primary (P-S-P), but it is not uncommon to encounter shell-form winding of P-S-P-S-P. The winding structure for both the primary and secondary windings is normally of the pancake-type winding structure [2]. 3.10.8.4 Proof of Design Concept The desire of the purchaser is to obtain a transformer at a reasonable price that will achieve the required performance for an extended period of time. The desire of the manufacturer is to construct and sell a product, at a profit, that meets the customer’s goals. The specification and purchase contract are the document that combines both purchaser’s requirements and manufacturer’s commitment in a legal format. The specification will typically address the transformer’s service condition, rating, general construction, control and protection, design and performance review, testing requirements, and transportation and handling. Since it is impossible to address all issues in a specification, the industry uses standards that are acceptable to purchaser and supplier. In the case of power transformers, the applicable standards would include those found in IEEE C57, IEC 76, and NEMA TR-1. 3.10.8.5 Standard Winding Tests ANSI/IEEE C57.12.00 [51] defines routine and optional test and testing procedures for power transformers. The following are listed as routine tests for transformers larger than 501 kVA: Winding resistance Winding-turns ratio Phase-relationship tests: polarity, angular displacements, phase sequence No-load loss and exciting current Load loss and impedance voltage Low-frequency dielectric tests (applied voltage and induced voltage) Leak test on transformer tank The following are listed as tests to be performed on only one of a number of units of similar design for transformers 501 kVA and larger: Temperature-rise tests Lightning-impulse tests (full and chopped wave) Audible sound test Mechanical test from lifting and moving of transformer Pressure tests on tank Other tests are listed [51] that include, for example, short-circuit forces and switching-surge-impulse tests. Additionally, specific tests may be required by purchasers base on their application or field experience. The variety of transient voltages a transformer may see in its normal useful life are virtually unlimited [9]. It is impractical to proof test each transformer for every conceivable combination of transient voltages. However, the electrical industry has found that it is possible, in most instances, to assess the integrity of the transformer’s insulation systems to withstand transient voltages with the application of a few specific, aperiodic voltage waveforms. Figure 3.10.8 illustrates the forms of the full, chopped, and switching surge waves. IEEE C57-1990 [51] contains the specific wave characteristics, relationships and acceptable methods and connections required for these standard tests. Each of these tests is designed to test the insulation structure for a different transient condition. The purpose of applying this variety of tests is to substantiate adequate performance of the total insulation system for all the various transient voltages it may see in service. The insulation integrity for steady-state and dynamic voltages is also assessed by factory tests called out in ANSI/IEEE C57.12.00 [51]. One should not be lulled to sleep and think that a transformer that has passed all factory voltage tests (both impulse and low frequency) can withstand all transient voltages to which it may be subjected by the system. One should always assess the environment the transformer is applied in and determine if there may be unusual transient-voltage excitation present in an application that is not covered in the standards (see [9,10,31]). 3.10.8.6 Design Margin The actual level of insulation requested, e.g., basic impulse insulation level (BIL) and switching impulse level (BSL), is determined by recognizing the system within which the transformer will operate and the arrester’s level of protection. Normally, a minimum protective margin of 15 to 20% between the arrester peak voltage and the transformer capability at three sigma (3) is established. This is illustrated in Figure 3.10.9 for a 230-kV transformer at 750-kV BIL protected with a 180-kV-rated arrester [52]. The curve designated A in Figure 3.10.9 is used to represent the transformer’s insulation-coordination characteristic (insulation capability) when subjected to aperiodic and oscillatory wave forms. The curve to be used to represent the transformer volt–time insulation-coordination characteristic when subjected to aperiodic wave forms with a time to failure between 0.1 and 2000 sec is to be based on five points [53]. The five points are: 1. Front-of-wave voltage plotted at its time of crest (about 0.5 sec). If the front-of-wave voltage is not available, a value of 1.3 times the BIL should be plotted at 0.5 sec. 2. Chopped-wave voltage at its time of crest (about 3.0 sec) 3. Full-wave voltage (or BIL) plotted at about 8 sec 4. Switching-surge voltage (or BSL) plotted at about 300 sec 5. A point at 2000 sec, where its magnitude is established with the following expression: © 2004 by CRC Press LLC © 2004 by CRC Press LLC
