[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Recommendations

Info

FIGURE 3.6.17 Circuit for measuring winding resistance. FIGURE 3.6.15 Test circuit for load-loss measurement. FIGURE 3.6.16 Equivalent circuit and phasor diagram for load-loss test. recently been developed by a working group of the IEEE PES <strong>Transformer</strong>s Committee. When generally available, it will provide a complete background and the basis for carrying out measurements and calibrations to ensure accurate values of reported losses as required by ANSI and IEEE standards [1]. Because the magnitudes of load losses and impedance depend upon the tap positions of the deenergized tap changer (DETC) and the load tap changer (LTC), if present, load-loss and impedancevoltage measurements are usually carried out in the rated voltage connection and at the tap extremes. If the transformer under test has multiple MVA ratings that depend on the type of cooling, the tests are normally carried out at all ratings. 3.6.6.3 Winding Resistance Measurements 3.6.6.3.1 Purpose of Winding Resistance Measurements Measurements of dc winding resistance are of fundamental importance because they form the basis for determining the following: Resistance measurements, taken at known temperatures, are used in the calculation of winding conductor I 2 R losses. The I 2 R losses at known temperatures are used to correct the measured load losses to a standard reference temperature. Correction of load losses is discussed in section 3.6.6.2. Resistance measurements, taken at known temperatures, provide the basis to determine the temperature of the same winding at a later time by measuring the resistance again. From the change in resistance, the change in temperature can be deduced. This measurement is employed to determine average winding temperatures at the end of heat run tests. Taking resistance measurements after a heat run test is discussed in section 3.6.6.4. Resistance measurements across the transformer terminals provide an assessment of the quality of internal connections made to the transformer windings. Loose or defective connections are indicated by unusually high or unstable resistance readings. 3.6.6.3.2 Nature of the Quantity Being Measured The dc winding resistance differs from the value of resistance indicated for the resistor shown in Figure 3.6.16 or the resistors that appear in textbook illustrations of the PI or T equivalent circuits of transformers to represent the resistance of the windings. The resistors in the equivalent circuits include the effects of winding I 2 R loss, eddy loss in the windings, stray losses in structural parts, and circulating currents in parallel conductors — namely, they represent the resistive components of the load loss. The resistors shown in the equivalent circuits can be thought of as representing an equivalent ac resistance of the windings. The dc resistance of the windings is a different quantity, one that is relevant for calculating I 2 R, for determining average winding temperature, and for evaluating electrical connections. 3.6.6.3.3 How Winding-Resistance Measurements Are Made The measurement of power transformer winding resistance is normally done using the voltmeter-ammeter method or using a ratiometric method to display the voltage–current ratio directly. A circuit for the measurement of winding resistance is shown in Figure 3.6.17. A dc source is used to establish the flow of steady direct current in the transformer winding to be measured. After the R-L transient has subsided, simultaneous readings are taken of the voltage across the winding and the current through the winding. The resistance of the winding is determined from these readings based on Ohm’s law. 3.6.6.3.4 Discussion of the Measurement Process If a dc voltage is applied as a step to a series R-L circuit, the current will rise exponentially with a time constant of L/R. This is familiar for the case where both resistance and inductance remain constant during the transient period. For a transformer winding, however, it is possible for the true resistance, the apparent resistance, and the inductance of the winding to change with time. The true resistance may change if the direct current is of high-enough magnitude and is applied long enough to heat the winding substantially, thereby changing its resistance during the measurement. The inductance changes with time because of the nonlinear B-H curve of the core steel and varies in accordance with the slope of the coresteel saturation curve. In addition, there is an apparent resistance, Ra, during the transient period. V Ra R L dI (3.6.3) I I dt Note that the apparent resistance, Ra, is higher than the true resistance, R, during the transient period and that the apparent resistance derived from the voltmeter and ammeter readings equals the true resistance only after the transient has subsided. Resistance measurement error due to heating of the winding conductor is usually not a problem in testing transformers, but the possibility of this effect should be taken into consideration, especially for © 2004 by CRC Press LLC © 2004 by CRC Press LLC
