Home » News » The Ultimate Guide to What is a oil-immersed (liquid-filled) transformer

The Ultimate Guide to What is a oil-immersed (liquid-filled) transformer

Views: 0     Author: Site Editor     Publish Time: 2023-08-21      Origin: Site


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1.What is an oil-immersed (liquid-filled) transformer?

2.What are the classifications of oil-immersed transformers?

3.oil-immersed transformers categories and applications

4.What are the advantages of oil-immersed transformers?

5.What determines the price of oil-immersed transformers?

6.Production process of oil-immersed transformer

7.Structure of oil-immersed transformer

8.Transformer bushing

9.Transformer Tap Changer

10.Transformer Core

11.What is the no-load loss (iron loss) of a transformer?

12.How to reduce the iron loss (no-load loss) of the transformer?

13.Why should the transformer core be grounded?

14.Why can't the transformer core be grounded at multiple points?

15.Transformer Winding

16.How to reduce the winding loss (load loss) of the transformer?

17.Transformer leads

18.Transformer tank

19.Transformer Oil conservator

20.Transformer radiator

21.Transformer Cooling fan

22.Cooling method of oil-immersed transformer

23.Transformer oil

24.Transformer Grounding bolt

25.Transformer Oil drain valve

26.Transformer Oil sample valve

27.Transformer Nameplate

28.What information is on the transformer nameplate?

29.Transformer Oil level gauge

30.Transformer explosion-proof tube

31.Transformer Breather

32.Transformer thermostat

33.Transformer Oil purifiers

34.Transformer Gas relay (gas relay)

35.Transformer Pressure relief valve

36.The structure of oil-immersed Hermetically Sealed distribution transformer

37.Advantages and applications of pad-mounted transformers

38.Single phase pole mounted transformer construction and drawings

39.Three phase pole mounted transformer construction and drawings

40.oil-immersed single phase and three phase pole mound transformer application and advantage

41.single phase pad mounted transformer construction and drawings

42.Three phase pad mounted transformer construction and drawings

43.What tests should be done before the oil-immersed transformer leaves the factory?

44.What should be paid attention to when installing oil-immersed transformers?

45.What does the daily maintenance of transformers generally include?


1.What is an oil-immersed (liquid-filled) transformer?
• Oil-immersed transformers are widely used in industries that require power distribution and transformation. Oil-type transformers are transformers that impregnate the iron core and windings in insulating oil. Due to the loss of magnetic flux in the windings and iron cores during the operation of the transformer, the temperature of the iron core rises. At this time, the transformer oil acts as a liquid medium to reduce the heat generated by the transformer during operation. It also plays the role of insulation, heat dissipation, and anti-corrosion, thereby prolonging the life of the transformer.

2.What are the classifications of oil-immersed transformers?

• Oil-immersed transformers are classified according to the number of phases: the number of phases of the transformer is determined by the number of windings, and the selection of the iron core structure is determined by comprehensive factors such as the reasonable layout of the windings, saving materials, and meeting the transportation height. The bypass can reduce the fifth and seventh harmonics in the leakage flux and excitation current, thereby reducing losses.

Single-phase: single-phase two-column stacked iron core, single-phase single-column side-yoke four-column core, single-phase double-column stacked iron core, single-phase radial stacked iron core, etc., usually used in low-voltage, high-current or small-capacity transformers.

Three-phase: Three-phase column-type stacked iron core, three-phase side-yoke-type five-column iron core, three-phase double-frame stacked iron core, three-phase reactor stacked iron core and other structures, which can be used to raise and lower the voltage of the three-phase system at the same time.


3.Oil immersion type transformer application categories and applications:

油浸式变压器文章配图 拷贝

Power transformer: Electric voltage converter can be used in various scenes of voltage increase and voltage reduction voltage power station and voltage change voltage station, comprehensive voltage station, solar power item and other power-intensive work, power voltage adjustment capacity is insufficient, and the root demand can be reduced. There is a problem with low power transportation, and we also provide long-distance power transportation solutions.

Distribution transformer: 10kv-35kv or less voltage transformer is required for distribution voltage. Suitable for various types of electric power, special distribution area, suitable for large load, large load, high voltage, high stress, and high voltage. The design is robust, flexible, and has a long service life.

Hermetically Sealed Distribution Transformer: Hermetically Sealed Distribution Transformer is normally used in environments such as humidity, dust, ash, and corrosive gases. Because the pressure oil does not come into contact with the external air, the pressure oil does not change, and the service life of the pressure changer is extended, so this kind of pressure changer is practically free.

4.What are the advantages of oil-immersed transformers?

1. Low operating costs: Oil-immersed transformers are cheaper than dry-type transformers and have a wider range of applications.

2. High efficiency: Transformer oil has good insulation and anti-oxidation ability, which can effectively reduce losses and increase the service life of transformers.

3. Fast heat dissipation: insulating oil and radiator improve the heat dissipation efficiency of the transformer and increase the load capacity of the transformer.

4. Strong insulation: Transformer oil can effectively isolate the inner winding of the transformer and the influence of the external environment, ensuring the safety and stability of the transformer operation.

5. Low noise: The operation noise of the oil-immersed transformer is relatively small.

6. Safe and reliable: Transformer oil is non-flammable and has strong insulating ability, and it also acts as a shock-absorbing medium to improve the mechanical strength of the transformer.

7. Easy maintenance: simple maintenance, only need to do regular inspection of accessories, cleaning and replacement of transformer oil.

8. Recyclability: Transformer oil can be reused after purification, and its internal materials can be recycled and reused.

5.What determines the price of oil-immersed transformers?

• Selection of insulating medium: Usually, transformer oil is used as the main filling of transformers. Its high ignition point insulating oil remains stable at high temperature and has excellent electrical insulation performance. Due to different insulation levels, the cost increases with the increase of insulation level. (Dielectric strength TG8, viscosity, freezing point and acid are the main indicators that determine the properties of insulating oil.)

• Selection of winding material: Typically, copper and aluminum are used to make windings or coils. Although copper is a better conductor of electricity, aluminum coil transformers are low cost and light weight. For the same rated current, a copper conductor with a smaller section is used as the winding material in the transformer. Copper coils have higher mechanical strength than aluminum coils.

