[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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educing expenses by reducing the frequency of equipment inspections and by reducing or delaying active interventions (repair, replacement, etc.) on the equipment. Strategic benefits are based on the ability to prevent (or mitigate) failures or to avoid catastrophe. These benefits can be substantial, since failures can be very damaging and costly. Benefits in this category include better safety (preventing injuries to workers or the public in the event of catastrophic failure), protection of the equipment, and avoiding the potentially large impact caused by system instability, loss of load, environmental cleanup, etc. 3.13.1.2 Direct Benefits 3.13.1.2.1 Maintenance Benefits Maintenance benefits represent resources saved in maintenance activities by the application of on-line monitoring as a predictive maintenance technique. On-line monitoring can mitigate or eliminate the need for manual time-based or operation-based inspections by identifying problems early and allowing corrective actions to be implemented. 3.13.1.2.2 Equipment Usage Benefits Equipment usage benefits arise because additional reinforcement capacity may be deferred because online monitoring and diagnostics allow more effective utilization of existing equipment. On-line monitoring equipment can continuously provide real-time capability limits, both operationally and in terms of equipment life. 3.13.1.3 Strategic Benefits Strategic benefits are those that accrue when the results of system failures can be mitigated, reduced, or eliminated. A key feature of on-line monitoring technology is its ability to anticipate and forestall catastrophic failures. The value of the technology is its ability to lessen the frequency of such failures. 3.13.1.3.1 Service Reestablishment Benefits Service reestablishment benefits represent the reduced need for repair and/or replacement of damaged equipment because on-line monitoring has been able to identify a component failure in time for planned corrective action. Unscheduled repairs can be very costly in terms of equipment damage and its potential impact on worker safety and public relations. 3.13.1.3.2 System Operations Benefits System operations benefits represent the avoidance of operational adjustments to the power system as a result of having identified the component failure prior to a general failure. System adjustments, in the face of a delivery-system breakdown, can range from negligible to significant. An example of a negligible adjustment is when the failure is in a noncritical part of the network and adequate redundancy exists. Significant adjustments are necessary if the failure causes large, baseload generation to experience a forced outage, or if contractual obligations to independent generators cannot be met. These benefits are driven in part by the duration of the resulting circuit outage. 3.13.1.3.3 Outage Benefits Outage benefits represent the impact of component failure and resulting system breakdown on end-use customers. A utility incurs direct revenue losses as a result of a system or component failure. A utility’s customers, in turn, may also experience losses during failures. The magnitude and/or frequency of such losses may result in the customer’s loss of significant revenues. 3.13.2 On-Line Monitoring Systems The characteristics of transformer on-line monitoring equipment can vary, depending on the number of parameters that are monitored and the desired accessibility of the data. An on-line monitoring system typically records data at regular intervals and initiates alarms and reports when preset limits are exceeded. The equipment required for an on-line transformer monitoring system consists of sensors, data-acquisition units (DAU), and a computer connected with a communications link. 3.13.2.1 Sensors Sensors measure electrical, chemical, and physical signals. Individual sensor types and monitoring methods are discussed in Section 3.13.3, On-Line Monitoring Applications. Standard sensor output signal levels are 4 to 20 mA, 0 to 1 mA, and 0 to 10 V when an analog representation is used. The sensors can be directly connected to the data acquisition unit(s). Another category of sensors communicates in serial format, as is characteristic of those implemented within intelligent electronic devices (IED). Information/data about a function or status that is being monitored is captured by a sensor that can be attached directly to the transformer or within the control house. Once captured, the data are transferred to a data-acquisition unit (DAU) that can also be attached to the transformer or located elsewhere in the substation. The transfer is triggered by a predefined event such as a motor operation, a signal reaching a threshold, or the changing state of a contact. The transfer can also be initiated by a timebased schedule such as an hourly measurement of the power factor of a bushing, or any other such quantity. The method of data collection depends on the characteristics of the on-line monitoring system. A common characteristic of all systems is the need to move information/data from the sensor level to the user. The following represent examples of possible components in a data-collection system. 3.13.2.2 Data-Acquisition Units A data-acquisition unit collects signals from one or more sensors and performs signal conditioning and analog-to-digital conversions. The DAU also provides electrical isolation and insulation between the measured output signals and the DAU electronics. For example, a trigger could cause the DAU to start recording, store information about the event, and send it to a substation computer. 3.13.2.3 DAU-to-Computer Communications Line The data-collection process usually involves transferring the data to a computer. The computer could be located within the DAU, elsewhere in the substation, or off-site. The data can be transferred via a variety of communications networks such as permanent direct connection, manual direct connection, local-area networks (LAN), or wide-area networks (WAN). 3.13.2.4 Computer At the computer, information is held resident for additional analysis. The computer may be an integral part of the DAU, or it may be located separately in the station. The computer is based on standard technology. From a platform point of view, software functions of the substation computer program include support of the computer, the users, communications systems, storage of data, and communications with users or other systems, such as supervisory control and data acquisition (SCADA). The computer manages the DAUs and acts as the data and communications server to the user-interface software. The computer facilitates expert-system diagnostics and contains the basic platform for data acquisition and storage. 3.13.2.5 Data Processing The first step in data processing is the extraction of sensor data. Some types of data can be used in the form in which it is acquired, while other types of data need to be processed further. For example, a transformer’s top-oil temperature can be directly used, while a bushing’s sum current waveform requires additional processing to calculate the fundamental frequency (50 or 60 Hz) phasor. The data are then compared with various reference values such as limits, nameplate values, and other measurements, depending on the user’s application. In situations where reference data are not available, a learning period may be used to generate a baseline for comparison. Data are accumulated during a specified period of time, and statistical evaluation is used to either accept or reject the data. In some applications, the rejected data are still saved, but they are not used in the calculation of the initial benchmark. In other applications, the initial benchmark is determined using only the accepted data. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
