PRODUCT

4.1 Iron Core
The material performance is not less than 30ZH110 cold rolled silicon steel sheet; Three-phase three-column three-dimensional symmetrical structure has the most balanced three-phase magnetic circuit; Each group (coil) core is wound by thin steel strip on a special core winding machine, and the pressure is uniform and tight; The core frame is annealed by vacuum to eliminate stress and improve electromagnetic performance. Core surface coated with epoxy resin paint, moisture-proof, curing.

For an iron core, the core sheet is required to be flat, no waves, should be vertical after standing up, not skewed, the order is clear, the joint is tight, correct, the edge of the piece without short circuit, etc., in order to ensure the electromagnetic performance of the iron core. If the core pack quality is not good, it will affect the electromagnetic and mechanical properties of the core.

The deflection of the core will cause difficulties in winding installation and may also cause insufficient insulation distance in the winding.

The joint of the core sheet causes large no-load current, and the mechanical strength is weakened.

The insulation resistance of power transformers in operation is qualified by: 10KV class and below, the insulation value is greater than 300 megohm: 35KV class, the insulation value is greater than 400 megohm (the ambient temperature is 20℃).

Through the measurement of insulation resistance, power transformers can effectively find some insulation problems and other problems of transformers, such as winding touching the shell, touching the core, short circuit between the coils, etc. So during regular maintenance or after overhaul. Always measure insulation resistance. When measuring the insulation resistance of the transformer, it is generally necessary to measure the insulation resistance between the high voltage coil and the shell, the low voltage coil and the low voltage coil, and the insulation resistance of the core screw to the iron core should be measured when the hanging core is repaired. 2500V megohm meter is generally used to measure the insulation resistance of power transformers, but 1000 megohm meter is generally used to measure the insulation resistance of the core screw to the iron core.

The insulation resistance of power transformer is greatly affected by humidity and temperature. When the humidity increases, the surface and interior absorb water, the leakage current increases, and the insulation resistance decreases. When the temperature rises, the charged particles are easily conductive due to the strengthening of thermal motion, the leakage current increases, and the insulation resistance decreases. Therefore, the resistance value of the insulation resistance measured at different temperatures is different, and the higher the temperature, the lower the insulation resistance.

Standard for insulation resistance of transformers

1. Insulation should be measured before the transformer is put into operation after newly installed or overhauled and after long-term disuse (three weeks).
2, the voltage level of more than 1000V winding use 2500V dial, 1000V dial winding below 1000V. The resistance value is specified (20℃) for 300MΩ in 3-10KV, 400MΩ in 20-35kV, 800MΩ in 63-220KV, and 3000MΩ in 500KV. The resistance of 500V dial for transformers with voltage 0.4KV and below is ≥0.5MΩ.
3, when the resistance value is less than 50% of the previous value, notify the maintenance treatment, if necessary, measure the dielectric loss and absorption ratio of the transformer. Absorption ratio R60 "/R15", the ratio is not less than 1.3.

Name of insulation resistance:

High to low ground: (primary winding to secondary winding and housing) high voltage winding to low voltage winding and housing insulation resistance;
Low to high ground: (secondary winding to primary winding and housing) insulation resistance of low voltage winding to high voltage winding and housing;

Insulation resistance qualified value standard

(1) The insulation resistance value measured this time is compared with the last measured value at the same temperature, and the value should not be reduced by 30% compared with the last value;

(2) The absorption ratio R60/R15 (the ratio of insulation resistance at 60 seconds and 15 seconds in telemetry), should be 1.3 and above at 10 ~ 30℃ :

(3) For a transformer with a primary side voltage of 10kV, the minimum qualified value of its insulation resistance is related to temperature. Relation between insulation resistance of transformer and temperature at test time
Transformer insulation resistance calculation formula: Use the formula to calculate the insulation resistance at each temperature "rising ten half, reducing ten double, good multiplied by one and a half" absorption ratio: R20 = Rt X 10t-20/40 temperature rises 10OC, Rt X 2/3 times. Rt X 1.5 times for every 10OC drop in temperature.

4, the newly installed and overhauled transformer, its insulation resistance qualified value should comply with the above provisions. Transformers in operation are no less than 10 megohm.

The types of transformers are varied, but as far as their working principle is concerned, they are made according to the principle of electromagnetic induction. In general, the classification of commonly used transformers can be classified as follows.

