[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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substation enclosing a transformer or step-voltage regulators. The implementations illustrated accomplish bus-voltage regulation on a three-phase (Figure 3.7.1a) or single-phase (Figure 3.7.1b) basis. Another common application is to use voltage regulators on the distribution feeders. A principal argument for the use of single-phase regulators is that the voltages of the three phases are controlled independently, whereas a three-phase transformer or regulator controls the voltage of all phases based on knowledge of the voltage of only one phase. For Figure 3.7.1: S = source, the utility network at transmission or subtransmission voltage, usually 69 kV or greater Z = impedance of the source as “seen” looking back from the substation L = loads, distributed on the feeders, most often at 15 kV to 34.5 kV The dominance of load-tap-changing apparatus involves either 33 voltage steps of 5/8% voltage change per step, or 17 voltage steps of 1 1 / 4% voltage change per step. In either case, the range of voltage regulation is 10%. FIGURE 3.7.1A Three-phase bus regulation with three-phase LTC transformer. FIGURE 3.7.1B Independent-phase bus regulation with three single-phase step-voltage regulators. shunt capacitors can also be used for this purpose, but in the context of this discussion, shunt capacitors are presumed to be applied with the objective of improving the system power factor. The control of a tap changer is much more involved than simply responding to a voltage excursion at the transformer secondary. Modern digital versions of LTC control include so many ancillary functions and calculated parameters that it is often used with its communications capability to serve as the means for system-condition monitoring. The control of the tap changer in a transformer or step-voltage regulator is essentially the same. Unless stated otherwise, the use of the term “transformer” in this section applies equally to step-voltage regulators. It should be recognized that either type of product can be constructed as single-phase or threephase apparatus, but that transformers are more often three phase, while step-voltage regulators are more commonly single phase. 3.7.2 System Perspective, Single <strong>Transformer</strong> This discussion is patterned to a typical utility distribution system, the substation and the feeder, although much of the material is also applicable to transmission applications. This first discussion of the control considers that the control operates only one LTC in isolation, i.e., there is not the opportunity for routine operation of transformers in parallel. The system can be configured in any of several ways according to the preference of the user. Figure 3.7.1 depicts two common implementations. In the illustrations of Figure 3.7.1, the dashed-line box depicts a 3.7.3 Control Inputs 3.7.3.1 Voltage Input The voltage of the primary system is unimportant to the LTC control. The system will always include a voltage transformer (VT) or other means to drop the system voltage to a nominal 120 V for use by the control. Because of this, the control is calibrated in terms of 120 V, and it is common to speak of the voltage being, say, 118 V, or 124 V, it being understood that the true system voltage is the value stated times the VT ratio. Presuming a single tap step change represents 5/8% voltage, it is easily seen that a single tap step change will result in 0.00625 per unit (p.u.) 120 V = 0.75-V change at the control. The control receives only a single 120-V signal from the voltage transformer, tracking the line-ground voltage of one phase or a line-line voltage of two phases. For the case of three-phase apparatus, the user must take great care, as later described, in selecting the phase(s) for connecting the VT. 3.7.3.2 Current Input The current transformers (CT) are provided by the transformer manufacturer to deliver control current of “not less than 0.15 A and not more than 0.20 A … when the transformer is operating at the maximum continuous current for which it is designed.” (This per ANSI Std. C57.12.10, where the nominal current is 0.2 A. Other systems may be based on a different nominal current, such as 5.0 A.) As with the voltage, the control receives only one current signal, but it can be that of one phase or the cross connection (the paralleling of two CTs) of two phases. 