[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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Substation A Substation B FIGURE 3.7.7A Step-voltage regulator in routine application. Substation A FIGURE 3.7.7B Step-voltage regulator in reverse power-flow application. 52A 52B 52A 52B Open L A1 L B1 L A1 L B1 or often up to three different percentages, using different taps on the fooler transformer. Having the sensed voltage boosted by, say, 5% without changing the voltage set point of the control will cause the control to run the tap position down by 5% voltage, thus accomplishing the desired voltage reduction. The percentage of three reduction steps, most commonly 2.5, 5.0, and 7.5%, is preestablished by the design of the fooler transformer. Digital controls do the same function much more conveniently, more accurately, and more quickly. The voltage reduction applicable to steps 1, 2, and 3 are individually programmed and, upon implementation, effectively lower the voltage set point. Control based on the new set point is implemented without intentional time delay, reverting to panel time delay after the voltage reduction has been implemented. Most often, controls provide for three steps of voltage reduction, each step individually programmed up to 10.0%. 3.7.6.3 Reverse Power Flow Voltage regulators as used on distribution feeders are sometimes subjected to reverse power flow due to system switching. A common practice is to install the distribution system per the illustration of Figure 3.7.7. Normally, step-voltage regulators A and B both usually “see” forward power flow, as in Figure 3.7.7a. Line switching, such as might be necessary due to service required in Substation B, causes Feeder B to be served from Substation A and the Regulator B to operate in reverse power flow mode, as in Figure 3.7.7b. Were special precautions not taken, Regulator B would operate incorrectly, inevitably running to a “raise” or “lower” tap limit. This would have occurred because the regulator would continue to attempt to operate based on knowledge of the voltage at its load terminal, but now that terminal is, in fact, the source. Tap-changer operation does nothing to correct that voltage, but it severely disturbs the voltage to loads L B1 between Regulator B and Substation B. The remedy is to operate Regulator B based on the voltage at its usual source (now the load) terminal. Analog controls have been provided to automatically switch between the load and source terminals upon FPF V S V L Reg A V S FPF V L Reg B FPF V S V S Reg A RPF Reg B V L V L L A2 L B2 L A2 L B2 recognition of reverse power flow, but the expense of doing so, involving in most cases the addition of a new VT, seldom justified the avoidance of simply manually switching off automatic operation during the planned power-reversal event. Digital controls accomplish this feature automatically with no additional hardware. These controls calculate the voltage at the normal source terminal using knowledge of the measured load terminal voltage, the recognized tap position plus, in some cases, an approximation of the internal regulation of the regulator. Accomplishing voltage regulation with Regulator B during reverse power-flow operation in this manner adds only about 0.5% to the error of the control vis a vis having a supplemental VT for the application. The apparatus and procedures defined above for step-voltage regulators are not correct for most transformer applications where reverse power flow can occur. The basic difference is that the feederregulator application remains a radial system after the line switching is complete. Reverse power in transformers is more likely to occur on a system where the reverse power is due to a remote generator that is operating continuously in parallel with the utility. The proper operation of the LTC in this case must be evaluated for the system. Perhaps the preferred operation would be to control the LTC so as to minimize the var exchange between the systems. Some systems are simply operated with the LTC control turned off of automatic operation during reverse power flow so that the LTC says fixed on position until forward power flow resumes. 3.7.7 Introduction to Control for Parallel Operation of LTC Transformers and Step-Voltage Regulators For a variety of reasons, it may be desirable to operate LTC transformers or regulators in parallel with each other. This may be done simply to add additional load-handling capability to an existing overloaded transformer, or it may be by initial design to afford additional system reliability, anticipating that there may be a failure of one transformer. Most common paralleling schemes have the end objective of having the load tap changers operate on the same, or on nearly the same, tap position at all times. For more-complex schemes, this may not be the objective. A knowledge of the system is required to assess the merits of the various techniques. 3.7.7.1 The Need for Special Control Considerations To understand why special control consideration needs to be given to paralleling, consider two LTC transformers operating in parallel, i.e., the primary and secondary of the transformers are bused together as in Figure 3.7.8. If the transformers are identical, they will evenly divide the load between them at all times while they are operating on the same tap position. Consider that the voltage at the secondary bus goes out of band. Even if the controls are set the same, they are never truly identical, and one will command tap-change operation before the other. Later, when the voltage again changes so as to require a second voltage correction, the same control that operated more quickly the first time would be expected to do so again. This can continue indefinitely, with one LTC doing all of the operation. As the tap positions of the LTC transformers digress, the current that circulates in the substation increases. This current simply circulates around the loop formed by the buses and the two transformers doing no useful work, but it causes an increase in losses, perhaps causing one or both of the transformers to overheat. To put the matter in perspective, consider the case of a distribution substation where there are two 5/8%-step, 15- MVA LTC transformers of 8.7% impedance in parallel. The secondary voltage is 12.47 kV. For a tap difference of only one step, a circulating current of 25 A flows in the substation LV bus. This current magnitude increases linearly with tap-position difference. In the illustration above for LTC transformers, the circulating current was limited by the impedances of the transformers. It is very important to recognize that the same procedure cannot be done with stepvoltage regulators, as the impedance of a regulator is very low at even the extreme tap positions, and it may be essentially zero at the neutral tap position. In this case, if one regulator is on neutral and the other moves to position 1R, the circulating current will be expected to be sufficiently large to cause catastrophic failure of the regulators. The user is cautioned: Step-voltage regulators can only be operated in parallel when there is adequate supplemental impedance included in the current loop. This is most © 2004 by CRC Press LLC © 2004 by CRC Press LLC
