[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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Given the sound-pressure-level requirement of L pD dB at a distance D from the transformer, the maximum allowable sound power L W (in watts) for the transformer can be estimated using Equation 3.9.14: L W = L pD + 10 log (2 D 2 ) (3.9.14) Therefore, the maximum allowable sound-pressure level L pT at the measurement surface near the transformer will be: L pT = L W – 10 log (measurement surface area) (3.9.15) where measurement surface area, m 2 = 1.25 transformer tank height measurement contour length 3.9.5 Factors Affecting Sound Levels in Field Installations To ensure repeatability, the factory measurements are made under controlled conditions specified by sound-measurement standards. Every effort is made to maintain core excitation voltage constant and sinusoidal to ensure the presence of a known core induction while making sound-level evaluations in transformers. The effects of ambient sound levels and reflections must be subtracted from the overall sound-level measurements in order to determine the true sound power of a transformer. Therefore, it is essential that the ambient sound levels are constant and that they are measured accurately. Many times it becomes necessary to make these measurements in a sound chamber, where ambient sound level is very low and the effects of reflecting surfaces can also be eliminated. The sound-level measurements at the transformer site can be drastically different, depending on its operating conditions. The effects of the following factors should therefore be considered while defining the sound-level requirements for transformers. 3.9.5.1 Load Power Factor In the factory, core and winding sound levels are measured separately at rated voltage and full-load current, respectively. These sound levels are produced by core and winding vibrations of twice the power frequency and its even harmonics. It is assumed that these vibrations are in phase with each other, and therefore their power levels are added to predict the overall sound level of the transformer. However, this assumption applies only when the transformer is carrying purely resistive load. Under actual operating conditions, depending on the power factor of the load, the phase angle between voltage and load current may induce a change in the factory-predicted transformer sound level. 3.9.5.2 Internal Regulation The magnitude and the phase angle of the load currents also change the internal voltage drop in the transformer windings. The transformer loading conditions therefore can change the core induction level and significantly influence the core sound levels. 3.9.5.3 Load Current and Voltage Harmonics During factory tests, only sinusoidal load current is simulated for measuring winding noise. This noise is produced by magnetic forces that are proportional to the square of the load current. However, harmonic content in the load current has a larger impact on the sound level than might be expected from the amplitude of the harmonic currents alone, since they interact with the power-frequency load current. In such cases, the magnetic force is proportional to the cross product between the power-frequency current and the harmonic current in addition to the force that is proportional to the square of load current and the square of the harmonic current. Thus, the highest contribution to the sound level due to the harmonic current occurs when the product of the load current and the harmonic current reaches the maximum. The resulting audible tones are made up of frequencies of the harmonic current the fundamental power frequency. Current harmonics are a major source of increase in sound levels in HVDC and rectifier transformers. Nonlinear loads cause harmonics in the excitation voltage, resulting in an increase in core sound levels. This influence must be considered when specifying sound level for a transformer. 3.9.5.4 DC Magnetization Even a moderate dc magnetization of a transformer core will result in a significant increase in the transformer audible sound level. In addition to increasing the power level of the normal harmonics in the transformer vibrations (i.e., even harmonics of the power frequencies), dc magnetization will add odd harmonic tones to the overall sound level of the transformer. Modern cores have high remnant flux density. Upon energization, the core sound levels may be as much as 20 dB higher than the factory test value. It is therefore recommended that a transformer should be energized for approximately six hours before evaluating its sound levels. Traditionally, circuits like dc feeders to the transportation systems have been a source of dc fields in transformers. However, with the increased used of power electronic equipment in power transmission systems and in the industry, the number of possible sources for dc magnetization is increasing. Geomagnetic storms can also cause severe dc magnetization in transformers connected to long transmission lines. 