[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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for a period of 10 to 15 min as the windings cool down toward the surrounding average oil temperature. The average oil temperature is itself falling, but at a much slower rate (with a longer time constant). Data for the changing resistance vs. time is then plotted and extrapolated back to the instant of shutdown. With computers, the extrapolation can be done using a regression-based curve-fitting approach. The extrapolated value of winding resistance at the instant of shutdown is used to calculate winding temperature, using the method discussed below, with some correction for the drop in top-oil rise during the 1- h loading at rated current. The winding temperature, thus determined, minus ambient temperature is equal to the winding temperature rise at a given loading. Similar tests are repeated for all ratings for which temperature rise tests are required. 3.6.6.4.5 Determining the Average Temperature by Resistance The average winding temperature, as determined by the following method, is sometimes called the “average winding temperature by resistance.” The word measurement is implied at the end of this phrase. The conversion of measured winding resistance to average winding temperature is accomplished as follows. Initial resistance measurements are made at some time before commencement of the heat run when the transformer is in thermal equilibrium. When in equilibrium, the assumption can be made that the temperature of the conductors is uniform and is equal to that of the transformer oil surrounding the coils. Initial resistance measurements are made and recorded, along with the oil temperature. This measurement is sometimes called the cold-resistance test, so the winding resistance measured during the test will be called the cold resistance R c , and the temperature will be called the cold temperature T c . At the end of the heat run, R h , the hot resistance is determined from the time series of measured resistance values by extrapolation to the moment of shutdown. The formula given below is used to determine T h , the hot temperature, knowing the hot resistance, cold resistance, and the cold temperature. This calculated temperature is the average winding temperature by resistance. Rh T R T T T h [ c k] k c (3.6.5) where T h is the “hot” temperature T c is the “cold” temperature R h is the “hot” resistance R c is the “cold” resistance T k is a material constant: 234.5C for copper, 225C for aluminum Accurate measurements of R c and T c during the cold-resistance test, as well as accurate measurements of hot winding resistance, R h , at the end of the heat run are extremely critical for accurate determination of average winding temperature rises by resistance. The reason for this will be evident by analyzing the above formula. The following discussion illustrates how measurement errors in the three measured quantities, R h , R c , and T c , affect the computed quantity, T h , via the functional relationship by which T h is computed. Shown in Table 3.6.2 are computed values for T h and the resulting error, e Th , in the computation of T h for sample sets of the measured quantities R h , R c , and T c , measured in error by the amounts e Rh , e Rc , and e Tc , respectively. Let us examine this table row by row. The way that measurement error propagates in the calculation is shown in the formula below for copper conductors. ( T e ( Rh eRh) ) ( ) [( T e R e ) . ] . h Th c Tc 234 5 234 5 c Rc (3.6.6) TABLE 3.6.2 Effect of Measurement Error in Average Winding Temperature by Resistance Row T h + e Th (C) R h + e Rh () R c + e Rc () T c + e Th (C) 1 87.375 + 0 0.030 + 0 0.024 + 0 23.0 + 0 2 87.375 – 3.187 = 0.030 + 0 0.024 + 0.00024 = 23.0 + 0 84.188 (–3.6%) 0.02424 (+1.0%) 3 87.375 + 3.218 = 0.030 + 0.0003 = 0.024 + 0 23.0 + 0 90.594 (+3.7%) 0.0303 (+1.0%) 4 87.375 + 1.25 = 88.625 (+1.43%) 0.030 + 0 0.024 + 0 23.0 + 1.0 = 24.0 (4.35 %) In row 1 of Table 3.6.2, there is no measurement error. The sample shows a set of typical measured values. The value for T h in this row can be considered the “correct answer.” In row 2 the cold-resistance measurement was 1% too high, causing the calculated value of T h to be 3.6% too low. This amplification of the relative measurement error is due to the functional relationship employed to perform the calculated result. This example illustrates the importance of measuring the resistance very carefully and accurately. Similarly, in row 3 a hot-resistance reading 1% too high results in a calculated hot temperature that is 3.7% too high. Row 4 shows the result if the cold-resistance reading is 1C too high. The result is that the determined hot resistance is 1.25C too high. In this case, while there is a reduction in the error expressed as percent, the absolute error in C is in fact greater than the original temperature error in the cold-temperature reading. These examples show that all three measured quantities — R h , R c , and T c — must be measured accurately to obtain an accurate determination of the average winding temperature. Other methods for correction to the instant of shutdown based on W/kg, or W/kg and time, are given in the IEEE test code [2]. The cooling-curve method, however, is preferred. 3.6.7 Other Tests 3.6.7.1 Short-Circuit-Withstand Tests 3.6.7.1.1 Purpose of Short-Circuit Tests Short-circuit currents during through-fault events expose the transformer to mechanical stresses caused by magnetic forces, with typical magnitudes expressed in thousands of kilograms. Heating of the conductors and adjacent insulation due to I 2 R losses also occurs during a short-circuit fault. The maximum mechanical stress is primarily determined by the square of the peak instantaneous value of current. Hence, the short-circuit magnitude and degree of transient offset are specified in the test requirements. Fault duration and frequency of occurrence also affect mechanical performance. Therefore, the number of faults, sometimes called “shots,” during a test and the duration of each fault are specified. Conductor and insulation heating is for the most part determined by the rms value of the fault current and the fault duration. Short-circuit-withstand tests are intended primarily to demonstrate the mechanical-withstand capability of the transformer. Thermal capability is demonstrated by calculation using formulas provided by IEEE C57.12.00 [1]. 3.6.7.1.2 Transformer Short-Circuit Categories The test requirements and the pass-fail evaluation criteria for short-circuit tests depend upon transformer size and construction. For this purpose, transformers are separated into four categories as shown in Table 3.6.3. The IEEE standards [1] and [2] refer to these categories while discussing the test requirements and test results evaluation. 3.6.7.1.3 Configurations A short circuit is applied using low-impedance connections across either the primary or the secondary terminals. A secondary fault is preferred, since it more directly represents the system conditions. The fault can be initiated in one of two ways: © 2004 by CRC Press LLC © 2004 by CRC Press LLC
