[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<br />
line to main tank for equalizing the pressure. Do not operate the LTC mechanism while the unit is on<br />
vacuum, as severe damage could occur to the mounting board.<br />
If the LTC requires oil, do not add the oil while the main tank is on vacuum as the unequal pressure<br />
could damage the LTC. The process of adding oil to the external LTC tank will put pressure inside the<br />
tank. This added positive pressure along with the negative gage pressure of the main tank could cause<br />
the LTC barrier boards to rupture. No additional work should be attempted while the main transformer<br />
tank is under vacuum.<br />
Look for loose hardware or any misalignment of the contacts. Operate the LTC through all positions<br />
and check each contact for alignment. Refer to the supplier’s instruction manual for the allowable<br />
variance. Perfect “center line” alignment during the complete range of operation from 16R to 16L will<br />
be difficult to achieve.<br />
3.11.1.8 Positive Pressure System<br />
This system consists of a cabinet with regulating equipment and alarms with an attached nitrogen bottle.<br />
A positive supply of nitrogen is kept on the transformer. With positive pressure on the tank, the possibility<br />
of moisture entering is reduced. Loss of nitrogen pressure, without any oil spillage, is usually found in<br />
the transformer “gas space” or the nitrogen supply system.<br />
3.11.1.9 Control Cabinet<br />
All control equipment must be inspected for loose wiring or problems caused by the shipping. The fan,<br />
gauges, LTC controls, and monitoring equipment must be tested or calibrated. Information for the<br />
installation and/or the calibration should be supplied by the manufacturer.<br />
3.11.1.10 Accessories<br />
There are many items that could be required for your particular transformer. A few are listed below.<br />
1. HV and LV arrestors<br />
2. External current transformers<br />
3. Discharge meters<br />
4. Neutral grounding resistors<br />
5. Bushing potential devices<br />
6. Cooling fan (hi speed)<br />
7. Gas monitors (various types)<br />
Caution: All oil handling equipment, transformer bushings, and the transformer should be grounded<br />
before starting the vacuum oil cycle. Special requirements are needed for vacuum oil filling in cold<br />
weather. Check your manufacturer’s manual.<br />
3.11.1.11 Vacuum Cycle<br />
Pulling vacuum on a transformer is usually done through the mechanical relief flange or a special vacuum<br />
valve located on the cover of the transformer. A vacuum se nsor, to send a sig nal t o the v acuum r ecording<br />
gauge, should be at the hig hest location on the t ransformer’s cover. This position reduces the risk of<br />
the sensor being contaminated with oil, which would let the vacuum gauge give a false reading. All<br />
readings from this gauge should be recorded at least every hour. Note: All radiator and cooler valves<br />
should be open prior to starting the vacuum cycle.<br />
3.11.1.12 Vacuum Filling System<br />
Manufacturers differ on the duration of vacuum required and the method to add oil to the unit. It is<br />
important that the vacuum crew doing this process follow the correct procedure as stated by the<br />
manufacturer or the warranty can be invalid. Good record-keeping during this process is just as important<br />
for your information as it is for supplying the manufacturer with information that validates the warranty.<br />
The length of time (pulling vacuum) will vary as to the exposure time to atmospheric air, the transformer<br />
rating, and the dew point/moisture calculations. Most of the needed information as to the vacuum<br />
cycle time, should be furnished in the installation book. (See Figs. 3.11.1 and 3.11.2.)<br />
Always consider that some contractors have used their equipment on older and/or failed transformers.<br />
The equipment needs to be thoroughly cleaned with new transformer oil and a new filter medium added<br />
to the oil filtering equipment. The vacuum oil pump should have new vacuum oil installed and it should<br />
be able to “pull-down” against a closed valve to below 1 mm of pressure.<br />
3.11.1.13 <strong>Transformer</strong> Oil<br />
The oil supplied should be secured from an approved source and meet the IEEE C57 106-1991 guide for<br />
acceptance. When requested, an inhibitor can be added to the oil, to a level of 0.3% by weight.<br />
3.11.1.14 Adding the Oil<br />
All oil from tankers should be field tested for acceptable dielectric level prior to pumping oil through<br />
the oil handling equipment. The oil temperature should be at least 0°C while pumping directly from an<br />
oil tank car. A superior method that will assist in the removal of moisture involves heating the oil to 50<br />
to 70°C and passing the oil through a filter. Oil filling a conservator transformer takes a lot more time<br />
as the piping and the conservator have to be slowly filled while air is “bled” out of the piping, bushings,<br />
CT turrets, and gas monitor. Methods vary for adding the oil to the conservator because of the risk to<br />
the oil bladder. Weeks later, the air should be “bled” again. If this is not done, you could receive a false<br />
signal that may take the transformer out of service.<br />
3.11.1.15 Field Test<br />
After the vacuum oil filling cycle, the transformer should be field tested and compared to some of the<br />
factory tests. The field test also will give you a baseline record for future reference.<br />
These are a few of the tests that are routinely done:<br />
1. Current transformer ratio<br />
2. Turns ratio of the transformer, including tests at al LTC tap positions<br />
3. <strong>Power</strong> factor bushings<br />
4. <strong>Power</strong> factor transformer<br />
5. Winding resistance<br />
6. Core ground<br />
7. Lab test of the oil<br />
8. Test all gauges<br />
9. Test all pressure switches<br />
A field test report may be required by the manufacturer in order to validate the warranty. Any<br />
questionable test values should be brought to the attention of the transformer manufacturer.<br />
3.11.2 <strong>Transformer</strong> Maintenance<br />
The present maintenance trend is to reduce cost, which in some cases means lengthening the intervals<br />
of time to do maintenance or eliminating the maintenance completely. The utility, or company, realizes<br />
some savings on manpower and material by lengthening the maintenances cycle, but by doing this, the<br />
risk factor is increased. A few thousand dollars for a maintenance program could save your utility or<br />
company a half-million dollar transformer. Consider the following:<br />
1. The length of time to have a transformer rebuilt or replaced<br />
2. The extra load on your system<br />
3. Rigging costs to move the transformer<br />
4. Freight costs for the repair, or buying a new one<br />
5. Disassembly and reassembly costs<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC