[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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installation book for information concerning vacuum oil filling of the unit. Some LTCs require a vacuum line to main tank for equalizing the pressure. Do not operate the LTC mechanism while the unit is on vacuum, as severe damage could occur to the mounting board. If the LTC requires oil, do not add the oil while the main tank is on vacuum as the unequal pressure could damage the LTC. The process of adding oil to the external LTC tank will put pressure inside the tank. This added positive pressure along with the negative gage pressure of the main tank could cause the LTC barrier boards to rupture. No additional work should be attempted while the main transformer tank is under vacuum. Look for loose hardware or any misalignment of the contacts. Operate the LTC through all positions and check each contact for alignment. Refer to the supplier’s instruction manual for the allowable variance. Perfect “center line” alignment during the complete range of operation from 16R to 16L will be difficult to achieve. 3.11.1.8 Positive Pressure System This system consists of a cabinet with regulating equipment and alarms with an attached nitrogen bottle. A positive supply of nitrogen is kept on the transformer. With positive pressure on the tank, the possibility of moisture entering is reduced. Loss of nitrogen pressure, without any oil spillage, is usually found in the transformer “gas space” or the nitrogen supply system. 3.11.1.9 Control Cabinet All control equipment must be inspected for loose wiring or problems caused by the shipping. The fan, gauges, LTC controls, and monitoring equipment must be tested or calibrated. Information for the installation and/or the calibration should be supplied by the manufacturer. 3.11.1.10 Accessories There are many items that could be required for your particular transformer. A few are listed below. 1. HV and LV arrestors 2. External current transformers 3. Discharge meters 4. Neutral grounding resistors 5. Bushing potential devices 6. Cooling fan (hi speed) 7. Gas monitors (various types) Caution: All oil handling equipment, transformer bushings, and the transformer should be grounded before starting the vacuum oil cycle. Special requirements are needed for vacuum oil filling in cold weather. Check your manufacturer’s manual. 3.11.1.11 Vacuum Cycle Pulling vacuum on a transformer is usually done through the mechanical relief flange or a special vacuum valve located on the cover of the transformer. A vacuum se nsor, to send a sig nal t o the v acuum r ecording gauge, should be at the hig hest location on the t ransformer’s cover. This position reduces the risk of the sensor being contaminated with oil, which would let the vacuum gauge give a false reading. All readings from this gauge should be recorded at least every hour. Note: All radiator and cooler valves should be open prior to starting the vacuum cycle. 3.11.1.12 Vacuum Filling System Manufacturers differ on the duration of vacuum required and the method to add oil to the unit. It is important that the vacuum crew doing this process follow the correct procedure as stated by the manufacturer or the warranty can be invalid. Good record-keeping during this process is just as important for your information as it is for supplying the manufacturer with information that validates the warranty. The length of time (pulling vacuum) will vary as to the exposure time to atmospheric air, the transformer rating, and the dew point/moisture calculations. Most of the needed information as to the vacuum cycle time, should be furnished in the installation book. (See Figs. 3.11.1 and 3.11.2.) Always consider that some contractors have used their equipment on older and/or failed transformers. The equipment needs to be thoroughly cleaned with new transformer oil and a new filter medium added to the oil filtering equipment. The vacuum oil pump should have new vacuum oil installed and it should be able to “pull-down” against a closed valve to below 1 mm of pressure. 3.11.1.13 Transformer Oil The oil supplied should be secured from an approved source and meet the IEEE C57 106-1991 guide for acceptance. When requested, an inhibitor can be added to the oil, to a level of 0.3% by weight. 3.11.1.14 Adding the Oil All oil from tankers should be field tested for acceptable dielectric level prior to pumping oil through the oil handling equipment. The oil temperature should be at least 0°C while pumping directly from an oil tank car. A superior method that will assist in the removal of moisture involves heating the oil to 50 to 70°C and passing the oil through a filter. Oil filling a conservator transformer takes a lot more time as the piping and the conservator have to be slowly filled while air is “bled” out of the piping, bushings, CT turrets, and gas monitor. Methods vary for adding the oil to the conservator because of the risk to the oil bladder. Weeks later, the air should be “bled” again. If this is not done, you could receive a false signal that may take the transformer out of service. 3.11.1.15 Field Test After the vacuum oil filling cycle, the transformer should be field tested and compared to some of the factory tests. The field test also will give you a baseline record for future reference. These are a few of the tests that are routinely done: 1. Current transformer ratio 2. Turns ratio of the transformer, including tests at al LTC tap positions 3. Power factor bushings 4. Power factor transformer 5. Winding resistance 6. Core ground 7. Lab test of the oil 8. Test all gauges 9. Test all pressure switches A field test report may be required by the manufacturer in order to validate the warranty. Any questionable test values should be brought to the attention of the transformer manufacturer. 3.11.2 Transformer Maintenance The present maintenance trend is to reduce cost, which in some cases means lengthening the intervals of time to do maintenance or eliminating the maintenance completely. The utility, or company, realizes some savings on manpower and material by lengthening the maintenances cycle, but by doing this, the risk factor is increased. A few thousand dollars for a maintenance program could save your utility or company a half-million dollar transformer. Consider the following: 1. The length of time to have a transformer rebuilt or replaced 2. The extra load on your system 3. Rigging costs to move the transformer 4. Freight costs for the repair, or buying a new one 5. Disassembly and reassembly costs © 2004 by CRC Press LLC © 2004 by CRC Press LLC