• Choose a core material with low hysteresis loss: The choice of core material is critical in transformer design. The core material must have high magnetic permeability and low hysteresis loss. Generally, silicon steel, CRGO, etc. are used to achieve the minimum hysteresis loss and high magnetic permeability. The iron loss (i.e. no-load loss) of the new amorphous alloy transformer is 70-80% lower than that of the traditional transformer with silicon steel sheet as the core. As losses are reduced, so is the need to generate electricity, with a corresponding reduction in emissions of greenhouse gases such as carbon dioxide.

• Selection of accessories: external connection accessories (bushing, tap changer), protection device (oil conservator (oil conservator), radiator, cooling fan, grounding bolt, oil sample valve, nameplate, oil level gauge, explosion-proof pipe, moisture absorber, thermometer, oil purifier, gas relay (gas relay), pressure release valve), dispersion accessories (bracket, gasket, screw, etc.)

6.Production process of oil-immersed transformer:

• Raw material selection→oil tank and clamp manufacturing→iron core manufacturing→insulation manufacturing→coil manufacturing→body assembly→transformer assembly→vacuum oil filling→testing→warehousing.

7.Structure of oil-immersed transformer.

• The structure of the power transformer:

结构 拷贝

8.Transformer bushing:

• The role of the transformer bushing: In the power system, both the primary winding and the secondary winding of the transformer are connected to the high-voltage line. Since the winding has to bear the high voltage of the transmission line, in order to avoid the flashover caused by the direct contact between the high-voltage connection and the transformer, it is necessary to insulate the current-carrying part from the non-current-carrying part. The transformer bushing serves as a special insulation device to provide mechanical and electrical insulation support for the transformer winding.


Transformer bushing classification: The two main types of bushing construction are solid type (bulk type) bushing and capacitance graded (capacitor type) bushings.

Solid type transformer bushing: Usually made of a center conductor and porcelain or epoxy resin insulators at both ends, mainly used for voltages below 25 kV. Solid bushings are usually used for everything from small distribution transformers and circuit switches to large generator step-up transformers

Capacitance grading bushing: Applied at a rated voltage higher than 25Kv system voltage. Usually used for large power transformers.

•  Capacitance graded bushings involve more technical and manufacturing details than solid bushings and are therefore more expensive. These details include the insulation, the conductive layer system, the equipment for winding the capacitor core, and the oil that impregnates the paper insulation.

According to voltage classification:  (HV) high voltage bushing (used for transformer low voltage winding, usually a solid bushing with porcelain or epoxy resin insulator) and (LV) low voltage bushing (used for transformer high voltage winding, usually a rated voltage bushing with higher capacitance classification.).

According to standard classification: DIN Standard Bushing、EN Standard Bushing、BS Standard Bushing、ANSI Standard Bushing、AS Standard Bushing...

According to material classification: Transformer Porcelain Bushing、Oil-Impregnated Paper (OIP) Transformer Bushings、Resin Impregnated Paper (RIP) Transformer Bushings、Resin Impregnated Synthetic (RIS) Transformer Bushings、Resin Bonded Paper (RBP) Transformer Bushings、Cast Epoxy Transformer Bushings.

According to the end insulation medium: Air-to-oil bushing、Air-to-air bushing、Air-to-SF6 bushings、SF6-to-oil bushings、Oil-to-oil bushings、Air-to-oil bushing.

According to the inner insulation of the bushing: Air Insulated Bushings、Oil Insulated or Oil Filled Bushings、Oil Impregnated Paper Insulated Bushings、Cast Insulation Bushings、Gas Insulated Bushings

• An air-to-oil bushing has air insulation at one end of the bushing and oil insulation at the other

• Air-to-SF6 bushings, usually used in SF6-insulated circuit breakers;

• SF6-to-oil bushings used as transitions between SF6 bus ducts and oil-filled apparatus;

• Oil-to-oil bushings, used between oil bus ducts and oil-filled apparatus.

9.Tap Changer

The role of the tap changer: In the process of transmitting electric energy in the power system, two parts of voltage and power will be lost. At this time, by adjusting the tap changer to increase or decrease the number of winding turns to change the voltage ratio, control the power flow or adjust the load current to supply a stable voltage. The tap changer of the transformer is divided into two types, off-excitation tap changer and on-load tap changer.

• On-load tap-changer: Transformer on-load tap-changer structure is divided into two types: composite and combined on-load tap-changer

Composite type: Combine the functions of the diverter switch and the tap selector together to form a selector switch, which not only undertakes the selection of the tap but also the switching of the load current, forming an integral plug-in structure.

Combined type: composed of two separate parts, the diverter switch and the tap selector placed in the oil chamber are combined. The tap selector is installed under the diverter switch oil chamber, and the tap changer is hung on the transformer oil tank cover as a whole.

• Off-load tap-changer: Off-load tap-changer can change the tap of the transformer to change the voltage ratio of the transformer under the condition that the transformer does not apply voltage. Off-excitation tap-changers can be divided into single-phase and three-phase according to the number of phases; according to the voltage regulation part, they can be divided into three types: neutral point voltage regulation, middle part voltage regulation and line end voltage regulation.

Transformer Active Part: The Active Part of the transformer is mainly composed of iron core, winding and lead wire. The iron core conducts magnetic flux, the winding converts the input voltage into output voltage, and the lead wire is connected to the current conversion device

The transformer body has sufficient insulation strength, mechanical strength and heat resistance.


10.Transformer Core:

Transformer core: The transformer core is one of the most important parts of the transformer. It is a static device. The core is generally made of high-permeability magnetic material sheets and formed into a stepped shape (graded cylindrical type).


Working principle of transformer core: The transformer works by the principle of mutual inductance. When the alternating current passes through the primary coil (primary winding), magnetic flux is generated, and when the magnetic field passes through the secondary coil (secondary winding), an electromotive force is generated, thereby changing the current or voltage.

