OVERHAULING OF TRANSFORMER The word “Overhauling” means where each and every part of the equipment is inspected after its complete dismantling and abnormalities whatever observed are rectified with the view to bring back the equipment to its original condition for totally trouble free and safe operation. It differs from routine preventive and breakdown maintenance where inspection is limited to important part or parts, which are likely to fail or have failed. The recommended practices and procedures during periodic overhauling of transformer or any major electrical equipment are discussed. Compliance with these recommendations will go a long way for ensuring safer plant operation. For overhauling, complete technical details of the equipment should be documented. The man-power engaged for the overhauling should be competent to understand all the technical details pertaining to the equipment. Since design of electrical equipment changes from manufacturer to manufacturer and from time to time, there should be a standard checklist for inspection items during overhauling. The checklists provide here under supplement the requirement laid down by manufacturer/supplier of the equipment. 1.1
General Requirements : Before commencement of overhauling activity, the following preparatory jobs & availability of facilities in working condition should be completed/ensured : 1.
Operation/maintenance installation/interconnection equipment are available.
instruction drawings and
2.
Detailed scheduling requirements.
jobs
3.
Right type of instruments, testing equipment, tools & tackles and consumables such as white spirit, solvents, tintless muslin cloth, etc sould be arranged as per jobwise requirements, based on instruction manuals.
4.
Testing instruments such as milli-volt, milli-ohm meters, motorised megger, HV testing kit, etc.
5.
Lifting facilities such as overhead crane, slings, etc.
6.
Planning & scheduling the jobs with other groups such as Mechanical to avoid any interference.
7.
Spares such as gaskets, insulating materials, insulation varnishes, etc.
8.
Temporary site office with facility to record all the measurements, test results, consumption of manhours, spares and consumables etc for future reference.
9.
Jigs, fixtures and such devices required for overhauling.
of
listing
manuals, as other documentation
out
all
activities
10. Personnel engaged to handle the job are experienced and skilled.
and
of
built the
resource
11. Usage of spring washers and double nuts for securing locking of terminals. 12. In case of water cooled equipment, cooling water is isolated and cooler drained. 13. Wherever paint has come off or pulled off or any signs of corrosion observed all such metallic structures should be painted. 14. All valves such as drain/sampling and isolation valves should be serviced for proper operation. 15. Incoming electric power and auxiliary supplies are de-energised and are positively isolated from all possible sources, lockedout through a permit system throughout the duration of overhauling work and a caution board indicating the nature of work should be displayed near the isolating point. 16. While using flammable materials such as solvent, following precautions should be taken : a. No hot job is going on nearby b. Area is well ventilated. c. Winding is protected from such inflammable materials. d. Other precautions such as periodic tests to ensure absence of gas concentration 17. The fitting of antifriction bearing for drive mechanism unit shall be carried out by process of controlled heating only (preferably induction heating). 18. Oil, grease lubricant specified shall be of proper grade and free from contamination. 19. All works should be carried out with covered enclosures for weather and environmental protection, to avoid entry of moisture, contamination, dust, etc in the equipment being overhauled. The work should not be planned in open, in rainy season and in the water logged areas. 20. Use of any combustible material like bamboos, coir rope and tarpaulin should be avoided. 21. Fasteners should be checked before use and be replaced if necessary. 22. All safety and operational interlocks should be checked for proper operation. Control circuit operations shall be checked by simulating fault conditions. 23. In case of equipment used in hazardous areas during overhauling, only approved types of gaskets, fasteners, and other accessories should be used, appropriate to the area classification. 24. Frequency of overhauling shall be generally as recommended by manufacturer. However as a guideline, for a transformer it shall be after every 10 years.
1.2
Overhauling Checklist : After the compliance for general requirements detailed in General Requirements, the following activities and checks should be carried out :
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Initial Jobs : 1. Check and record oil dielectric value, acidity, resistivity, moisture content, and dissolved gas analysis. 2. Check condition of all oil drain valve gland/gaskets. 3. Check and record IR value and polarisation index. 4. Check for visible oil leakage on all parts of transformers and mark them for attending them during overhauling. Note : Based on the above, decision should be taken on the nature and quantum of work involve, such as drying out, oil changing and filtering and required facilities should be planned for. 5. Check Terminal Box for : a. Condition of end terminal epoxy/compound. b. Sign of any hot spot. c. Gasket of terminal box. d. Bushing for any oil leakage. e. Loose connection, cracked bushings, hot spot, etc. 6. Check Oil level/Temperature gauges for : a. Calibration. b. The capillary tubing for tightness & the dial glasses for mechanical damages, if any c. Explosion vent diaphragm for any damage and also whether wire mesh provided at the mouth of the explosion vent is in tact. 7.
Ensure the following before untanking : a. Working personnel do not carry loose items on their persons which are likely to fall inside. b. Availability of spare gaskets for the top cover before lifting it. c. All accessories and cable connectors are dismantled so that cover is free for lifting. d. Core earthing is disconnected as per manufacturer’s recommended procedure. e. Pipe work from the cover to the conservator has been removed including explosion vent pipe. f. CT secondary leads are disconnected and marked. g. Switch handle assembly of Off-load tap-changer has been dismantled. h. The lifters are rigidly fixed to core clamps before lifting core and winding.