6. Costs to set up and use a mobile transformer 7. Costs for oil handling of a failed unit 8. Vacuum oil filling of the rebuilt or new transformer 9. Customer’s dissatisfaction with outage 10. Labor costs, which usually cover a lot of overtime or employees pulled away from their normal work schedule You will have to answer questions such as: Why did this happen? Could it have been prevented? Time spent on a scheduled maintenance program is well worth the expense. There are many systems available to monitor the transformer which can assist you in scheduling your maintenance program. Many transformer manufacturers can supply monitoring equipment that alerts the owner to potential problems. However, relying solely on monitoring equipment may not give your notice or alert you to mechanical problems. Some of these problems can be: fans that fail, pressure switches that malfunction, or oil pumps that cease to function. You could also have oil leaks that need to be repaired. Annual inspections can provide a chance for correcting a minor repair before it becomes a major repair. 3.11.2.1 Maintenance Tests There are two important tests that could prevent a field failure. Using an infrared scan on a transformer could locate “hot spots”. The high temperature areas could be caused by a radiator valve closed, low oil in a bushing, or an LTC problem. Early detection could allow time to repair the problem. Another important test is dissolved gas analysis test of the oil by a lab. A dissolved gas analysis lab test will let you know if high levels of gases are found and they will inform you as to the recommended action. Following the lab report could let you plan your course of action. If there seems to be a problem, it would be worthwhile to take a second dissolved gas in oil sample and send it to a different lab and compare the results (IEEE C57 104-1991). Maintenance inspection and tests can be divided into two sections: (1) minor and, after a set period of years, (2) major inspection. Annual tests are usually done while the transformer is in service, and consist of the following: 1. Check the operation of the LTC mechanism for misalignment or excessive noise. 2. Take an oil sample from the LTC. 3. Change silica gel in breathers. 4. Inspect fan operation. 5. Take an oil sample from the main tank. 6. Check oil level in bushings. 7. Check tank and radiators for oil leaks. 8. Check for oil levels in main tank and the LTC. 9. Make sure all control heaters are operating. 10. Check all door gaskets. 11. Record the amount of LTC operations and operate through a couple of positions. 12. Most importantly, have your own check-off list and take time to do each check. This record (checkoff list) can be used for future reference. Major inspections require the transformer to be out of service. Both primary and secondary bushings should be grounded before doing the work. Besides the annual inspection checks that should be made, the following should also be done: 1. Power factor the bushings and compare to the values found during the installation tests. 2. Power factor the transformer and check these valves. 3. Make a complete inspection of the LTC and replace any questionable parts. If major repair is required during this inspection, a turns ratio test should be done. 4. Painting rusty areas may be necessary. 5. Test all pressure switches and alarms. 6. Check the tightness of all bolted connections. 7. Check and test the control cabinet components. The lists for annual and major inspections may not be complete for your transformer and you may want to make a formal record for your company’s reference. Some transformer installation/installation books furnished by the manufacturer may have a list of their inspections areas which you should utilize in making your own formal record. References Waukesha Electric Systems Instruction Book. 3.12 Problem and Failure Investigation Wallace Binder and Harold Moore 3.12.1 Introduction The investigation of transformer problems or failures is in many respects similar to medical procedures. The health of a transformer can be monitored using the many diagnostic tools available today. Ignoring a minor problem can lead to a more severe failure. Documenting and recording the results of operation and diagnostic testing is essential to a complete evaluation. Early detection and mitigation of developing problems can, in the long term, save the cost of major repairs. Many transformer operators with significant numbers of transformers perform field diagnostics and share this information with other operators to establish benchmark performance for transformers in different applications and with different designs. Elements of transformer design, application, and operation are involved in failure investigation. The elements to be investigated depend on the nature and the severity of the problem. If a failure is involved, all of the elements are usually investigated. The analyses required can be quite complex and involved. It would be impossible to describe the many details involved in complex failure investigations within this one section; indeed, a complete book could be written on this subject. In the space allotted here, it will only be possible to describe the processes involved in such investigations. Excellent references are ANSI/IEEE C57.117, IEEE Guide for Reporting Failure Data for Power Transformers and Shunt Reactors on Electrical Power Systems, and ANSI/IEEE C57.125, IEEE Guide for Failure Investigation, Documentation, and Analysis for Power Transformers and Shunt Reactors. The following steps are involved in problem and failure investigations. • Collect pertinent background data. • Visit the site to obtain application and operational data. • Interview all persons that may have relevant knowledge. • Inspect the transformer and perform a partial or complete dismantling if a failure is involved. • Analyze the available information and transformer history. • Prepare a preliminary report and review with persons who are involved or who have a direct interest to generate additional inputs. • Write a final report. As a general statement, no details should be neglected in such investigations. Experience has indicated that what may appear to be minor details sometimes hold the essential clues for solutions. Collection of all relevant data is the most important aspect of problem and failure investigation. © 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
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FIGURE 3.3.19 Effect of the leakage
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References Goosen, P.V. , Transform
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If i 1 is the full-load current rat
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TABLE 3.4.1 Loading Recommendations
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3.5.4 Three-Phase Transformer Conne
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FIGURE 3.5.4 Interconnected star-gr
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3.6.1.1 Standards ANSI standards fo
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ANSI standards. LTC control setting
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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: All connections should be cleaned a
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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