[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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3.6.6.1.2 Nature of the Quantity Being Measured Transformer no-load loss, often called core loss or iron loss, is the power loss in a transformer excited at rated voltage and frequency but not supplying load. The no-load loss comprises three components: 1. Core loss in the core material 2. Dielectric loss in the insulation system 3. I 2 R loss due to excitation current in the energized winding FIGURE 3.6.13 Test circuit for partial-discharge measurement during the induced-voltage test. • The magnitude of partial discharge activity does not exceed 100 V (500 pC). • After the PD level has stabilized, any increase in partial discharge activity does not exceed 30 V (150 pC). • The partial discharge levels during the 1-h period do not show an increasing trend. • There is no sustained increase in partial discharge level during the last 20 min of the test. The test circuit for PD measurement during the induced-voltage test is shown in Figure 3.6.13. The bushing capacitance tap is connected to a coupling impedance unit, which provides PD signals to the PD detection unit. Methods for measurement and calibration are described in detail in the IEEE test code [2] and in the IEEE partial discharge measurement guide [5]. If there is significant level of PD activity during the test, it is desirable to identify the location of the PD source inside the transformer so that the problem can be corrected. This will ensure PD-free operation. Methods for location of PD sources inside the transformer often are based on triangulation. Ultrasonic acoustic waves arriving at transformer tank surfaces due to PD activity are monitored at various locations on the tank walls. Knowing wave propagation velocities and travel paths, it is possible to locate sources of PD activity with reasonable accuracy. The no-load loss of a transformer is primarily caused by losses in the core steel (item 1, above). The remaining two sources are sometimes ignored. As a result, the terms no-load loss, core loss, and iron loss are often used interchangeably. Core loss and iron loss, strictly speaking, refer only to the power loss that appears within the core material. The following discussion on no-load loss, or core loss, will explain why the average-voltage voltmeter method, to be described later, is recommended. The magnitude of no-load loss is a function of the magnitude, frequency, and waveform of the impressed voltage. These variables affect the magnitude and shape of the core magnetic flux waveform and hence affect the value of the core loss. It has been verified through measurements on power and distribution transformers that core loss also depends, to some extent, upon the temperature of the core. According to the IEEE test code [2], the approximate rate of change of no-load loss with core temperature is 0.00065 per-unit core loss increase for each C reduction in core temperature. The two main components of the core loss are hysteresis loss and eddy-current loss. The change in eddy-current loss, due to a change in the resistivity of the core steel as temperature changes, appears to be one factor that contributes to the observed core-loss temperature effect. The hysteresis loss magnitude is a function of the peak flux density in the core-flux waveform. When the impressed voltage waveform is distorted (not a pure sine wave), the resulting peak flux density in the flux waveform depends on the average absolute value of the impressed voltage wave. Eddy-current loss is a function of the frequency of the power source and the thickness of the core-steel laminations. Eddy loss is strongly influenced by harmonics in the impressed voltage. The IEEE transformer test code [2] recommends the averagevoltage voltmeter method, to be described below, for measuring no-load loss. 3.6.6.1.3 How No-Load Loss Is Measured The measurement of no-load loss, according to the average-voltage voltmeter method, is illustrated in Figure 3.6.14. Voltage and current transformers are required to scale the inputs for voltmeters, ammeters, and wattmeters. Three-phase no-load-loss measurements are carried out the same way, except that three sets of instruments and instrument transformers are utilized. The test involves raising voltage on one winding, usually the low-voltage winding, to its rated voltage while the other windings are in open circuit. Two voltmeters connected in parallel are employed. The voltmeter labeled V a in Figure 3.6.14 represents an average-responding, rms-calibrated voltmeter. The voltmeter labeled V r represents a true rms-responding voltmeter. Harmonics in the impressed voltage will cause the rms value of the waveform to be different from the average-absolute (rms-scaled) value, and the two voltmeter readings will differ. When the voltage reading, as measured by the average-responding voltmeter, reaches a value corresponding to the rated 3.6.6 Performance Characteristics 3.6.6.1 No-Load Loss and Excitation-Current Measurements 3.6.6.1.1 Purpose of No-Load Loss Measurements A transformer dissipates a constant no-load loss as long as it is energized at constant voltage, 24 hours a day, for all conditions of loading. This power loss represents a cost to the user during the lifetime of the transformer. Maximum values of the no-load loss of transformers are specified and often guaranteed by the manufacturer. No-load-loss measurements are made to verify that the no-load loss does not exceed the specified or guaranteed value. FIGURE 3.6.14 Test circuit for no-load-loss measurement. © 2004 by CRC Press LLC © 2004 by CRC Press LLC
voltage of the excited winding, readings are taken of the rms current, the rms voltage, and the no-load power. The ratio of the measured rms current to the rated load current of the excited winding, expressed in percent, is commonly referred to as the percent excitation current. The measured no-load loss is corrected to a sine-wave basis by a formula given in the IEEE test code [2], using the readings of the two voltmeters. The correction is shown below. The corrected value is reported as the no-load loss of the transformer. Pm Pc Vr P 2 1 P V 2 (3.6.1) where P c is the corrected (reported) value of no-load loss P m is the measured value of no-load loss V a is the reading of the average-responding, rms-calibrated voltmeter V r is the reading of the true-rms-responding voltmeter P1 and P2 are the per-unit hysteresis and per-unit eddy-current losses, respectively According to the IEEE test code [2], if the actual values of P1 and P2 are not available, it is suggested that the two loss components be assumed equal in value, assigning each a value of 0.5 p.u. 3.6.6.2 Load Loss and Impedance Measurements 3.6.6.2.1 Purpose of Load Loss Measurements A transformer dissipates a load loss that depends upon the transformer load current. Load loss is a cost to the user during the lifetime of the transformer. Maximum values of the load loss of transformers at rated current are specified and often guaranteed by the manufacturer. Load-loss measurements are made to verify that the load loss does not exceed the specified or guaranteed value. 