The next data-processing step is to determine if variations suggest actual apparatus problems or if they are due to ambient fluctuations (such as weather effects), power-system variables, or other effects. A combination of signal-processing techniques and/or the correlation of the information obtained from measurements from locations on the same bus can be used to eliminate both the power-system effects and temperature influences. The next step in processing depends on the sophistication of the monitoring system. However, the data generally need to be interpreted, with the resulting information communicated to the user. One common approach is to compare the measured parameter with the previous measurement. If the value has not changed significantly, then no data are recorded, saved, or transmitted. 3.13.3 On-Line Monitoring Applications Various basic parameters of power transformers, load tap changers, instrument transformers, and bushings can be monitored with available sensor technologies. 3.13.3.1 Power Transformers Transformer problems can be characterized as those that arise from defects and develop into incipient faults, those that derive from deterioration processes, and those induced by operating conditions that exceed the capability of the transformer. These problems may take many years to gestate before developing into a problem or failure. However, in some cases, undesirable consequences can be created quite precipitously. Deterioration processes relating to aging are accelerated by thermal and voltage stresses. Increasing levels of temperature, oxygen, moisture, and other contaminants significantly contribute to insulation degradation. The deterioration is particularly exaggerated in the presence of catalysts and/or throughfaults and by mechanical or electromechanical wear. Characteristics of the deterioration processes include sludge accumulation, weakened mechanical strength of insulation materials such as paper-wrapped conductor, shrinkage of materials that provide mechanical support, and improper alignment of tapchanger mechanisms. Excessive moisture accelerates the aging of insulation materials over many years of operation. During extreme thermal transients that can occur during some loading cycles, high moisture content can result in water vapor bubbles. The bubbles can cause serious reduction in dielectric strength of the insulating liquid, resulting in a dielectric failure. The processes causing eventual problems (e.g., shrinkage of the insulation material or excessive moisture) may take many years to develop, but the consequences can appear suddenly. Continuous monitoring permits timely remedial action: not too early (thus saving valuable maintenance resources) and not too late (which would have costly consequences). Higher loading can be tolerated, as continuous automated evaluation will alert users of conditions that could result in failure or excessive aging of critical insulation structures (Griffin, 1999). Table 3.13.1 lists the major transformer components along with their associated problems and the parameters that can be monitored on-line to detect them. 3.13.3.1.1 Dissolved-Gas-in-Oil Analysis 3.13.3.1.1.1 Monitored Parameters — Dissolved gas-in-oil analysis (DGA) has proved to be a valuable and reliable diagnostic technique for the detection of incipient fault conditions within liquid-immersed transformers by detecting certain key gases. DGA has been widely used throughout the industry as the primary diagnostic tool for transformer maintenance, and it is of major importance in a transformer owner’s loss-prevention program. Data have been acquired from the analysis of samples from electrical equipment in the factory, laboratory, and field installations over the years. A large body of information relating certain fault conditions to the various gases that can be detected and easily quantified by gas chromatography has been developed. The gases that are generally measured and their significance are shown in Table 3.13.2 (Griffin, 1999). Griffin provides methods for interpreting fault conditions associated with various gas concentration levels and combinations of these gases (Griffin, 1999). TABLE 3.13.1 Main Tank Transformer Components, Failure Mechanisms, and Measured Signals Component General Specific Phenomenon Measured Signals Noncurrentcarrying metal components Core Overheating of laminations Top and bottom temperatures Ambient temperature Line currents Voltage Hydrogen (minor overheating) Multigas, particularly ethane, ethylene, and methane (moderate or severe overheating) Winding insulation Frames Clamping Cleats Shielding Tank walls Core ground Magnetic shield Cellulose: Paper, pressboard, wood products Overheating due to circulating currents, leakage flux Floating core and shield grounds create discharge Local and general overheating and excessive aging Severe hot spot Overheating Moisture contamination Bubble generation Partial discharge Top and bottom temperatures Ambient temperature Line currents Voltage Multigas, particularly ethane, ethylene, and methane Hydrogen or multigas Acoustic and electric PD Top and bottom temperatures Ambient temperature Line currents RS moisture in oil Multigas, particularly carbon monoxide, carbon dioxide, and oxygen Top and bottom temperatures Ambient temperature Line currents Moisture in oil Multigas, particularly carbon monoxide, carbon dioxide, ethane, hydrogen, and oxygen Top and bottom temperatures Ambient temperature Relative saturation of moisture in oil Top and bottom temperatures Ambient temperature Total percent dissolved gas-in-oil Line currents Relative saturation of moisture in oil Hydrogen Acoustic and electric PD Hydrogen or multigas Acoustic and electric PD Liquid insulation Moisture contamination Top and bottom temperatures Ambient temperature Relative saturation of moisture in oil Partial discharge Hydrogen Acoustic and electric PD Arcing Hydrogen and acetylene Local overheating Ethylene, ethane, methane Cooling system Fans Pumps Temperaturemeasurement devices Electrical failures of pumps and fans Motor (fan, pump) currents Top-oil temperature Line currents — continued © 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
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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
Page 169 and 170: TABLE 3.6.3 Transformer Short-Circu
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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
Page 201 and 202: where [A] = state matrix [B] = inpu
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 and 214: 6. Costs to set up and use a mobile
Page 215 and 216: Records of relay operation must be
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Page 219: a reference value) of the currents
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
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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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