[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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trend analyses, and prognoses. Monitoring application is a judgment of transformer size and importance and of maintenance and equipment costs. FIGURE 3.3.22 Overvoltage protection devices arranged within the arcing switch. Nonlinear resistors are solutions for high-power transformers and for all transformers where the service conditions would cause the spark gaps to respond frequently. In the early stages, silicon-carbide (SiC) elements were installed. The specific characteristics of this material did not allow full-range application. When high-power zinc-oxide (ZnO) varistors came on the market, the application of these elements for overvoltage protection was studied in detail with good results. For more than 20 years, ZnO varistors have solely been in use. 3.3.7 Maintenance of Load Tap Changers LTC maintenance is the basis for the regulating transformer’s high level of reliability. The background for maintenance recommendations is as follows: For LTCs where oil is used for arc-quenching, the arcing at the arcing switch or arcing tap switch contacts causes contact erosion and carbonization of the arcing switch oil. The degree of contamination depends upon the operating current of the LTC, the number of operations, and to some degree the quality of the insulating oil. For LTCs using vacuum interrupters for arc-quenching, contact life of the vacuum interrupters and the mechanically stressed parts of the device are the key indicators for the maintenance recommendations. The overall performance of vacuum-type LTCs leads more and more toward maintenance-free LTC designs. Maintenance and inspection intervals depend on the type of LTC, the LTC rated through-current, the field experience, and the individual operating conditions. They are suggested as periodical measures with respect to a certain number of operations or after a certain operating time, whichever comes first. The recommended maintenance intervals for an individual LTC type are given in the operating and inspection manuals available for each LTC type. Normally, maintenance of an LTC can be performed within a few hours by qualified and experienced personnel, provided that it has been properly planned and organized. In countries with tropical or subtropical climate, the humidity must also be taken into consideration. In some countries, customers decide to start maintenance work only if the relative humidity is less than 75%. Economical factors are taken more and more into consideration by users of large power transformers in distribution networks when assessing the operating parameters for cost-intensive operating equipment. While users are aiming at cost reduction for transformer maintenance, they are also demanding higher system reliability. Besides the new generation of LTCs with vacuum switching technology, modern supervisory concepts on LTCs (LTC monitoring) offer a solution for the control of these divergent development tendencies. Today a few products are on the market that differ significantly in their performance. A state-of-the-art LTC on-line monitoring system should include an early-fault-detection function and information on condition-based maintenance, which requires an expert-system of the LTC manufacturer. The data processing and visualization should provide information about status-signal messages, 3.3.8 Refurbishment/Replacement of Old LTC Types With regard to system planning of power utilities, the lifetime of regulating transformers is normally assumed to be 25 to 30 years. The actual lifetime is, however, much longer. Due to economic aspects and aging networks, as well as the requirement to improve reliability, refurbishment/replacement is becoming a major policy issue for utility companies. Refurbishment includes a complete overhaul of the regulating transformer plus other improvements regarding loading capability, an increase in insulation levels, a decrease in noise levels, and the possible replacement of the bushings and of the LTC or a complete overhaul of the LTC. This overhaul should be performed by specialists from the LTC manufacturer in order to avoid any risk when judging the condition of the LTC components, when deciding which components have to be replaced, and with regard to the disassembly and the reassembly as well as the cleaning of insulation material. The replacement of an old risky LTC (for which neither maintenance work nor spare parts are available) by a new LTC may economically be justified, compared with the expenses for a new regulating transformer, even if the transformer design has to be modified for that reason. The manufacturer of the new LTC must, of course, guarantee maintenance work and spare parts for the foreseeable future. 