[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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FIGURE 2.7.28 Voltage profile using voltage-level setting without LDC. 2.7.7.4 Line-Drop Resistive and Reactive Compensation Quite often regulators are installed some distance from a theoretical load center or the location at which the voltage is regulated. This means the load will not be served at the desired voltage level due to the losses (voltage drop) on the line between the regulator and the load. Furthermore, as the load increases, line losses also increase, causing the lowest-voltage condition to occur during the time of heaviest loading. This is the least desirable time for this to occur. To provide the regulator with the capability to regulate at a “projected” load center, a line-dropcompensation feature is incorporated in the control. Of all the devices in the step-voltage regulator control system, none is more important — and at the same time less understood — than line-drop compensation. This circuitry consists of a secondary supply from the internal current transformer, proportional to the load current, and resistive and reactive components through which this current flows. As the load current increases, the resulting secondary current flowing through these elements produces voltage drops, which simulate the voltage drops on the primary line. This causes the “sensed” voltage to be correspondingly altered; therefore, the control responds by operating upon this pseudo load-center voltage. To select the proper resistive and reactive values, the user must take into account several factors about the line being regulated. When line-drop compensation is not used, the step-voltage regulator reads the voltage at its own terminals and compares it with a reference voltage level and bandwidth setting. If the sensed voltage is outside the set bandwidth, the regulator automatically responds to bring the load-side voltage in line with the programmed settings. This is the normal operation of a step-voltage regulator that is not using line-drop compensation. Adjusting the voltage level at the regulator to compensate for a low voltage away from the regulator near a load center is only a temporary solution for a specific load. Figure 2.7.28 shows an example of a power-system voltage profile using the voltage-level setting in lieu of line-drop compensation to improve voltage at a load center. Therefore, the step-voltage regulator, without line-drop compensation, can hold a reasonably constant voltage at the regulator location only. This is not likely to be the best scenario for most applications that need a reliable voltage for the entire length of the feeder. The ideal situation is to provide the value of the voltage-level setting at the primary side of the distribution transformer for all consumers. Since this is not realistically attainable, the objective would be to provide each consumer a voltage as close as possible to the voltage-level value for all loads. To do this, the point on the line at which the voltage is regulated should not be at the regulator but at an area at the center of the majority of the consumers, the load center. It is not always possible to locate a regulator at the exact location where the regulation is most needed. Also, the system changes as loads are added and removed over time, and the desired point of FIGURE 2.7.29 Voltage profile using voltage-level setting with LDC. regulation may change. However, it may not be feasible to relocate the regulator bank. By having linedrop-compensation devices, the regulation point can be changed without having to physically move a bank of regulators. In any mode, the regulator monitors a specific voltage and changes taps as needed based on that voltage level and the existing settings. With line-drop compensation, this monitored voltage can be modified in such a way as to simulate the voltage at some point further out on the distribution system. Knowing the peak-load current expected on the line and the size and length of the line to the load center, the voltage drop at the load center due to resistive and reactive components can be calculated. Inside the regulator control, the line-drop-compensation device will model that portion of the system between the load center and the step-voltage regulator. Figure 2.7.29 shows an example of a power-system voltage profile resulting from a regulator set up with the line-drop-compensation feature. When line-drop compensation is used, the correct polarity of the resistance and reactance components is necessary for proper regulation. On four-wire wye-connected systems, the polarity selector is always set for +X and +R values. On delta-connected systems, however, the line current is 30 displaced from the line-to-line voltage (assuming 100% power factor). On open-delta-connected regulator banks, one regulator is 30 leading, the other is 30 lagging. On closed-delta regulator banks, all regulators in a given bank will be either leading or lagging. As a result of this displacement, the polarity of the appropriate resistive or reactive element must sometimes be reversed. The setting of the selector switch would be set on the +X+R, –X+R, or +X–R setting. A number of publications are available from manufacturers to assist in determining the variables needed and the resulting calculations. 2.7.8 Unique Applications Most step-voltage regulators are installed in circuits with a well-defined power flow from source to load. However, some circuits have interconnections or loops in which the direction of power flow through the regulator may change. For optimum utility system performance, step-voltage regulators, installed on such circuits, have the capability of detecting this reverse power flow and then sensing and controlling the load-side voltage of the regulator, regardless of the direction of power flow. In other systems, increasing levels of embedded (dispersed) generation pose new challenges to utilities in their use of step-voltage regulators. Traditionally, distribution networks have been used purely to transport energy from the transmission system down to lower voltage levels. A generator delivering electricity directly to the distribution network can reverse the normal direction of power flow in a regulator. Options in the electronic control of the step-voltage regulator are available for handling different types of scenarios that give rise to reverse power-flow conditions. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