3.6.6.2.2 Nature of the Quantity Being Measured The magnitude of the load loss is a function of the transformer load current. Its magnitude is zero when there is no load on the transformer. Load loss is always given for a specified transformer load, usually at rated load. Transformer load loss, often called copper loss or winding loss, includes I 2 R losses due to load current in the winding conductors and stray losses in various metallic transformer parts due to eddy currents induced by leakage fields. Stray losses are produced in the winding conductors, in core clamps, in metallic structural parts, in magnetic shields, and in tank walls due to the presence of leakage fields. Stray losses also include power loss due to circulating currents in parallel windings and in parallel conductors within windings. Because winding resistance varies with conductor temperature, and because the resistivities of the structural parts producing stray losses vary with temperature, the transformer load losses are a function of temperature. For this reason, a standard reference temperature (usually 85C) for reporting the load loss is established in ANSI and IEEE standards [1]. To correct the load-loss measurements from the temperature at which they are measured to the standard reference temperature, a correction formula is provided in the IEEE test code [2]. This correction involves the calculation of winding I 2 R losses, where I is the rated current of the winding in amperes, and R is the measured dc resistance of the winding. The I 2 R losses and the stray losses are separately corrected and combined in the formula given in the standard. The measurement of the dc winding resistance, R, is covered in Section 3.6.6.3. Stray losses are determined by subtracting the I 2 R losses from the measured total load losses. All of this is covered in detail in the IEEE test code [2]. The formula for this conversion is stated in general in Equation 3.6.2: W LL 2 2 2 2 ( I pRp IsRs)( Tk Tr) ( WmI pRp IsRs)( Tk Tm) ( T T ) ( T T ) k m a k r (3.6.2) where T r is the reference temperature (C) T m is the temperature at which the load loss is measured (C) W m is the load loss (W) as measured at temperature T m (C) W LL is the load loss (W) corrected to a reference temperature T r (C) T k is a winding material constant: 234.5 for copper, 225 for aluminum R p and R s are, respectively, the primary and secondary resistances () I p and I s are the primary and secondary rated currents (A) It can be seen from Equation 3.6.2 that the measurement of load losses and correction to the reference temperature involves the measurement of five separate quantities: 1. Electric power (the load loss as measured) 2. Temperature (the temperature at time of test) 3. Resistance of the primary winding (at a known temperature) 4. Resistance of the secondary winding (at a known temperature) 5. Electric current (needed to adjust the current to the required values) Another characteristic of transformer load loss is its low power factor. Most often, when considering electric-power measurement applications, the power factor of the load being measured is relatively high, usually exceeding 80%. For large modern power transformers, the power factor can be very low, in the range from 1 to 5%. The measurement of electric power at very low power factor requires special consideration, as discussed in the next section. 3.6.6.2.3 How Load Loss Is Measured Load losses are normally measured by connecting one winding, usually the low-voltage winding, to a short circuit with adequately sized shorting bars and connecting the other winding, usually the highvoltage winding, to a power-frequency voltage source. The source voltage is adjusted until the impressed voltage causes rated current to flow in both windings. Input rms voltage, rms current, and electric power are then measured. Figure 3.6.15 shows a circuit commonly used for measurement of load losses of a single-phase transformer. Three-phase measurement is carried out in the same way but with three sets of instruments and instrument transformers. Precision scaling devices are usually required because of the high magnitudes of current, voltage, and power involved. The applied test voltage when the transformer is connected as in Figure 3.6.15, with rated currents in the windings, is equal to the impedance voltage of the transformer. Hence impedance, or impedance voltage, is also measured during the load-loss test. The ratio of the measured voltage to the rated voltage of the winding, multiplied by 100, is the percent impedance voltage of the transformer. This quantity is commonly called “percent impedance” or, simply, “impedance.” 3.6.6.2.4 Discussion of the Measurement Process The equivalent circuit of the transformer being tested in the load-loss test and the phasor diagram of test voltage and current during the test are shown in Figure 3.6.16. The load-loss power factor for the load-loss test is cos = E R /E Z . Because the transformer leakage impedance, consisting of R and X in Figure 3.6.16, is mainly reactive, and more so the larger the transformer, the power factor during the load-loss test is very low. In addition, there is a trend in modern transformers to create designs with lower losses due to increased demands for improved efficiency and transformer load-loss evaluations for optimal life-cycle costs. Transformer designs for low values of load loss lead to reductions in the equivalent resistance shown in Figure 3.6.16, and hence to low values for the quantity, E R , which translates to lower values for the load-loss power factor in modern transformers. In fact the load-loss power factors for large modern transformers are often very low, in the range from 0.01 to 0.05 per unit. Under circuit conditions with very low power factor, the accurate measurement of electric power requires special scaling devices having very low phase-angle error and power measuring instruments having high accuracy at low power factor. In addition to the IEEE test code [2], a Transformer Loss Measurement Guide, C57.123, has © 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
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 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: FIGURE 3.6.10 Standard switching-im
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
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
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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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[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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