(1) By use:

1) Power transformer: used to boost or buck the power system, is one of the most common and most commonly used transformers:
2) Test transformer: generate high voltage, high voltage test with electrical equipment:
3) Instrument transformers: such as voltage transformers, current transformers, used for measuring instruments and relay protection devices:
4) Transformers for special purposes: furnace transformers for smelting, rectifier transformers for electrolysis, welding transformers for welding, voltage regulation transformers for testing, etc.
(2) According to the number of phases:

1) Single-phase transformer: for single-phase load and three-phase transformer group:

2) Three-phase transformer: used for raising and lowering voltage of three-phase system.

(3) According to the winding form:

1) Autotransformer: used to connect ultra-high voltage, large capacity power systems:
2) Double-winding transformer: used to connect two voltage levels of the power system;

3) Three-winding transformer: used to connect three voltage levels, generally used in regional substations of power systems

(4) According to the core form:

1) Core transformer: for high voltage power system:
2) Shell type transformer: special transformer for high current, such as electric furnace transformer and welding transformer, etc. : or power transformer for electronic instruments and TV, radio, etc

(5) According to the cooling method:

1) Oil-immersed transformer: such as oil-immersed self-cooling, oil-immersed air cooling, oil-immersed water cooling, forced oil circulation and water internal cooling, etc. :
2) Dry transformer: rely on air convection for cooling, the current indoor power transformer, the sample is cooled by fan:
3) Inflatable transformer: use special gas (SF6) instead of transformer oil heat dissipation;
4) Evaporative cooling transformer: use special liquid instead of transformer oil for insulation and heat dissipation.

(1) The purpose of vacuum oil injection

Vacuum oil injection can greatly reduce the content of transformer oil and water, thus greatly enhancing the breakdown strength of transformer oil, that is, reducing the oil gap distance of the body insulation, resulting in greatly reduced transformer design costs.

Attention:

Transformer oil filtration and oil injection should be carried out in fine weather. Transformer drain valve port should be matched with the oil filter inlet pipe port.

Oil filter should be filled with vacuum tank oil standard tube 1/2 qualified oil.

(2) transformer oil filter in operation

Open the oil pillow cover of the transformer, insert the oil filter outlet pipe under the oil level on the oil pillow, and then seal the oil pillow cover with clean filter paper or film.

Insert the inlet line of the oil filter into the oil drum prepared with qualified oil.

Open the oil drain valve at the bottom of the transformer and drain 3-5% of the oil of the transformer (3-5% of the oil is mainly impurities and water, and should be stored separately.

After the oil drain valve at the bottom of the transformer is discharged by 3-5%, the oil filter is pumped into a high vacuum (not less than -0.09Mpa) and the oil level of the transformer oil pillow meets the requirements, the oil inlet pipe of the oil filter is connected to the oil drain valve. At this time, the oil filter can work normally.

After the hot oil circulation of the transformer is completed and the oil injection is completed, it should be left standing for a period of time before applying the voltage. 110KV needs to stand for 24h; 220KV needs to stand for 48h.

Note: After the transformer is resting, it should be vented several times from the transformer's bushing, elevating seat, cooling device, gas relay and pressure release device, and should start the submersible oil pump until the residual gas is exhausted.

(3) Oil injection of newly installed transformer

Oil filter and newly installed transformer vacuum

After all transformer accessories are installed. Open all valves of the connecting body of the accessories and components, and vacuum all accessories (including coolers and radiators) except the oil storage tank and gas relay. If the oil storage tank is designed in full vacuum, the oil storage tank and gas relay are also vacuumed.

When vacuuming, close the vacuum valve on the transformer body, check that the vacuum pipe system does not leak, and open the vacuum valve to vacuum the transformer. After the vacuum degree of the transformer reaches the specified value, the vacuum holding time can be filled with vacuum oil only after the specified time is reached (generally keeping the vacuum for 3 to 8 hours is conducive to moisture volatilization).

Vacuum oiling

The transformer oil must be injected into the transformer after the oil sample test is qualified according to the current national standard "Electrical Equipment installation Engineering Electrical equipment handover test Standard".

After checking that all parts are normal, open the oil inlet valve of the oil filter device and the oil outlet valve of the oil tank. After the insulating oil enters the oil tank of the oil filter device, turn on the heater.

After the liquid level in the oil tank reaches the observation window position, open the oil outlet valve and the oil filling valve of the transformer, and open the oil outlet pump to fill the transformer with oil (in order to ensure that the oil is clean and free of impurities, a small amount of oil can be discharged before filling the transformer to clean the tubing and connecting accessories).

The oil injection speed should not be greater than 1800L/h, and the transformer is vacuumed while the oil is injected.

Fill the oil to the specified oil level of the oil storage tank, and the corresponding temperature position should be slightly higher according to the temperature curve of the oil level.

The oil filling is finished, and the shutdown operation is the same as that of oil filtering.

After oil injection, it should continue to maintain direct air, holding time :110kV transformer shall not be less than 2h, 220KV transformer shall not be less than 4h.

Transformer hot oil cycle operation

Hot oil circulation must be carried out after vacuum oil injection for transformers with voltage grades of 220kV and above. Cycle time: 110kV transformer general hot oil cycle 24 hours, 220kV transformer should not be less than 48 hours.

Connect the oil filter to the transformer, connect the oil filter inlet pipe to the bottom valve of the transformer, the outlet pipe to the top valve of the transformer, and the hot oil is injected from the top of the transformer.

Start the oil filter vacuum pump, first open the oil filter side inlet valve and outlet valve, use the oil filter vacuum pump to evacuate the air in the oil pipe, and then open the valve at the bottom of the transformer for circulating transformer heating.

Hot oil circulation precautions:

Before hot oil circulation in full oil state, check that the moisture absorber is correctly installed and breathing is smooth.

When the hot oil is circulating, the outlet oil temperature of the oil filter should not be lower than 50 ° C, and the temperature in the tank should not be lower than 40 ° C.

After the oil sampling inspection of hot oil circulation, it should meet the provisions of the current national standard "Electrical equipment handover test Standard for Electrical equipment Engineering".

After the hot oil circulation of the transformer is completed and the oil injection is completed, it should be left standing for a period of time before applying the voltage. 110KV needs to stand for 24h; 220KV needs to stand for 48h.

After the transformer is static, it should be vented several times from the transformer bushing, elevating seat, cooling device, gas relay and pressure release device, and should start the submersible oil pump until the residual gas is exhausted.

The main function of insulating oil is insulation and heat dissipation, insulating oil first requires good insulation performance, that is, high breakdown voltage, small dielectric factor, which affects the breakdown voltage of the main factors are water, impurities, gas and so on. Transformer oil in operation and the newly installed oil in the long-term storage process is difficult to avoid water intrusion, the operation of the oil under the action of the arc is also easy to produce carbon black, while due to the aging of the oil is also easy to produce impurities such as sludge. The performance of insulating oil is reduced and the safe operation of transformer is affected. At this time, it is necessary to purify the insulating oil to remove the water and various impurities in the oil.

The vacuum oil filter device is a device for the purification of insulating oil, which can efficiently remove water, gas and impurity particles in the oil, improve the insulation strength and oil quality, ensure the safe operation of electrical equipment, and has the functions of hot oil circulation, vacuum oil injection and vacuum pumping for electrical equipment.

1. Working principle of vacuum oil filter device

Before oil filtration, the coarse impurities are filtered from the oil inlet through the initial filter under the action of pressure difference, and the impurities containing particles are heated into the vacuum separation tank through the heating tube. Under the action of the special degassing element of the vacuum cylinder, the insulating oil is fully thin film, and the oil is divided and recombined, so that the tiny water is condensed together into a larger volume of water in the condenser. When the vacuum degree is -0.09Mpa, the boiling point of water is only about 40 ° C, and the oil has been heated and stabilized at 60 ° C, the water in the oil boils out, the oil and water are separated, and the rest of the water vapor and the harmful gases in the oil are discharged by the vacuum pump. The water removed oil is removed by the discharge pump through the fine filter to filter out the particulate impurities, complete a working cycle, after a short cycle, the water, gas and impurities in the oil will be removed to meet the use standard.

