[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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I OP Pos. Thres. Neg. Thres. (A) Inrush Current t A FIGURE 3.8.11 Differential-relay blocking based on recognizing the duration time of low-current intervals. the differential current with given thresholds in two different polarized elements. Both elements must pick up to produce a trip. Rockefeller [16] suggested extending this idea to a digital relay. Another alternative [44] is to use the difference of the absolute values of the positive and negative semicycles of the differential current for restraint. It has also been proposed [44] to use the amplitude of the negative semicycles of the differential current as the relay operating quantity. The negative semicycle is that having the opposite polarity with respect to the dc component. More recently, Wilkinson [17] proposed making separate percentage-differential comparisons on both semicycles of the differential current. Tripping occurs if an operation condition similar to Equation 3.8.7 is true for both semicycles. 3.8.5 An Improved Approach for <strong>Transformer</strong> Protection t The evaluation in the previous section of existing harmonic-restraint/blocking methods makes clear that independent restraint/blocking methods may fail to ensure security for some real-life inrush conditions. Common harmonic restraint/blocking could provide solutions, but the behavior of these methods for internal faults combined with inrush currents requires further study. Combining restraint and blocking into an independent restraint/blocking method provides an improved approach to transformer differential protection. Even harmonics of the differential current provide restraint, while both the fifth harmonic and the dc component block relay operation. 3.8.5.1 Even-Harmonic Restraint In contrast to the odd harmonics ac CT saturation generates, even harmonics are a clear indicator of magnetizing inrush. Even harmonics resulting from dc CT saturation are transient in nature. It is important to use even harmonics (and not only the second harmonic) to obtain better discrimination between inrush and internal fault currents. Tests suggest use of even harmonics (second and fourth) in a restraint scheme that ensures security for inrush currents having very low second-harmonic current. The operation equation is: I OP (B) Internal Fault Current A + A + A + A + I SLP I K I K I OP RT 2 2 4 4 t B A - t (3.8.19) 3.8.5.2 Fifth-Harmonic Blocking It is a common practice to use the fifth harmonic of the operation current to avoid differential-relay operation for transformer overexcitation conditions [13]. A preferred solution may be a harmonic blocking scheme in which the fifth harmonic is independently compared with the operation current. In this scheme, a given relay setting, in terms of fifth-harmonic percentage, always represents the same overexcitation condition. In a fifth-harmonic restraint scheme, a given setting may represent different overexcitation conditions, depending on the other harmonics that may be present. Relay tripping in this case requires fulfillment of Equation 3.8.19 and not Equation 3.8.13. 3.8.5.3 DC Blocking The improved method of even-harmonic restraint and fifth-harmonic blocking provides very high relay security for inrush and overexcitation conditions. There are, however, some inrush cases in which the differential current is practically a pure sine wave. One of the real cases analyzed later exhibits such a behavior. Any harmonic-based method could cause relay misoperation in such extreme inrush cases. The dc component of inrush current typically has a greater time constant than that for internal faults. The presence of dc offset in the inrush current is an additional indicator that can be used to guarantee relay security for inrush. This waveshape recognition method is relatively easy to apply in a digital relay, because extraction of the dc component is a low-pass filtering process. The improved method splits the differential current into its positive and negative semicycles and calculates one-cycle sums for both semicycles. Then, the method uses the ratio of these sums to block relay operation. The one-cycle sum of the positive semicycle is proportional to the area A + (see Figure 3.8.11); the one-cycle sum of the negative semicycle is proportional to the area A – . The sum, S + , of the positive-current samples is given by the following equations: S i i 0 where i k represents the current samples, and N is the number of samples per cycle. The sum S – of the negative-current samples is given by: N k 1 k (3.8.20) (3.8.21) (3.8.22) (3.8.23) Calculate the dc ratio, DCR, according to Equation 3.8.24, to account for both positive and negative dc offsets: (3.8.24) Equation 3.8.24 gives a DCR value that is normalized (the value of DCR is always between 0 and 1) and also avoids division by zero. By comparing DCR with a 0.1 threshold, the relay dc-blocking method is implemented: k S 0 0 N k 1 i k S i i 0 k k S 0 0 i k DCR Min S S ( , ) Max ( S , S ) DCR 01 . (3.8.25) © 2004 by CRC Press LLC © 2004 by CRC Press LLC
