[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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3.3.4 Rated Characteristics and Requirements for Load Tap Changers The rated characteristics of an LTC are as follows: • Rated through-current 1 • Maximum rated through-current 1 • Rated step voltage 1 • Maximum rated step voltage 1 • Rated frequency • Rated insulation level The basic requirements for LTCs are laid down in various standards (IEEE Std. C57.131-1995 [1995]; IEC 60214 [2003]; and IEC 60542 [1988]). The main features to be tested during design tests are: • Contact life: IEEE Std. C57.131-1995-6.2.1.1; IEC 60214-1-5.2.2.1 50,000 operations at the max rated through-current and the relevant rated step voltage shall be performed. The result of these tests may be used by the manufacturer to demonstrate that the contacts used for making and breaking current are capable of performing, without replacement of the contacts, the number of operations guaranteed by the manufacturer at the rated throughcurrent and the relevant rated step voltage. • Temporary overload: IEEE Std. C57.131-1995-6.1.3 and 6.2.2; IEC 60214-1-4.3, 5.2.1, and 5.2.2.2 At 1.2 times maximum rated through-current, temperature-rise tests of each type of contact carrying current continuously shall be performed to verify that the steady-state temperature rise does not exceed 20°C above the temperature of the insulating fluid, surrounding the contacts. In addition, breaking-capacity tests shall be performed with 40 operations at a current up to twice the maximum rated through-current and at the relevant rated step voltage. LTCs that comply with the said design tests, and when installed and properly applied to the transformer, can be loaded in accordance with the applicable IEEE or IEC loading guides. • Mechanical life: IEEE Std. C57.131-1995-6.5.1; IEC 60214-1-5.2.5.1 A mechanical endurance test of 500,000 tap-change operations without load has to be performed. During this test the LTC shall be assembled and filled with insulating fluid or immersed in a test tank filled with clean insulating fluid, and operated as for normal service conditions. Compared with the actual number of tap-change operations in various fields of application (Table 3.3.1) it can be seen that the mechanical endurance test covers the service requirements. • Short-circuit-current strength: IEEE Std. C57.131-1995-6.3; IEC 60214-1-5.2.3 All contacts of different design that carry current continuously shall be subjected to three shortcircuit currents of 10 times maximum rated through-current (valid for I > 400 A), with an initial peak current of 2.5 times the rms value of the short-circuit current, each current application of at least 2-sec duration. • Dielectric requirements: IEEE Std. C57.131-1995-6.6; IEC 60214-1-5.2.6 The dielectric requirements of an LTC depend on the transformer winding to which it is to be connected. The transformer manufacturer shall be responsible not only for selecting an LTC of the appropriate insulation level, but also for the insulation level of the connecting leads between the LTC and the winding of the transformer. The insulation level of the LTC is demonstrated by dielectric tests — in accordance with the standards (applied voltage, basic lightning impulse, switching impulse, partial discharge, if applicable) — on all relevant insulation spaces of the LTC. The test and service voltages of the insulation between phases and to ground shall be in accordance with the standards. The values of the withstand voltages of all other relevant insulation spaces of an LTC shall be declared by the manufacturer of the LTC. • Oil tightness of arcing-switch oil compartment: IEC 60214-1-5.2.5.4 The analysis of gases dissolved in the transformer oil is an important, very sensitive, and commonly used indication for the operational behavior of a power transformer. To avoid any influence of the switching gases produced by each operation of the arcing switch, on the results of the said gas-in-oil analysis, the arcing-switch oil compartment has to be oil tight. Furthermore, the arcing switch conservator tank must be completely separated from the transformer conservator tank on the oil and on the gas side. Vacuum- and pressure-withstand values of the oil compartment shall be declared by the manufacturer of the LTC. TABLE 3.3.1 Number of LTC Switching Operations in Various Fields of Application FIGURE 3.3.14 Characteristics of the arcing switch — through-current, step voltage, switching capacity. 1 Within the maximum rated through-current of the LTC, there may be different combinations of values of rated through-current and corresponding rated step voltage. Figure 3.3.14 shows that relationship. When a value of rated step voltage is referred to as a specific value of rated through-current, it is called the relevant rated step voltage. Transformer Data Number of On-Load Tap Power, Voltage, Current, Changer Operations Per Year Transformer MVA kV A Min. Medium Max. Power station 100–1300 110–765 100–2000 500 3,000 10,000 Interconnected 200–1500 110–765 300–3000 300 5,000 25,000 Network 15–400 60–525 50–1600 2,000 7,000 20,000 Electrolysis 10–100 20–110 50–3000 10,000 30,000 150,000 Chemistry 1.5–80 20–110 50–1000 1,000 20,000 70,000 Arc furnace 2.5–150 20–230 50–1000 20,000 50,000 300,000 © 2004 by CRC Press LLC © 2004 by CRC Press LLC