Transformer Core Losses: The transformer always works in the AC state. At this time, the current passes through the winding to form a resistance to form a magnetic field in the iron core. During the magnetization process of the iron core, hysteresis loss and eddy current loss are generated. This loss is usually called "iron loss" (no-load loss). The reason affecting the loss is related to the size of the iron core volume, material, current and voltage.

11.What is the no-load loss (iron loss) of a transformer?

Hysteresis loss: core loss is controlled by hysteresis, which refers to the hysteresis phenomenon produced by the transformer in the AC state. Due to the hysteresis of the magnetization on the applied magnetic field, the core energy loss caused by the periodic rotational friction of the magnetic domain, the higher the magnetic field, the higher the hysteresis loss, and it is linearly related to the frequency. The hysteresis loss is proportional to the frequency; it is proportional to the power of the hysteresis coefficient of the maximum magnetic flux density.

Eddy current loss: Eddy current loss is the heat loss caused by the induced current generated by the alternating magnetic flux in the iron core. It changes with the induced current generated by the magnetic flux in the iron core. Its size is related to the resistivity of the iron core material and the thickness of the iron core. The eddy current loss is proportional to the product of the frequency, the maximum magnetic flux density, and the thickness of the magnetic material. The most important way to reduce the eddy current loss is to increase the resistivity of the magnetic element.

12.How to reduce the iron loss (no-load loss) of the transformer?

Improve the manufacturing process of the iron core: select high-quality burr-free iron core materials with high magnetic permeability, such as high-silicon steel sheets, amorphous alloy sheets, etc., to reduce hysteresis and reduce the total amount of iron cores, and use step stacking, plasma treatment, laser irradiation, mechanical indentation and other treatment methods to further reduce loss.

Improve the structure of the iron core: the iron core is not punched, and the glass adhesive tape is not bound, the end surface is coated with cured paint, and the interphase iron yoke is bound with high-strength steel strips. Non-magnetic steel plates are used for the pull plates connecting the upper and lower clamps on both sides of the stem. When assembling the iron core of the transformer, a wound iron core or a three-dimensional iron core can be used. The wound iron core is continuously wound according to the orientation of the silicon steel sheet. Compared with the traditional laminated iron core, the wound iron core does not have 4 sharp corners. After the annealing process is fully and continuously wound, the additional loss can be effectively reduced.

Reduce the overlapping area: According to the core test, for every 1% increase in the overlapping area, the no-load loss of the 45° joint will increase by 0.3%. At the corners of the core laminations, the lap width of the joints between the core pieces and the transverse pieces has a certain influence on the no-load performance of the transformer. If the overlapping area is large, the area through which the magnetic flux passes will increase accordingly, resulting in an increase in no-load loss.

Adopt other methods: such as reducing the size of the core window to reduce the quality of the core, designing a non-resonant core to reduce noise and energy saving, improving the structure of the core to reduce the width of the core overlap and loss, etc., can effectively reduce the core loss.

13.Why should the transformer core be grounded?

• During the operation of the transformer, the iron core, fixed iron core, winding metal structure, parts, components, etc. are all in a strong electric field. Under the action of the electric field, they have a high ground potential. If the iron core is not grounded, there will be a potential difference between it and the grounded clamp and fuel tank. Under the action of the potential difference, intermittent discharge may occur. Under the action of the applied voltage, when the potential difference between two points or a point to the ground is greater than the dielectric withstand voltage between the two, that is, when the discharge voltage is exceeded, spark discharge will occur. As a result, the insulating oil is decomposed or the solid insulating medium is destroyed, causing accidents. Therefore, in order to avoid transformer discharge, the core of the transformer must be grounded.

14.Why can't the transformer core be grounded at multiple points?

• If the transformer core is grounded at multiple points, an eddy current path will be formed through the grounding point. When the main magnetic track passes through this closed circuit, it will cause local heating of the core or even burn the metal parts and insulation layer, resulting in a short circuit fault between the iron chips, which will increase the iron loss and seriously affect the performance of the transformer. In fact, although the silicon steel sheet is coated with insulating paint, its insulation resistance is small, which can only block eddy currents, but cannot prevent high-voltage induced currents.

15.Transformer Winding:

• The coil is the electrical circuit through which the transformer inputs and outputs electrical energy and is the basic component of the transformer. The transformer has two windings, which are divided into a primary winding and a secondary winding. The windings consist of paper-insulated current-carrying conductors wound on various parts of the core; the winding leads are the insulated conductors that connect one winding to another, a tap changer, or an outlet terminal (bushing).

• The arrangement of the high and low voltage windings of the transformer is determined by many factors. As far as most transformers are concerned, the low-voltage winding stage is arranged inside the high-voltage winding. It is because of the insulation considerations. Since the iron core of the transformer is grounded, the low-voltage winding is close to the iron core to easily meet the insulation requirements. In theory, no matter how the high-voltage winding or low-voltage winding is arranged, it can play a role in voltage transformation.


Structure of Transformer Windings

• The coil type is mainly determined according to the coil voltage level and capacity. At the same time, the electrical strength, mechanical strength, heat dissipation and the feasibility of the manufacturing process should also be considered. Transformer coils can be roughly divided into two types: layer type and cake type. Pie coils can be divided into spiral, continuous, entangled, entangled and continuous, inner shielded continuous and staggered.

Transformer winding materials

• Winding wires can be divided into copper and aluminum according to the conductor material, round wire and flat wire according to the conductor shape, and paper, lacquer, and glass wire according to the insulating material. Power transformers usually use paper-wrapped flat copper wire. Copper has higher mechanical strength and electrical conductivity, while aluminum material is less costly and lighter than copper.

16.How to reduce the winding loss (load loss) of the transformer?

1. Use oxygen-free copper rods with higher conductivity than electrolytic copper to improve conductivity.

2. Properly reduce the current density, improve the insulation structure, use half oil channels, prefabricated insulation parts, complete winding transposition, self-adhesive wire, self-adhesive paper, reduce insulation volume, increase winding filling factor, and reduce winding size through optimized design.

3. Reduce the loss of other components

Improve the core structure, design and control the leakage flux of the winding, adjust the ampere-turn balance, and reduce the stray loss of structural parts such as fuel tanks.