3.9.5.5 Acoustical Resonance Dry-type transformers are most frequently applied inside buildings. In a room with walls of low soundabsorption coefficient, the sound from the transformer will reflect back and forth between walls, resulting in a buildup of sound level in the room. The number of dBs by which the sound level at the transformer will increase can be approximated using Equation 3.9.16: dB buildup = 10 log [1 + 4(1 – a) A T /(a A U )] (3.9.16) where A T = surface area of the transformer A U = area of the reflecting surface a = average absorption coefficient of the surfaces In a room with concrete walls (with an absorption coefficient of 0.01) and with sound-reflecting surface area four times that of the transformer (A U /A T = 4), the increase in sound level at the transformer can be 20 dB. However, covering the reflecting surfaces of this room with sound-absorbing material with absorption coefficient of 0.3 will reduce this buildup to 5.5 dB. Sound propagation is affected by many factors, such as atmospheric absorption, interceding barriers, and reflective surfaces. An explanation of these factors is beyond the scope of this text; however, they are mentioned to make the reader aware of their potential influence. If the existing site conditions will influence the sound propagation, the reader is advised to reference textbooks dealing with the subject of acoustic propagation or to consult an expert in conducting more accurate sound-propagation calculations. References Anon., Measuring Sound, Brüel & Kjaer, Nerum, Denmark, 1984. Anon., Sound Intensity, Brüel & Kjaer, Nerum, Denmark, 1993. Beranek, L., Ed., Noise and Vibration Control, Institute of Noise Control Engineering, Washington, D.C. Fanton, J.P. et al., Transformer noise: determination of sound power level using the sound intensity measurement method, IEC SC 12-WG 12 (April 1990), Electra, 144, 1992. Gade, S., Sound Intensity Theory, Technical Review No. 3, Brüel & Kjaer, Nerum, Denmark, 1982. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
Gade, S., Sound Intensity Instrumentation and Applications, Technical Review No. 4, Brüel & Kjaer, Nerum, Denmark, 1982. IEEE Audible Sound and Vibration Subcommittee Working Group, Guide for Sound Level Abatement and Determination for Liquid Immersed Power Transformers and Shunt Reactors Rated to be 500kVA, IEEE C57.156 (draft 9), Institute of Electrical and Electronics Engineers, Piscataway, NJ. Replinger, E., Study of Noise Emitted by Power Transformers Based on Today’s Viewpoint, CIGRE Paper 12-08, Siemens AG, Transformatorenwerk, Nürnberg, Germany, 1988. Specht, T.R., Noise Levels of Indoor Transformers, Westinghouse Corp. Transformer Division, 1955. Westinghouse Electric Corp., Bolt Beranek, and Newman Inc., Power Transformer Noise Abatement, ESERC Report EP 9-14, Empire State Electric Energy Research Corp., New York, 1981. 3.10 Transient-Voltage Response Robert C. Degeneff 3.10.1 Transient-Voltage Concerns 3.10.1.1 Normal System Operation Transformers are normally used in systems to change power from one voltage (or current) to another. This is often driven by a desire to optimize the overall system characteristics, e.g., economics, reliability, or performance. To achieve these system goals, a purchaser must specify — and a designer must configure — the transformer to meet a desired impedance, voltage rating, power rating, thermal characteristic, short-circuit strength, sound level, physical size, and voltage-withstand capability. Obviously, many of these goals will produce requirements that are in conflict, and prudent compromise will be required. Failure to achieve an acceptable characteristic for any of these goals will make the overall transformer design unacceptable. Transformer characteristics and the concomitant design process are outlined in the literature [1–4]. Normally, a transformer operates under steady-state voltage excitation. Occasionally, a transformer (in fact all electrical equipment) experiences a dynamic or transient overvoltage. Often, it is these infrequent transient voltages that establish design constraints for the insulation system of the transformer. These constraints can have a far-reaching effect on the overall equipment design. The transformer must be configured to withstand any abnormal voltages covered in the design specification and realistically expected in service. Often, these constraints have great impact on other design issues and, as such, have significant effect on the overall transformer cost, performance, and configuration. In recent years, engineers have explored the adverse effect of transient voltages on the reliability of transformers [5–7] and found them to be a major cause of transformer failure. 3.10.1.2 Sources and Types of Transient-Voltage Excitation The voltages to which a transformer’s terminals are subjected can be broadly classed as steady state and transient. The transient voltages a transformer experiences are commonly referred to as dynamic, transient, and very fast transients. The majority of the voltages a transformer experiences during its operational life are steady state, e.g., the voltage is within 10% of nominal, and the frequency is within 1% of rated. As power-quality issues grow, the effect of harmonic voltages and currents on performance is becoming more of an issue. These harmonics are effectively reduced-magnitude steady-state voltages and currents at harmonic frequencies (say 2nd to the 50th). These are addressed in great detail in IEEE Std. 519, Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems [8]. Strictly speaking, all other voltage excitations are transients, e.g., dynamic, transient, and very fast transient voltages. Dynamic voltages refer to relatively low-frequency (60 to 1500 Hz), damped oscillatory voltage. Magnitudes routinely observed are from one to three times the system’s peak nominal voltage. Transient voltage refers to the class of excitation caused by events like lightning surges, switching events, and line faults causing voltages of the chopped waveform [9]. Normally, these are aperiodic waves. Occasionally, the current chopping of a vacuum breaker will produce transient periodic excitation in the 10- to 200-kHz range [10]. The term very fast transient encompasses voltage excitation with rise times in the range of 50 to 100 sec and frequencies from 0.5 to 30 MHz. These types of voltages are encountered in gasinsulated stations. The voltages produced within the transformer winding structure by the system is a part of the problem that must be addressed and understood if a successful insulation design is to be achieved [11]. Since transient voltages affect system reliability, in turn affecting system safety and economics, a full understanding of the transient characteristic of a transformer is warranted. 3.10.1.3 Addressing Transient-Voltage Performance Addressing the issue of transient-voltage performance can be divided into three activities: recognition, prediction, and mitigation. By 1950 over 1000 papers had been written to address these issues [12–14]. The first concern is to appreciate that transient-voltage excitation can produce equipment responses different than one would anticipate at first glance. For example, the addition of more insulation around a conductor may, in fact, make the transient-voltage distribution worse and the insulation integrity of the design weaker. Another example is the internal voltage amplification a transformer experiences when excited near its resonant frequency. The transient-voltage distribution is a function of the applied voltage excitation and the shape and material content of the winding being excited. The capability of the winding to withstand the transient voltage is a function of the specific winding shape, the material’s voltage-vs.- time characteristic, the past history of the structure, and the statistical nature of the voltage-withstand characteristic of the structure. The second activity is to assess or predict the transient voltage within the coil or winding. Today, this generally is accomplished using a lumped-parameter model of the winding structure and some form of computer solution method that facilitates calculation of the internal transient response of the winding. Once this voltage distribution is known, its effect on the insulation structure can be computed with a two- or three-dimensional finite element method (FEM). The resultant voltages, stresses, and creeps are examined in light of the known material and geometrical capability of the system, with consideration for desired performance margins. The third activity is to establish a transformer structure or configuration that — in light of the anticipated transient-voltage excitation and material capability, variability, and statistical nature — will provided acceptable performance margins. Occasionally, nonlinear resistors are used as part of the insulation system to achieve a cost-effective, reliable insulation structure. Additionally, means of limiting the transient excitation include the use of nonlinear resistors, capacitors, and snubbers. 3.10.1.4 Complex Issue to Predict The accurate prediction of the transient-voltage response of coils and winding has been of interest for almost 100 years. The problem is complex for several reasons. The form of excitation varies greatly. Most large power transformer are unique designs, and as such each transformer’s transient-response characteristic is unique. Each has its own impedance-vs.-frequency characteristic. As such, the transientresponse characteristic of each transformer is different. Generally, the problem is addressed by building a large lumped-parameter model of inductances, capacitances, and resistances. Constructing the lumpedparameter model is challenging. The resultant mathematical model is ill-conditioned, e.g., the resultant differential equation is difficult to solve. The following sections outline how these challenges are currently addressed. It should be emphasized that the voltage distribution within the winding is only the first component of the insulation design process. The spatial distribution of the voltages within the winding must be determined, and finally the ability of the winding configuration in view of its voltage-vs.-time characteristic must be assessed. © 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
Page 145 and 146: FIGURE 3.3.19 Effect of the leakage
Page 147 and 148: References Goosen, P.V. , Transform
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: H 2m 1. Vertical Forced Air 2. Prin
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
Page 217 and 218: • Has heating occurred as the res
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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[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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