[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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1000 10 100 10 10 5 1.0 5 0.1 5 0.01 Time in Seconds Rated Current Bayonet Max Clear Bayonet Min Melt Transformer Withstand Inrush and Cold Load Pickup Fuse Withstand Bolted Fault Current Minimum Interrupting Current Rating Per Unit Current Crossover Maximum Interrupting Current Rating FIGURE 2.2.38 Time–current-curve crossover coordination. (By permission of ABB Inc., Jefferson City, MO.) 2.2.13.4.2.1 Fuse Curves — The main tool used for coordination is a graph of time vs. current for each fuse or breaker, as seen in Figure 2.2.38. The graph is displayed as a log–log plot and has two curves for any particular fuse. The first curve is called the minimum-melt curve, and this represents time-current points where the fuse element just starts to melt. The other curve is a plot of points at longer times (to the right of the minimum-melt curve). The latter curve is called the maximum-clear or sometimes the average-clear curve. The maximum-clear curve is where the fuse can be considered open and capable of sustaining full operating voltage across the fuse without danger of restrike. Even if a fuse has melted due to a fault, the fault current continues to flow until the maximum-clear time has passed. For expulsion fuses, there is a maximum interrupting rating that must not be exceeded unless a current-limiting or other backup fuse is present. For partial-range current-limiting fuses, there is a minimum interrupting Partial-Range Min Melt current. Above that minimum current, clearing occurs in about 0.25 cycles, so the maximum-clear curve is not actually needed for most cases. 2.2.13.4.2.2 Transformer Characteristics — Each transformer has characteristics that are represented on the time-current curve to aid in the coordination process: • Rated current = primary current at rated kVA • Bolted fault current (I SC ) = short-circuit current in the primary with secondary shorted • Inrush and cold-load-pickup curve: • Inrush values are taken as 25 times rated current at 0.01 s and 12 times rated current at 0.1 s. • Cold-load-pickup values are presumed to be six times rated current at 1 s and three times rated current at 10 s. • Through-fault duration or short-circuit withstand established by IEEE C57.109. For most transformers, the curve is the plot of values for I 2 t = 1250 or 50 times rated current at 0.5 s, 25 times rated current at 2 s, and 11.2 times rated current at 10 s. Values longer than 10 s are usually ignored. 2.2.13.4.2.3 Fuse Coordination Steps — Select an expulsion fuse such that: • The minimum-melt curve falls entirely to the right of the inrush/cold-load-pickup curve. For most fuses, the minimum-melt curve will always be to the right of 300% of rated load, even for very long times. • The maximum-clear curve will fall entirely to the left of the through-fault-duration curve at 10 s and below. Select a partial-range current-limiting (PRCL) fuse such that its minimum-melt curve: • Crosses the expulsion-fuse maximum-clear curve to the right of the bolted fault line, preferably with a minimum 25% safety margin • Crosses the expulsion-fuse maximum-clear curve at a current higher than the PRCL minimum interrupting rating • Crosses the expulsion-fuse maximum-clear curve at a current below the maximum interrupting rating of the expulsion fuse. It is not a critical issue if this criterion is not met, since the PRCL will quickly clear the fault anyway. There are additional considerations, such as checking for a longtime recross of the two fuse characteristics or checking for a recross at a “knee” in the curves, as might occur with a dual-sensing fuse or a low-voltage circuit breaker with a high-current magnetic trip. 2.2.13.4.3 Low-Voltage Oil-Breaker Coordination The coordination of an oil breaker with an expulsion fuse is slightly different than the previous example. The oil-breaker current duty is translated to the high-voltage side and is sized in a manner similar to the expulsion fuse in the previous example. The expulsion fuse is then selected to coordinate with the breaker so that the minimum melt falls entirely to the right of the breaker’s maximum clear for all currents less than the bolted fault current. This ensures that the breaker will protect against all secondary faults and that the internal expulsion fuse will only open on an internal fault, where current is not limited by the transformer impedance. 2.2.13.5 Internal Secondary Circuit Breakers Secondary breakers that are placed in the bulk oil of a transformer can protect against overloads that might otherwise cause thermal damage to the conductor-insulation system. Some breakers also have magnetically actuated trip mechanisms that rapidly interrupt the secondary load in case of a secondary fault. When properly applied, secondary breakers should limit the top-oil temperature of a transformer to about 110˚C during a typical residential load cycle. Breakers on overhead transformers are often equipped with a red signal light. When this light is on, it signifies that the transformer has come close to tripping the breaker. The light will not go off until a lineman resets the breaker. The lineman can also © 2004 by CRC Press LLC © 2004 by CRC Press LLC
