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Key words: transformer; valve; no oil replacement valve;
0 Preface:
Take the No. 2 main unit of a power plant as an example, the model is (DFP-240000/500) single-phase transformer, each phase rated capacity is 240MVA, and two oil filling valves are designed and installed at the bottom of each phase. The valve type is DN100 brass ball valve, running The two oil filling valves are closed.
The main phase A of the main transformer is cracked in one of the oil filling valve bodies at the bottom of the operation, and the length of the crack is about 300 mm. Although no leakage of insulating oil has been found, serious safety hazards have been left. (pictured)
1 The disadvantages of the old process method According to the previous process requirements, if the transformer valve is broken or leaking oil during operation, the transformer must be taken out of operation and the oil in the body should be placed below the valve to be replaced. If you are replacing the valve at the bottom of the transformer, you need to remove almost all of the oil, and then remove the components (such as air coolers, unions, etc.) to replace the valve.
The main transformer has 35.2 tons of oil. Therefore, before replacing the oil filling valve, not only must the oil filter be prepared, but also at least one oil tank with a volume of 40 tons, and the inner wall of the oil tank should be cleaned, otherwise the transformer oil will be insulated. Performance is declining. According to this method, it takes at least 3 to 4 people to replace the oil filling valve for 3 days. After the replacement, refill the oil, hot oil circulation, static release, and carry out relevant tests according to the standard after the overhaul of the transformer. If the test fails. It is also necessary to continue the oil filter and hot oil circulation until it is qualified. Therefore, if the rupture oil filling valve is replaced by the previous process, the small problem will be complicated, and the winding may be exposed to moisture and the oil quality of the insulating oil may be lowered, so it is not accepted.
2 After the implementation of the new process method, we decided to replace the copper valve with vacuuming negative pressure by using part of the transformer oil. However, the degree of vacuum reaches the amount of transformer oil that does not flow out during the replacement process and needs to be determined by calculation.
According to the basic equation of hydrostatics, the mechanical conditions for ensuring that oil does not flow out of the pipe joint during the replacement of the butterfly valve are:
P true = P gas Ï oil · g · h
The readings converted to vacuum gauges are:
P table '=P gas-P true=Ï oil·g·h
In order to ensure the reliability of the work, the safety factor K should be considered for vacuuming. In fact, the reading value of the vacuum gauge should be:
P table = K · P table '
In general, K takes 1.3 to 1.4.
Among them, P---vacuum degree vacuum degree P---standard atmospheric pressure, 0.101MPa
Ï oil - transformer oil density at -20 ° C, 882 kg / m3
g---gravity acceleration 9.807m/s2
h---oil level to the height of the butterfly valve to be replaced, m
P table, P table '---vacuum table reading on the spot to measure the oil level to the height of the valve that needs to be replaced, so by calculation can be:
P table = K · P table '
=1.3×Ï油·g·h=0.022 Mpa (vacuum table is negative)
Operating procedures:
1. Power outages and appropriate safety measures.
2. Connect vacuum oil filter and oil tank drain line.
3. Close the lower valve of the oil conservator and the valve at both ends of the gas relay. From the other well-filled oil valve at the bottom of the tank, through the vacuum oil filter, put the transformer oil into the phase oil storage cabinet to lower the oil level. It is 100-200mm away from the lid.
4. Vacuum the main body from the butterfly valve at the end of the upper main pipe of the transformer (the special butterfly valve for vacuuming), and the vacuum degree is about 0.03MPa.
5. Loosen the bolt of the cracking valve diagonally. Try to move the valve while loosening the bolt to observe whether the valve body is affected by the vacuum. At the same time, observe the oil leakage state of the cracked part, confirm the valve is suctioned, and install the disassembly valve. Bolts, only the bottom end of the valve is reserved and loosened. If the oil leakage is large at this time and the construction cannot be continued, replace the oil.
6. Pry the valve body outwards, and quickly insert the steel plate between the valve body and the tank side mounting flange. After confirming the suction force to absorb the steel plate, loosen the reserved bolts and remove the cracking valve. At this time, care should be taken to evenly stress the steel plate to avoid damage to the vacuum and oil leakage. (pictured)
7. Stick the gasket of the new valve to the sealing flange of the valve body, start to install the bolt at the bottom end of the valve body, slightly move the steel plate, and then install the bolt with the nearest distance. After confirming that the valve body can be fixed, the steel plate can be quickly extracted. Push the valve body to seal it with the flange at the tank, and install and tighten all the bolts.
8. Continue to vacuum and increase the vacuum to -0.1 MPa. Depending on the treatment time and intake conditions, it is decided to maintain this vacuum and continuously evacuate: 2-4 hours.
9. Stop vacuuming, close the pumping valve, slowly open the lower part of the oil conservator and the valves at both ends of the gas relay, put the transformer oil in the oil conservator into the transformer, and adjust the oil level to the appropriate position. After the end of the replenishment, open the venting plug on the upper part of the casing riser, the connecting pipe, etc. to deflate, and close the plug when the oil overflows.
10. The product is allowed to stand for 72 hours: during the process of standing, the venting plug of the upper part of the casing and the upper part of the pipe is periodically opened for deflation, and the plug is closed when the oil overflows.
3 Note 1. In the process of replacing the valve, the oil filter, vacuum pump and power box must be guarded by the personnel, pay close attention to the operating status of the oil filter and vacuum pump, and report the abnormality to the person in charge immediately.
2. When operating any valve such as butterfly valve or oil drain valve during oil injection and vacuuming, it must be lightly opened and closed to prevent damage to the valve caused by excessive force.
3. The new valve to be replaced must be tested in advance for metal detection to determine that the valve is free of defects.
4. Adhere the sealing gasket of the new valve to the groove of the valve gasket in advance to confirm that the paste is firm.
5. When the oil in the body is injected into the oil conservator, the air in the oil pipeline must be drained before the pipeline is connected to the oil storage valve of the oil conservator to prevent gas from entering the oil conservator.
6. Closely observe the oil level of the oil conservator and the body during the oil draining process to ensure that the windings are not exposed to the air.
7. When disassembling the old valve, the lowermost bolt cannot be removed. When the old valve does not leave the flange, install a new valve. When the new valve can be fixed on the flange, remove the old valve to prevent the vacuum process. Sudden oil leakage, new and old valves are not on the flange, causing a large amount of insulating oil leakage.
4 Conclusions The practice proves that the butterfly valve is easy to use and has good effect. It can easily and quickly replace the oil-damping butterfly valve in the field. It has obvious effect in shortening the maintenance period and reducing the cost, and has the promotion value.
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Oil-immersed transformer does not drain oil replacement valve technology field application
Abstract: Large power transformers are usually equipped with valves of various functions, such as accidental oil drain valves, oil filling valves, vacuum valves, etc. Most valves do not have two valves. Once the valve body has cracks and other faults, it will cause a large amount of leakage of insulating oil. The oil is polluted, which seriously affects the safe operation of transformers and even the power grid. In the past, replacing the valve often required to discharge the insulating oil in the transformer below the valve. Not only the workload was large, but also the oil quality of the insulating oil and the protection of the winding were very high, and the process was cumbersome, so the method of replacing the valve without draining oil was ready. The author of this paper proves that the transformer non-oil discharge valve technology is a scientific and feasible method through personal experiments.