[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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3.12.2 Background Investigation The background investigation usually involves exploring the following. • Identification of failure. How did the transformer problem manifest itself or how did the failure occur? This starting point assumes that some event has occurred, such as a trip, malfunction, or abnormal diagnostic test results. • The original transformer design • A history of similar transformer designs • The original manufacturing and testing of the transformer • System application and operational data 3.12.2.1 Transformer Records 3.12.2.1.1 Transformer Application Cooperation between parties involved will speed the investigation and lead more easily to a correct conclusion. The manufacturer should be contacted even when the transformer is many years old. Most equipment manufacturers track problems with their designs, and awareness of problems can help them improve designs and manufacturing processes. The manufacturer may also be able to provide information leading to a solution. Application of the transformer in the power system should be determined. A party with expertise in transformer application should confirm that the transformer was properly rated for the purpose for which it was intended and that subsequent operation has been consistent with that intent. Transformer problems can lead to service outages. Careful analysis of the situation before restoring service can avoid a repetition of the outage and avoid increasing damage to the transformer. The performance of transformers made by the manufacturer should be studied. Industry records of transformers of similar rating and voltage class may be helpful in establishing base data for the investigation. For example, there have been transformers made by certain manufacturers that have a history of short-circuit failures in service. At one time, the failure record of extra high voltage (EHV) transformers designed with three steps of reduced basic impulse insulation level (BIL) had a higher failure rate than those with higher BILs. 3.12.2.1.2 Transformer Design The specifications for the transformer, instruction books and literature, nameplate, and drawings such as the outline and internal assembly should be examined. If failures involving the core and windings have occurred, the investigative process is much like a design review in which the details of the insulation design, winding configuration, lead configurations and clearances, short-circuit capability, and the core construction and normal operating induction must be studied. If components are involved, bushings and bushing clearances, tap changers, heat exchanger equipment, and control equipment should be investigated. 3.12.2.1.2.1 Design Review Process — An explanation of the design review process would require a separate chapter and is beyond the scope of this subject. The process involves a detailed study of the winding and insulation designs, short-circuit capability, thermal design, magnetic-circuit characteristics, leakage flux losses and heating analysis, materials used, oil preservation systems, etc. In some cases, it may be desirable to conduct part or all of a design review as a part of a problem investigation. 3.12.2.1.2.2 Determine if Similar Designs Experienced Problems or Failures — This can be difficult, since there is no agency that collects and distributes data on all industry problems. It has been made even more difficult by the closing of several major transformer manufacturing plants. However, some information can usually be obtained by discussions with users having transformers made by the same manufacturer. 3.12.2.1.3 Manufacturing and Testing of the Transformer The manufacturing and test records should be studied to determine if discrepancies occurred. Of particular importance are deviations from normal manufacturing specifications or practices. Such deviations could be involved in the problem or failure. All parts of the test data must be studied to determine if discrepancies or deviations existed. Partial-discharge and impulse test data that have any deviation from good practices should be recorded. Approvals of deviations made during testing, especially relating to dielectric and other test standards, are sometimes indications of difficulty during testing. 3.12.2.1.4 Transformer Installation The records of installation and initial field tests should be used as a benchmark for any future test results. These initial field tests are more easily compared with subsequent field tests than with factory tests. Factory tests are performed at full load current or full voltage, and adjustments must be made to field test results to account for differences in losses at reduced load and differences in excitation at the operating voltage. 3.12.2.2 Transformer Protection The protection of the transformer is as important a part of the application as the rating values on the transformer. Entire texts, as well as Chapter 3.8 in this text, are devoted to the subject of transformer protection. When investigating a failure, one should collect all the protection-scheme application and confirm that the operation of any tripping function was correct. 