Replace the tube radiator with corrugated oil tank, sheet radiator and heat pipe, and replace the old radiator with a new structure radiator to improve the heat dissipation efficiency.

Adopt fan to improve efficiency and reduce noise.

Use magnetic shielding or electric shielding to reduce the stray loss of the fuel tank, and use non-magnetic materials as binding parts or flux separators to increase stray loss.

17.Transformer leads:

• The wires connected externally to the winding terminals of the transformer winding are called lead wires. The external power supply power is input into the transformer through the lead wires, and the electric energy transmitted through the lead wires is output from the transformer to the outside.

• There are mainly the following types of leads:The lead wire connecting the winding wire end and the bushing 2. The connecting lead wire between the winding ends 3. The tap lead wire connecting the winding branch and the switch

Lead materials generally include:

1. Bare copper rod, scope of application: 10kV class 6300kVA and below transformers;

2. Paper-wrapped round copper rod, applicable scope: 10-35kV small-capacity transformer;

3. Bare copper bars, scope of application: 10kV and below low-voltage winding leads;

4. Copper stranded wire, scope of application: all voltage levels, especially lead wires of 110kV and above;

5. Copper tube, scope of application: 220kV and above transformer leads.

• In order to ensure that the insulation distance is sufficient, the leads are insulated by laminated wood and cardboard parts, which must meet the requirements of electrical performance, mechanical strength, and temperature rise. The selection of leads is also selected according to the electric field strength and mechanical strength, as well as the temperature rise during short circuit and the temperature rise during long-term load.

2.Connection of leads:

1. The connection forms of transformer leads are: brazing, gas welding, cold pressure welding and bolt connection.

2. The electrode for brazing should be phosphor copper alloy, which is used for the connection between the winding outlet wire and the lead wire and the lead wire.

3. Gas welding is used for the welding of copper lead wires and the welding of cable-through sleeve joints.

4. Cold pressure welding is to insert the two terminals connected by the lead wire into a metal tube, and then squeeze the metal tube with a mold to press the two terminals tightly together. Cold pressure welding does not require heating, welding is relatively safe, there is no virtual welding, and burns lead wires and other parts insulation, extrusion quality, and good tensile strength. Therefore, cold pressure welding is the main lead wire connection method for large transformers at present.

5. The bolt connection is mainly used for the lead wire connected with the guide rod bushing. The lead wire can be disassembled and can compensate for the deviation of the lead wire length. Usually, a curved curved lead wire structure that can be freely stretched is used, also known as a soft connection.

3.Fastening of leads

In order to ensure the insulation distance of the leads, and to withstand the vibration and impact of electrodynamic force during operation and short circuit without displacement and deformation, clamps must be used to fasten the leads.

The clamping part of the lead wire should have sufficient mechanical strength and electrical strength. For this reason, the structure of the clamping part of the lead wire generally adopts a wooden bracket structure. When the clamping part is fixed with the metal parts of the transformer body, metal bolts can be used to improve the mechanical strength. Insulating cardboard should be added to the place where the lead wire is clamped as additional insulation to prevent the lead wire from being jammed.

18.Transformer. tank:

• The transformer oil tank is a container for the transformer body and transformer oil. Its main functions are:

1. Hold the transformer oil, avoid the direct contact of the transformer oil with the atmosphere and be polluted by moisture, acid, free carbon, oxides, etc., and prevent the aging of the transformer oil.

2. Grounding protection, providing insulation protection for the transformer. Preventing transformer accidents due to potential differences

3. Fix the transformer to improve the mechanical strength and environmental adaptability of the transformer.


19.Oil conservator: Oil conservators can be divided into two types: open type and sealed type, among which sealed type can be divided into capsule type, diaphragm type, and metal corrugated capsule type oil conservators: mainly composed of the cabinet body, capsule, gas collection chamber (the gas collection chamber has components such as main oil tank pipeline, injection and discharge pipeline, exhaust pipeline and sewage pipeline, small tube oil gauge, etc.), moisture absorber and pipeline, vent plug, oil drain plug, oil level gauge, and other components.

Atmoseal Type(Capsule)Conservator:This type of oil conservator tank uses capsules as seals to isolate transformer oil from the atmosphere. The outer side of the capsules is transformer oil, and its inner chamber is connected to the atmosphere through a moisture absorber pipeline, making the pressure inside the oil storage tank the same as the atmosphere. Among the three types of sealed oil storage tanks, capsule type oil storage tanks are the most commonly used and have advantages such as good manufacturing process and convenient operation compared to diaphragm type oil storage tanks


Diaphragm Sealed Conservator:This type of oil conservator uses a diaphragm between the upper and lower cabinets as a seal to isolate transformer oil and the atmosphere, which has the same function and material as the capsule. The sealing surface between the upper and lower sections of this type of oil storage tank is large and rectangular, making it difficult to make and ensuring the flatness of the sealing surface, which can easily cause oil leakage. When filling the oil storage tank, it is necessary to open the hand hole on the upper section of the tank body and exhaust the gas in the transformer oil through the exhaust hose on the diaphragm. The operation is more complicated, while the capsule type oil storage tank only needs to unscrew the exhaust plug on the tank body to exhaust.


Corrugated type oil conservator: This kind of oil conservator uses a metal corrugated expander made of stainless steel sheet as a seal to isolate transformer oil from the atmosphere. Metal corrugated oil conservator can be divided into two types: external oil type and internal oil type.


20.Transformer radiator:

As one of the main equipment for transformer heat dissipation, the radiator transfers heat energy from one medium to another to speed up the cooling of the transformer oil. Usually, the radiator is usually installed on the upper and lower sides of the pipeline to connect with the transformer.


The working principle of the radiator: During the full-load operation of the transformer, the current passes through the winding and iron core to cause loss and generate heat, which causes the temperature of the transformer oil to rise and expand. At this time, the oil level rises and flows into the transformer heat sink to form a convective circulation flow to quickly dissipate heat from the transformer oil. At the same time, prevent the transformer from being in a high temperature state for a long time, causing insulation aging and affecting the power supply reliability of the equipment.

Since the rating of electrical equipment depends on its allowable temperature rise limit, if the temperature rise of the transformer insulating oil is controlled, the capacity or rating of the transformer can be expanded.

