Drying of Transformers The dry out of transformer is necessary in following cases: a- On first commissioning. b- After prolonged storage at side without nitrogen. c- After detection of free moisture/high moisture content in oil. d- Due to exposure of core and coil assembly for 48 hours or more in case of inspection. Various methods can be adopted for drying out a transformers depending upon the facilities available at site. The drying out the transformer by vacuum pulling, N2 filling is most effective and quickest too, but cannot be applied in case of transformer not designed for withstanding the vacuum pressure.
A- Material and equipments required The dry out of transformer is important and precious process. The supervision and inspection charges of the proprietary are about to Rs. 20,000 to 30,000 per day. Minor precaution and management of material before calling any manufacturer’s supervisor may save lacs of rupees of department. Before taking the transformer for drying out process, the following equipments and material should be available at site, so that delay on account of minor thing/material may be avoided. Sl. Description Nos. No. 01 Filter machine 01 Capacity 6000 litters per hour 01 Capacity 2000 litters per hour (for filtration of oil of storage tank if available it curtails the time period of filtration of oil) 02 Nitrogen Gas and Associated Equipment a Nitrogen Gas Iolar grade –I Cylinders of 6.5 CUM capacity Dew Point of N2 gas should be of order of or below -50 °C For 315MVA, 400/220/33kV transformer ( for draining of 18 oil/ each cycle of N2 gas) For 100 MVA, 220/66/11kV transformer ( for draining of 12 oil /each cycle of N2 gas) b Nitrogen gas cylinders opener 01 c Gas regulators with two gauges having scale 0 - 02 210Kg/cm2 & 0- 20 Kg/cm2 for measuring the pressure of cylinder and of outlet pipe. d Dew point meter- for checking the dew point of N2 gas. 01 e PVC Pipe suitable for filling the N2 Gas 5mtrs 03 Oil & associated equipments a Oil testing Equipment (Motorised) 01 suitable for checking the Electric strength (BDV) of oil up to 100kV across 2.5 mm gap. b Spare Transformer Oil 20KL c Oil Storage Tank (according to oil capacity of transformer 85 KL tank) each of 20KL to 30 KL capacity. Oil Storage tank
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b c 05 a b c d 06 a b c d 07 a b c d e f g
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should have silica-gel breather and inlet/outlet valve for oil circulation. Oil hose 10 meters, 50NB flexible hose flanged at both end capable of withstanding vacuum 10-2 torr and shall be compatible with hot transformer oil at 120 °C Venyl hose ( Transparent PVC pipe) 12 mm diameter , 3 mm thick wall for use as oil level gauge connected at the top & bottom valve of transformer Adjustable fasting clips for above pipe Flanged with reducing pipe for connecting the 12 mm PVC pipe and with top & bottom valve of transformer Oil sampling cans or bottles Vacuum system Vacuum Pump - The vacuum pump should have sufficient capacity to evacuate a tank of 69 KL capacity and having a leak rate of 0.5 Torr/hr to an ultimate vacuum of 0.1 Torr in 2 hrs. Vacuum hose 5 meters, 75NB flexible hose flanged at both end capable of withstanding vacuum 10-2 torr. Vacuum hose 10 meters, 50NB flexible hose flanged at both end capable of withstanding vacuum 10-2 torr. GAUGES Compound Gauge with centre pointer and pressure range -1 to + 1 kg/cm2 Air pressure gauge - Scale 0 to 0.1 Kg/cm2 Air pressure gauge- 0 - to 1 Kg/cm2 Vacuum gauge 0 - 760 mm Hg. Testing Meters Multi-meter Volt meter 0 - 300 Volts Insulation tester (Megger) 2500/5000 Volts Insulation tester (Megger) 500 Volts Miscellaneous Items Pipe wrench ( 3 inches) Spanner & Screw driver of different sizes (sets) Electric hand lamp-100Watts 230 VAC with supply board. Knife Flanges Different Sizes Gaskets Synthetic nitrile rubber/nitrile rubber bonded cork 10mm, 08 mm, 06 mm sheet each Small gaskets /O rings 3, 5, 8 mm each Round rubber cord of diameter 8, 12, or 19 mm each Blanking plates OLTC & Main Tank Equalising Pipes Cotton rope Tripal to cover the transformer Heater 2kW 230 volts AC Tent for staff Lighting arrangement for night,
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20 Mtrs. 12 02 07 01
03 02
01 01 01 03 01 01 01 01
01 02 02 01 12 02 each 04 nos 20 mtrs 04 03 02 06 20 01 250 watts HPMV/SV 02
lamp , 230 Volts AC Source of supply 440 Volts AC for filter machine 02 Register and pen for recoding the day to day activities by 01 staff.