3. Circulating current (current balance): The most common method in use in the U.S. today; about 90% of new installations use this procedure. It has been implemented with technical variations by several sources. Advantages: Generally reliable operation for any reasonable number of paralleled transformers. Uses the same CT as that provided for LDC, but it operates independently of the line-drop compensation. Disadvantages: Control circuits can be confusing, and they must be accurate as to instrument transformer polarities, etc. Proper operation is predicated on the system being such that any significant difference in CT currents must be due only to circulating current. Matched transformers will at times operate unbalanced, i.e., at differing tap steps under normal conditions. FIGURE 3.7.8 LTC transformers in parallel with no interconnections. often the impedance of the transformers that are in series with each regulator bank, or it could be a series current-limiting reactor. 3.7.7.2 Instrument Transformer Considerations Refer again to Figure 3.7.2. Most techniques for paralleling use the voltage and current signals derived for line-drop compensation. It is essential that the paralleled transformers deliver voltage and current signals that are in phase with each other when the system has no circulating current. This means that the instrument transformers must deliver signals from equivalent phases. Further, the ratios of the instrument transformers must be the same, except under very special conditions that dictate otherwise. 3.7.8 Defined Paralleling Procedures Numerous procedures have been identified over the years to accomplish LTC transformer paralleling using electronic control. These are listed with some limited description along with alternative names sometimes used: 1. Negative reactance (reverse reactance): Seldom used today except in some network applications, this is one of the oldest procedures accomplished by other than mechanical means. This means of paralleling is the reason LTC controls are required by the standard to provide negative X capability on the line-drop compensation. Advantages: Simplicity of installation. The system requires no apparatus other than the basic control, with LDC X set as a negative value. There is no control interconnection wiring, so transformers can be distant from each other. Disadvantages: Operation is with a usually high –X LDC set point, meaning that the bus voltage will be lowered as the load increases. This can be sufficiently compensated in particular cases with the additional use of +R LDC settings. 2. Cross-connected current transformers: Unknown in practice today, the system operates on precepts similar to the negative reactance method. The LDC circuits of two controls are fed from the line CT of the opposite transformer. Advantages: System requires no apparatus other than the basic control, but it does require CT circuits to pass between the transformers. Disadvantages: System may need to be operated with a value of +X LDC much higher than that desired for LDC purposes, thereby boosting the voltage too much. The system can be used on two transformers, only. 4. Master/follower (master/slave, electrical interlock, lock-in-step): Used by most of the 10% of new installations in the U.S. that do not use circulating current. It is used much more commonly elsewhere in the world than in the U.S. Advantages: Matched transformers will always be balanced, resulting in minimal system losses. Disadvantages: As usually implemented, involves numerous auxiliary relays that may fail, locking out the system. 5. Reactive current balance (delta var): Generally used only when special system circumstances require it. Operates so as to balance the reactive current in the transformers. Advantages: Can be made to parallel transformers in many more-complex systems where other methods do not work. Disadvantages: May be more expensive than more common means. The two most common paralleling procedures are master/follower and circulating-current minimization. The negative-reactance method also deserves summary discussion. 3.7.8.1 Master/Follower The master/follower method operates on the simple premise: Designate one control as the master; all other units are followers. Only the master needs to know the voltage and need for a tap change. Upon recognition of such a need, the master so commands a tap change of the LTC on the first transformer. Tap-changer action of #1 activates contacts and relays that make the circuit for LTC following action in all of the followers, temporarily locking out further action by the master. The LTC action of the followers in turn activates additional contacts and relays, thus freeing the master to make a subsequent tap change when required. 3.7.8.2 Circulating-Current Method All of the common analog implementations of the circulating-current method provide an electronic means (a “paralleling balancer”) of extracting the load current and the circulating current from the total transformer current. These currents are then used for their own purposes, the load current serving as the basis for line-drop compensation and the circulating current serving to bias the control to favor the next LTC action that will tend to keep the circulating current to a minimum. Consider Figure 3.7.9. The balancers receive the scaled transformer current and divide it into the components of load current and circulating current. The load-current portion of the transformer current is that required for line-drop compensation. The circulating-current portion is essentially totally reactive and is the same in magnitude in the two controls, but of opposite polarity. Presuming a lagging powerfactor load, the control monitoring the transformer on the higher tap position will “see” a more lagging current, while that on the lower tap position sees a less lagging, or perhaps leading, current. This circulating current is injected into the controls. The polarity difference serves to bias them differently. The next LTC to operate is the one that will correct the voltage while tending to reduce the circulating current, i.e., the one that brings the tap positions into closer relation to each other. © 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
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 and 170: TABLE 3.6.3 Transformer Short-Circu
Page 171 and 172: FIGURE 3.7.3 Control for voltage re
Page 173: FIGURE 3.7.6A Feeder with power-fac
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
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
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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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