1. Working principle of vacuum oil filter device

Before oil filtration, the coarse impurities are filtered from the oil inlet through the initial filter under the action of pressure difference, and the impurities containing particles are heated into the vacuum separation tank through the heating tube. Under the action of the special degassing element of the vacuum cylinder, the insulating oil is fully thin film, and the oil is divided and recombined, so that the tiny water is condensed together into a larger volume of water in the condenser. When the vacuum degree is -0.09Mpa, the boiling point of water is only about 40 ° C, and the oil has been heated and stabilized at 60 ° C, the water in the oil boils out, the oil and water are separated, and the rest of the water vapor and the harmful gases in the oil are discharged by the vacuum pump. The water removed oil is removed by the discharge pump through the fine filter to filter out the particulate impurities, complete a working cycle, after a short cycle, the water, gas and impurities in the oil will be removed to meet the use standard.

2. Vacuum oil filter operation process

(1) Check and prepare before starting

The equipment is placed smoothly, and the oil inlet pipe of the equipment is connected with the oil outlet of the oil tank to be filtered, and the oil outlet of the equipment is connected with the oil inlet of the oil storage barrel. If there is a large amount of precipitation in the tank or oil drum, do not directly insert the tubing to the bottom, if necessary, install pre-filtration.

The cooler is connected to the cooling power supply, in accordance with the principle of the bottom into the top out, small flow water circulation. If it is used for a short time, the water content in the oil is not high, or when it is used in winter, it can also be used without cooling water.

Open the electric control box, select the corresponding three-phase cable according to the total power, switch on the power and ground the vacuum oil filter reliably.

Start the power supply, the power indicator lights up; If the alarm alarm, it means that the phase sequence of the incoming line power line is reversed, (some do not have an alarm, you can observe the positive and negative rotation of the motor, if the motor is reversed, the phase sequence is also reversed) Only need to replace any two wires in the three-phase line.

Safety fences should be placed in the working area, and fire fighting water sources and fire fighting equipment should be provided.

(2) vacuum oil filter operation

The oil storage tank can be filtered by pouring the tank oil filter, and the oil to be filtered through the high vacuum oil filter is all filtered into the oil storage tank, and the oil storage tank is self-circulating, and the oil tank should be sealed. Moisture and magazines in the air are prohibited from entering the oil storage tank.

First, close the inlet and outlet oil valves and other blowdown valves connected with the outside world, sampling valves, water discharge valves and air mixing valves, etc., and then start the vacuum pump on the control panel to begin to vacuum the oil filter device. Vacuum degree rises to set value.

Observe the vacuum gauge on the device, when the vacuum degree is -0.06-0.08mpa, slowly open the oil inlet valve and start to feed oil.

Note: When opening the valve, it should be opened gradually and slowly to avoid damage to the initial filter element due to sudden increase in impact force.

Observe the observation window on the vacuum tube. When the oil enters the vacuum tank and the oil level reaches the middle position of the observation window, open the oil outlet valve first, then start the oil pump, and the oil outlet starts to discharge oil.

Note: If the oil valve is not opened, starting the oil pump first will cause excessive pressure at the oil outlet, resulting in device shutdown and even damage to the filter device.

Adjust the inlet and outlet valves properly, when the inlet and outlet oil reach the basic balance, open the heating switch, start the heater, adjust the temperature control value, generally 55-65℃ is appropriate. If it is a two-stage heating device, it can be decided whether to turn on another set of heaters according to the amount of oil treated and the ambient temperature. The temperature of the secondary heater is slightly higher than that of the primary heating temperature by 3-5℃. (Two stages of heating can also be opened at the early stage of the cycle to facilitate rapid heating, and a group can be closed after the overall oil temperature reaches more than 50 ° C to save energy).

After keeping the circulation filtration for a period of time, open the sampling valve and take the oil sample. (Before sampling, drain a small amount of oil to flush the sampling port and sampling tubing, and the sampling bottle also needs to be cleaned with oil). Until the insulation oil test qualified.

(3) Shutdown operation

Before stopping, turn off the heater 3-5 minutes in advance and keep the oil circulating for a period of time, so that the temperature drops below 50 ° C, otherwise it is easy to damage the heater.

Close the outlet valve of the oil tank first, and then close the inlet valve of the vacuum oil filter. After discharging the remaining oil in the equipment as much as possible, close the oil pump first, and then close the oil valve.

Close the vacuum pump after draining the oil from the tubing. Open the intake valve and remove the vacuum.

Open the valve at the lower end of the condenser, discharge the residual oil or water inside, if you do not need to drain the cooling water for a long time (please be sure to drain the cooling water in winter to avoid freezing). Close the valve when drained.

