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Oct 31 2019

Structure of Transformer

I Overview of the Structure of the Transformer

The power transformer is an electrical device manufactured according to the principle of electromagnetic induction. Therefore, the power transformer should at least be able to efficiently utilize the core and winding of the electromagnetic induction.

The main parts of the power transformer are the core, the windings, the insulation, the outer casing and the necessary components. Due to the difference in capacity and voltage, the structure of the core, winding, insulation, outer casing and necessary components of the power transformer may be different.

From the basic principle of the transformer, the transformer is mainly composed of the core and the primary and secondary windings wound around the core. Therefore, the windings and core are the most basic components of the transformer, which is called the electromagnetic part. The following is a focus on windings and cores.


 Core Structure of the Transformer

2.1 The Role of Core

In principle: the magnetic conductor of the core is the magnetic circuit of the transformer. It converts the electrical energy of a primary circuit into magnetic energy, and converts its own magnetic energy into electrical energy of a secondary circuit, which is a dielectric for energy conversion.

In structure: the structural part of the core not only makes the magnetic conductor a mechanically complete structure, almost all the components inside the transformer are installed on it. It is equivalent to the skeleton of the transformer.

2.2 The Type of Core

(1) Shell type: coils surrounded by core;

(2) Heart type: except the shell type, the rest is a heart-typed core, the heart-type core generally has three-phase three-column, three-phase five-column and single-phase core.

Advantages of the heart-typed core:

a. The process is better, the coil is round and easy to wind.

b. Good economy, which makes full use of space and save raw materials.

c. Good mechanical strength.

Most power transformer companies use heart-shaped cores. Shell-typed cores are used for individual transformers, such as motor car transformers.

2.3 The Composition of the Core

The core is the basic component of the power transformer, and is composed of a core lamination, an insulating component, a core structure component, and a core grounding portion.

The core structural component is composed of a clip foot, an upper beam, a pull plate, a side beam, a pressing nail and the like. The structural component ensures sufficient clamping of the laminations to form a complete and strong core structure. There are insulating components between the lamination and the clip, the foot, the upper beam, the side beam, the drawstring and the pull plate.

The core laminations and the grounding lead wires of the clips are reliably grounded to the outside through the oil tank, and the core is not allowed to be grounded at multiple points.

2.4 Core Lamination

2.4.1 Materials of the Core

1) Silicon lamination

2) Amorphous alloy

2.4.2 Formation of Eddy Current

When the block metal is placed in a changing magnetic field, or when moving in a magnetic field, an induced current will be generated in the metal. This current is self-contained in the metal, which is like a vortex of water so It is called eddy current. Because the resistance of the block metal is small, the eddy current is very strong, causing a large amount of heat generation of the block metal, and the electric energy is greatly wasted.

In order to reduce the hysteresis and eddy current loss of the core, the core is punched into several different sizes with a silicon lamination having a thickness of 0.23 to 0.3 mm, and stacked in accordance with a certain rule. At present, the silicon laminations used by the company are generally 0.27mm and 0.3mm.

Since the silicon lamination has a larger electrical resistance than ordinary steel, the core made of silicon lamination can further reduce the eddy current loss.

2.4.3 Core Lamination

Large oil-immersed power transformers are mostly laminated cores, and in the form of lap joints, each seam is staggered and pressed, which has two advantages:

a. Insulation: avoiding the possibility of inter-chip short circuit;

b. Easy to clamp: good mechanical strength


1) Number of seams: generally have two levels, three levels, four levels, five levels, six levels

2) Factors determining the number of seams: a. loss; b. noise; c. no-load current; d. craftability

3) Angle: At present, the domestic manufacturing plants are both 45°, because the cold-rolled electrical steel sheets are oriented, and the 45° oblique seams use magnetic flux to circulate in the rolling direction, and the magnetic resistance is small.

4) Size of seams: In the core stacking, the size of seams also has a certain influence on the loss of the transformer core. The loss of the magnetic domain subdivided silicon lamination model under different seam conditions.

2.4.4 Core Column and the Shape of Iron Yoke

2.Core Structural Components

1) Clips

2) Core pull plate

3) Core tie

4) Other structural components

2.6. Insulation of the Core

1) insulation of the core lamination;

2) Insulation of the core lamination by the core structural components

2.7 The Cooling Oil Passage of the Core

In order to prevent overheating of the core, the large-capacity transformer needs to be equipped with a cooling oil passage. The oil passage has two types and they are longitudinal cooling oil passages and horizontal cooling oil passages. At present, the company only uses longitudinal cooling oil passages. There are two types of oil passages in the company, one is cardboard strip structure and the other is a button structure. The oil passage material needs to be made of insulating material and has a certain mechanical strength. When an oil passage is provided, the core laminations separated by the oil passage need to be connected by connecting copper sheets to maintain no potential suspension.