mounted transformers can have arresters too, especially those at the end of a radial line, and they can be inside the tank, plugged into dead-front bushings, or at a nearby riser pole, where primary lines transition from overhead to underground. FIGURE 2.2.39 Cutaway showing CSP components. (By permission of ABB Inc., Jefferson City, MO.) set the breaker on its emergency position, which allows the transformer to temporarily supply a higher overload until the utility replaces the unit with one having a higher kVA capacity. The secondary oil breaker is also handy to disconnect load from a transformer without touching the primary connections. 2.2.13.6 CSP ®2 Transformers Overhead transformers that are built with the combination of secondary breaker, primary protective link, and external lightning arrester are referred to generically as CSPs (completely self-protected transformers). This protection package is expected to prevent failures caused by excessive loads and external voltage surges, and to protect the system from internal faults. The breaker is furnished with a signal light and an emergency control as described above. The protective link is often mounted inside the high-voltage bushing insulator, as seen in Figure 2.2.39. 2.2.13.7 Protection Philosophy CSP transformers are still in use, especially in rural areas, but the trend is away from secondary breakers to prevent transformer burnouts. Continued growth of residential load is no longer a foregone conclusion. Furthermore, utilities are becoming more sophisticated in their initial transformer sizing and are using computerized billing data to detect a transformer that is being overloaded. Experience shows that modern distribution transformers can sustain more temporary overload than a breaker would allow. Most utilities would rather have service to their customers maintained than to trip a breaker unnecessarily. 2.2.13.8 Lightning Arresters Overhead transformers can be supplied with primary lightning arresters mounted nearby on the pole structure, on the transformer itself, directly adjacent to the primary bushing, or within the tank. Pad- 2 CSP ® is a registered trademark of ABB Inc., Raleigh, NC. 2.2.14 Economic Application 2.2.14.1 Historical Perspective Serious consideration of the economics of transformer ownership did not begin until the oil embargo of the early 1970s. With large increases in the cost of all fuels, utilities could no longer just pass along these increases to their customers without demonstrating fiscal responsibility by controlling losses on their distribution systems. 2.2.14.2 Evaluation Methodology An understanding soon developed that the total cost of owning a transformer consisted of two major parts, the purchase price and the cost of supplying thermal losses of the transformer over an assumed life, which might be 20 to 30 years. To be consistent, the future costs of losses have to be brought back to the present so that the two costs are both on a present-worth basis. The calculation methodologies were published first by Edison Electric Institute and recently updated in the form of a proposed ANSI standard (IEEE, 2001).The essential part of the evaluation method is the derivation of A and B factors, which are the utility’s present-worth costs for supplying no-load and load losses, respectively, in the transformer as measured in $/W. 2.2.14.3 Evaluation Formula The proposed ANSI guide for loss evaluation expresses the present value of the total owning cost of purchasing and operating a transformer as follows (in its simplest form): TOC Transformer Cost A No Load Loss B Load Loss (2.2.8) where A = loss-evaluation factor for no-load loss, $/W B = loss-evaluation factor for load loss, $/W The guide develops in detail the calculation of A and B factors from utility operating parameters as shown in Equation 2.2.9 and Equation 2.2.10, respectively: SC EC HPY A FCR 1000 SC RFEC LSF HPY PL B FCR 1000 where SC = GC + TD SC = avoided cost of system capacity GC = avoided cost of generation capacity TD = avoided cost of transmission and distribution capacity EC = avoided cost of energy HPY = hours per year FCR = levelized fixed-charge rate RF = peak responsibility factor LSF = transformer loss factor PL = equivalent annual peak load 2 (2.2.9) (2.2.10) © 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 and 28: is installed so that the tank never
Page 29 and 30: above which persons might be burned
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: voltage in each winding simultaneou
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
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
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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
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
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
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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
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
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
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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
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
Page 229 and 230:
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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