Winding 1 Secondary Currents Data Acquisition Fundamental Frequency Filter Tap 1 Scaling Connection Compensation I1W1F1C I2W1F1C I3W1F1C I1W1F1C I1W2F1C · | å IWnC| IOP1 + 2 U87P _ 87U1 Second Harmonic Filter Tap 1 Scaling Connection Compensation I1W1F2C I2W1F2C I3W1F2C O87P + 3 _ 87O1 Fourth Harmonic Filter Tap 1 Scaling Connection Compensation FIGURE 3.8.12 Data acquisition, filtering, scaling, and compensation for Winding 1 currents. I1W1F4C I2W1F4C I3W1F4C Relay tripping requires the fulfillment of Equation 3.8.19 but not Equation 3.8.13 or Equation 3.8.25. Selecting a value for the threshold in Equation 3.8.25 means deciding on a compromise between security and speed. A high value (near 1) affords high security but is detrimental to speed. From tests, a value of 0.1 is selected as a good solution. The delay is practically negligible for system X/R ratios as great as 40. The response of this dc blocking method depends on the dc signal information apart from the harmonic content of the differential current. For example, the method ensures dependability for internal faults with CT saturation and maintains its security during inrush conditions with low even-harmonic content. I1W1F2C I1W2F2C I1W1F4C I1W2F4C |IW1F1C| |IW2F1C| · · · 1 2 IRS1 I1F2 |å I1WnF2C| |åI1WnF4C| I1F4 IRT1 + 1 _ 100/PCT2 100/PCT4 IOP1 IOP1 1 S1 2 SLP1 (SLP1 - SLP2) IRS1 +IRT1 SLP2 _ 5 + + _ 6 · 2 nd Harmonic Blocking S2 4 th Harmonic Blocking IRT1á f(SLP) · Enable + 4 _ 87R1 3.8.6 Current Differential Relay The relay consists of three differential elements. Each differential element provides percentage-differential protection with independent even-harmonic restraint and fifth-harmonic and dc blocking. The user may select even-harmonic blocking instead of even-harmonic restraint. In such a case, two blocking modes are available: (1) independent harmonic and dc blocking and (2) common harmonic and dc blocking. 3.8.6.1 Data Acquisition, Filtering, Scaling, and Compensation Figure 3.8.12 shows the block diagram of the data acquisition, filtering, scaling, and compensation sections for Winding 1 currents. The input currents are the CT secondary currents from Winding 1 of the transformer. The data-acquisition system includes analog low-pass filters and analog-to-digital converters. The digitalized current samples are the inputs to four digital band-pass filters. These filters extract the samples corresponding to the fundamental component and to the second, fourth, and fifth (not shown) harmonics of the input currents. A dc filter (not shown) also receives the current samples as inputs and forms the one-cycle sums of the positive and negative values of these samples. The outputs of the digital filters are then processed mathematically to provide the scaling and connection compensation required by the power and current transformers. 3.8.6.2 Restraint-Differential Element Figure 3.8.13 shows a schematic diagram of one of the percentage-differential elements with evenharmonic restraint (Element 1). Inputs to the differential element are the filtered, scaled, and compensated sets of samples corresponding to the fundamental component and second and fourth harmonics of the currents from each of the transformer windings. The magnitude of the sum of the fundamentalcomponent currents forms the operating current, IOP1, according to Equation 3.8.1. The scaled sum of the magnitudes of the fundamental-component currents forms the restraint current, IRT1, according to Equation 3.8.3, with k = 0.5. The magnitudes of the sums of the second-and fourth-harmonic currents represent the second- (I1F2) and fourth- (I1F4) harmonic restraint currents. Restraint current, IRT1, is scaled to form the restraint quantity IRT1 (SLP). Comparator 1 and switch S1 select the slope value as a function of the restraint current to provide a dual-slope percentage If the Comparator 1 Output Is True, Close S1 to 2 Otherwise, Close S1 to 1 S2 Closed if HRSTR=Y, Else Open FIGURE 3.8.13 Even-harmonic restraint, 87R1, and unrestraint, 87U1, differential elements. characteristic. Harmonic-restraint currents are scaled to form the second- and fourth-harmonic restraint quantities. The scaling factors 100/PCT2 and 100/PCT4 correspond to K 2 and K 4 , respectively (Equation 3.8.19). Comparator 4 compares the operating current to the sum of the fundamental and harmonic restraint quantities. The comparator asserts for fulfillment of Equation 3.8.19. Comparator 3 enables Comparator 4 if the operating current, IOP1, is greater than a threshold value, O87P. Assertion of Comparator 3 provides the relay minimum pickup current, I PU . Switch S2 permits enabling or disabling of evenharmonic restraint in the differential element. Comparators 5 and 6 compare the operating current to the second- and fourth-harmonic restraint quantities, respectively, to generate the second- and fourth-harmonic blocking signals. Comparison of the operating current with the fifth-harmonic restraint quantity (not shown) permits generation of the fifth-harmonic blocking signal (5HB1). The differential element includes an unrestrained, instantaneous differential overcurrent function. Comparator 2, which compares the operating current, IOP1, with a threshold value, U87P, provides the unrestrained differential overcurrent function. Figure 3.8.14 depicts the operating characteristic of the restraint-differential element. The characteristic can be set as either a single-slope, percentage-differential characteristic or as a dual-slope, variable percentage-differential characteristic. Figure 3.8.14 shows recommended setting values. 3.8.6.3 DC Filtering and Blocking Logic Figure 3.8.15 shows a schematic diagram of the dc blocking logic for Element 1. The positive, S + , and negative, S – , one-cycle sums of the differential current are formed. Then determine the minimum and © 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
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 and 182: current transformer having two prim
Page 183: 87R1 87BL1 (A) Independent Harmonic
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
Page 233 and 234: 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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