[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Recommendations

Info

3.12.3.2.2.4 Resistance — Such measurements must be made using suitable high-accuracy instruments; these measurements are not easy to make under service conditions. 3.12.3.2.2.5 Insulation Power Factor, Resistance, and Capacitance — A polarization index can be determined in the same manner. Polarization index results that are less than 2.0 indicate deterioration of the insulation. 3.12.3.2.2.6 Low-Voltage Exciting Current (if Previous Data Are Available) — This information is an excellent indication of winding movement, but the test must be performed under circumstances similar to the benchmark test to be of value. 3.12.3.2.2.7 Short-Circuit Impedance (if Faults Have Been Involved) — This test is an indicator of winding movement that may also give indication of shorted turns. 3.12.3.2.3 Internal Inspection If the problem cannot he identified from the test data and behavior analyses, an internal inspection may be necessary. In general, internal inspections should be avoided because the probability of failure increases after persons have entered transformers. The following items should be checked when the transformer is inspected. • Is there evidence of carbon tracks indicating flashovers or severe partial discharges? • Check leads for evidence of overheating. The insulation will be tan, brown, or black in extreme cases. • Check for evidence of partial-discharge trees or failure paths. Do odors indicate burned insulation or oil? • Check bolted connections for proper tightness. • Are leads and bushing lower shields in position? Is the lead insulation tight? • Inspect the windings for evidence of distortion or movement. • Check the end insulation on core-form designs for evidence of movement or looseness. • Are the support members at the ends of the phases tight in shell-form designs? • Are coil clamping devices tight and in position? • Check tap changers for contact deterioration. Is there evidence of problems in the operating mechanisms? • In core-form units, check visible parts of the core for evidence of heating. After the inspection has been completed, look over everything again for evidence of “does anything look abnormal.” Such final inspections frequently reveal something of value. 3.12.4 Failure Investigations The same processes are required in failure investigations as for problem analyses. In fact, it is recommended that the approaches described for problem investigations be performed before dismantling of the transformer to determine possible causes for the failure. Performing this work in advance usually results in hypotheses for the failure and frequently indicates directions for the dismantling. Thus, the only additional steps to be performed following a failure are to dismantle the transformer and make a determination whether it should be repaired. There is never a 100% certainty that the cause of failure can be determined. All theories should be tested against the facts available, and when assumptions come into doubt, the information must be replaced with data that is confirmed by the other facts available. As a general rule, all steps of the process should be documented with photographs. All too often it happens that something that was not considered relevant in the early stages of the investigation is later determined to be important. A photographic record will ensure that evidence that has been destroyed in the dismantling process is available for later evaluation. 3.12.4.1 Dismantling Process Complete dismantling is usually performed when a failure has occurred. In a few instances, such operations are performed to determine the cause for a problem, such as excessive gassing that could not be explained by other investigations. The following steps are recommended for this process. 3.12.4.1.1 Transformer Expert The process should be directed by a person or persons having knowledge of transformer design. If it is done in the original manufacturer’s plant, the manufacturer’s experts will usually be available. However, it is recommended that the user have experts available if the failure mechanism is in doubt. It is good to have two experts available because they may look into the failure from different perspectives and will provide the opportunity to discuss various aspects of the investigation from differing viewpoints. 3.12.4.1.2 Inspection before Untanking Inspect the tank for distortion resulting from high internal pressures that sometimes result from failures. Check the position of leads and connections. Determine if there has been movement of bushings. 3.12.4.1.3 Inspection after Removal of Core and Windings • Make a detailed inspection of the top ends of windings and cores. • Inspect the mechanical and electrical condition of the interphase insulation and the insulation between the windings and the tank. • Check the core ground. • Check lead entrances to windings for mechanical, thermal, and electrical condition. • Check the general condition of leads, connections, and tap changers. • Check the leakage flux shields on tank walls or on frames for heating or arcing. • Examine the wedging between phases and from the windings to the core on shell-form units and the winding clamping structures on core-form units to determine if there is still pressure on the windings. 3.12.4.1.4 Detailed Inspection of the Windings • Is there evidence of electrical failure paths from the windings over the major insulation to ground? • Is there evidence of distortion of the coils or windings resulting from short-circuit failures? • Has electrical failure occurred between turns, and has there been mechanical distortion of the turns? • Have failures occurred between windings or between coils? Are there weaknesses in nonfailed portions that could affect the electrical strength? Such instances include damage to turn insulation, distorted disks or coils, or insulation pieces not properly assembled. • Is there evidence of metallic contamination on the insulation and windings? • Is there evidence of hot spots at the ends of windings or in cap leads inside the disks or coils? • Is there any evidence of partial discharges or overheating in the leads or connections? • Were the windings properly supported for short circuit? • Are spacers in alignment? • Is insulation between windings and between the inner winding and the core tight? 3.12.4.1.5 Examination of the Magnetic Circuit • Have electrical arcs occurred to the core? If so, determine if the fault current flowed from the failure point to ground, resulting in damage to core laminations in this path. Note that any such damage will usually require scrapping of the laminations. • Is there evidence of leakage flux heating in the outer laminations? © 2004 by CRC Press LLC © 2004 by CRC Press LLC