3.3.5 Selection of Load Tap Changers The selection of a particular LTC will render optimum technical and economical efficiency if requirements due to operation and testing of all conditions of the associated transformer windings are met. In general, usual safety margins may be neglected as LTCs designed, tested, selected, and operated in accordance with IEEE standards (1995) and IEC standards (1976; 2003) are most reliable. The details for the selection of LTCs are described in Krämer, 2000. To select the appropriate LTC, the following important data of associated transformer windings should be known: • MVA rating • Connection of tap winding (for wye, delta, or single-phase connection) • Rated voltage and regulating range • Number of service tap positions • Insulation level to ground • Lightning-impulse and power-frequency voltage of the internal insulation The following LTC operating data may be derived from this information: • Maximum through-current: Imax • Step voltage: Ust • Switching capacity: Pst = Ust Imax and the appropriate tap changer can be determined: • LTC type • Number of poles • Nominal voltage level of LTC • Tap selector size/insulation level • Basis connection diagram If necessary, the following characteristics of the tap changer should be checked: • Breaking capacity • Overload capability • Short-circuit current (especially to be checked in case of Figure 3.3.12 applications) • Contact life In addition to that, the following two important LTC stresses resulting from the arrangement and application of the transformer design have to be checked. 3.3.5.1 Voltage Connection of Tap Winding during Changeover Operation During the operation of the reversing or coarse changeover selector, the tap winding is disconnected momentarily from the main winding. It thereby takes a voltage that is determined by the voltages of the adjacent windings as well as by the coupling capacities to these windings and to grounded parts. In general, this voltage is different from the voltage of the tap winding before the changeover selector operation. The differential voltages are the recovering voltages at the opening contacts of the changeover selector and, when reaching a critical level, they are liable to cause inadmissible discharges on the changeover selector. If these voltages exceed a certain limit value (for special product series, said limit voltages are in the range of 15 to 35 kV), measures regarding voltage control of the tap winding must be taken. Especially in case of PSTs with regulation at the line end (e.g., Figure 3.3.12), high recovery voltages can occur due to the winding arrangement. Figure 3.3.15a illustrates a typical winding arrangement of PST according to Figure 3.3.12. Figure 3.3.15b gives the phasor diagram of that arrangement without limiting measures. As it can be seen, the recovery voltages appearing at the changeover selector contacts are in the range of the system voltages on the source and the load side. FIGURE 3.3.15 Phase-shifting transformer, circuit (as shown in Figure 3.3.12). Top) typical winding arrangement with two tap windings; bottom) recovery voltages (U r+ , U r– ) for tap windings 1 and 2 (phasor diagram). It is sure that an LTC cannot be operated under such conditions. This fact has already been taken into account during the planning stage of the PST design. There are three ways to solve the above-mentioned problem: 1. Install screens between the windings. These screens must have the voltage of the movable changeover selector contact 0 (Figure 3.3.12). See Figs. 3.3.16a and b. 2. Connect the tap winding to a fixed voltage by a fixed ohmic resistor (tie-in resistor) or by an ohmic resistor, which is only inserted during changeover selector operation by means of a voltage switch. This ohmic resistor is usually connected to the middle of the tap winding and to the current take-off terminal of the LTC (Figure 3.3.17). 3. Use an advance–retard switch (ARS) as changeover selector (Figure 3.3.18). This additional unit allows the changeover operation to be carried out in two steps without interruption. With this arrangement, the tap winding is connected to the desired voltage during the whole changeover operation. As this method is relatively complicated, it is only used for high-power PSTs. The common method for the voltage connection of tap windings is to use tie-in resistors. The following information is required to dimension tie-in resistors: • All characteristic data of the transformer such as power, high and low voltages with regulating range, winding connection, insulation levels • Design of the winding, i.e., location of the tap winding in relation to the adjacent windings or winding parts (in case of layer windings) © 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
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 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: FIGURE 3.3.10 LTC with delta connec
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
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.,
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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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[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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