3.7.3.3 Phasing of Voltage and Current Inputs In order for the control to perform all of its functions properly, it is essential that the voltage and current input signals be in phase for a unity power factor load or, if not, that appropriate recognition and corrective action be made for the expected phasing error. Figure 3.7.2 depicts the three possible CT and VT orientations for three-phase apparatus. Note that for each of the schemes, the instrument transformers could be consistently shifted to different phases from that illustrated without changing the objective. The first scheme, involving only a line-to-groundrated VT and a single CT, is clearly the simplest and least expensive. However, it causes all control action to be taken solely on the basis of knowledge of conditions of one phase. Some may prefer the second scheme, as it gives reference to all three phases, one for voltage and the two not used for voltage to current. The third scheme is often found with a delta-connected transformer secondary. Note that the current signal derived in this case is 3 times the magnitude of the individual CT secondary currents. This must be scaled before the signal is delivered to the control. The user must assure the proper placement of the VT(s) to be consistent with the CT(s) provided inside the transformer. This is not a concern with step-voltage regulators, where all instrument transformers are provided (per ANSI/IEEE C57.15) internal to the product. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
FIGURE 3.7.3 Control for voltage regulation of the bus. FIGURE 3.7.2 Phasing of voltage and current inputs. 3.7.4 The Need for Voltage Regulation Referring to the circuits of Figure 3.7.1, note that system conditions will change over time, with the result that the voltage at the substation bus, and as delivered to the load, will change. From Figure 3.7.1, the source voltage, the source impedance, and the load conditions will be expected to change with time. The most notable of these, the load, must be recognized to consist of two factors — the magnitude and the power factor, or what is the same point, the real (watt) and reactive (var) components. 3.7.4.1 Regulation of the Voltage at the Bus Many times, the object of the LTC is simply to hold the substation bus voltage at the desired level. If this is the sole objective, it is sufficient to bring only a VT signal that is representative of the bus voltage into the control. The secondary of the VT is usually 120 V at the nominal bus voltage, but other VT secondary voltages, especially 125 V, 115 V and 110 V, are used. Figure 3.7.3 shows the circuit. The figure shows a motor (M) on the LTC, which is driven in either the raise or lower direction by the appropriate output (M R or M L ) of the control. This first control, Figure 3.7.3, is provided with only three settings, these being those required for the objective of regulating the substation secondary-bus voltage: 3.7.4.1.1 Voltage Set Point The voltage set point on the control voltage base, e.g., 120 V, is the voltage desired to be held at the load. (The load location for this first case is the substation secondary bus because line-drop compensation is not yet considered and is therefore zero.) This characteristic is also commonly referred to as “voltage band center,” this being illustrative of the point that there is a band of acceptable voltage and that this is the midpoint of that band. If line-drop compensation (LDC, as detailed later) is not used, the set point will often be somewhat higher than 120 V, perhaps 123 V to 125 V; with use of LDC the setting will be lower, perhaps 118 V. 3.7.4.1.2 Bandwidth The bandwidth describes the voltage range, or band, that is considered acceptable, i.e., in which there is not a need for any LTC corrective action. The bandwidth is defined in the ANSI/IEEE C57.15 standard as a voltage with one-half of the value above and one-half below the voltage set point. Some other controls adjust the bandwidth as a percentage of the voltage set point, and the value represents the band on each side of the band center. The bandwidth voltage selected is basically determined by the LTC voltage change per step. Consider a transformer where the voltage change per step is nominally 0.75 V (5/8% of 120 V). Often this is only the average; the actual voltage change per step can differ appreciably at different steps. Clearly, the bandwidth must be somewhat greater than the maximum step-change voltage, because if the bandwidth were less than the voltage change per step, the voltage could pass fully across the band with a single step, causing a severe hunting condition. The minimum suggested bandwidth setting is twice the nominal step-change voltage, plus 0.5 V, for a 2.0-V minimum setting for the most common 5/8% systems. Many users choose somewhat higher bandwidths when the voltage is not critical and there is a desire to reduce the number of daily tap changes. © 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
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 and 168: FIGURE 3.6.18 Current, voltage, and
Page 169: TABLE 3.6.3 Transformer Short-Circu
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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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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