Note : 1. Oil-filled bushing should be stored vertical/inclined position only. Oil may come out if stored in horizontal position.
2. HV bushings are generally hermetically sealed. Oil inside the bushing shall have very low Tan ? . There is normally a pressure injection system through which oil sample can be taken and tested. ?
Core & Winding : 1. Check that the oil passages are not clogged due to deposition of any sludge in the winding block. 2. Check for tightness of core bolt & condition of core bolt insulation. 3. Check that winding coils are not distorted or discoloured. Also ensure winding insulation paper remains tightly wrapped. 4. Check all separators between windings are firmly in position and have no damage/dis-colouration. 5. Check that the core is not apparently distorted. 6. Check for any slackness of winding and if necessary tie rod/coil clamping screw may be tightened. 7. Check and clean sludge et. From the bottom of the tank. 8. Check for any corrosion on the tank internal and clean and apply protective coating if necessary. Note : Care should be taken that air release valves/plugs be kept open while filling oil into transformer, and filling to be from bottom only.
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Buchholz Relay, Conservator, Radiator & Explosion vent : 1. Check for any sludge and other impurities in the conservator and flush it with clean transformer oil. 2. If the conservator is fitted with separator (Air Cell), it should be tested for leakage as mentioned below : ?? Constant Oil Pressure System (Separator inside Conservator) : This is one of the modern developments in the field of transformers. The working system of Constant Oil Pressure System (COPS) is that it performs two main functions : First, it separates the transformer insulating oil from the atmosphere thus preventing gas or moisture contamination of the oil. Second, it maintains a constant atmospheric pressure on the transformer oil. A separator (Figure-1) is installed inside the conservator for this purpose, which is a sealed envelope made from nylon fabric coated with synthetic elastomer nitrile and hot vulcanised. It is fitted to the conservator through flange connected to the plate of man hole. The separator is also called as “Flexible Air Cell”, is connected to the top of conservator tank through a gasket joint. The air line from this connection is connected to dehydrating breather which ensures the dry atmosphere inside the air cell. Under normal
operation, the air cell is completely surrounded by oil & floats as high as it can in the conservator. As the transformer oil volume changes the air cell inflates or deflates by equivalant volume. The float of magnetic oil level gauge makes contact with the underside of air cell and follows the motion up and down thereby giving an indication of the oil level inside the conservator.
Figure-1 Conservator with Separator (Air Cell) In the event that air cell becomes damaged and fills with oil, it will sink and activate the low level alarm on the oil level gauge. The system will then function as the normal conservator and it will not effect the operation of the transformer. Before installation, separators are rolled and wrapped in cardboard cylinders. They can remain inside their packing as long as necessary under shelter at an ambient temperature approximately 20oC. ?
On-load Tap-changer (OLTC) : ? Selector Switch Unit : 1. Check fixed and moving contacts. If worn out, replace with new contacts. 2. Check that all nuts, screws and circlips are secure. 3. Check that the insulating oil confirms to IS:335. The oil should be filtered after every 30000 operations. The electrical withstand strength of oil should not be less than 30 KV at any time. The selector switch should be tested every 6 months. Note : At regular intervals preferably during filtration, the switch unit should be washed with clean oil and all traces of carbon should be removed. ? Drive Mechanism Unit :
1. Replace the grease (special long life grease) of all ball and roller bearings. 2. Check that cam faces, gear rollers and wiping surfaces are lubricated with proper quality grease. 3. Check for any breakage in the torque limiting shear pins. If damaged, replace with new ones in which case ensure synchronisation between tap-changer position indicator and actual position of tap-changer. 3. Check for the proper functioning of heater circuit with its ON-OFF switch, provided in the drive mechanism to eliminate any condensation due to changes in ambient temperature. 5. Check the condition of air filters. Note : No oil should be applied to any part of the mechanism. ? Functional Checks : 1. Check for proper electrical and manual operation of OLTC and ensure that limit and directional sequence switches, motors, contactors, and connected linkages of driving mechanism are functioning as per design, for all tap positions in both RAISE & LOWER modes. Ensure correct response in both local & remote control operations. 2. Ensure voltage readings for each tap position correspond with nameplate values. Also the position indicator in the remote relay panel should tally with OLTC’s mechanical tap position indicator physically. 3. Check that Buchholz relay operation is satisfactory, by injecting air through lower level petcock. 4. Check the diaphragm of the explosion vent fitted at the exposed end for any mechanical damage. 5. Level gauge in the conservator tank shall be cleaned. 6. Check that oil level is upto the fill level or approximately 2/3 of the level gauge after opening the valve between Conservator radiator and Buchholz relay. 7. Ensure that transformer body has been earthed before filling oil. ?