3.12.2.2.1 Surge Arresters Surge arrester protective level must be coordinated with the BIL of the transformer. Their purpose, to state what may seem obvious, is to protect the transformer from impulse voltages and high-frequency transients. Surge arresters do not eliminate voltage transients. They clip the voltages to a level that the transformer insulation system is designed to tolerate. However, repeated impulse voltages can have a harmful effect on the transformer insulation. 3.12.2.2.2 Overcurrent Protection Overcurrent devices must adequately protect the transformer from short circuits. Properly applied, the time–current characteristic of the device should coordinate with that of the transformer. These characteristics are described in IEEE C57.109-1993, Guide for Liquid-Immersed Transformer Through-Fault Duration. Overcurrent devices may be as simple as power fuses or more complex overcurrent relays. Modern overcurrent relays contain recording capability that may contain valuable information on the fault being investigated. 3.12.2.2.3 Differential Protection Differential relays, if applied, should be coordinated with the short-circuit current available, the transformer turns ratio and connection, and the current transformers employed in the differential scheme. If differential relays have operated correctly, a fault occurred within the protected zone. One must determine if the protected zone includes only the transformer, or if other devices, such as buswork or circuit breakers, might have faulted. 3.12.2.3 Recording Devices The occurrence of unusual trends or events that indicated a possible problem should be recorded. Operation of relays or protective equipment that indicated a failure should be studied. Copies of oscillographic or computer records of the events surrounding the problem or failure should be obtained. Records of events immediately prior to the observation of a problem or immediately prior to a trip are often only the final events in a series that led to the failure. Investigations at the transformer location should concentrate on collecting such data as relay targets, event recordings, and on-line monitoring records as well as making observations of the condition of the transformer and associated equipment. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
Records of relay operation must be analyzed to determine the sequence of events. A failure of a transformer due to a through fault, if uncorrected, can lead to the failure of a replacement unit. This can be a very expensive oversight. Based on initial observations and recordings, a preliminary cause may be determined. Subsequent tests should focus on confirming or refuting this cause. 3.12.2.4 Operational History 3.12.2.4.1 Diagnostic Testing The operation records of the transformer should be examined in detail. Records of field tests such as insulation resistance and power factor, gas-in-oil analyses, oil test data, and bushing tests should be studied. Any trends from normal such as increasing power factor, water in oil, or deterioration of oil properties should be noted. Any internal inspections that may have been performed, changes in oil, and any repairs or modifications are of interest. Maintenance records should be examined for evidence of either good or poor maintenance practices. Proper application of arresters and other protective schemes should be verified. 3.12.2.4.2 Severe Duty Operational history such as switching events, numbers and type of system short circuits, known lightning strikes, overloads, etc., should be recorded. 3.12.2.4.2.1 Overloads — The effect of loading a transformer beyond nameplate is not apparent without disassembly. The history of transformer operation will be the first indication of damage from overloads. The transformer may have some inherent capability to handle loading in excess of nameplate rating. However, the result of loading beyond nameplate can result in accelerated loss of life. This loss of life is cumulative and cannot be restored. Loading recommendations are more fully described in IEEE C57.91- 1995, Guide for Loading Mineral-Oil-Immersed Transformers, and in IEEE C57.96-1999, Guide for Loading Dry-Type Distribution and Power Transformers. 3.12.2.4.2.2 Overvoltages — Maximum continuous transformer operating voltage should not exceed the levels specified in ANSI C84.1-1995. Overexcitation will result in core heating and subsequent damage. The results of core heating may not be readily discerned until a transformer is untanked. Examination of the operating records may provide insight into this as a cause of damage. 3.12.2.4.2.3 Short Circuits — IEEE C57.12.00-2000, IEEE Standard General Requirements for Liquid- Immersed Distribution, Power, and Regulating Transformers, and IEEE C57.12.01, IEEE Standard General Requirements for Dry-Type Distribution and Power Transformers Including Those with Solid-Cast and/or Resin-Encapsulated Windings, state limits for short-circuit durations and magnitude. Not all short-circuits reach the magnitude of those limits. However, like overloads, the effect can be cumulative. This effect is further expanded in IEEE C57.109-1993, IEEE Guide for Liquid-Immersed Transformer Through-Fault-Current Duration. The effect is described as an “extremely inverse time-current characteristic” upon which the overcurrent protection of the transformer should be based. 