[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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1. Design tests 2. Routine tests 3. Special tests FIGURE 3.2.9 Lapp test terminal. (Photo courtesy of Lapp Insulator Company, Leroy, NY.) At higher voltages, excessive amounts of corona may occur due to the relatively sharp edges present on the top or both ends of the Lapp test terminal. Large-diameter corona rings are therefore placed over one or both ends of the terminal. 3.2.7.3 Draw-Lead Conductors Normally, bushings have current ratings of 1200 A or higher. Some applications of bushings mounted in transformers with lower MVA ratings do not require these current-carrying capacities. In these cases, it is practical to run a smaller-diameter cable inside the hollow central tube in the bushing and connect it directly to the transformer winding. If the bushing must be removed for some reason, the transformer oil can be lowered, as necessary, to a level below the top of the transformer tank or turret. Then, the top of the draw-lead is unfastened from the top terminal of the bushing. The bushing can then be lifted out of the transformer and replaced with a new one, and the top terminal is reinstalled to the draw-lead. Finally, the oil can be adjusted to the proper level in the transformer. The use of a draw-lead therefore enables much faster replacement of bushings and eliminates the need for time-consuming processing of oil for the transformer. Current-carrying capabilities of draw-leads are established by transformer manufacturers and are not standardized at this time. In general, the capability will increase with the cross-sectional area of the cable and decrease with the length of the cable. For these reasons, larger-sized holes are placed in the central tubes of bushings with higher voltage ratings. Table 3.2.4 gives the maximum current ratings of drawlead applications, the current rating of the same bushing for bottom-connected applications, and the minimum hole size in the central conductor. [6] 3.2.8 Tests on Bushings 3.2.8.1 Categories of Tests Standards [1] designate three types of tests to be applied to bushings: TABLE 3.2.4 Current Ratings of Bushings Capable of Being Used with Draw-Leads Nominal System Voltage, kV Maximum Draw-Lead Current Rating, A Bottom-Connected Current Rating, A Minimum Diameter inside Tube, mm 34.5–69 400 1200 22 138–230 800 1200 41 345–765 800 1200 51 3.2.8.1.1 Design Tests Design, or type, tests are only made on prototype bushings, i.e., the first of a design. The purpose of design tests is to ascertain that the bushing design is adequate to meet its assigned ratings, to ensure that the bushing can operate satisfactorily under usual or special service conditions, and to demonstrate compliance with industry standards. These tests need not be repeated unless the customer deems it necessary to have them performed on a routine basis. Test levels at which bushings are tested during design tests are higher than the levels encountered during normal service so as to establish margins that take into account dielectric aging of insulation as well as material and manufacturing variations in successive bushings. Bushings must withstand these tests without evidence of partial or full failure, and incipient damage that initiates during the dielectric tests is usually detected by comparing values of power factor, capacitance, and partial discharge before and after the testing program. Standards [1] prescribe the following design tests: Low-frequency wet-withstand voltage on bushings rated 242 kV maximum system voltage and less Full-wave lightning-impulse-withstand voltage Chopped-wave lightning-impulse-withstand voltage Wet-switching-impulse-withstand voltage on bushings rated 345 kV maximum system voltage and greater Draw-lead bushing-cap pressure test Cantilever-withstand test Temperature test at rated current 3.2.8.1.2 Routine Tests Routine, or production, tests are made on every bushing produced, and their purpose is to check the quality of the workmanship and the materials used in the manufacture. Standards [1] prescribe the following routine tests: Capacitance and power-factor measurements at 10 kV Low-frequency dry-withstand test with partial-discharge measurements Tap-withstand voltage test Internal hydraulic pressure test 3.2.8.1.3 Special Tests Special tests are for establishing the characteristics of a design practice and are not part of routine or design tests. The only special test currently included in standards [1] is the thermal-stability test, only applicable to extra high voltage (EHV) bushings, but other tests could be added in the future. These include short-time, short-circuit withstand and seismic capabilities. 3.2.8.2 Dielectric Tests 3.2.8.2.1 Low-Frequency Tests There are two low-frequency tests: 1. Low-frequency wet-withstand voltage test 2. Low-frequency dry-withstand voltage test 3.2.8.2.1.1 Low-Frequency Wet-Withstand Voltage Test — The low-frequency wet-withstand voltage test is applied on bushings rated 242 kV and below while a waterfall at a particular precipitation rate and conductivity is applied. The values of precipitation rate, water resistivity, and the time of application vary in different countries. American standard practice is a precipitation rate of 5 mm/min, a resistivity © 2004 by CRC Press LLC © 2004 by CRC Press LLC