3.3.9 Future Aspects For the time being, no alternative to regulating transformers is expected. The LTC will therefore continue to play an essential part in the optimum operation of electrical networks and industrial processes in the foreseeable future. With regard to the future of LTC systems, one can say that a static LTC, without any mechanical system and consisting only of power electronics, leads to extremely uneconomical solutions, and this will not change in the near future. Therefore, the mechanical LTC will still be used. Conventional LTC technology has reached a very high level and is capable of meeting most requirements of the transformer manufacturer. This applies to the complete voltage and power fields of today, which will probably remain unchanged in the foreseeable future. It is very unlikely that, due to new impulses given to development, greater power and higher voltages will be required. Today the main concern goes to service behavior as well as reliability of LTCs and how to keep this reliability at a consistently high level during the regulating transformer’s life cycle. At the present time and for the foreseeable future, the proper implementation of the vacuum switching technology in LTCs provides the best formula of quality, reliability, and economy achievable toward a maintenance-free design. The vacuum switching technology entirely eliminates the need for an on-line filtration system and offers reduced down-times with increased availability of the transformer and simplified maintenance logistics. All this translates into substantial savings for the end-user. Consequently, today’s design concepts of LTCs — resistance- and reactance-type LTCs — are based more and more on vacuum interrupters. The vacuum switching technology — used in OLTCs — is in fact “state of the art” design today and tomorrow. Another target of development is the insulation and cooling media with low or no flammability for regulating transformers, mainly relevant in the field of medium-power transformers (
References Goosen, P.V. , Transformer accessories, (on behalf of Study Committee 12), CIGRE, Volume No., 12-104, 1996. IEC International Standard 60214-1:2003, Tap-Changers, Part I: Performance requirements and test methods, 2003. IEC Std. Publication 60542, Application Guide for On-Load Tap Changers, 1976, first amendment 1988. IEEE Std. C57.135-2001, IEEE Guide for the Application, Specification, and Testing of Phase-Shifting Transformers, 2001. IEEE Std. C57.131-1995, IEEE Standard Requirements for Load Tap Changers, 1995. Krämer, A., On-Load Tap-Changer for Power Transformers: Operation, Principles, Applications and Selection, MR Publications, 2000. Krämer, A. and Ruff, J., Transformers for phase angle regulation, considering the selection of on-load tap changers, IEEE Trans. Power Delivery, 13, 1998. 3.4 Loading and Thermal Performance Robert F. Tillman, Jr. 3.4.1 Design Criteria ANSI standards collection for the C57 series sets forth the general requirements for the design of power transformers. With respect to loading, the requirements of concern are those that define the transformer’s thermal characteristics. The average ambient temperature of the air in contact with the cooling equipment for a 24-hr period shall not exceed 30˚C, and the maximum shall not exceed 40˚C (IEEE, 1994). For example, a day with a high temperature of 35˚C and a low of 25˚C has an approximate average ambient temperature of 30˚C. The average winding-temperature rise above ambient shall not exceed 65˚C for a specified continuousload current (IEEE, 1994). The industry uses this criterion rather than the more relevant hot-spot temperature rise because manufacturers can obtain it by simply measuring the resistance of the windings during temperature-rise tests. The manufacturer must guarantee to meet this requirement. The hottest-conductor temperature rise (hot spot) in the winding shall not exceed 80˚C under these same conditions (IEEE, 1994). This 80˚C limit is no assurance that the hot-spot rise is 15˚C higher than the average winding-temperature rise. In the past, using the 15˚C adder to determine the hot-spot rise produced very conservative test results. The greatest thermal degradation of cellulose insulation occurs at the location of the hottest spot. The hot-spot location is near the top of the high- or low-voltage winding. The most common reason for hot spots to occur is that these regions have higher localized eddy losses because the leakage flux fringes radially at the winding ends. Standards specify requirements for operation of the transformer above rated voltage. At no load, the voltage shall not exceed 110% (IEEE, 1994). At full load, the voltage shall not exceed 105% (IEEE, 1994). Modern transformers are usually capable of excitation beyond these limits without causing undue saturation of the core. However, this causes the core to contribute greater than predicted heating. Consequently, all temperature rises will increase. While not critical at rated load, this becomes important when loading transformers above the nameplate rating. 