3.12.3 Problem Analysis where No Failure Is Involved 3.12.3.1 Transformer Components Component failures can lead to major failures if left uncorrected. All of these should be corrected as soon as identified to avoid letting them develop into problems that are more serious. 3.12.3.1.1 Transformer Tank Transformer tank welds can crack after thermal cycling or even after withstanding the stresses of throughfaults. Bushing and radiator gaskets can deteriorate over time, leading to leaks. Left unattended, these leaks can result in moisture entering the transformer insulation. 3.12.3.1.2 Transformer Radiators and Coolers Radiators can leak, leading to ingress of moisture, or they can clog, reducing their cooling efficiency. Fan motors can fail, resulting in a loss of cooling. Today’s modern low-loss transformer designs can have a dual rating ONAN/ONAF with as few as one or two cooling fans. Loss of a large percentage of the fans necessitates using the ONAN rating for transformer capacity. (See Section 2.1.2.3, Cooling Classes, for a discussion of cooling-class terminology.) 3.12.3.1.3 Transformer Bushings Bushings occasionally develop problems such as leaks or high power factor and thermal problems. Problems detected during periodic diagnostic testing should be remedied as quickly as possible. 3.12.3.1.4 Transformer Gauges Gauges and indicating devices should be repaired or replaced as soon as it is suspected that they are giving false indication. Confirm that the abnormal indication is false and replace the device. Letting a false indication continue may disguise the development of a more severe problem. 3.12.3.2 Severe Duty Investigations It is advisable to make some investigations after severe operating events such as direct lightning strikes (if known), high-magnitude short-circuit faults, and inadvertent high-magnitude overloads. Problem investigations are sometimes more difficult than failure analyses because some of the internal parts cannot be seen without dismantling the transformer. The same data-collection process is recommended for problems as for failures. It is recommended that a plan be prepared when the investigation is initiated, including a checklist to guide the study and to ensure that no important steps are omitted. A recommended checklist is in ANSI/IEEE C57.125. Specific steps that are recommended are as follows: 3.12.3.2.1 Communication with Persons Involved or Site Visit • Obtain the background of the events indicating a problem. • If there is any external evidence such as loss of oil, overheated parts, or other external indications, a site visit may be advisable to get first-hand information and to discuss the events with persons who operated the transformer. • Determine if there have been any unusual events such as short circuits, overvoltages, or overloads. 3.12.3.2.2 Diagnostic Testing If the utility has a comprehensive field-testing program, obtain and study the following data. If the data are not available, arrange for tests to be made. 3.12.3.2.2.1 Gas-in-Oil Analyses — Obtain test results for several years prior to the event, if possible. Many good papers and texts have discussed the interpretation of dissolved gas-in-oil analysis (DGA) results. One must also remember that once an internal failure has occurred, the initiating cause may be masked by the resulting fault gases. IEEE C57.104-1991, IEEE Guide for Interpretation of Gases Generated in Oil-Immersed Transformers, can provide the latest interpretation of DGA results. 3.12.3.2.2.2 Oil Test Data — Dielectric strength in accordance with ASTM D 1816 • Water in oil • Power factor at 25 and 100˚C, if available • General characteristics such as color, inter-facial tension (IFT), etc. 3.12.3.2.2.3 Turns Ratio — This test should match the factory results and be within the standard 0.5% of calculated value (except as noted in Clause 9.1 of IEEE C57.12.00-2000). Any deviation indicates a partially shorted turn. © 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
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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
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
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Page 197 and 198: Gade, S., Sound Intensity Instrumen
Page 199 and 200: nected. This steady-state voltage d
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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: 6. Costs to set up and use a mobile
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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
Page 225 and 226: 3.13.3.2 Instrument Transformers Th
Page 227 and 228: FIGURE 3.13.5 Analysis of bushing s
Page 229 and 230: temperature. Under abnormal conditi
Page 231 and 232: 3.14.1.2 Accredited Standards Commi
Page 233 and 234: FIGURE 3.14.4 IEC technical-committ
Page 235 and 236: TABLE 3.14.2 Relevant Documents for
Page 241: TABLE 3.14.13 Relevant Documents fo
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