of 178 ohm-m, and a test duration of 10 sec, whereas European practice is 3 mm/min, 100 ohmm, and 60 sec, respectively [15]. If the bushing flashes over externally during the test, it is allowed that the test be applied one additional time. If this attempt also flashes over, then the test fails and something must be done to modify the bushing design or test setup so that the capability can be established. 3.2.8.2.1.2 Low-Frequency Dry-Withstand Voltage Test — The low-frequency dry-withstand test was, until recently, made for a 1-min duration without the aid of partial-discharge measurements to detect incipient failures, but standards [1] currently specify a one-hour duration for the design test, in addition to partial-discharge measurements. The present test procedure is: Partial discharge (either radio-influence voltage or apparent charge) shall be measured at 1.5 times the maximum line-ground voltage. Maximum limits for partial discharge vary for different bushing constructions and range from 10 to 100 V or pC. A 1-min test at the dry-withstand level, approximately 1.7 times the maximum line-ground voltage, is applied. If an external flashover occurs, it is allowed to make another attempt, but if this one also fails, the bushing fails the test. No partial-discharge tests are required for this test. Partial-discharge measurements are repeated every 5 min during the one-hour test duration at 1.5 times maximum line-ground voltage required for the design test. Routine tests specify only a measurement of partial discharge at 1.5 times maximum line-ground voltage, after which the test is considered complete. Bushing standards were changed in the early 1990s to align with the transformer practice, which started to use the one-hour test with partial-discharge measurements in the late 1970s. Experience with this new approach has been good in that incipient failures were uncovered in the factory test laboratory, rather than in service, and it was decided to add this procedure to the bushing test procedure. Also from a more practical standpoint, bushings are applied to every transformer, and transformer manufacturers require that these tests be applied to the bushings prior to application so as to reduce the number of bushing failures during the transformer tests. 3.2.8.2.2 Wet-Switching-Impulse-Withstand Voltage This test is required on bushings rated for 345-kV systems and above. The test waveshape is 250 s timeto-crest and 2500 s time-to-half-value with tolerance of 30% on the time to crest and 20% on the time-to-half-value. This is the standard waveshape for testing insulation systems without magnetic-core steel present in the test object and is different than the waveshape for transformers. Three different standard test procedures are commonly used to establish the wet-switching-impulsewithstand voltage of the external insulation: Fifteen impulses of each polarity are applied, with no more than two flashovers allowed. Three impulses of each polarity are applied. If a flashover occurs, then it is permitted to apply three additional impulses. If no flashovers occur at either polarity, then the bushing passes the test. Otherwise, the bushing fails the test. The 90% (1.3 ) level is established from the 50% flashover tests. 3.2.8.2.3 Lightning-Impulse Tests The same waveshapes are used to establish the lightning-impulse capability of bushings and transformers. The waveshape for the full wave is 1.2 s for the wavefront and 50 s for the time-to-half-value, and the chopped wave flashes over at a minimum of 3.0 s. One of the same procedures as described above for the wet-switching-impulse tests is followed to establish the full-wave capability for both polarities. The chopped-wave capability is established by applying a minimum of three chopped impulses at each polarity. 3.2.8.3 Mechanical Tests IEEE Std. C57.19.01 [6] specifies the static-cantilever-withstand forces to be applied separately to the top and bottom ends of outdoor-apparatus bushings. The forces applied to the top end range from 68 kg (150 lb) for the smaller, lower-voltage bushings to 545 kg (1200 lb) for the larger, higher-voltage or current bushings, and the forces applied to the lower end are generally about twice the top-end forces. The test procedure is to apply the specified forces perpendicular to the bushing axis, first at one end then at the other, each application of force lasting 1 min. Permanent deflection, measured at the bottom end, shall not exceed 0.76 mm, and there shall be no oil leakage at either end at any time during the test or within 10 min after removing the force. 