2.8 The Grounding of the Core 

In order to avoid the discharge caused by the suspension of the structural components, the core must be grounded, and all the live parts and the insulation of these parts are designed and manufactured according to the ground insulation.

The core can only be grounded at one point. If there are two or more groundings, a closed loop can be formed between the grounding points. When the main magnetic flux passes through the closed loop, a circulating current will be generated therein, causing internal heat accidents.

Grounding method of the core:

1) The small transformer is connected with the fuel tank in the transformer tank through the structural components.

2) Large transformer usually takes the core grounding piece out of the transformer tank through the bushing and ground it outside the transformer tank. This structure is for checking the insulation state of the core. By opening the external ground, the core can be ungrounded and insulated.

2.9 Performance Parameters of the Core

The no-load performance of the transformer core is one of the main performance indicators of the transformer. The no-load performance includes no-load loss, no-load current, magnetizing inrush current and hysteresis noise when the secondary side is unloaded to the primary side.

2.10 The Influence of the Manufacturing Process of Core on the No-load Performance

The manufacturing process of the core directly affects the no-load performance of the core. The machining of silicon lamination, such as core punching, burrs, seam size, clamping and bending of the core sheets, affects no-load losses and no-load currents:

1) Silicon lamination deformation and mechanical stress can increase the no-load loss;

2) Burr and insulation damage and no-load performance of the core piece;

3) The influence of the size of the core seam on the no-load loss.


III Winding Structure of Transformer

The winding is the most basic component of the transformer. It is called the power transformer body together with the core. It is the part of the circuit that establishes the magnetic field and transmits the electric energy. The power transformer winding consists of a high voltage winding, a low voltage winding, a grounding insulation layer (main insulation), an insulation component between the high and low voltage windings, an oil passage formed by a dovetail pad and a struts, a high voltage lead, and a low voltage lead.

3.1 Conductor:

A material in a transformer that causes a current to flow is called a conductor. For example, silver, copper, aluminum and other materials can be used.

3.2 The Winding Wire:

1) Round wire;

2) Flat wire;

3) Combined wire;

4) Transposed wire.

3.3 Classification and Structure of Windings

Transformer winding structure can be generally divided into two categories: layered and crossover structures, the breakdown of which is shown in the table below.









Layer Type


Cylinder Type

Single-layer Cylindrical type, Double-layer Cylindrical Type, Multi-layer Cylindrical Type, Layered Cylinder Type

Foil Type

General Foil Type, Segmented Foil Type



Crossover Type

Continuous Type

General Continuous Type, Semi Continuous Type, Inner Shield Continuous Type


Tangled Type

Ordinary Tangled Type, Flower Entangled Type, Tangled Continuous Type


Spiral Type

Single-screw Type, Single-semi-helical Type, Double-spiral Type, Double-semi-helical Type, Triple-helical Type, Four-screw Type

Interlaced Type

Continuous Spiral Arrangement

Shell Transformer

With a single pie or double pie

Spiral winding transposition:

Figure 1. One Standard Transposition

Figure 1. One Standard Transposition

Figure 2. 2-1-2 Transposition

Figure 2. 2-1-2 Transposition

2-4-2 Transposition

Figure 3. 2-4-2 Transposition

Figure 4. 4-2-4 Transposition

Figure 4. 4-2-4 Transposition

IV Insulation Structure of Transformer

4.1 Classification and Structure of Insulation of Oil-immersed Transformer

4.1.1 Inner Insulation

1) Winding insulation: main insulation, between the windings of the same phase, between the windings of different phases, the winding pair oil tank, the winding pair core column, the iron yoke, the core side column. Longitudinal insulation, between winding turns, between winding wires, between winding layers

2) Lead insulation: main insulation, lead-to-ground, and lead-to-phase windings. Longitudinal insulation, between different leads of the same winding

3) Switch insulation: main insulation, switch to ground, switch between contacts of different phase winding lead. Longitudinal insulation, between the contacts of same phase winding lead.

4) Outer insulation: the bushing is placed between the grounding parts and the bushings.

4.1.2 Typical Insulation Structure inside the Transformer

(1) Pure oil clearance: The smaller the φ/D between the leads of the low voltage level, the more uneven the electric field.

(2) Pure oil gap: between the low voltage level busbars, between the busbars and the fuel tank or other structural components.

(3) Leads are between the fuel tank or other structural components.

(4) Insulation between winding turns and between cakes: Insulation between cakes can make oil, paper or cardboard, and wires have different thicknesses of insulation.