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Note - Please note following:1- Air cell is made of nylon fabric coated oil resistant nitrile rubber. N2 pressure in air cell should not be filled up more than 0.035 kg/cm2. 2- PRV relay is set at the pressure of 0.56 kg/cm2, so that during the filling/draining of oil, the pressure of N2 gas in main tank of transformer should not be exceed the 0.17 ± 0.02 kg/cm2 and maximum 0.2 kg/cm2. 3- Do not allow WTI/OTI temperature to exceed 65 °C during dry out of transformer and filter machine temperature beyond 70°C.
B- Preparation of site The power transformer specially 400 KV class and above is very important & sensitive equipments. The minor laps may damage transformer and it may make all effort made by you to zero. Keep patience, be dedicated, be confident, don’t take it granted, don’t tense, but don’t be careless, prepare yourself, and prepare & plan your staff for round the clock working in shifting duties for at least for 30 days otherwise you will feel tire and you will not get your goal. 1- First of all, made available all above material at site. 2- Clean perfectly oil storage tank from inside, drain pipe and valve with transformer oil. 3- Clean magnetic filter and paper filter of filter machine with transformer oil. 4- Clean and fill up the oil in vacuum pumps as recommended by manufacturer.
C- Draining oil 1- Before draining of oil, transport required nos of nitrogen gas cylinders
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from supplying agency and made available at site to avoid any delay on account of transportation, traffic jam/no entry, non availability of N2 gas, since this will interrupt process of N2 gas filling. Check the dew point of Nitrogen gas; it should be of order of or below -50 °C. Connect the transparent PVC pipe at top and bottom valves of transformer with help of reducing flange and adjustable fasting clips for watching the oil level and open the both valves. Connect the inlet oil hose pipe of filter machine to bottom valve of transformer through flange and outlet hose pipe to oil storage tank. Connect one end of N 2 gas pipe at the top of EHV bushing’s air vent plug of transformer with help of reducing pipe and another end through regulator to gas cylinder. Close the butter fly and gate valves of cooling banks and both side of buchholz relay on conservator pipe. Put the suitable gasketted blanking
plates/ dummy plates at cooler banks pipe,s valves and at valve provided between main tank and buchholz relay in conservator pipe, so that minor oil leakage can be stopped and safety of buchholz relay can be done during creation of vacuum otherwise it will be damaged. 7- Start the oil filter machine 8- Start draining the oil by opening outlet & inlet of oil filter machine and simultaneously purging N2 gas through air vent plug of EHV bushing till oil come to below the level of N2 purging top valve of transformer and close the air vent plug of EHV bushing and inlet valve of oil filter machine. 9- Connect equalising pipe between main tank and OLTC tank to avoid excessive vacuum/pressure on wall of OLTC tank. It may be damaged due to excessive pressure difference between OLTC tank & main tank. 10-Now remove N 2 gas pipe from air release plug of EHV bushing and close air release plug. Connect one end of gas pipe to top side valve of transformer with help of reducing flange and another end through regulator to gas cylinder. The valve of transformer and regulator of gas cylinder should be kept closed. 11-Connect Compound Gauge (with centre pointer and pressure range -1 to + 1 kg/ cm2) at top valve of transformer for watching vacuum & pressure of tank during draining of oil. 12- Start draining the oil by opening outlet & inlet of oil filter machine and simultaneously purging N2 gas through top side valve of transformer. 13- The care should be taken that oil should be drained at the speed as N2 gas entering the tank and N2 pressure of tank should not be raised above 0.10 Kg/cm2. 14- After drainage of oil from transformer tank, open bottom drain plug for drainage of oil from bottom tank with suitable pipe connector and fill up the oil in drums. 15- After draining the oil from bottom tank of transformer, close the drain plug and fill up the N2 gas up to 0.17± 0.02 kg/cm2 in transformer tank and close the nitrogen filling valve. Watch N2 pressure for 24 hrs for leakage and attend the same. D- Inspection 1- Please see the weather condition before opening inspection cover of transformer. It is should be preferred that weather should be clear and sun heat is available. 2- Prepared the oil of storage tank by filter machine making BDV 70 kV or above with temperature above 50°C. 3- Check that the oxygen content inside the transformer tank and it should be minimum 20%, if a person is about to enter into the tank. 4- Send two trained person inside the tank with help of ropes. Inspect the inside all joint and winding for any looseness, excessive heating points, and any other abnormality. 5- Clean bottom and side wall of transformer with help of good quality dry cotton (white colour).
6- Connect 1.5 meter length GI pipe of ½ inches diameter to outlet hose
pipe of filter machine and wash out the windings and inside of transformer with hot oil from storage tank with help of pipe and filter machine. 7- Drain & collect the oil accommodated at bottom of transformer due to washing/flashing of winding with help of suitable pipe through drain plug in oil drums. Close the drain valve. 8- Open inspection cover one by one and replace gasket of inspection cover and leakage points. 9- Close the inspection and tight the nut bolts and studs with uniform torque.