The oil-immersed transformer cooling device includes a radiator and a cooler. The one without strong oil circulation is called a radiator, and the one with strong oil circulation is called a cooler.

21.Transformer Cooling fan: The cooling fan is generally installed on the side of the transformer. When the transformer is overloaded or faulty, the temperature of the transformer oil rises rapidly. When natural cooling cannot meet the cooling requirements of the transformer, it is necessary to use the oil pump to force the oil to accelerate circulation, and then the radiator fan cools the transformer oil.

22.Cooling method of oil-immersed transformer:

Oil Natural Air Natural (ONAN):The working principle of oil-immersed self-cooling is to use the natural convection of oil to drive the heat generated by the transformer to the surface of the oil tank wall and the position of the heat dissipation pipe, and then dissipate the heat under the action of air convection and air heat conduction. There is no specially prepared cooling equipment for such a cooling system.

Oil Natural Air Forced (ONAF):On the basis of the working principle of oil-immersed self-cooling, some fans are installed on the wall of the oil tank or the heat dissipation pipe, so that the transformer can be cooled by the fan blower. After installing the fan, the capacity of the transformer and the working load can be increased by nearly 35%.

Oil Forced Air Forced (OFAF):Forced oil circulation air cooling system is used for large capacity transformers. This cooling system is based on the oil-immersed air-cooled type, and a submersible pump is installed on the connecting pipe between the main casing of the oil tank and the radiator with fan (also known as the cooler). When the oil pump is running, the oil in the forced oil tank is sucked into the radiator from the upper part, and then enters the oil tank from the lower part of the transformer to realize forced oil circulation. The cooling effect is related to the oil circulation rate.

Oil Natural Water Forced (ONWF):Use the oil pump to drive it into the cooler and then repeatedly make it into the oil tank. The oil cooler needs to be made into some special shapes that are easier to dissipate heat, so that the medium can be cooled by the blowing effect of the electric fan, thereby taking away the heat generated by the transformer. If such a heat dissipation method triples the speed of oil circulation, it can increase the capacity of the transformer by nearly 30%.

Oil Directed Air Forced(ODAF)A cooling method that uses forced oil circulation to make cold oil pass through the winding and the inside of the core along a certain path to improve heat dissipation efficiency.

Oil Directed water Forced(ODWF):This method is to use the oil pump to force the oil to accelerate the circulation, and to dissipate heat through the water cooler to cool the transformer oil.

23.Transformer oil:

•  Transformer oil tank: The role of the transformer oil tank is to hold transformer oil, dissipate heat, and protect insulation (isolate insulation from the atmosphere, avoid absorbing moisture and gas, and prevent oil aging).

•  The transformer oil tank is generally welded by iron or stainless steel. After hot-dip galvanizing and external paint coating, salt spray test, humidity test, water immersion, acid leaching, and peeling test are carried out. The inside of the tank is cleaned and thermally sprayed with anti-pollution and oil-resistant paint, and then dried again.

1. Insulation: Transformer oil has a much higher dielectric strength than air. The insulating material is soaked in oil, which not only improves the insulation strength, but also protects against moisture.

2. Heat dissipation effect: Transformer oil has a large specific heat and is often used as a coolant. The heat generated during the operation of the transformer causes the oil close to the iron core and winding to expand and rise. Through the convection of the oil up and down, the heat is dissipated through the radiator to ensure the normal operation of the transformer.

3. Arc suppression effect: On the oil circuit breaker and the on-load tap changer of the transformer, an arc will be generated when the contacts are switched. Due to the good thermal conductivity of the transformer oil and the high temperature of the arc, it can decompose a large amount of gas and generate a large pressure, thereby improving the arc extinguishing performance of the medium and extinguishing the arc quickly.

4. Anti-corrosion: Transformer oil can keep the original chemical and physical properties of wood and paper insulation, and make metal anti-corrosion, so that the transformer insulation can be kept in good condition.

Transformer Tank accessories:

24.Transformer Grounding bolt: Grounding protection and zero connection protection are collectively referred to as protective grounding, which is to prevent accidents such as personal electric shock, ensure the normal operation of electrical equipment, and improve the stability, reliability and safety of power supply in the power distribution system.

Protection principle: The basic principle of grounding protection is to limit the leakage current of leakage equipment to the ground so that it does not exceed a certain safety range. Once it exceeds a certain set value, the protector can automatically cut off the power supply; the principle of zero-connection protection is to use the short-circuit current to prompt the protection device on the line to act quickly when the equipment touches the shell after insulation damage to form a single-phase metal short circuit by means of the zero-connection circuit.

25.Transformer Oil drain valve: The transformer oil drain valve is installed at the lower part of the transformer oil tank to release or replace the transformer oil.

26.Transformer Oil sample valve: Before the transformer is put into use, the oil sample valve is required to sample the transformer oil, and test whether the electrical strength (breakdown voltage) of the transformer oil is qualified. After the transformer is put into use, check whether the moisture, acid, free carbon, oxides, etc. in the transformer meet the standards

27.Transformer Nameplate: The nameplate of the transformer is used to record detailed technical information such as the manufacture and design of the transformer, which determines the scope of application of the transformer.

28.What information is on the transformer nameplate?

1.Model 2.rated capacity 3.rated voltage 4.rated current 5.no-load current 6.no-load loss 7.short-circuit loss 9.impedance voltage 10.rated frequency and phase number 11.connection group label 12.temperature rise 13.cooling method  14.insulation level 15.weight 16.manufacturer 17.production date.


29.Transformer Oil level gauge:

magnetic oil level indicator or magnetic oil gauge(MOG)

magnetic oil level indicator Function: The magnetic oil level gauge is usually used in the power transformer oil conservator to detect the oil level in the transformer oil tank. Since the temperature rise or fall during the working process of the transformer will affect the expansion or contraction of the transformer oil, the float of the oil level gauge changes with the oil level.

