[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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FIGURE 2.9.8 230-kV bus-tie reactors. A method to evaluate the merits of using either phase reactors or bus-tie reactors is presented in Section 2.9.3.2, Phase Reactors vs. Bus Tie Reactors. 2.9.2.2.3 Neutral-Grounding Reactors Neutral-grounding reactors (NGR) are used to control single line-to-ground faults only. They do not limit line-to-line fault-current levels. They are particularly useful at transmission-voltage levels, when autotransformers with a delta tertiary are employed. Figure 2.9.9 shows a typical neutral-grounding reactor. These transmission-station transformers can be a strong source of zero-sequence currents, and as a result, the ground-fault current may substantially exceed the 3-fault current. These devices, normally installed between the transformer or generator neutral and ground, are effective in controlling single line-to-ground faults, since the system short-circuit impedance generally is largely reactive. NGRs reduce short-circuit stresses on station transformers for the most prevalent type of fault in an electrical system. If the objective is to reduce the single line-to-ground (1) fault, then Equations 2.9.14 and 2.9.14a must be used and, after algebraic manipulations, the required neutral reactor impedance, in Ohms, is calculated as: X NGR = 3 V LL [(1/I SCA ) – (1/I SCB )]/3 (2.9.3) where, X NGR = reactance of the neutral grounding reactor, V LL = system line-to-line voltage kV I SCA = required single line-to-ground short circuit current after the installation of the neutral reactor, kA. I SCB = available single line-to-ground short curcuit current before installation of neutral reactor, kA FIGURE 2.9.9 Typical neutral-grounding-reactor connection. where the parameters are defined as before, with the exception that the short-circuit currents in question are the single line-to-ground fault currents expressed in units of kA. A factor to be taken into consideration when applying NGRs is that the resulting X 0 /X 1 may exceed a critical value (X 0 10X 1 ) and, as a result, give rise to transient overvoltages on the unfaulted phases. (For more information, see IEEE Std. 142-1991, Green Book.)[14] Because only one NGR is required per three-phase transformer and because their continuous current is the system-unbalance current, the cost of installing NGRs is lower than that for phase CLRs. Operating losses are also lower than for phase CLRs, and steady-state voltage regulation need not be considered with their application. The impedance rating of a neutral-grounding reactor can be calculated using Equation 2.9.1, provided that the short-circuit currents before and after the NGR installation are single line-to-ground faults. Although NGRs do not have any direct effect on line-to-line faults, they are of significant benefit, since most faults start from line-to-ground, some progressing quickly to a line-to-line fault if fault-side energy is high and the fault current is not interrupted in time. Therefore, the NGR can contribute indirectly to a reduction in the number of occurrences of line-to-line faults by reducing the energy available at the location of the line-to-ground fault. 2.9.2.2.3.1 Generator Neutral-Grounding Reactors — The positive-, negative-, and zero-sequence reactances of a generator are not equal, and as a result, when its neutral is solidly grounded, its line-to-ground short-circuit current is usually higher than the three-phase short-circuit current. However generators are usually required to withstand only the three-phase short-circuit current, [1] and a grounding reactor or a resistor should be employed to lower the single line-to-ground fault current to an acceptable limit. Other reasons for the installation of a neutral-grounding device are listed below: • A loaded generator can develop a third-harmonic voltage, and when the neutral is solidly grounded, the third-harmonic current can approach the generator rated current. Providing impedance in the grounding path can limit the third-harmonic current. • When the neutral of a generator is solidly grounded, an internal ground fault can produce large fault currents that can damage the laminated core, leading to a lengthy and costly repair procedure. The ratings of generator neutral-grounding reactors can be calculated as follows: 1. The reactance value required for limiting a single line-to-ground fault to the same value as a threephase fault current can be calculated by the following formula: X NGR X" X d 3 m0 (2.9.4) where X NGR = reactance of the neutral-grounding reactor, X d = direct axis subtransient reactance of the machine, X m0 = zero-sequence reactance of the machine, Equation 2.9.4 assumes that the negative-sequence reactance is equal to X d . In the absence of complete information, the value of reactance calculated using Equation 2.9.4 is satisfactory. Equations presented in chapter 19 of the Westinghouse <strong>Electric</strong>al Transmission and Distribution Reference Book [2] can also be used when the negative-sequence reactance of the generator is not equal to X d . 2. The short-circuit-current rating of the grounding reactor is equal to the three-phase generator short-circuit current when the machine is an isolated generator or operating in a unit system (Figure 2.9.10). When more than one generator is connected to a shared bus and not all of them are grounded by a reactor with a reactance calculated from Equation 2.9.4, the short-circuitcurrent rating of the reactor in question should be calculated by using proper system constants for a single line-to-ground fault at the terminal of the machine being grounded. Equations in © 2004 by CRC Press LLC © 2004 by CRC Press LLC