FIGURE 3.6.18 Current, voltage, and apparent resistance with time. some low-current distribution transformer windings where the dc current can be significant compared with the rated current. It is more likely that errors will occur because of meter readings taken before core saturation is achieved. The process involved in core saturation is described below. Compared with the exponential current-vs.-time relationship for the R-L circuit with constant R and constant L, the current in a transformer winding, when a dc voltage is first applied, rises slowly. The slow rate of rise comes about because of the high initial impedance of the winding. The initially high impedance results from the large effective inductance of the winding with its iron core. As the current slowly increases, the flux density in the core slowly rises until the core begins to saturate. At this point, the winding no longer behaves like an iron-core coil and instead behaves like an air-core coil, with relatively low inductance. The rate of rise of the current increases for a period as the core saturates; then the current levels off at a steady-state value. Typical shapes for the voltage, current, and apparent resistance are shown in Figure 3.6.18. The magnitude of the dc voltage affects the rate at which flux builds up in the core, since V = N(d/dt). The higher the magnitude of the dc voltage, the shorter is the time to saturation because of a higher value for d/dt. At the same time, though, the coil must be able to provide the required magnetomotive force in ampere turns, N I, needed to force the core into saturation, which leads to a minimum value for the dc current. Of course, there is an upper limit to the value for dc current, namely the point at which conductor heating would disturb the resistance measurement. Note the time scale of the graph in Figure 3.6.18. It is very important that the steady-state dc current be attained before meter readings are taken. If this is not done, errors in excess of 20% are easily realized. 3.6.6.3.5 Winding Resistance and Average Winding Temperature Two of the three purposes listed above for measuring the dc resistance of a transformer winding inherently involve a concomitant measurement of temperature. When measuring resistance for the purpose of calculating I 2 R at a given temperature, the I 2 R value obtained will be used to determine the load-loss value at a different temperature. When the winding resistance is measured before and during a heat run, the determination of average winding temperature at the end of a heat run test requires knowledge of winding resistance at two temperatures. The winding dc resistance at two temperatures, T1 and T2, will have values of R1 and R2, respectively, at the two temperatures. The functional relationship between winding resistance and average temperature is shown in Equation 3.6.4: R1 T1 Tk R2 T2 T k (3.6.4) where R1 is the value of winding resistance, corresponding to average winding temperature of T1 R2 is the value of winding resistance, corresponding to average winding temperature of T2 T k is 234.5C for copper, 225C for aluminum Correction of load loss for temperature is covered in section 3.6.6.2. Determination of average winding temperature in a heat run test is covered in section 3.6.6.4. 3.6.6.4 Heat Run Tests 3.6.6.4.1 Purpose of Heat Run Tests The maximum allowable average and hottest-spot temperature rises of the windings over ambient temperature and the maximum allowable temperature rise of the top oil of the transformer are specified by ANSI and IEEE standards and are guaranteed by the manufacturer. The purpose of temperature-rise tests is to demonstrate that the transformer will deliver rated load without exceeding the guaranteed values of the temperature rises of the windings and oil. According to the ANSI and IEEE standards, these tests are performed at the minimum and maximum load ratings of a transformer. 3.6.6.4.2 Test Methods For factory testing, it is not practical to connect the transformer to a load impedance with full rated secondary voltage applied to the simulated load. Although this would most directly simulate service conditions, most of the total test input power would dissipate in the load impedance. The load power, which equals the load rating of the transformer, is much larger than the sum of no-load and load losses that dissipate in the transformer. The electrical heating of the load would not contribute to transformer heating. <strong>Electric</strong> power consumption for the test would be excessive, and the test would not be practical for routine testing. In factory tests according to methods specified in the ANSI and IEEE standards, several artificial loading schemes can be used to simulate heat dissipation caused by the load and no-load losses of the transformer. The back-to-back loading method, described in the IEEE test code [2], requires two identical transformers and is often used for heat runs of distribution transformers. For power transformers, it is most common to use the short-circuit loading method, as specified in the IEEE test code [2]. The test setup is similar to that used for measurement of load loss and impedance voltage. One winding is connected to a short circuit, and sufficient voltage is applied to the other winding to result in currents in both windings that generate the required power loss to heat the oil and windings. 