How the magnetic oil gauge Works: When the oil level of the transformer oil conservator rises or falls, it drives the float to swing. The float arm drives the bevel gear device. The bevel gear is connected to the driver magnet, which in turn affects the driven magnet on the other side of the tank wall. Finally, the gauge hands are connected to a follower magnet which moves in unison with the float and gives a proportional response as the float rises and falls. When the oil level fails, the oil level gauge will send out an alarm.

Advantages: The advantage of the magnetic level gauge is that the internal system is completely isolated from the external instrument by magnetic coupling. Since it does not penetrate the tank wall or pipeline, it ensures that no contamination will occur due to gasket leakage, etc. It realizes automatic monitoring and improves the safety and reliability of the transformer.

The importance of monitoring the oil level: If the oil level is too low, it may cause the action of the gas relay, causing the transformer to open circuit. If the oil level is too high, it will cause oil spill and make the respirator invalid.

30.Transformer explosion-proof tube: Anti-riot tube is a kind of safety protection device for transformers. It is installed on the large cover of the transformer. The anti-riot tube is connected to the atmosphere. When a fault occurs, the heat will vaporize the transformer oil, triggering the gas relay to send an alarm signal or cut off the power supply to prevent the oil tank from bursting.

31.Transformer Breather: The transformer respirator is installed on the air inlet of the transformer or transformer oil conservator. According to the model, it is divided into ordinary type, double breathing type and explosion-proof type.


The function of the transformer silica gel breather: To avoid the air flow in the transformer oil tank when the temperature of the transformer insulating oil rises or falls, which will bring moisture and dust into the transformer oil tank. In order to ensure the insulation strength of the transformer oil and avoid the aging of the transformer.

Silica gel desiccant: The shape is granular, and the color changes according to the degree of water absorption, from blue to light purple, and light pink when saturated with water, which is convenient for substation maintenance personnel to inspect and observe daily. The main ingredients are silica gel desiccant and a small amount of cobalt dichloride.

32.Transformer thermostat: The service life of a transformer depends on its winding temperature, which plays a decisive role in the performance of insulating materials. In order to prevent the accelerated aging of the transformer caused by excessive oil temperature of the transformer, a temperature controller is required to provide functions such as temperature measurement and cooling control. When the temperature exceeds the allowable range, an alarm or trip signal is provided to ensure the life of the equipment. Temperature controllers include oil level temperature controllers and winding temperature controllers.


There are two types of temperature controllers for transformers, namely the oil surface temperature controller and the winding temperature controller.

The working principle of the oil surface thermostat: the thermostat is mainly composed of elastic elements, capillaries and temperature bulbs. The closed system composed of these three parts is filled with temperature-sensing liquid. When the measured temperature changes, due to the "thermal expansion and contraction" effect of the liquid, the volume of the temperature-sensing liquid in the temperature sensor also changes linearly. system to achieve the purpose of controlling the temperature rise of the transformer.

Working principle of winding thermostat: winding thermometer is a special monitoring (control) instrument that is suitable for thermal analog measurement technology to measure the hottest point temperature of power transformer winding. The so-called thermal simulation measurement technology is based on the easy-to-measure transformer top layer oil temperature TO, and then applies an additional temperature rise △T for the change of transformer load current, so the sum of the two T=TO+△T can simulate the hottest point temperature of the transformer.

Oil temperature alarm temperature: the maximum oil temperature at the top layer of transformers with strong Oil Forced Air Forced (OFAF) and strong oil-directed Oil Directed Air Forced(ODAF) is 80°C, and the maximum temperature of oil at the top layer of transformers with Oil Natural Air Forced (ONAF) and Oil Natural Air Natural (ONAN) (capacity 180MVA and below) is 90°C. To ensure the safety of the main transformer operation, it is generally set to 80°C. Limit temperature: When the transformer operates under various over-rated current modes, if the highest oil temperature of the top layer is 100°C, the load should be reduced immediately, and the trip should be reliable when it reaches 105°C.

33.Transformer Oil purifiers: Oil purifiers are also called thermosiphon filters, which use the principle of oil circulation caused by oil differences. The interior is equipped with silica gel or activated alumina adsorbents to maintain good electrical and chemical properties for oil purification, prolong service life, reduce maintenance frequency, and reduce use costs. Oil purifiers are usually installed on the side of the transformer. The purification efficiency is high when installed on the top, and it is easy to replace when installed on the bottom.

Working principle: When the transformer is running, the temperature difference causes the transformer oil to circulate and convect. When the insulating oil flows through the oil purification device, the moisture, acid, free carbon, oxides, etc. in the oil are adsorbed by the adsorbent. The forced oil circulation cooler is generally used on transformers that require forced oil circulation cooling, which is beneficial to the forced circulation of the transformer oil by the oil pump to achieve purification. The working status is related to the pressure of the oil pump and has nothing to do with the temperature difference.

34.Transformer Gas relay (gas relay): monitor and protect the internal safety of oil-immersed transformer equipment, reflect the failure of the internal circuit of the oil tank, such as multi-phase short circuit in the oil tank, short circuit between winding turns, short circuit between winding and iron core or between the casing and iron core failure, oil level drop or oil leakage, poor contact of tap changer or poor wire welding, etc.

Function: monitor and protect the internal safety of oil-immersed transformer equipment, reflect the failure of the internal circuit of the oil tank, such as multi-phase short circuit in the oil tank, short circuit between winding turns, short circuit between winding and core or between the shell and core fault, oil level drop or oil leakage, poor contact of tap changer or poor wire welding, etc.


There are two types of gas relays: main transformer body gas relays and on-load tap changer gas relays.

Working principle: When a fault occurs in the transformer oil tank, due to the short-circuit current and the arc at the short-circuit point, the transformer oil and other insulating materials will be decomposed due to heat to produce gas, which will flow through the gas relay during the process of rising to the upper oil conservator; When the open contact closes the heavy gas, the relay protection trip will be started directly, the circuit breaker will be disconnected, and the faulty transformer will be protected.

The on-load tap-changer for oil arc extinguishing should use the oil flow speed relay. When the tap-changer is switching gears, the arc will cause the transformer oil to decompose and generate gas. If there is a light gas alarm function, frequent exhaust operations may be required after switching the tap-changer.