3.2.8.4 Thermal Tests There are two thermal tests. The first is the thermal test at rated current, and it is applied to all bushing designs. The second test is the thermal-stability test [11], and it is applied for only EHV bushings. 3.2.8.4.1 Thermal Test at Rated Current This test demonstrates a bottom-connected bushing’s ability to carry rated current. The bushing is first equipped with a sufficient number of thermocouples, usually placed inside the inner diameter of the hollow-tube conductor, to measure the hottest-spot temperature of the conductor. The bushing is then placed in an oil-filled tank, the oil is heated to a temperature rise above ambient air of 55C for transformers and 40C for circuit breakers, and rated current is passed through the central conductor until thermal equilibrium is reached. The bushing passes the test if the hottest-spot temperature rise above ambient air does not exceed 65C. 3.2.8.4.2 Thermal-Stability Test [16] Capacitive leakage currents in the insulating material within bushings cause dielectric losses. Dielectric losses within a bushing can be calculated by the following equation using data directly from the nameplate or test report: P d = 2 f C V 2 tan (3.2.9) where P d = dielectric losses, W f = applied frequency, Hz C = capacitance of bushing (C 1 ), F V = operating voltage, rms V tan = dissipation factor, p.u. A bushing operating at rated voltage and current generates both ohmic and dielectric losses within the conductor and insulation, respectively. Since these losses, which both appear in the form of heat, are generated at different locations within the bushing, they are not directly additive. However, heat generated in the conductor influences the quantity of heat that escapes from within the core. A significant amount of heat generated in the conductor will raise the conductor temperature and prevent losses from escaping from the inner surface of the core. This causes the dielectric losses to escape from only the outer surface of the core, consequently raising the hottest-spot temperature within the core. Most insulating materials display an increasing dissipation factor, tan , with higher temperatures, such that as the temperature rises, tan also rises, which in turn raises the temperature even more. If this cycle does not stabilize, then tan increases rapidly, and total failure of the insulation system ensues. Bushing failures due to thermal instability have occurred both on the test floor and in service. One of the classic symptoms of a thermal-stability failure is the high internal pressure caused by the gases generated from the deteriorating insulation. These high pressures cause an insulator, usually the outer one because of its larger size, either to lift off the flange or to explode. If the latter event occurs with a porcelain insulator, shards of porcelain saturated with oil become flaming projectiles, endangering the lives of personnel and causing damage to nearby substation equipment. Note from Equation 3.2.9 that the operating voltage, V, particularly influences the losses generated within the insulating material. It has been found from testing experience that thermal stability only becomes a factor at operating voltages 500 kV and above. © 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
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: the conductivity of the water-solub
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 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
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Winding 1 Secondary Currents Data A
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Y Y 20 18 3 rd Harmonic CTR1=40 CTR
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230 kV 3 180 MVA 138 kV 5 CTR1 = 2
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29. Concordia, C. and Rothe, F.S.,
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FIGURE 3.9.1 Measurement of pressur
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H 2m 1. Vertical Forced Air 2. Prin
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Gade, S., Sound Intensity Instrumen
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nected. This steady-state voltage d
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where [A] = state matrix [B] = inpu
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where C = capacitance between the t
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L ijo = N i N j ijo (3.10.31) 1.4
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% VOLTAGE % VOLTAGE 100 BIL 90 50 3
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29. Degeneff, R.C., Reducing Storag
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All connections should be cleaned a
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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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