TABLE 3.6.3 Transformer Short-Circuit Test Categories Category Single-Phase, kVA Three-Phase, kVA I 5 to 500 15 to 500 II 501 to 1,667 501 to 5,000 III 1,668 to 10,000 5,001 to 30,000 IV above 10,000 above 30,000 1. Starting with an open circuit at the terminal to be faulted, energize the transformer and then apply the short circuit by closing a breaker across the terminal. 2. Starting with a short circuit across the terminal to be faulted, close a breaker at the source terminal to energize the prefaulted transformer. The first fault-initiation method is the preferred method. Given in the order of preference as listed in C57.12.90 [2], the following fault types are permissible: • Three-phase source, three-phase short circuit. • Single-phase source, single phase-to-ground short circuit. • Single-phase source, simulated three-phase short circuit. • Single-phase source, single-phase short circuit, applied one phase at a time The simulated three-phase short circuit, using single-phase source, is conducted as follows, depending on whether the connection is delta or wye. For wye-connected windings, the fault or source is applied between one terminal and the other two terminals connected together. For delta-connected windings, the fault or source is connected between two line terminals, with no connection to the other terminal, which must be repeated for each of the three phases. 3.6.7.1.4 Short-Circuit Current Duration, Asymmetry, and Number of Tests The required short-circuit current magnitude and degree of asymmetry of the first cycle of current is given in IEEE C57.1200 [1]. Each phase of the transformer is subjected to a total of six tests that satisfy the symmetrical current requirements. At least two of the six must also satisfy the asymmetrical current tests. Five of the six tests must have a fault duration of 0.25 sec. One of the tests, which is sometimes called the “long duration test,” must be of longer duration for Category I, II, and III transformers. The longer duration must be determined as follows: For Category I transformers, t = 1250/I 2 where t = duration, sec I = fault current, per unit of load current For Category II transformers, t = 1.0 sec For Category III transformers, t = 0.5 sec Clause 7 of IEEE C57.12.00-1993 [1] and Clause 12 of IEEE C57.12.90-1993 [2] provide a detailed and excellent description of the requirements and procedures for short-circuit testing. 3.6.7.1.5 Evaluation of Short-Circuit Test Results Clause 12.5 of IEEE C57.12.90-1993 [2], Proof of Satisfactory Performance, spells out the requirements for passing or failing the tests. These include: • Visual inspection • Dielectric tests following short-circuit tests • Observations of terminal voltage and current waveforms — no abrupt changes • Changes in leakage impedance are limited to values specified in the standards • Low-voltage impulse tests may be performed • Increases in excitation current are limited by the standards Clause 12.5 of the test code discusses these criteria in detail. 3.6.7.2 Special Tests Many additional tests are available to obtain certain information about the transformer, usually to address a specific application issue. These are listed below. • Overload heat run • Gas-in-oil sampling and analysis (in conjunction with other tests) • Extended-duration no-load-loss tests • Zero-sequence impedance measurements • Tests of the load tap changer • Short-circuit-withstand tests • Fault-current capability of enclosures (overhead distribution transformers) • Telephone line voice frequency electrical noise (overhead distribution transformers) • Tests on controls These tests and others, of a specific nature, are beyond the scope of this general article. Interested readers will find more information about these and other tests in the national and international standards dealing with transformers. References 1. IEEE, Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, IEEE Std. C57.12.00-1993, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1993. 2. IEEE, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers; and Guide for Short-Circuit Testing of Distribution and Power Transformers (ANSI), IEEE/ANSI Std. C57.12.90-1992, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1992. 3. IEEE, The New IEEE Standard Dictionary of Electrical and Electronics Terms, Std. 100-1992, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1992. 4. IEEE, Guide for Transformer Impulse Tests, IEEE Std. C57.98-1993, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1993. 5. IEEE, Guide for Partial Discharge Measurement in Liquid-Filled Power Transformers and Shunt Reactors, IEEE Std. C57.113-1991, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1991. 3.7 Load-Tap-Change Control and Transformer Paralleling James H. Harlow 3.7.1 Introduction Tap changing under load (TCUL), be it with load-tap-changing (LTC) power transformers or step-voltage regulators, is the primary means of dynamically regulating the voltage on utility power systems. Switched © 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 = (
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 and 128: FIGURE 3.2.1 Solid-type bushing. ma
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: FIGURE 3.6.18 Current, voltage, and
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 and 206: L ijo = N i N j ijo (3.10.31) 1.4
Page 207 and 208: % VOLTAGE % VOLTAGE 100 BIL 90 50 3
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
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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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