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Evacuation 1- Keep Isolate the main tank of transformer from cooler banks and
conservator by shutting off gate valve /butter fly valve 2- The transformer tank (excluding cooler banks and conservator) shall be evacuated to pressure of 1 torr. 3- Connect vacuum pump hose pipe to tank through vacuum valve and create the vacuum up 760 mm and attend the leakage. After ascertaining that there is no leakage in transformer from gasket/valve etc, pull out the vacuum and keep the transformer under near absolute vacuum (1torr or less) for 48 or more running the vacuum pump continuously. Afterward hold the vacuum of 760 mm for 24 hours and attend leakage if any.
Nitrogen gas Cycling
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1- The purging of Nitrogen gas is normally done in two or three cycling or
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till required result is obtained, but not less than two N2 cycle even though required dew point has been achieved. The dew point of nitrogen gas after absorbing the moisture shall not be less than -30 °C. Connect N2 gas filling arrangement and break the vacuum with dry nitrogen and fill it up to 0.15± 0.02 Kg/cm2 and close the valve of N2 filling at tank and of cylinder. When the pressure of N2 filling cylinder falls to 10 kg/cm2 this should be considered as empty and immediately its valve should be closed and another cylinder should be connected. After N2 filling, transformer are heated externally by using the heater below and fastening with transformer tank. Temperature at wall should be order of 60 °C to 70 °C. Cover the transformer by tripal, so that heat cannot be escaped/wasted. Keep pressurised the tank with N2 gas for 36 hours and attend the leakage. Initially one or two nitrogen cycles may be kept for 36hrs. After that it may be kept for 48 hrs depending upon the dew point being achieved.
8- After holding nitrogen gas pressure for 36hrs, release the nitrogen gas
through outlet and measure the dew point of N2 gas at this outlet. If it is more than -30 °C (say -28 or -29°C) the repeat cycle again. 9- Drain & collect the oil accommodated at bottom of transformer during vacuuming of tank & winding with help of suitable pipe through drain plug in oil drums. Close the drain valve. 10-Another cycle, vacuum the transformer tank for 24 hrs by running vacuum pump continuously and holds it for 24 hrs then break the vacuum by nitrogen gas. Fill up nitrogen gas to 0.17± 0.02 kg/cm2. Hold this nitrogen pressure for 24 hrs. 11-The timing of creating and holding vacuum and N2 pressure are depend upon the dew point measured at outlet. 12-The cycling to be continued till dew point of -30 °C or below is achieved. 13-It will be better to achieve the dew point of N2 gas below -35°C. 14-After achieving the dew point of N2 gas -35°C or below, stop releasing the N2 gas from tank and create the vacuum 760 mm and prepare oil and fill the oil under vacuum.
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Filling of oil in main tank 1- Keep separate the cooling bank/radiators and tank by isolating valves. 2- Already prepared oil by heating & filtering by filter machine shall be
heated to temperature of 50 °C to 70 °C measured at filter outlet valve. The pressure during the filling shall be max 1 torr. During the filling of oil, a transparent plastic tube which has a wall thickness of 5-8 mm can be used as an oil level gauge to observe level. The transparent PVC tube shall be connected to top and bottom valve of the transformer. 3- Oil filling in the tank is to be done through bottom valve of the transformer with maximum speed maximum of 4-5 kL/hour. The pressure in oil pipes shall be kept positive. When oil level has reached about 50 mm below the top cover the vacuum pump is stopped. It is preferred that dry nitrogen is introduced in tank by N2 gas filling arrangement already connected to tank and fill up oil. The oil may be filled up in transformer by holding vacuum ( vacuum pump stopped) up to buchholz level & isolating valve tank side. 4- Remove the equalising pipe provided at OLTC s and main tank and put dummy plate at their places. H- Filling of oil in cooling bank 1- Create slightly vacuum / fill up the N2 gas in cooling bank. 2- Keep it up to 2 hrs. 3- Open vent plugs provided at top of cooling bank header. 4- Fill up the oil by filter machine and when oil over flow through vent valve, close the vent plugs. Stop the filter machine. I-
Draining of oil from conservator tank 1-
Open vent plug provided at both end at the top of conservator.
