[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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TABLE 2.6.7 Materials/Construction for Low- and Medium-Voltage Classes Class, kV Indoor Applications Materials/Construction Outdoor Applications Materials/Construction 0.6 Tape, varnished, plastic, cast, or potted Cast or potted 1.2 - 5.0 Plastic, cast Cast 8.7 Cast Cast 15.0 Cast Cast or tank/oil/porcelain 25.0 Cast Cast or tank/oil/porcelain 34.5 Cast Cast or tank/oil/porcelain 46 Not commonly offered Cast or tank/oil/porcelain 69 Not commonly offered Cast or tank/oil/porcelain Note: the term cast can imply any polymeric material, e.g., butyl rubber, epoxy, urethane, etc. Potted implies that the unit is embedded in a metallic housing with a casting material. Environmental requirements will help define the insulation medium. In indoor applications, the instrument transformer is protected from external weather elements. In outdoor installations, the transformer must endure all weather conditions from extremely low temperatures to severe UV radiation and be impervious to moisture penetration. The outer protection can range from fabric or polyester tape, varnish treatment, or thermoplastic housings to molding compounds, porcelain, or metal enclosures. Table 2.6.7 identifies, by voltage rating, the commonly used materials and construction types. All installations above 69 kV are typically for outdoor service and are of the tank/oil/SF 6 /porcelain construction type. 2.6.2.7 Thermal Ratings An important part of the insulation system is the temperature class. For instrument transformers, only three classes are generally defined in the standard, and these are listed in Table 2.6.8A. This rating is coordinated with the maximum continuous current flow allowable in the instrument transformer that will limit the winding heat rise accordingly. Of course, other classes can be used to fit the application, especially if the instrument transformer is part of an apparatus that has a higher temperature class, e.g., when used under hot transformer oil or within switchgear, bus compartments, and underground network devices, where ambient temperatures can be 65 to 105C. In these cases, a modest temperature rise can change the insulation-system rating. These apply to the instrument transformer under the most extreme continuous conditions for which it is rated. The insulation system used must be coordinated within its designated temperature class (Table 2.6.8B). It is not uncommon for users to specify a higher insulation system even though the unit will never operate at that level. This may offer a more robust unit at a higher price than normally required, but can also provide peace of mind. 2.6.2.8 Primary Winding The primary winding is subjected to the same dynamic and thermal stresses as the rest of the primary system when large short-circuit currents and voltage transients are present. It must be sized to safely carry the maximum continuous current without exceeding the insulation system’s temperature class. 2.6.3 Voltage <strong>Transformer</strong> The voltage transformer (VT) is connected in parallel with the circuit to be monitored. It operates under the same principles as power transformers, the significant differences being power capability, size, operating flux levels, and compensation. VTs are not typically used to supply raw power; however, they do have limited power ratings. They can often be used to supply temporary 120-V service for light-duty maintenance purposes where supply voltage normally would not otherwise be available. In switchgear compartments, they may be used to drive motors that open and close circuit breakers. In voltage regulators, they may power a tap-changing drive motor. The power ranges are from 500 VA and less for low-voltage VT, 1–3 kVA for medium-voltage VT, and 3–5 kVA for high-voltage VT. Since they have such low power ratings, their physical size is much smaller. The performance characteristics of the VT are based on standard burdens and power factors, which are not always the same as the actual connected burden. It is possible to predict, graphically, the anticipated performance when given at least two reference points. Manufacturers typically provide this data with each VT produced. From that, one can construct what is often referred to as the VT circle diagram, or fan curve, shown in Figure 2.6.10. Knowing the ratio-error and phase-error coordinates, and the values of standard burdens, the graph can be produced to scale in terms of VA and power factor. Other power-factor lines can be inserted to pinpoint actual circuit conditions. Performance can also be calculated using the same phasor concept by the following relationships, provided that the value of the unknown burden is less than the known burden. Two coordinates must be known: at zero and at one other standard burden value. TABLE 2.6.8A Temperature Class (IEEE C57.13) 30C Ambient 55C Ambient Temperature Class Temperature Rise Hot-Spot Temperature Rise Temperature Rise 105C 55C 65C 30C 120C 65C 80C 40C 150C 80C 110C 55C TABLE 2.6.8B Temperature Class (General) Temperature Class Class 90 (O) Class 105 (A) Class 130 (B) Class 155 (F) Class 180 (H) Class 220 (C) Hot-Spot Temperature Rise @ 30C Ambient (40C Maximum) 50C 65C 90C 115C 140C 180C FIGURE 2.6.10 Voltage transformer circle diagram (fan curves). © 2004 by CRC Press LLC © 2004 by CRC Press LLC