Assembly of Transformer : 1. After cleaning and painting of tank, core may be inserted (while inserting core, care shall be taken to avoid damages to core and winding). 2. All internal connections shall be restored. 3. All the flange joints shall be thoroughly cleaned and gaskets shall be shaped and fixed for proper sealing.
4. The busducts shall be fixed back (with phase segregators wherever provided) after thorough inspection and cleaning. 5. It shall be ensured that neutral earthing conductor is insulated from tank body and other supporting structure. 6. Put back the covers, insulators, cable box, marshalling box. (All the unused openings in Marshalling box shall be sealed). 7. Wheel stoppers should be put back in position. 8. All earthing connections shall be restored. 9. Before oil filling, all inspection and operation checks (which includes tap checking) should be carried out (in case of offload tap-changer). 10.Before tanking OLTC, operation OLTC should be checked with indications (both local & remote). 11.Painting of the transformer should be carried out. ?
Drying of Windings & Reconditioning of Oil : 1. Check all gasket joints, valve glands for any leakage of oil. 2. Check that IR value has become constant before stopping filtration. 3. During filtration ensure that the earthing of the transformer coil is not disconnected. Note : If IR value is very less, drying out by short circuit method should be carried out. Ensure rise in oil temperature is less than 100C per Hour and maximum oil temperature does not exceed 600C.
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Pre-Test Inspection : 1. Check that trapped air is released through air release plugs & valves fitted for the purpose on radiator, bushing caps, tank cover and Buchholz relay as well as gas relay on the OLTC tank. 2. Check whether gas operated relay is mounted at an angle by placing a spirit level on the top of the relay. Ensure for proper direction of oil flow. (The contact operation may be checked by manual testing key provided). 3. Check alarm and trip contact by simulation of WTI, Dial Thermometer, MOG etc., and reset to normal setting. 4. Check that off load switch handle is locked at the desired tap position. 5. Check that all valves except drain, filter and sampling valves are opened (such as radiator valves, Buchholz relay pipe line etc.)
6. Check the condition replace/reactivate.
of
silicagel
in
the
breather
and
if
necessary
7. Check that transformer oil is filled in the silicagel breather upto the required level. 8. Check that external electrical connections to bushings, are tight. 9. Check Arcing Horn gap on bushings. 10.Check that neutrals are effectively earthed. 11.Check and record winding resistance for HV, LV, and tertiary windings. 12.Check protection devices and circuits of the transformer for correct operation. 13.Conduct core balance and ratio tests as per IS:2705/IS:2706. 14.Check final IR value along with cable before charging the transformer. 1.3
Requisite Conditions for Putting Overhauled Transformer into Service Without Drying-out : After overhauling a transformer as explained in earlier topic, it can be taken into service without drying-out, provided the insulation characteristics comply with the following (Refer Table -1.3): Table-1.3: Voltage class & Capacity
Insulation Characteristic R60” ? Must not decrease during repair / overhauling by more than 50% of the Up to 35 KV inclusive with a value specified by the manufacturer, or must not be lower than the value capacity of upto 10,000 KVA inclusive. indicated in Table-4.3.1. R 60”/ R 15” ? Must not be lower than 1.3 at a temperature of 10 to 300C. R60” ? Must not decrease during repair / overhauling by more than 40% of the value specified by the manufacturer, or must not be lower than the value indicated Table-
4.3.1. Up to 35 KV inclusive with a capacity of more than 10,000 KVA; 110 KV and above irrespective of capacity inclusive.
R60”/ R15” ? Must not be lower than 1.3 at a temperature of 10 to 300C. Tan ? ? Must not increase during repair by more than 30% of the value specified by the manufacturer, or must not be higher than indicated in Table-4.3.2. C2/C50 ? Must not increase during repair by more than 20% of the value specified by the manufacturer, or must not be higher than indicated in Table-4.3.2. ? C/C ? Must not be higher than indicated in Table4.3.2.
Table-1.3.1 : Transformer Voltage Class ( KV ) Up to 35 KV > 35 KV to 110 KV
10 450 900
Winding Temperature ( 0C) 20 30 40 50 60 300 200 130 90 60 600 400 260 180 120
70 40 80
Table-1.3.2 : Transformer Voltage Class Insulation Winding Temperature (0C) and Capacity Characteristic 10 20 30 40 50 60 Up to 35 KV inclusive with a capacity of 10,000 KVA & above; 110 to 220 KV irrespective of capacity. 1.8 2.5 3.5 5.0 7.0 10.0 Tan ? , % 330 to 500 KV irrespective of capacity. 1.0 1.3 1.6 2.0 2.5 3.2 Upto 35 KV inclusive irrespective of capacity. C 2/C50 1.2 1.3 1.4 1.5 1.6 1.7 110 to 150 KV irrespective of capacity. 1.1 1.2 1.3 1.4 1.5 1.6 110 KV and over irrespective of capacity. 8 12 18 29 44 ? C/C Difference in ? C/C between the end and the start of repair, i.e (? C/C)f(? C/C)i, 4 5 8.5 13 corrected to the 3 same temperature. * - The listed data refer to all transformer windings .
70
14.0 4.0 1.8 1.7 -
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