35.Transformer Pressure relief valve

The pressure relief valve is a pressure protection device for the transformer. When there is a serious fault inside the transformer, the oil will decompose and generate a large amount of gas instantly. resulting in a sharp rise in internal pressure. If the pressure cannot be released outward, it will cause the risk of rupture and explosion of the transformer oil tank..


Working principle: When a short circuit or fault occurs inside the transformer, the winding will generate huge heat to instantly vaporize the oil and generate gas continuously, causing the pressure in the oil tank to rise very quickly. At this time, the pressure relief valve will be opened in time to remove the gas and part of the transformer oil and reduce the pressure in the oil tank. After the pressure in the fuel tank decreases, the pressure relief valve will automatically close to keep the fuel tank sealed.

36.Advantages and applications of pad-mounted transformers:

Pad mounted transformers are used as an important part of underground power distribution,suitable for residential applications, tourist sites、hotels、buildings、commercial、and industrial settings、among others.

Its advantages are reasonable internal structure design is reasonable, the operation is simple, the use is convenient, and the assembly is easy and fast、High application income、low noise, small size, and low loss.

37.The structure of oil-immersed Hermetically Sealed distribution transformer:

38.single phase pole mounted transformer construction and drawings:

single phase pole mounted transformer construction and drawings

39.Three phase pole mounted transformer construction and drawings:

Three phase pole mounted transformer construction and drawings

40.oil-immersed single phase and three phase pole mound transformer application and advantage:

• The transformer suitable for rural power grid, remote mountainous areas, scattered villages, agricultural production, lighting and power consumption, and can also be used for energy-saving transformation of railway and urban power grid.

• The transformer is designed with a high-efficiency, energy-saving, and environmentally-friendly coil core structure. It can reduce the length of low-voltage distribution lines and reduce line losses, and improve the quality and stability of power supply. Its advantages are small size, small infrastructure investment, and reduced radius of low-voltage power supply. The fully sealed structure has high mechanical strength, strong overload capacity, reliable and stable continuous operation, and simple maintenance.

41.single phase pad mounted transformer construction and drawings:

single phase pad mounted transformer construction and drawings

42.Three phase pad mounted transformer construction and drawings:

Three phase pad mounted transformer construction and drawings

43.What tests should be done before the oil-immersed transformer leaves the factory?

① insulation Resistance (Giga -OHM) ②Ratio Test  ③Winding resistance measurement at Amb ④No Load Loss  ⑤Load loss at tap(3) at Amb Temprature  ⑥Separate source power frequency withstand test  ⑦Induced over voltage withstand test  ⑧Polarity test  ⑨Iron core insulation resistance measurement   ⑪Vector Group (Dyn11)  ⑫BDV Transformer oil BDV.

Preparation before the test: whether the assembly is completed as required, and whether there is a lack of accessories necessary for the test, such as transformer grounding and iron core grounding devices.

1. Insulation resistance test: Keep the insulation dry before testing. The insulation resistance depends on the temperature, and is usually measured according to the ambient temperature during operation. If the measurement is performed at a higher ambient temperature, it must be considered before determining the insulation resistance value Positive coefficient. The insulation resistance is measured between the high voltage winding and the low voltage winding, the high voltage winding to ground, and the low voltage winding to ground. All other parts of the transformer not tested are always grounded when the rest is tested.

2. Winding resistance test: Before the test, the static time of the dry-type transformer at a constant temperature should be no less than 3 hours. The influence of the self-inductance effect during the test should be minimized. The connection and welding position between the parts of the purpose Whether it is good, whether the resistance and resistivity of each winding are balanced.

3. Voltage ratio test: The voltage ratio is defined as the ratio of the primary voltage to the secondary voltage (VR). The voltage of the winding is proportional to the number of turns of the coil. It is carried out with a transformer turns ratio tester (TTR) for the purpose of Make sure the turns ratio of primary turns to secondary turns is correct.

4. Polarity test: ① Add polarity - the windings of two or more transformers are connected, then the voltages of the connected transformers are added together. ② Polarity subtraction - if the opposite windings are connected, polarity subtraction will occur. If If the transformers are connected in reverse polarity in parallel operation, it will cause accidents and cause damage to the system.

5. No-load loss test: No-load loss is to apply the rated voltage (main tap) of the sinusoidal waveform at the rated frequency to the selected winding, and the remaining windings are open, and the measured active power input to the transformer is called no-load loss. No-load loss It occurs in the iron core, and the loss caused by the change of the magnetic flux in the silicon steel sheet with the AC frequency is dissipated in the form of heat. In addition, the no-load current flowing through the winding will generate the resistance loss I2R of the no-load current in the winding, but the loss I2R is usually small and generally negligible.

6. No-load loss includes two aspects:
Hysteresis loss: When the AC passes through the transformer, the direction and size of the magnetic field lines passing through the silicon steel sheet of the transformer change accordingly, causing the internal molecules of the silicon steel sheet to rub against each other and release heat energy, thus losing part of the electric energy.
Eddy current loss: When the transformer is working, there are magnetic force lines passing through the iron core, and an induced current will be generated on a plane perpendicular to the magnetic force lines. Since this current forms a closed loop and forms a vortex, it is called eddy current. The existence of eddy current Heat the iron core and consume energy.

7. Load loss test: The load loss U is the loss measured when one pair of windings of the transformer is short-circuited, the other winding is applied with voltage, and the rated current passes through. The rated frequency, sinusoidal The impedance when the winding on the other side of the rated current of the waveform is short-circuited is called the short-circuit impedance of the transformer, which is generally expressed as a percentage relative to a certain reference impedance. In the transformer load test, the load loss of the transformer and the short-circuit reactance of the transformer are measured in time.

8. High voltage test: ensure that the insulation performance of the transformer is sufficiently stable under rated electrical or overload conditions.

9. Dielectric test: Dielectric test is divided into three different types: applied potential test, induced potential test, pulse test (type test). The dielectric test ensures the overall insulation strength of the transformer and confirms that the transformer can withstand the conditions defined in the standard. test level. The full test voltage is applied for more than 60 seconds between the winding under test and all other windings, the transformer core, and the earthed case.