RCF x Bx Bt RCF t – RCF 0 cos t – x t – 0 sin t – x Bx x t – 0 cos t – x - RCF t – RCF 0 sin t – x Bt where RCF x = RCF of new burden RCF t = RCF of known burden RCF 0 = RCF at zero burden x = phase error of new burden, radians (to obtain x in minutes, multiply value from Equation 2.6.7 by 3438) t = phase error of known burden, radians 0 = phase error at zero burden, radians B x = new burden B t = known burden x = new burden PF angle, radians t = known burden PF angle, radians (2.6.9) (2.6.10) 2.6.3.1 Overvoltage Ratings The operating flux density is much lower than in a power transformer. This is to help minimize the losses and to prevent the VT from possible overheating during overvoltage conditions. VTs are normally designed to withstand 110% rated voltage continuously unless otherwise designated. IEEE C57.13 divides VTs into groups based on voltage and application. Group 1 includes those intended for line-to-line or line-to-ground connection and are rated 125%. Group 3 is for units with line-to-ground connection only and with two secondary windings. They are designed to withstand 173% of rated voltage for 1 min, except for those rated 230 kV and above, which must withstand 140% for the same duration. Group 4 is for line-to-ground connections with 125% in emergency conditions. Group 5 is for line-to-ground connections with 140% rating for 1 min. Other standards have more stringent requirements, such as the Canadian standard, which defines its Group 3 VTs for line-to-ground connection on ungrounded systems to withstand 190% for 30 sec to 8 h, depending on ground-fault protection. This also falls in line with the IEC standard. 2.6.3.2 VT Compensation The high-voltage windings are always compensated to provide the widest range of performance within an accuracy class. Since there is compensation, the actual turns ratio will vary from the rated-voltage ratio. For example, say a 7200:120-V, 60:1 ratio is required to meet 0.3 class. The designer may desire to adjust the primary turns by 0.3% by removing them from the nominal turns, thus reducing the actual turns ratio to, say 59.82:1. This will position the no-load (zero burden) point to the bottommost part of the parallelogram, as shown in Figure 2.6.10. Adjustment of turns has little to no effect on the phaseangle error. 2.6.3.3 Short-Circuit Operation Under no normal circumstance is the VT secondary to be short-circuited. The VT must be able to withstand mechanical and thermal stresses for 1 sec with full voltage applied to the primary terminals without suffering damage. In most situations, this condition would cause some protective device to operate and remove the applied voltage, hopefully in less than 1 sec. If prolonged, the temperature rise would far exceed the insulation limits, and the axial and radial forces on the windings would cause severe damage to the VT. 2.6.3.4 VT Connections VTs are provided in two arrangements: dual or two-bushing type and single-bushing type. Two-bushing types are designed for line-to-line connection, but in most cases can be connected line-to-ground with reduced output voltage. Single-bushing types are strictly for line-to-ground connection. The VT should never be connected to a system that is higher than its rated terminal voltage. As for the connection between phases, polarity must always be observed. Low- and medium-voltage VTs may be configured in delta or wye. As the system voltages exceed 69 kV, only single-bushing types are available. Precautions must be taken when connecting VT primaries in wye on an ungrounded system. (This is discussed further in Section 2.6.3.5, Ferroresonance.) Primary fusing is always recommended. Indoor switchgear types are often available with fuse holders mounted directly on the VT body. 2.6.3.5 Ferroresonance VTs with wye-connected primaries on three-wire systems that are ungrounded can resonate with the distributed line-to-ground capacitance (see Figure 2.6.11). Under balanced conditions, line-to-ground voltages are normal. Momentary ground faults or switching surges can upset the balance and raise the line-to-ground voltage above normal. This condition can initiate a resonant oscillation between the primary windings and the system capacitance to ground, since they are effectively in parallel with each other. Higher current flows in the primary windings due to fluctuating saturation, which can cause overheating. The current levels may not be high enough to blow the primary fuses, since they are generally sized for short-circuit protection and not thermal protection of the VT. Not every disturbance will cause ferroresonance. This phenomenon depends on several factors: • Initial state of magnetic flux in the cores • Saturation characteristics (magnetizing impedance) of the VT • Air-core inductance of the primary winding • System circuit capacitance One technique often used to protect the VT is to increase its loading resistance by (1) connecting a resistive load to each of the secondaries individually or (2) connecting the secondaries in a deltaconfiguration and inserting a load resistance in one corner of the delta. This resistance can be empirically approximated by Equation 2.6.11, FIGURE 2.6.11 VTs wye-connected on ungrounded system. R delta = (100 L A )/N 2 (2.6.11) © 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
Page 19 and 20: FIGURE 2.1.14 Layer windings (singl
Page 21 and 22: the transformer design, which may b
Page 23 and 24: consumer’s service circuit is a d
Page 25 and 26: FIGURE 2.2.3 Single-phase transform
Page 27 and 28: is installed so that the tank never
Page 29 and 30: above which persons might be burned
Page 31 and 32: FIGURE 2.2.16 Two-bushing subway. (
Page 33 and 34: carbon steel or the very expensive
Page 35 and 36: FIGURE 2.2.31 Radial-style dead fro
Page 37 and 38: 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 for CTs is TCF = RCF - (PA/2600
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 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
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aluminum shielding, i.e., an oil-im
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Leo J. Savio ADAPT Corporation Ted
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3.1.2.2.2 Heat Dissipation A second
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FIGURE 3.2.1 Solid-type bushing. ma
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1.6 cm. This means that most of the
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where HS = steady-state bushing ho
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the conductivity of the water-solub
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of 178 ohm-m, and a test duration o
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2. Easley, J.K. and Stockum, F.R.,
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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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Page 239 and 240:
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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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