3.6.6.4.3 Determination of the Top, Bottom, and Average Oil Rises This discussion applies to the short-circuit loading method. Initially, the test current is adjusted to provide an input power loss equal to the no-load loss plus the load loss. This can be called the total loss. The total loss is corrected to the guaranteed temperature rise plus ambient temperature. During this portion of the heat run, the windings provide a heat source for the oil and the oil cooling system. The winding temperatures will be higher than expected because higher-than-rated currents are applied to the windings, but here only the oil temperature rises are being determined. For an oil-filled power transformer, the total power loss for a given rating is maintained until the top-oil temperature rise is stabilized. Stabilization is defined as no more than 1C change in three consecutive 1-h periods. After stabilization is achieved, the top- and bottom-oil readings are used to determine the top, bottom, and average oil temperature rises over ambient at the specified load. 3.6.6.4.4 Determination of the Average Winding-Temperature Rise After the top, bottom, and average oil temperature rises are determined, the currents in the windings are reduced to rated value for a period of 1 h. Immediately following this 1-h period, the ac power leads are disconnected, and resistance measurements are carried out on both windings. The total time from disconnection of power and the first resistance readings should be as short as possible, typically less than 2 min — certainly not more than 4 min. Repeated measurements of winding resistance are carried out © 2004 by CRC Press LLC © 2004 by CRC Press LLC
Page 1 and 2:
ELECTRIC POWER TRANSFORMER ENGINEER
Page 3 and 4:
I wish to recognize the interest of
Page 5 and 6:
Contents 1 Theory and Principles Ch
Page 7 and 8:
1.3 Equivalent Circuit of an Iron-C
Page 9 and 10:
designer starts to make a design fo
Page 11 and 12:
• Transient voltages generated du
Page 13 and 14:
the transformer’s bushings and, m
Page 15 and 16:
% regulation = [(V NL - V FL )/V FL
Page 17 and 18:
In core-form transformers, the wind
Page 19 and 20:
FIGURE 2.1.14 Layer windings (singl
Page 21 and 22:
the transformer design, which may b
Page 23 and 24:
consumer’s service circuit is a d
Page 25 and 26:
FIGURE 2.2.3 Single-phase transform
Page 27 and 28:
is installed so that the tank never
Page 29 and 30:
above which persons might be burned
Page 31 and 32:
FIGURE 2.2.16 Two-bushing subway. (
Page 33 and 34:
carbon steel or the very expensive
Page 35 and 36:
FIGURE 2.2.31 Radial-style dead fro
Page 37 and 38:
FIGURE 2.2.36 Complete transformer
Page 39 and 40:
voltage in each winding simultaneou
Page 41 and 42:
mounted transformers can have arres
Page 43 and 44:
V S + V S I V L Z=R+jX a) V L V S V
Page 45 and 46:
Z = jX (2.3.19) 0.7 Then the curren
Page 47 and 48:
X1 L* X1 S =X1 L X2 L* X3 L* a) S L
Page 49 and 50:
150 V(%) 100 50 0 -50 40 80 T(s) 12
Page 51 and 52:
2.3.8.1.2 Series Connection • The
Page 53 and 54:
A six-pulse single-way transformer
Page 55 and 56:
The degree of coupling also influen
Page 57 and 58:
where I h = current for the hth har
Page 59 and 60:
In the case of liquid-filled transf
Page 61 and 62:
FIGURE 2.4.21 A 36-pulse system mad
Page 63 and 64:
Vacuum-pressure encaps ulated — T
Page 65 and 66:
FIGURE 2.6.1 Typical wiring and sin
Page 67 and 68:
a) H1 b) Ip X2 FLUX H2 LEAKAGE FLUX
Page 69 and 70:
and for CTs is TCF = RCF - (PA/2600
Page 71 and 72:
RCF x Bx Bt RCF t - RCF 0 co
Page 73 and 74:
2000 1000 5% Vex BANDWITH FROM NOMI
Page 75 and 76:
This may be more useful when operat
Page 77 and 78:
also some uncertainty in repeatabil
Page 79 and 80:
FIGURE 2.6.25 Standard two-element
Page 81 and 82:
2.7 Step-Voltage Regulators Craig A
Page 83 and 84:
Source Bypass Switch Source Bypass
Page 85 and 86:
2.7.4 Theory A step-voltage regulat
Page 87 and 88:
FIGURE 2.7.22 Equalizer winding inc
Page 89 and 90:
TABLE 2.7.4 Increase in Ampere Load
Page 91 and 92:
References IEEE, Standard Requireme
Page 93 and 94:
300% FIGURE 2.8.4 Schematic of a co
Page 95 and 96:
TABLE 2.8.1 Typical Sizing Workshee
Page 97 and 98:
Input with Interruption Ferro Outpu
Page 99 and 100:
FIGURE 2.8.20A Performance of a rid
Page 101 and 102:
• Input frequency or frequency ra
Page 103 and 104:
References 1. Sola/Hevi-Duty Corp.,
Page 105 and 106:
FIGURE 2.9.5 Typical phase-reactor
Page 107 and 108:
FIGURE 2.9.10 Two generator arrange
Page 109 and 110:
• Overloading of lines • Increa
Page 111 and 112:
FIGURE 2.9.23 34-kV, 25-MVAR (per p
Page 113 and 114:
V 90 ( X X ) s T (2.9.10a) where,
Page 115 and 116: Therefore, Line reactive losses = (
Page 117 and 118: ate of rise of transient-recovery v
Page 119 and 120: the LLCR is provided in IEEE Std. C
Page 121 and 122: aluminum shielding, i.e., an oil-im
Page 123 and 124: Leo J. Savio ADAPT Corporation Ted
Page 125 and 126: 3.1.2.2.2 Heat Dissipation A second
Page 127 and 128: FIGURE 3.2.1 Solid-type bushing. ma
Page 129 and 130: 1.6 cm. This means that most of the
Page 131 and 132: where HS = steady-state bushing ho
Page 133 and 134: the conductivity of the water-solub
Page 135 and 136: of 178 ohm-m, and a test duration o
Page 137 and 138: 2. Easley, J.K. and Stockum, F.R.,
Page 139 and 140: FIGURE 3.3.5 Reactance-type LTC, in
Page 141 and 142: FIGURE 3.3.10 LTC with delta connec
Page 143 and 144: 3.3.5 Selection of Load Tap Changer
Page 145 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: voltage of the excited winding, rea
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
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 237 and 238:
Page 239 and 240:
Page 241:
TABLE 3.14.13 Relevant Documents fo
show all

[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?