(5) Between the leads or between the windings of the small-capacity transformer. Depending on the voltage level, the wires may be wrapped with different thicknesses of insulation or covered with other insulating materials.

(6) Solid insulation Fix the terminals or leads of different potentials (there may be insulation outside the leads).

(7) Oil-partition insulation: The main insulation structure between windings (including between in-phase and out-of-phase windings).

(8) Oil-partition insulation: the winding is on the oil tank, the winding is on the core, and the winding end is on the iron yoke.

4.2 Main Insulation Material

4.2.1 Liquid Insulation Material

1) Transformer oil: It is a fraction of natural petroleum in the refining process which is refined by refining and adding appropriate stabilizer. Its main components are cycloalkanes, alkanes, aromatic hydrocarbons. Transformer oil fills the entire space in the transformer tank, which acts as both insulation and heat transfer (heat dissipation). For transformer oil, it can also play the role of arc extinguishing, for example for switching.

2) Other liquid insulation materials: alpha oil, beta oil, complex sensitive insulating liquid, polychlorinated biphenyl, silicone oil.

4.2.2 Gas Insulation Materials

1) Air----mainly as external insulation

2) SF6 gas-----applies to circuit breakers, combination electric, high voltage transformers and transformers.

4.2.3 Solid Insulation Material

1) Insulating paper, insulating paperboard and paper products

2) Wood and wood products (laminated wood)

3) Rubber cardboard, tape board, plastic paper tube, tape tube

    For example: phenolic paperboard, phenolic replacement, epoxy cloth board, epoxy glass cloth board, phenolic paper tube, epoxy glass cloth tube

4) Fiber products

    For example: straight cloth tape, twill tape, heat shrinkable polyester tape

5) Chemicals

4.3 Oil, Paper Insulation Structure: 

1) Covering;

2) Insulating layer;

3) Insulating partition

4.4 Typical Electric Field inside the Transformer

1) A slightly uneven electric field:

a) A plate-shaped uniform electric field;

b) A cylindrical slightly uneven electric field;

c) A coaxial cylindrical slightly uneven electric field;

d) A slightly uneven electric field perpendicular to the cylindrical shape.

2) Extremely uneven electric field:

e) Tip-to-pointed pole non-uniform electric field;

f) Tip-to-plate pole non-uniform electric field.

4.5 The External Electric Field of the Transformer

The external electric field of the transformer is mainly the electric field of the bushing to the ground and between the bushing. This electric field can be described by a typical tip-to-pointed and pointed-to-plate electric field. The insulation between the electrodes is air, which has both a pure air gap and creepage along the surface of the bushing.

4.6 The Main Insulation of the Transformer

1) Large oil passage thick paper tube: It is characterized by allowing the oil passage to discharge under the power frequency and impact test voltage. All the voltage is received by the thick paper tube and is not broken down. However, this cooperation does not guarantee that the solid insulation is not damaged under the test voltage. Therefore, it is no longer used on transformers of higher voltage levels.

2) Thin paper tube small oil passage: The characteristic is that the oil pressure resistance increases when the oil volume is reduced. Therefore, it is generally used on transformers with higher voltage levels. Due to the limitations of insulation materials and process manufacturing conditions, as well as winding heat dissipation requirements, it is not possible to make the oil passages small. According to the characteristics of the main insulation structure, the weak point of the insulation is oil. Therefore, an effective way to improve the insulation strength is to increase the insulation strength of the transformer oil.

4.7 The Type of Voltage on the Transformer

A) Normal working voltage

B) Power frequency voltage rises:

    1) When a single-phase ground fault occurs during operation, the voltage of the healthy phase will rise from the phase voltage to the line voltage;

    2) Line resonance overvoltage;

    3) External overvoltage (atmospheric overvoltage);

    4) Internal overvoltage (operating overvoltage);

    5) Fast transient overvoltage

4.8 Partial Discharge of the Transformer

4.8.1 Definition: 

Internal insulation of the transformer, due to various reasons, local and repeated breakdown and extinction occur inside the body under a certain applied voltage.

4.8.2 Features: 

This partial discharge occurs in one or several small spaces, and the energy of discharge is small. The presence of partial discharge does not affect the short-term insulation strength of electrical equipment. However, a transformer has a partial discharge phenomenon in the unrecoverable insulation under the operating voltage. These weak discharged energies and some adverse effects caused by it can slowly damage the insulation, and over time, eventually lead to the entire insulation breakdown.