10. Partial discharge test: Under the same electric field, bubbles and impurities with small dielectric constants bear higher electric field strength. Therefore, when the applied voltage reaches a certain value, these parts tend to discharge first. This is partial discharge.
The cause of partial discharge: For the insulation structure of the transformer, there may be bubbles (air gaps), impurities, etc. inside, which are inevitable. Under the same electric field, bubbles and impurities with small dielectric constants withstand higher electric field strength.
Purpose of the test: whether there are local defects in the insulating material used, whether air bubbles, moisture or various impurities are mixed in the insulating structure, whether there are bad links, and whether the electric field strength in individual areas is too high.

11. Magnetizing current test: The magnetizing current test ensures that the magnetic circuit has sufficient reluctance to establish flux in the core. It can indicate and locate defects in core construction, problems in tap changers, turn-to-turn insulation faults, winding displacements, and more. Measure the DC resistance before performing the test.

12. Power factor test: This test is to determine the power loss of the transformer insulation system. It indicates failure or degradation of transformer insulation and is the power angle between the applied AC voltage and the resulting current.

13. Wiring group test: The wiring group of the three-phase transformer and the polarity of the lead wire of the single-phase transformer should be consistent with the parameters on the transformer nameplate. It can judge whether the connection of the transformer winding is correct, so as to ensure the secondary protection circuit and measurement circuit Correctness of wiring

14. Noise test: The noise of the dry-type transformer is caused by the hysteresis deformation of the iron core and the electromagnetic force in the winding and magnetic shielding. The noise generated by the magnetic field-induced vibration of the core laminations along the longitudinal direction is the main component of the transformer noise. The amplitude of the vibration is related to the magnetic flux density in the core laminations and the magnetic properties of the core material, but has little to do with the load current. In addition, the leakage magnetic field can also cause vibration of structural parts.

15. Climate test: The transformer is suitable for operation at an ambient temperature not lower than -5°C to -25°C, but the ambient temperature test during its transportation and storage.

Temperature rise test: The temperature rise value is determined by a combination of short-circuit test (providing load loss) and no-load test (providing load loss). It is suitable for non-closed, closed or fully enclosed thousand-type self-cooling AN or wind Cold AF Transformer.

16. Combustion performance test: F0 does not need to consider the fire hazard. Except for the characteristics inherent in the design of the transformer, no special measures are taken to limit its flammability, F2 flammability and the degree of toxic substances and opaque smoke released during combustion.

17.Transformer oil test: test the appearance, color, density, moisture, acid value, oxidation stability, breakdown voltage, dielectric loss factor, interfacial tension, sludge and sediment, flash point, gas component content in oil, water-soluble acid, freezing point, pour point, volume resistivity, and kinematic viscosity of transformer oil.

44.What should be paid attention to when installing oil-immersed transformers?

• Check whether the information on the nameplate matches the working environment before installation

Installation Location:

1. It should be as close to the load center as possible.

2. It is convenient for incoming and outgoing lines, and has ample access and exit corridors.

3. There are traffic passages for transporting transformers and other equipment.

4. It does not hinder the expansion and development of power consumption units.

5. The surrounding environment should be clean, located on the upwind side, not in polluted areas, and the terrain should not be threatened by floods.

Before installation, the transformer should be tested and tested:

1. Measure the DC resistance of the coil together with the bushing.

2. Check the transformer ratio of all taps.

3. Check the wiring group of the three-phase transformer and the polarity of the single-phase transformer.

4. AC withstand voltage test of the measuring coil together with the bushing.

5. Measure the insulation resistance of the core-through bolt (accessible) yoke iron clamp, binding steel strip to the iron yoke, iron core, fuel tank and coil pressure ring (equipment without body inspection is not acceptable).

6. Insulating oil test in the oil tank.

7. Check phase.

Install lightning arresters and grounding devices:

Lightning arrester: Lightning strike is a natural disaster, which is very destructive. Lightning strike may cause power transformer damage, burning and explosion, endangering human life. In order to prevent power transformer from lightning strike, lightning arrester must be installed when the power grid is offline. The lightning arrester should be connected in parallel with the high-voltage end of the protected power transformer.

Grounding device: When installing a grounding device, the natural grounding body should be fully utilized first, because many natural grounding bodies are reliably connected to the earth. If the grounding resistance of the natural grounding body used cannot meet the requirements, an artificial grounding body should be installed to make up for the lack of natural grounding.

Power transmission and commissioning after installation:

1. Transformer no-load input impact test. Full-voltage impact closing, generally can be input from the high-voltage side when impact closing. After the transformer receives power for the first time, the duration should not be less than 10 minutes, and there should be no abnormalities.

2. The inspection method of transformer no-load operation is mainly to listen to the sound. When it is normal, it emits a buzzing sound, but when it is abnormal, the following situations occur: when the sound is relatively loud and uniform, it may be that the applied voltage is relatively high;

3. Transformer commissioning and operation. After the no-load impact test, it can be operated under no-load for 24-28 hours, and it can be operated with half-load if no abnormality is confirmed. After the half-load energization and debugging operation of the transformer meets the safety operation regulations, the full-load debugging operation is carried out. The transformer has been commissioned and operated for 48 hours at full load, and the transformer temperature rise, oil level, oil leakage, and cooler operation are checked again. After passing the full-load test, the handover procedures can be handled before it can be put into operation.

45.What does the daily maintenance of transformers generally include?

Items to be inspected and monitored for routine maintenance generally include:

1. Whether the transformer has abnormal sound, such as uneven sound or discharge sound, etc.

2. Whether the oil level is normal, whether there is leakage or oil leakage.

3. Whether the oil temperature is normal (the upper layer oil temperature generally cannot exceed 85°C).

4. Whether the bushing is clean, whether there are cracks, damage and discharge.

5. Is there any heating phenomenon in the joint?

6. Whether the explosion-proof membrane of the explosion-proof tube is complete.

7. Whether the Buchholz relay is leaking oil and whether the interior is full of oil.

8. Whether the respirator is unblocked, whether the oil level of the oil seal respirator is normal, and whether the silica gel in the respirator is saturated with moisture.

9. Whether the cooling system is operating normally.

10. Whether the shell ground wire is intact.

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