FIGURE 2.9.10 Two generator arrangements where a reactor can be used as a neutral-grounding device: unit system (left) and three-wire system (right). chapter 19 of the Westinghouse <strong>Electric</strong>al Transmission and Distribution Reference Book [2] can be employed for this case. 3. The rated short-circuit duration for the grounding reactor shall also be specified. When reactors are used for a single isolated generator or in a unit system, a 10-sec rating is usually employed. When grounding reactors are used in systems having feeders at generator voltage, a 1-min rating is usually employed to accommodate for repetitive feeder faults. 4. The rated continuous current of the grounding reactor should be specified considering the allowable unbalance current and third-harmonic current. In the absence of this information, 3% of the reactor short-circuit rating can be specified as the continuous-current rating when the duration of the rated short-circuit current is 10 sec. In cases when the rated short-circuit duration is 1 min, 7% of rated short-circuit current is recommended as the rated continuous current. See IEEE Std. 32-1972 for more information. [3] 5. Insulation class and associated BIL (basic impulse insulation level) rating should be specified based on (1) the reactor voltage drop during a single line-to-ground fault and (2) the nominal system voltage. Refer to Table 4 of IEEE Std. 32-1972. [3] Transient overvoltages are another important consideration in the application of generator neutralgrounding reactors. When the neutral of a generator is not solidly grounded, transient overvoltages can be expected. These overvoltages are usually caused by phase-to-ground arcing faults in air or by a switching operation followed by one or more restrikes in the breaker. When a grounding reactor is used for generator neutral grounding, the X 0 /X 1 ratio at the generator terminals shall be less than three to keep transient overvoltages within an acceptable level. (X 0 and X 1 are the resultant of the generator and system zero- and positive-sequence reactances, respectively.) When a neutral-reactor installation is intended only for reduction of a single line-to-ground fault to the three-phase fault level, X 0 /X 1 is equal to unity, which is a safe ratio in terms of imposed transient overvoltages. 2.9.2.2.4 Arc-Suppression Reactors (Petersen Coils) An arc-suppression coil is a single-phase, variable-inductance, oil-immersed, iron-core reactor that is connected between the neutral of a transformer and ground for the purpose of achieving a resonant neutral ground. The zero-sequence impedance of the transformer is taken into consideration in rating the inductance of the arc-suppression coil. The adjustment of inductance is achieved in steps by means of taps on the winding, or inductance can be continuously adjusted by varying the reluctance of the magnetic circuit. The length of an air gap is adjusted by means of a central moveable portion of the core (usually motor driven). See Figure 2.9.11. The inductance is adjusted, in particular during nongroundfault conditions, to achieve cancellation of the capacitive ground-fault current, so that in the case of a single line-to-ground fault, cancellation of the capacitive fault current is achieved with an inductive current of equal magnitude. Current injection by an active component (power converter) into the neutral, usually through an auxiliary winding of the arc-suppression coil, can also provide cancellation of the resistance component of the fault current. See Figure 2.9.12 and Figure 2.9.13, which illustrate this FIGURE 2.9.11 Arc-suppression reactor. FIGURE 2.9.12 Single line-to-ground fault in nongrounded system. principle for nongrounded and resonant-grounded systems, respectively. Resonant grounding is used in distribution systems in Europe, parts of Asia and in a few areas of the U.S. The type of system ground employed is a complex function of system design, safety considerations, contingency (fault) operating practices, and legislation. Arc-suppression reactors are typically used to the best advantage on distribution systems with overhead lines to reduce the intermittent arcing-type single line-to-ground faults that may otherwise occur on ungrounded systems. © 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
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
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: FIGURE 2.9.5 Typical phase-reactor
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 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
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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
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FIGURE 3.6.10 Standard switching-im
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voltage of the excited winding, rea
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FIGURE 3.6.18 Current, voltage, and
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TABLE 3.6.3 Transformer Short-Circu
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FIGURE 3.7.3 Control for voltage re
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FIGURE 3.7.6A Feeder with power-fac
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3. Circulating current (current bal
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FIGURE 3.7.10 Control block diagram
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Primary Current (Amps) 60 40 20 0 -
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current transformer having two prim
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87R1 87BL1 (A) Independent Harmonic
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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
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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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