References IEEE, Standard Requirements, Terminology, and Test Code for Step-Voltage Regulators, C57.15-1999, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1999. ANSI/IEEE, IEEE Guide for Loading Liquid-Immersed Step-Voltage and Induction-Voltage Regulators, C57.95-1987, Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1987. Colopy, C.A., Grimes, S., and Foster, J.D., Proper Operation of Step Voltage Regulators in the Presence of Embedded Generation, presented at CIRED conference, Nice, France, June 1999. Cooper Power Systems, How Step Voltage Regulators Operate, Bulletin 77006 2/93, Waukesha, WI, 1993. Cooper Power Systems, Voltage regulator instruction and operating manuals and product catalogs, Waukesha, WI. Cooper Power Systems, Distribution Voltage Regulation and Voltage Regulator Service Workshops, Waukesha, WI. Day, T.R. and Down, D.A., The Effects of Open-Delta Line Regulation on Sensitive Earth Fault Protection of 3-Wire Medium Voltage Distribution Systems, Cooper Power Systems, Waukesha, WI, June 1995. Foster, J.D., Bishop, M.T., and Down, D.A., The Application of Single-Phase Voltage Regulators on Three-Phase Distribution Systems, Paper 94 C2, presented at IEEE Conference, Colorado Springs, CO, April 1994. General Electric Co., Voltage regulator instruction and operating manuals and product catalogs, Shreveport, LA. Kojovic, L.A., Impact of DG on Voltage Regulation, presented at IEEE/PES summer meeting, Chicago, IL, 2002. Kojovic, L.A., McCall, J.C., and Colopy, C.A., Voltage Regulator and Capacitor Application Considerations in Distribution Systems for Voltage Improvements, Systems Engineering Reference Bulletin SE9701, Franksville, WI, January 1997. Siemens Energy and Automation, Voltage regulator instruction and operating manuals and product catalogs, Jackson, MS. 2.8 Constant-Voltage Transformers Arindam Maitra, Anish Gaikwad, Ralph Ferraro, Douglas Dorr, and Arshad Mansoor 2.8.1 Background Constant-voltage transformers (CVT) have been used for many years, primarily as a noise-isolation device. Recently, they have found value when applied as a voltage-sag protection device for industrial and commercial facilities. The purpose of this section in the handbook is to give power-system engineers and facility engineers who are unfamiliar with the CVT technology (also known as ferroresonant transformers) the insights and information necessary to determine the types of electric-service-supply events that CVTs can mitigate. Items covered in this chapter include operation, characteristics, applications, specifications, and sizing guidelines of CVTs. The goal here is not to duplicate information currently available but, rather, to collect information into a single location and then supplement it to provide: • Adequate information and procedures to applications personnel in effectively selecting CVTs for voltage-sag ride-through protection • Application notes to demonstrate how CVTs can improve process voltage-sag ride-through 2.8.1.1 History of Constant-Voltage Transformers The industrial use of constant-voltage transformers (also called CVTs and ferroresonant transformers) goes back to the early 1940s. Living in the U.S. during the 1930s, Joseph G. Sola, a German-born engineer, discovered the CVT technology [1,2] based on a single transformer rather than an arrangement of FIGURE 2.8.1 Typical constant-voltage transformer. transformers, separate filters, and capacitors. This innovation provides several important advantages: its inherent robustness (CVT consists of just three or four windings and a high-reliability capacitor), its imperviousness to continuous short circuits (whether it is turned on into a short circuit or from full load), and its capability to maintain output-voltage stabilization on a cycle-to-cycle basis for significantly large overvoltages and undervoltages. Sola was both a farsighted inventor and successful businessman. Internationally recognized as the pioneer of transformer magnetics technology, his inventions and subsequent refinements of other electronic equipment were considered revolutionary by the electrical industry. Sola’s first transformers for furnace-ignition systems and neon lighting, based on the unique application of ferroresonant principles, led in 1938 to his invention of the CVT. This timely discovery was eagerly accepted by prime military contractors during World War II and established Sola as a world leader in voltage regulation. Sola was awarded a total of 55 U.S. patents during his lifetime, including five patents each for CVTs, Solatrons, and electronic power supplies, and 19 patents for high-intensity-discharge lighting ballasts. Throughout the last six decades, a series of applications has been found for products based on Sola’s CVT technology. It is one of the most cost-effective ac power conditioners available. 2.8.1.2 What Is a Constant-Voltage Transformer? A well-known solution for electrical “noise” in industrial plants has been the constant-voltage transformer, or CVT (see Figure 2.8.1). The typical components of a CVT are shown in Figure 2.8.2. The magnetic shunt on the central core has the following effects on the core’s reluctance. It reduces the reluctance of the core. This can be thought of as introducing more resistance in parallel to an existing resistance. The magnetic shunt in the CVT design allows the portion of the core below the magnetic shunt to become saturated while the upper portion of the core remains unsaturated. This condition occurs because of the presence of the air-gap between the magnetic shunt and the core limbs. Air has a much higher reluctance than the iron core. Therefore, most of the flux passes through the lower portion of the core, as shown by the thick lines in Figure 2.8.2. In terms of an electrical analogy, this configuration can be thought of as two resistances of unequal values in parallel. The smaller resistance carries the larger current, and the larger resistance carries the smaller current. The CVT is designed such that: • The lower portion of the central limb is saturated under normal operating conditions, and the secondary and the resonating windings operate in the nonlinear portion of the flux-current curve. • Because of saturation in the central limb, the voltage in the secondary winding is not linearly related to the voltage in the primary winding. © 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
Page 39 and 40: voltage in each winding simultaneou
Page 41 and 42: mounted transformers can have arres
Page 43 and 44: V S + V S I V L Z=R+jX a) V L V S V
Page 45 and 46: Z = jX (2.3.19) 0.7 Then the curren
Page 47 and 48: X1 L* X1 S =X1 L X2 L* X3 L* a) S L
Page 49 and 50: 150 V(%) 100 50 0 -50 40 80 T(s) 12
Page 51 and 52: 2.3.8.1.2 Series Connection • The
Page 53 and 54: 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: TABLE 2.7.4 Increase in Ampere Load
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 and 106: FIGURE 2.9.5 Typical phase-reactor
Page 107 and 108: FIGURE 2.9.10 Two generator arrange
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
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FIGURE 3.3.10 LTC with delta connec
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3.3.5 Selection of Load Tap Changer
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FIGURE 3.3.19 Effect of the leakage
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References Goosen, P.V. , Transform
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If i 1 is the full-load current rat
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TABLE 3.4.1 Loading Recommendations
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3.5.4 Three-Phase Transformer Conne
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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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