% VOLTAGE % VOLTAGE 100 BIL 90 50 30 0 0 T30 T90 Tp Th µsec 100 110% BIL Ttc ←→ Toc % UNDERSHOOT µsec 100 BSL (.83 BIL) Crest Voltage BIL (pu) 1.6 1.4 1.2 1 0.8 0.6 0.4 o B + C x D * A E o o + o x + * x * o o o + x * o 10 0 10 2 10 4 10 6 10 8 10 10 Tim e (microseconds) | | | | | - | - - - - - - - - - - - - - - - µ - | - - - - - - - - - - - - - - - 1σ - | - - - - - - - - - - - - - - - 2σ - | - - - - - - - - - - - - - - - 3σ | o + | x * % VOLTAGE 50 0 Tp FIGURE 3.10.8 Standard voltage waveforms for impulse tests: full, chopped, switching surge. logV 2000 logT BSL logT m 2000 where V 2000 is the voltage at 2000 sec, T 2000 ; V BSL is the BSL voltage (or 0.83 the BIL); and T BSL is equal to 300 sec. The value of m is established as the inverse of the slope of a straight line drawn on log-log paper from the BSL point to a point established by the peak of the 1-h induced test voltage plotted at a time the induced voltage exceeds 90% of its peak value (i.e., 28.7% of 3600 sec or 1033.2 sec). The connection between all points is made with a smooth continuous curve. The first four points in the curve establish an approximate level of insulation voltage capability for which one would anticipate only one insulation failure out of 1000 applications of that voltage level, e.g., at 3 the probability of failure is 1.0 – 0.99865 or 0.001. Experience has shown that the standard deviation for transformer insulation structures is on the order of 10 to 15%. Figure 3.10.9 assumes that is 10%. Curve B, or the 50% failure-rate curve, is established by increasing the voltage in Curve A by 30%. Therefore, for Curve B, on average the unit would be expected to fail one out of two times if it were subjected to this level of voltage. Curves C and D establish 1- and 2- curves, or 16% and 2.3% failure-rate curves, respectively. The inserted normal distribution on the right of Figure 3.10.9 illustrate this concept. All of this discussion is based on the assumption that the transformer is new. µsec 300 log logVBSL 2000 logV m BSL Th FIGURE 3.10.9 Voltage–time curve for insulation coordination. 3.10.8.7 Insulation Coordination In a field installation, an arrester is normally placed directly in front of the transformer to afford it protection from transient voltages produced on the system. Curve E in Figure 3.10.9 is a metal-oxidearrester protective curve established in a manner similar to that described in IEEE Std. C62.2. The curve is specified by three points: 1. The front-of-wave voltage held by the arrester plotted at 0.5 sec 2. The 8 20-sec voltage plotted at 8.0 sec 3. The switching-surge voltage plotted in straight line from 30 to 2000 sec The protective ratio is established by dividing the transformer insulation capability by the arrester protective level for the waveshape of interest. For example, in Figure 3.10.9 the protective level for a switching surge is on the order of 177% or (0.83/0.47) 100. 3.10.8.8 Additional System Considerations The current standards reflect the growing and learning within the industry, and each year they expand in breadth to address issues that are of concern to the industry. However, at present the standards are silent in regard to the effects of system voltage on transient response, multiphase surges, aging or mechanical movement of insulation structures, oscillatory voltage excitation, temperature variations, movement of oil, and loading history. A prudent user will seek the advice of users of similar products and explore their experience base. 3.10.9 Models for System Studies 3.10.9.1 Model Requirements The behavior of large power transformers under transient conditions is of interest to both transformer designers and power engineers. The transformer designer employs detailed electrical models to establish © 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
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2.7.4 Theory A step-voltage regulat
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FIGURE 2.7.22 Equalizer winding inc
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TABLE 2.7.4 Increase in Ampere Load
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References IEEE, Standard Requireme
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300% FIGURE 2.8.4 Schematic of a co
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TABLE 2.8.1 Typical Sizing Workshee
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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
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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
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V 90 ( X X ) s T (2.9.10a) where,
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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
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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
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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
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References Goosen, P.V. , Transform
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If i 1 is the full-load current rat
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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
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
Page 187 and 188: Y Y 20 18 3 rd Harmonic CTR1=40 CTR
Page 189 and 190: 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
Page 195 and 196: H 2m 1. Vertical Forced Air 2. Prin
Page 197 and 198: Gade, S., Sound Intensity Instrumen
Page 199 and 200: nected. This steady-state voltage d
Page 201 and 202: where [A] = state matrix [B] = inpu
Page 203 and 204: where C = capacitance between the t
Page 205: L ijo = N i N j ijo (3.10.31) 1.4
Page 209 and 210: 29. Degeneff, R.C., Reducing Storag
Page 211 and 212: 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
Page 223 and 224: temperature. As the transformer coo
Page 225 and 226: 3.13.3.2 Instrument Transformers Th
Page 227 and 228: FIGURE 3.13.5 Analysis of bushing s
Page 229 and 230: temperature. Under abnormal conditi
Page 231 and 232: 3.14.1.2 Accredited Standards Commi
Page 233 and 234: FIGURE 3.14.4 IEC technical-committ
Page 235 and 236: TABLE 3.14.2 Relevant Documents for
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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