• Has heating occurred as the result of large gaps at the joints, excessive burrs at slit, or cut edges and joints? • Does mechanical distortion exist in any parts of the core? • Is there evidence of heating in the lock plates used for mechanical support of the frames? • Is the core ground in good condition, with no evidence of heating or burning? Is there any evidence of a second (unintended) core ground having developed? 3.12.4.1.6 Mechanical Components • Is there distortion in the mechanical supports? • Is there evidence of leakage flux heating in the frames or frame shields? 3.12.5 Analysis of Information Information on interpretation of the data could take volumes, and there is much information on this subject in the technical literature. Some simple guides are listed below for reference. • The presence of high carbon monoxide and carbon dioxide are indications of thermal or oxidative damage to cellulose insulation. If there is high CO and there have been no overloads or previous indications of thermal problems, the problem may be excessive oxygen in the oil. • High oxygen is usually an indication of inadequate oil processing, gasket leaks, or leak of air through the rubber bag in expansion tanks. • Acetylene is an indication of arcing or very high temperatures. • Deterioration of oil dielectric strength usually results from particulate contamination or excessive water. • High power factor or low resistance between windings or from the windings to ground is usually the result of excessive water in the insulation. • High water in oil may result from excessive water in the paper. Over 95% of the total water in the system is in the paper, so that high water in oil is a reflection of the water in the paper. • Turns ratio different from previous measurements is an indication of shorts in a winding. The shorts can be between turns or between parts of windings, such as disk-to-disk. • A measurable change in the leakage impedance is an indication of winding movement or distortion. • Open circuits result from major burning in the windings or possibly a tap-changer malfunction. • High hydrogen, with methane being about 20% of the hydrogen, is an indication of partial discharges. “Spitting” or “cracking” noises noted prior to the failure are sometimes indicators of intense partial discharges. 3.12.5.1 Interpretation and Analysis of Information The most important part of the process is analysis of the information gathered. The objective is determining the cause of the problem or failure, and adequate analysis is obviously necessary if problems are to be solved and failures are to be prevented. There is no one process that is best for all situations. However, there are two helpful steps for reaching conclusions in such matters. • Make a systematic analysis of the data. • Compare data analyses to known problem and failure modes. 3.12.5.1.1 Systematic Analysis of Data • Prepare a list of known facts. List also the unknowns. Attempt to find answers for the unknowns that appear to be of importance. • Analyze known facts to determine if a pattern indicates the nature of the problem. • Prepare a spreadsheet of test data and observations, including inspections. Note items that appear to indicate the cause of the problem or failure. • Use problem-solving techniques. 3.12.5.1.2 Comparison to Known Problem and Failure Modes There are many recognized possible failure modes; a few are listed here as examples and for guidance. 3.12.5.1.2.1 Dielectric or Insulation Failures — Surface or creepage over long distances. If the design is shown to be adequate, this phenomenon is usually caused by contamination. If the design is marginal, slight amounts of contamination may initiate the discharges or failure. • Oil space breakdown. This can occur in any part of the insulation, since oil is the weak link in the insulation system. If the design is marginal, discharges can be initiated by particulate contamination or water in the oil. This type of breakdown usually occurs at interfaces with paper, such as at the edge of a radial spacer in a disk-to-disk space or at the edge of a spacer in a high-voltage winding to low-voltage winding space. • Oil breakdown over long distances, as from a bushing shield to tank wall or from a lead to ground. This problem type is usually caused by overstress in the large oil gap. It can occur in marginal situations if particles or gas bubbles are present in the gap. The dielectric strength of oil is lower at low temperatures if there is an appreciable amount of water in the oil. If such breakdowns occur in very low temperature conditions, investigate the oil strength at the low temperature as a function of the water in the oil. Consider also that the oil level may have been low by virtue of the very low temperature, causing parts normally under the oil to be exposed. • Turn-to-turn failures. If the design is adequate, such failures can result from mechanical weakness in the paper or from damage during short circuits if the paper is brittle due to thermal aging or oxidation. These failures usually are associated with fast transients such as lightning. • Extensive treeing in areas of high oil velocity, such as the oil entrance to the windings in forcedcooled designs. This can be associated with static electrification and usually occurs when the oil temperature is less than 40˚C and all pumps are in operation. • Discharges or failure originating from joints in leads. This type of failure usually results from the paper not being tight at the joint in the tape. Discharges start in the oil space at the surface of the cable and propagate out through the joint. IEEE C57.125-1991, IEEE Guide for Failure Investigation, Documentation, and Analysis for Power Transformers and Shunt Reactors, contains a comprehensive treatment of insulation system failure and analysis of the relative voltage stresses that can lead to discharges. 3.12.5.1.2.2 Thermal or Oxidation Failure Modes — Deteriorated insulation at the end turns of coreform transformers or on the outer turns of line coils in shell-form designs. Such deterioration is caused by local hot spots. The eddy losses are higher in these regions, and the designer may have used added insulation in some regions that have high electrical stress. • Overheated lap leads. This usually occurs because the designer has used added insulation on the leads. The leads may have added eddy loss because they are in a high leakage flux field. • Leads with brown or black paper at the surface of the conductor. This results from excessive paper insulation on the lead. • Joints with deteriorated paper. The resistance of the joint may be too high, or there may be leakage flux heating if the connector is wide. • Damaged paper or pressboard adjacent to the core or core supports. This type of heating is usually the result of leakage flux heating in the laminations or core joints. • Paper has lost much of its strength, but there have been no thermal stresses. This is the result of excessive oxygen in the oil. In the initial stages of the process, the outer layers of paper will have more damage than the inner layers. © 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 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 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: Records of relay operation must be
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
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?