Open the drain valve, drain the remaining oil in the tank, and then turn off the power.

■ Analysis and discussion on the leakage of transformer oil

With the development of the power industry, how to improve the performance of power equipment and ensure the reliable operation of the power system is an urgent problem that needs to be solved in power production. Therefore, whether the leakage of transformer oil can be timely and thoroughly dealt with has gradually become an important technical indicator to measure the development of the power industry. The phenomenon of oil leakage in oil-immersed power transformer is quite common in power system. If there is oil leakage in transformer, it may affect the safe and stable operation of transformer. How to solve the problem of leakage oil of transformer is a big problem in the production practice of power sector for many years.

1. Analysis of the causes of transformer oil leakage

According to operation experience, transformer leakage oil generally occurs in the following seven main parts: ① the body large cover connection surface; ② Radiator; ③ Sampling valve; ④ casing; ⑤ tubing flange face; ⑥ transformer base interface; ⑦ Gas relay, etc. It can be found that most of the leakage points are flange joints, seals and valves. Unreasonable flange structure, poor sealing material quality, poor processing technology, changes in the external environment temperature, pressure, etc. will cause leakage of the transformer. These factors complement each other and have a common impact. When there is a problem in one of the links, it will cause a chain reaction, resulting in the occurrence of leaking oil. In practice, the following specific analyses are carried out:

First, the flange surface manufacturing process is unreasonable, and the leakage point at the oil drain plug of the transformer radiator interface accounts for more than half of the total leakage point. The reasons of leakage are: (1) there are corrosion, burr, groove and other phenomena on the contact surface; (2) No sealing groove, unlimited pin; (3) Paint and other dirt on the contact surface; When dealing with paint debris, it may scratch scars on the sealing surface; (4) The position and tightness of the sealing ring are completely determined by experience, and too loose and too tight may lead to leakage.

Second, the most commonly used sealing material is butyl rubber, which is made of sealing rings and gaskets with poor oil resistance and faster aging speed, especially at high temperatures, which can easily cause aging cracking and loss of elasticity. In addition, due to the uneven finish of the sealing surface during the installation process, the compression amount is increased and the leakage is caused.

Third, the transformer shell manufacturing process and material quality, if the transformer shell there are pores, sand holes, welding, welding phenomenon will cause leakage of transformer oil.

Fourth, the connection surface of the plate butterfly valve is rough and thin, and there is only a single layer seal, which causes leakage of the transformer.

The fifth is the influence of temperature. When the external temperature is low, the transformer installed in the summer is not, after the summer (temperature up to 37 ° C) operation, the installation of a very tight seal due to the thermal expansion of the metal increases the compression amount, and when the winter temperature is reduced (the minimum can reach -4 ° C), the elasticity of the seal is reduced, and the compression is reduced, resulting in leakage.

Second, type analysis of transformer leakage oil

1. Air leakage. It's an invisible leak. For example, the leakage of air in and out of the casing head, the diaphragm of the oil storage tank, the glass of the safety airway, and the sand hole of the weld. The moisture and oxygen in the air slowly penetrate into the body through the leaking part, and the sealing inside and outside of the transformer is damaged, causing problems such as moisture insulation and accelerated aging of oil.

2. Oil leakage. One is internal leakage. Oil in the bushing or oil in the on-load tap changer chamber leaks into the transformer body. The second is external leakage. Weld leakage and seal leakage, this is the most likely and the most common leakage phenomenon.

1. Magnetic circuit optimization

(1) There is no joint between the three dimensional coil core layers, the magnetic circuit is evenly distributed everywhere, there is no obvious high resistance area, and there is no distortion of the magnetic flux density at the joint.

(2) The direction of magnetic flux is completely consistent with the crystal orientation of the silicon steel sheet.

(3) The length of the three-phase magnetic circuit is completely equal, and the sum of the length of the three-phase magnetic circuit is the shortest.

(4) The three-phase magnetic circuit is completely symmetrical, and the three-phase no-load current is completely balanced.

2, low loss, power saving effect is significant

(1) The magnetization direction of the three-dimensional coil core is completely consistent with the rolling direction of the silicon steel sheet, and there is no bypass connection between the core layers 槰, the magnetic flux distribution is uniform across the magnetic circuit, and there is no obvious high resistance zone and no distortion of the magnetic flux density at the joint. Under the premise of the same material, compared with the coil core and the laminated core, the iron loss process coefficient is reduced from 1.3-1.5 to about 1.05, and this alone can reduce the core loss by 10-20%.