3.4.2 Nameplate Ratings The transformer nameplate provides the voltage (excitation) rating for all tap positions. If a transformer has a four-step de-energized high-voltage tap changer (DETC) and a 32-step load tap changer (LTC) on the low-voltage winding, then it has five primary voltage ratings and 33 secondary voltage ratings. The excitation limits apply to all ratings. The cooling class affects the design of the cooling package. Different cooling classes have different thermal profiles. Therefore, gauge readings can be different for equivalently rated transformers under the same loading conditions. Consequently, the operator must understand the cooling classes and their thermal profiles in order to confirm that a given transformer is responding properly. ONAN cooling uses natural air flow through the radiators to dissipate heat. (See Section 2.1, Power Transformers, for description of cooling classes). ONAF cooling uses fans to force air through radiators in order to substantially increase the rate of cooling. There is natural flow by convection of the oil in the radiators from top to bottom with both ONAN and ONAF cooling. There may additionally be pumps used to draw the hot oil out of the top of the transformer and force it through the radiators into the bottom of the tank. If the design incorporates a cooling duct that directs the oil from the radiator outlet directly through the winding from bottom to top, it is classified as ODxx (Pierce, 1993). Medium–size power transformers, 12 MVA and larger, have three stages of cooling, i.e., natural convection plus two stages of forced air through the radiators. Large power transformers usually employ directed-flow cooling to the main windings. Some large power transformers, especially generator stepup (GSU) transformers, are rated for only forced oil and air cooling; the nameplate reveals no rating unless the auxiliary cooling equipment is operating. The nameplate kVA (or MVA) rating is the continuous load at rated voltage that produces an average winding-temperature rise within the 65˚C limit and hot-spot temperature rise within the 80˚C limit (IEEE, 1994). The rating of each cooling stage is given. The transformer design must meet temperaturerise guarantees for each cooling stage. Operators and planners generally express loading capability as a percent (or per unit) of the maximum MVA rating. The windings contribute most of the heat that produces the temperature rises. The winding heat results mostly from the I 2 R loss in the conductor. When users express loading as a percent on an MVA basis, they are assuming that the transformer is operating at rated voltage, both primary and secondary. Operators should express the load for thermal calculations as a percent (or per unit) of the full load current for a given set of primary and secondary tap positions (Tillman, 1998). 3.4.3 Other Thermal Characteristics Other thermal characteristics are dependent on the design, loss optimization, and cooling class. Top-oil temperature is the temperature of the bulk oil at the top of the tank. Users can directly measure this with a gauge mounted on the transformer tank. The temperature of the oil exiting the oil duct in the top of the winding is actually the important characteristic (Chew, 1978; Pierce, 1993). Users normally do not have a direct measurement of this in practice. Manufacturers can measure this during heat run tests, but that is not common practice. Bottom-oil rise is the temperature rise above ambient of the oil entering the bottom of the core and coil assembly. In all cooling classes, the bottom-oil temperature is approximately equal to the temperature of the oil exiting the radiators (Chew, 1978; Pierce, 1993). Manufacturers can easily measure it during heat run tests and provide the information on certified test reports. Users should require manufacturers to provide this information. In service, users can glean a measurement of the bottom-oil temperature with an infrared camera. Average oil rise is the key oil thermal characteristic needed to calculate the winding gradient (Chew, 1978). It is approximately equal to the average of the temperature rise of oil entering the bottom of the core and coil assembly and the oil exiting the top. The location of the average oil is approximately equal to the location of the mid-winding. The winding gradient is the difference between the average winding- and average oil-temperature rises (Chew, 1978). Designers assume that this gradient is the same from the bottom to the top of the winding except at the ends, where the hot spot exists. The hot-spot gradient is somewhat higher than the winding gradient. Designers calculate this gradient by determining the effects of localized eddy losses at the end of the winding. Computers allow designers to calculate this very accurately. An engineer can obtain an estimate of the hot-spot gradient by multiplying the winding gradient by 1.1 (Chew, 1978). © 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
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
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: FIGURE 3.3.19 Effect of the leakage
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 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
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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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