2Connect the inlet pipe of filter machine to drain pipe of conservator. 3Drain oil of conservator tank through drain pipe. 4After drainage of oil, open the inspection cover of conservator provided at the end of conservator where the MOG is provided. 5Provide the pressure gauge having scale from 0 to 0.1 kg/cm2 at top valve of conservator. 6See the condition of air cell; inflate the air cell by filling N2 gas up to pressure of 0.035 Kg/cm2 through valve provided at top middle of conservator and breather pipe. Close the top valve of conservator and check the pressure leakage and maintain pressure. Provide the inspection cover with gasket and tighten the same. 7Close vent plugs. Watch for 06 hrs for leakage. There should not be any leakage of gas. J-
Filling of oil in conservator tank 1- Open valves provided both side of buchholz relay and remove blanket
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Inflate. Open vent plug provided at both end at the top of conservator. Check the pressure at gauge provided at top of conservator. The pressure should not be more than 0.035 kg/cm2. Fill up the oil with help of filter machine from valve provided at the bottom of tank slowly till oil flows from conservator vents. Close the vent one by one after passing out the complete air from conservator. Stop filling the oil. Connect the air cell of conservator to breather pipe and remove the silica jell breather, so that there should not be any obstruction to air coming out from air cell during filling of oil. Place clean cloth to mouth to ingress of air inside the breather pipe. Start filling of oil with help of filter machine again through valve provided at the bottom of tank slowly. Remove cloth provided at the end of the breather and observe the air flow from cell. If air is coming out it means that air cell is functioning properly. Fill up the oil up to level of 35°C. Stop oil filling and close the bottom valve of tank.
K- Precautions 1- Oil filling in the conservator and also draining, whenever, required
must be done very slowly. During oil filling pressure in air cell should not exceed 0.1 kg/cm2 (1.5psi) at any condition. 2- Once all the air has been driven out during oil filling in the conservator, do not remove air release plugs provided at top of conservator. Otherwise air will be sucked inside the conservator. 3- If a pressure or vacuum is ever applied to main tank, the conservator must be disconnected by a blanking plate fitted to shut off valve. 4- Welding work should not be done on the conservator to avoid damage of air cell.
5- When assembling the rubber cord in groove, the cord length shall be
continuous. The gasket is given a small surplus length to compensate for shrinkage.
L- Filling the oil in OLTC 1- Connect oil outlet hose pipe of filter machine to sampling pipe to OLTC and fill up the oil in OLTC one by one from oil storage tank. 2- After filling of oil, stop filter machine and remove hose pipe connected to oil storage tank and reconnect it to common pipe of OLTC. 3- Circulate the oil through filter machine till BDV achieve 70 kV and above. 4- Cut off heater of filter machine & decrease the temperature slowly in the step of 10 °C to 20 °C. Finally switch off the heaters. 5- Stop the oil filtration machine.
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Hot Oil Circulation 1- To facilitate oil penetration and absorption of possible gas bubble, the
temperature of transformer shall after completed oil filling be increased by circulating the oil through vacuum filter and circulation direction from top to bottom and bottom to top. 2- Keep main tank and cooling banks isolated from each other. The oil circulation is done separately. 3- The temperature during oil circulation should not increase beyond 70 °C otherwise this may cause oxidation oil. 4- The oil temperature as measured by oil temperature indicator should be of the order of 55 °C and at the filter machine in no case exceeds 65 °C. 5- Keep oil circulation in main tank at 55 °C continue till insulation resistance and oil sample tests are satisfactory. Take IR value at interval of 2 hrs till IR value become constant for next 12 hrs and there is no further increase in IR value. 6- Initially IR values drop down as temperature goes up. The IR value will be low till moisture is coming out of insulation and start rising before steadying (First drops from higher value to lower value on increase of temperature then increase from lower to higher and become constant.) 7- Take BDV at interval of 12 hrs during oil circulation. 8- Note down the IR value and BDV and keep the record. As soon as IR value become constant and BDV more than 70 kV. Cut off heater of filter machine & decrease the temperature slowly in the step of 10 °C to 20 °C and finally switch off the heater. 9- Keep cold oil circulation for at least 04 hrs. 10Now connect oil filter hose to cooling bank and circulate oil increasing oil temperature slowly as similar manner as above and as soon as BDV more than 70 kV is achieved, cut off heater of filter machine & decrease the temperature slowly in the step of 10 °C to 20 °C. Finally switch off the heaters. 11Keep cold oil circulation for at least 04 hrs.
Take two samples from main tank; one from bottom and another from top, 03 Nos. from OLTC (One from each OLTC) and 02 Nos. from cooling bank (one from each). Test all test to carry out on oil i.e. BDV, Tan Delta at 90 °C, resistivity at 27°C, PPM etc. 13If test values are satisfactory, mix up the oil of main tank and cooling banks by opening isolating valve and circulate oil by pumps for 8hrs. 14Before energising the transformer by supply voltage, allow 48 hrs standing time for the transformer. 15After connecting necessary jumpers energise the transformer with the supply on no load for 12 hrs. 16Observe the noise and temperature during this period of 12 hrs. 17Finally take the load on the transformer. 12-
L P KUSHWAHA Manager (CWG) Project II AI 23-02-2010