(2) Due to the special three-dimensional structure, the material amount of the iron yoke part of the core is reduced by 25% compared with the traditional laminated core, and the reduced angular weight accounts for about 6% of the total weight of the core.

(3) The shear treatment of the silicon steel sheet will worsen its magnetic permeability, three-dimensional coil core by high temperature (800℃) vacuum nitrogen annealing treatment, not only eliminate the mechanical stress of the core, but also refine the magnetic domain of the silicon steel sheet, improve the secondary recrystallization capacity of the silicon steel sheet, so that the performance of the silicon steel sheet is much better than its factory performance.

(4) After testing, the no-load loss of the three-dimensional transformer is reduced by 25-35% compared with the national standard, and the no-load current can be reduced by up to 92%.

3, low noise

The source of noise generated by transformer body vibration is:

1) The magnetostriction of the silicon steel sheet causes vibration of the core and produces noise.

2) There is an electromagnetic attraction between the joint of the silicon steel sheet and the laminated sheet due to magnetic leakage, causing vibration of the core and noise.

3) The working magnetic density of the transformer is too high, close to or reach the saturation point, and the magnetic leakage is too large, resulting in noise.

Because the three-dimensional coil core is made of silicon steel sheet and strip material in a special core winding machine without interruption and tight continuous winding, there is no seam, and it will not produce noise caused by magnetic discontinuity such as laminated rush. At the same time, the three-phase magnetic circuit and magnetic flux are completely symmetrical, and the working magnetic density is reasonable, so the product noise is greatly reduced.

The type test sound level of SGB10-RL-2000/10 product is only 47dB, which is 19dB lower than the 66dB stipulated by the national standard, and almost reaches the environmentally friendly silent state, which is most suitable for indoor and residential use.

4. Strong overload ability

(1) The heat output of the product itself is very low: the no-load loss and no-load current of the coil core transformer are very small, and the heat output of the product itself is very low;

(2) As shown in the figure, the three-phase coil is arranged in a "product" shape, forming a central natural gas channel through the upper and lower between the coils -" exhaust flue ", due to the temperature difference between the upper and lower iron yoke 30-40℃, resulting in strong air convection, cold air from below to the central channel, heat from the upper iron yoke inner slope radiation out, the natural circulation quickly took away the heat generated by the transformer.

5, compact structure, small footprint

The special three-dimensional core makes the product compact structure and reasonable layout, the body plane occupation area is reduced by 10-15% compared with the traditional product, the body height is reduced by 10-20%, and the box variable volume can be reduced by nearly 1/4 if installed in the box-type substation

Dry type transformer is good or bad mainly depends on the following aspects:

1. Low noise and energy saving.

Low loss silicon steel sheet, stepped laminated iron core joint, further introduction of foil winding structure, noise research, environmental protection requirements, new materials, new processes, new technology and computer optimization design, its development can make the future dry transformer more energy saving and quiet.

2. High reliability.

For electrical products, especially dry transformers, its operational reliability is particularly important, and its reliability and safety is directly related to the safety and stability of daily electricity consumption, which can not be ignored.

3. Environmental protection feature certification.

With heat resistance, moisture resistance, stability, chemical compatibility, low temperature, radiation resistance and non-toxicity, high standard of safety, non-flammable resin.

4. Large capacity.

Dry type transformer is mainly used in urban residential areas, factories and mines and other large and medium-sized load centers, with the increase of urban power load, the capacity requirements of dry type transformer is also getting higher and higher, so, when choosing dry type transformer, the capacity size should also be considered.

The above are the main standards for the quality of common dry transformers, and I hope it will be helpful to everyone when choosing dry transformers.

The oil of the oil-immersed transformer is very important for the operation of the transformer, and the oil-immersed transformer without the transformer oil will be meaningless.

The importance of transformer oil is beyond doubt. It is worth reminding that with the use of oil-immersed transformers for a long time, transformer oil will appear aging

Like, this will inevitably affect the normal use of the transformer. So, how can we prevent the aging of oil-immersed transformer oil?

How to prevent aging of oil-immersed transformers: