[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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Many of the present voltage-regulation technologies can also provide mitigation of other power-quality problems (e.g., voltage-sag ride-through improvement or isolation for transient overvoltages). There are seven basic devices in use today. These include: • Motor-actuated voltage regulator — Generally inexpensive, these devices can handle high kVA loads, but they are slow to respond to changes in the electric-service supply and can only correct for gradual load changes. See Section 2.7, Step-Voltage Regulators. • Saturable reactor regulator — Relatively inexpensive and with a wide load-kVA range, these devices have a sluggish (five to ten cycles) response time, high output impedance, and are sensitive to a lagging-load power factor. • Electronic tap-changing transformers — These devices use triacs or silicon-controlled rectifiers to change taps quickly on an autotransformer. They respond in 0.5 cycle and are insensitive to load power factor and voltage unbalances. • Automatic voltage regulator — These devices function as an uninterruptible power supply with no energy storage. They have a fast response time (1 to 2 ms), but the need for a fully rated 60- Hz transformer can make their cost unacceptably high. • Hybrid electronic voltage regulator — These devices use a series transformer and a power converter to accomplish the voltage-regulation function. • Soft-switching automatic voltage regulator — These devices combine the high performance of active line filters with the lower cost of the more-conventional solutions. The electromagnetic interference generated by these units is low in spite of the high-frequency switching employed. • Constant-voltage transformer (ferroresonant transformers) — Appropriate application of a CVT can handle most low-frequency disturbances, except for deep sags or outages. Detailed descriptions of these devices are provided in the subsequent sections. 2.8.1.5 What Constant-Voltage Transformers Can and Cannot Do CVTs are attractive power conditioners because they are relatively maintenance-free; they have no batteries to replace or moving parts to maintain. They are particularly applicable to providing voltage-sag protection to industrial process-control devices such as programmable logic controllers (PLC), motorstarter coils, and the electronic control circuits of adjustable-speed drives. Ongoing research [3] has demonstrated power-quality attributes of CVTs that include filtering voltage distortion and notched waveforms. Figure 2.8.7 depicts typical distorted and notched input voltages versus the filtered CVT output. Also, a CVT can practically eliminate oscillating transients caused by capacitor switching and can significantly dampen impulsive transients caused by lightning (see Figure 2.8.7). To ensure full protection of sensitive electronic loads, CVTs may need to be coupled with other devices designed to mitigate dynamic disturbances. In addition, CVTs have been used for years for voltage isolation as well. Many plants install CVTs for voltage regulation. CVTs also offer protection for voltage swells. If properly sized, a CVT can regulate its output voltage during input voltage sags to 60% of nominal voltage for virtually any duration (see Figure 2.8.8). However, many commercial and industrial facilities are not aware of most of the CVT’s attractive features. At the same time, the CVT technology also has some negative characteristics, which in some applications may possibly outweigh its benefits. Some of these include: • CVTs are not effective during momentary voltage interruptions or extremely deep voltage sags (generally below 50% of nominal). • Because CVTs have relatively high output impedance, they produce large output drops during high current demands. As a result, conventionally sized CVTs cannot handle significant changes in current and are more attractive for constant, low-power loads. • Because CVTs are physically large devices, it is not always practical to install this type of device in either a small-office or home environment. • CVTs produce heat and noticeable operating hum and are sensitive to line-frequency variations. • CVTs can have relatively poor efficiency and high losses for light loading conditions. 2.8.2 Applications Distorted Input Notched Input Input with Oscillating Transient Input with Impulsive Transient FIGURE 2.8.7 Filtering ability of a constant-voltage transformer. Ferro Output Ferro Output Ferro Output Ferro Output FIGURE 2.8.8 Voltage regulations with a constant-voltage transformer during a voltage sag to 60%. Constant-voltage transformers have proven to be a reliable means of enhancing voltage-sag tolerance of industrial process-control elements such as relays, contactors, solenoids, dc power supplies, programmable logic controllers (PLCs), and motor starters. As mentioned earlier, while the CVT tends to work well for voltage sags, they are not a good solution for momentary interruptions. Additionally, © 2004 by CRC Press LLC © 2004 by CRC Press LLC
TABLE 2.8.1 Typical Sizing Worksheet for a Constant-Voltage Transformer FIGURE 2.8.9 Typical performance of a constant-voltage transformer (CVT) as a function of load; as the CVT load increases, the ability of the CVT to regulate its output voltage decreases there are sizing and fault-protection issues associated with proper application of the CVT, particularly when the output loads demand high-inrush starting currents. Specific issues that will be addressed in the subsequent sections include: • Relationship between CVT input and output voltage under steady-state and dynamic supply conditions • Relationship between CVT sizing and load size to enhance sag tolerance of a given load • Impact of load inrush current on CVT sizing; output performance during various loading conditions • Effect of different vendor designs in enhancing voltage-sag ride-through; three-phase designs versus single-phase designs 2.8.2.1 Application Considerations — Sizing Guidelines Because the type of loads connected to a single CVT can range widely, the startup and steady-state operational characteristics of each load must be well understood before deciding on the appropriate power rating of a CVT. A load draws inrush current when it is first turned on or when it cycles on and off during normal process operation. If a CVT is sized [8] without considering the inrush currents of all connected loads, the CVT may be inadequately sized for the inrush current. Thus, during the startup or cycling of a connected load, the CVT output voltage may sag, causing other sensitive loads connected to the same CVT output voltage to shut down. The ability of a CVT to regulate its output voltage is generally based upon two characteristics of the connected loads, both of which are related to current and both of which must be determined to properly size a CVT. The first characteristic is the amount of steady-state current drawn by all connected loads during their normal operation. As shown in Figure 2.8.9, the lower the ratio between the actual current drawn by the connected loads and the rated current of the CVT, the better the CVT can regulate its output voltage during dynamic load-switching events. As an illustration, a 1-kVA CVT loaded to 1 kVA will not mitigate voltage sags nearly as well as the same CVT loaded to 500 VA, and performance is even better if the same 1-kVA CVT is only loaded to 250 VA. Moreover, according to results of CVT testing at the Electric Power Research Institute’s Power Electronics Application Center (known as EPRI PEAC), a CVT rated at less than 500 VA may not be able to handle even moderate inrush current. Therefore, a minimum CVT rating of 500 VA is recommended. The second characteristic of a CVT load is the load’s inrush current. Values for inrush current and steady-state current of the connected loads will enable a CVT to be properly sized. CVT Circuit or Load Measured Steady-State Current, A rms Measured Peak Inrush Current, A (1 msec) Programmable logic controller 0.16 14.8 Programmable logic controller 0.36 10.8 5-V, 12-V power supply 1.57 29.1 24-V power supply 1.29 14.4 NEMA size 3 motor starter 0.43 9.9 NEMA size 0 motor starter 0.13 3.1 Ice-cube relay 0.05 0.2 Master control relay 0.09 1.8 Computational Section Sum of steady-state rms currents 4.08 Circuit voltage 120 Steady-state load VA = 490 2.5 = 1225 Highest peak inrush current 29.1 Circuit voltage 120 Inrush load VA = 3492 0.5 = 1746 Note: Use the larger of the steady-state load VA (1225 in this example) and the inrush load VA (1746 in this example) to determine the CVT size. A procedure to find the proper CVT size is described below: 1. Measure or estimate the total steady-state current drawn by the load and multiply this value by the circuit voltage to get steady-state VA. For optimum regulation during input-voltage sags, the VA rating of the CVT should be at least 2.5 times the steady-state VA calculated. 2. Measure the highest peak inrush current and multiply this value by the circuit voltage to get the worst-case inrush VA for all loads. For good sag regulation of the CVT output voltage during load starting or cycling, the VA rating of the CVT should be at least half of the maximum inrush VA. Recommended size of the CVT is based upon the larger of the two VA-rating calculations. 3. Add together all the steady-state currents and then multiply the resulting value by the circuit voltage to get the combined steady-state VA of all CVT loads. Then, select the highest peak-inrushcurrent measurement and multiply this value by the circuit voltage to get the worst-case inrush VA for all loads. For optimum regulation during input-voltage sags, the VA rating of the CVT should be at least 2.5 times the steady-state VA. For example, if the steady-state VA calculation is 490 VA, then the recommended size of the CVT would be 1225 VA or more. For good sag regulation of the CVT output voltage during load starting or cycling, the VA rating of the CVT should be at least half of the maximum inrush VA calculated. For example, if the maximum inrush VA is 3.49 kVA, then the optimum size of the CVT would be 1.75 kVA or more. A typical sizing worksheet for CVTs with measured data and calculations is shown in Table 2.8.1. 2.8.2.2 Application Considerations — Output Performance Under Varying Supply Conditions A series of structured tests have been performed using a 1000-VA, 120-V, single-phase CVT. The following results of these tests provide insight on the operational characteristics of CVTs. These tests were designed to characterize the regulation performance of a constant-voltage transformer during amplitude and frequency variations in the ac input voltage and to determine its ability to filter voltage distortion and notching. The tests were performed at the EPRI PEAC power-quality test facility [9]. The CVT was energized for more than 30 min before each test to stabilize its temperature. 2.8.2.2.1 Performance: Regulation The CVT was tested for its ability to regulate variations in input voltage amplitude at both half- and fullload levels for the three load types given in Table 2.8.2. The mixed nonlinear load was a combination of purely resistive loads and a 187-VA nonlinear load with a 0.85 power factor, resulting in a composite © 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
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 and 90: TABLE 2.7.4 Increase in Ampere Load
Page 91 and 92: References IEEE, Standard Requireme
Page 93: 300% FIGURE 2.8.4 Schematic of a co
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
Page 141 and 142: FIGURE 3.3.10 LTC with delta connec
Page 143 and 144: 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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