[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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(A) Differential Unit, DU (A) Harmonic Blocking DU Polarized Relay R 1 I RT SLP I OP + _ 87R1 87R Unfiltered Operating Current O R I 2 K 2 + _ 87BL1 Voltage Proportional to Winding 1 Current Voltage Proportional to Winding 2 Current I 5 K 5 + _ (B) Harmonic Blocking Unit, HBU (B) Harmonic Restraint I RT SLP L 3 I 2 K 2 å I OP + _ 87R1 Operating Current (w/o DC Offset) C 1 L 1 C 2 L 2 HBU Polarized Relay O R I 5 K 5 FIGURE 3.8.8 Two approaches to a differential element. å (C) Simplified Contact Logic HBU T DU FIGURE 3.8.7 <strong>Transformer</strong> differential relay with second-harmonic blocking. + - I 86 86 K I OP 2 2 I K I OP 5 5 (3.8.12) (3.8.13) Figure 3.8.8b depicts the logic diagram of a differential element using second- and fifth-harmonic restraint. Figure 3.8.9 shows the three-phase versions of the transformer differential relay with independent T harmonic blocking or restraint. The relay is composed of three differential elements of the types shown in Figure 3.8.8. In both cases, a tripping signal results when any one of the relay elements asserts. Note that in the harmonic-restraint element (see Figure 3.8.8b), the operating current, I OP , should overcome the combined effects of the restraining current, I RT , and the harmonics of the operating current. On the other hand, in the harmonic-blocking element, the operating current is independently compared with the restraint current and the harmonics. Table 3.8.2 summarizes the results of a qualitative comparison of the harmonic restraint (using all harmonics) and blocking methods for transformer differential protection. The comparison results given in Table 3.8.2 suggest use of the blocking method, if security for inrush can be guaranteed. However, it is not always possible to guarantee security for inrush, as is explained later in this chapter. Therefore, harmonic restraint is an alternative method for providing relay security for inrush currents having low harmonic content. Another alternative is to use common harmonic restraint or blocking. This method is simple to implement in a blocking scheme and is the preferred alternative in present-day digital relays. Figure 3.8.10 shows a logic diagram of the common harmonic blocking method. A method that provides a compromise in reliability between the independent- and common-harmonic blocking methods, described earlier, forms a composite signal that contains information on the harmonics of the operating currents of all relay elements. Comparison of this composite signal with the operating current determines relay operation. The composite signal, I CH , may be of the following form: 3 CH 2 2 n 3 3 n n 1 I K I K I ... (3.8.14) © 2004 by CRC Press LLC © 2004 by CRC Press LLC
87R1 87BL1 (A) Independent Harmonic Blocking (B) Independent Harmonic Restraint I CH may contain all or only part of the harmonics of the operating current. Another possibility is to calculate the root-mean-square (rms) value of the harmonics for each relay element, I Hn : 2 2 I I I ... Hn 2 n 3 n (3.8.15) 87R2 87BL2 87R3 87BL3 FIGURE 3.8.9 Three-phase differential relay with: (a) independent harmonic blocking and (b) independent harmonic restraint. 87R 87R1 87R2 87R3 87R The composite signal, I CH , can then be calculated as some type of an average value using Equation 3.8.16 or Equation 3.8.17. The relay blocking condition is the following: I I CH CH 1 3 1 3 3 n 1 3 I n 1 Hn I 2 Hn (3.8.16) (3.8.17) I K I OP CH CH (3.8.18) TABLE 3.8.2 Comparison of Harmonic Restraint and Blocking Methods Security for external faults All-Harmonic Restraint (HR) Harmonic Blocking (HB) Remarks higher lower HR always uses harmonics from CT saturation for additional restraint; HB only blocks if the harmonic content is high Security for inrush higher lower HR adds the effects of percentage and harmonic restraint; HB evaluates the harmonics independently Security for overexcitation higher lower Same as above; however, a fifth-harmonic blocking scheme is the best solution, as will be shown in a later section Dependability lower higher Harmonics from CT saturation reduce the sensitivity of HR for internal faults; the use of only even harmonics solves this problem Speed lower higher Percentage differential and harmonic blocking run in parallel in HB Slope characteristic harmonic dependent well defined HB slope characteristic is independent from harmonics Testing results depend upon harmonics straightforward Same as above 87R1 87R2 87R3 87BL1 87BL2 87BL3 FIGURE 3.8.10 Common harmonic blocking method. 87R Common-harmonic-blocking logic provides high security but sacrifices some dependability. Energization of a faulted transformer could result in harmonics from the inrush currents of the nonfaulted phases, and these harmonics could delay relay operation. 3.8.4.2 Wave-Shape Recognition Methods Other methods for discriminating internal faults from inrush conditions are based on direct recognition of the waveshape distortion of the differential current. Identification of the separation of differential current peaks represents a major group of waveshape recognition methods. Bertula [37] designed an early percentage-differential relay in which the contacts vibrated for inrush current (because of the low current intervals) and remained firmly closed for symmetrical currents corresponding to internal faults. Rockefeller [16] proposed blocking relay operation if successive peaks of the differential current fail to occur at about 7.5 to 10 msec. A well-known principle [14,38] recognizes the length of the time intervals during which the differential current is near zero. Figure 3.8.11 depicts the basic concept behind this low-current-detection method. The differential current is compared with positive and negative thresholds having equal magnitudes. This comparison helps to determine the duration of the intervals during which the absolute value of the current is less than the absolute value of the threshold. The time intervals are then electronically compared with a threshold value equal to one-quarter cycle. For inrush currents (Figure 3.8.11a), the low-current intervals, t A , are greater than one-quarter cycle, and the relay is blocked. For internal faults (Figure 3.8.11b), the low-current intervals, t B , are less than one-quarter cycle, and the relay operates. Using the components of the rectified differential current provides an indirect way to identify the presence of low-current intervals. Hegazy [39] proposed comparing the second harmonic of the rectified differential current with a given threshold to generate a tripping signal. Dmitrenko [40] proposed issuing a tripping signal if the polarity of a summing signal remains unchanged. This signal is the sum of the dc and amplified fundamental components of the rectified differential current. Another group of methods makes use of the recognition of dc offset or asymmetry in the differential current. Some early relays [15,41,42] used the saturation of an intermediate transformer by the dc offset of the differential current as a blocking method. A transient additional restraint based on the dc component was an enhancement to a well-known harmonic-restraint transformer differential relay [11]. Michelson [43] proposed comparing the amplitudes of the positive and negative semicycles of © 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
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 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: current transformer having two prim
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
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
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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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