4.8.3 Causes of Partial Discharge:

1) There is a concentration of local electric field strength in the transformer, causing discharge of transformer oil

2) Gas (usually air) in the insulating material or oil

4.8.4 How to Reduce Partial Discharge or No Partial Discharge:

1) Control the electric field strength of each part during design;

2) During the manufacturing process, pay special attention to the cleaning of the components in the body;

3) The insulating materials used is selected. In high electric fields, epoxy glass cloth sheets and other materials with large dielectric constant are avoided;

4) The suspension of each metal component in the assembly is absolutely not allowed;

5) After the total assembly of the transformer, it is necessary to fill the oil under high vacuum conditions, and the oil supplement should also be finished under high vacuum;

6) The injected transformer oil must be qualified transformer oil;

7) After the transformer is filled with oil, there is considerable time for static release, so that the gas remaining in the transformer can be fully absorbed by the transformer oil.

4.9 The Discharge Along the Insulation Surface of the Transformer

4.9.1 Main Factors Causing Surface Discharge:

1) There is a high field strength region or "point";

2) There are two components of the electric field on the insulating surface (that is, along the surface of the material and the surface of the vertical material);

3) Other factors: the oil flow rate is too large, the surface of the insulation board is damp.

4.9.2 Countermeasures to Prevent Surface Discharge:

1) Reduce the electric field strength at the "tip", such as the copper shield tube for 330kV products:

2) Reduce the electric field strength of the oil at the "tip";

3) Arrange the insulation structure as much as possible on the potential surface;

4) Extend the creepage path;

5) Select insulation materials with good creepage resistance;

6) When making insulation components, especially laminated insulation components, we must pay great attention to the compression and cleanness of the bonding surface.

4.10 The Insulation Level of the Transformer

4.10.1 Insulation Level of Transformer Windings and Lead Wires







Rated Lightning Impulse (internal 

and external insulation) Withstand 

Voltage kV (peak)

Rated Short duration Power Frequency (1min)    (internal and external 

insulation) Withstand Voltage

Rated Operating Impulse 

Withstand Voltage

     kV (effective value)

     Full Wave


               kV (effective value)

                   kV (peak)
































4.10.2 Insulation Level of the Neutral Point of the Graded Insulation Transformer Winding




/ kV




/ kV

Grounding Method Of Neutral Point

Lightning Shock Full Wave 

and Cut-off Withstand 

Voltage / kV

Short duration Power Frequency 

Withstand Voltage / kV



        Not Fixed Ground





            Fixed Ground



        Not Fixed Ground





            Fixed Ground



        Not Fixed Ground





            Fixed Ground



               Grounded                                   via Small Impedance




 Lead Structure of the Transformer

5.1 Lead

The wires connecting the terminals of the windings of transformer windings outside are called leads. The external power supply is input to the transformer through the leads, and the electric energy transmitted through the leads is also output from the transformer to the outside.

5.2 The Classification:

1) A lead wire connecting the winding wire terminal to the bushing;

2) Connecting leads between the winding terminals;

3) The tap leads connect the winding tap to the switch

5.3 Requirement:

1) Electrical performance;

2) Mechanical strength;

3) Temperature rise

5.4 Lead Materials

5.4.1 Conductor:

1) Bare copper rod, applicable range: transformer of 10kV class 6300kVA and below;

2) Paper-wrapped round copper rod, applicable range: small-capacity transformer of 10~35kV;

3) Bare copper row, applicable range: low voltage winding leads of 10kV and below;

4) Copper stranded wire, applicable range: lead wires of various voltage levels, especially 110kV and above;

5) Copper tube, applicable range: transformer leads of 220kV and above

5.4.2 Insulator: 

Laminated wood and cardboard

5.4.3 Connector: 

Connecting copper pipe, terminal block, lug, etc. (for conductive); bolt and nut (for conductor); non-conductive bolt and nut; bolt and nut for insulation

5.5 Lead Selection

1) According to electric field strength and mechanical strength

2) According to the temperature rise during short circuit and the temperature rise during long-term load

5.6 Problems Need to BNoted in the Lead:

1) Connection group;

2) Sufficient insulation distance;

3) Fasteners are fastened enough.


VI Structure of the Transformer Tank

6.1 Overview

The fuel tank of the transformer is not only the outer casing of the transformer body and the oil-filled container, but also the skeleton of the external components of the transformer. At the same time, the heat generated by the loss of the body is dissipated into the atmosphere by convection and radiation.

— As an oil container, the fuel tank should be guaranteed not to leak or seep oil.

— As the outer casing and skeleton, the fuel tank should have a certain mechanical strength.

— As a heat dissipating component, the structure of the fuel tank varies with the increase in capacity.

6.2 The Classification of the Fuel Tank

1) According to the shape: bell-type, barrel-type, clamp-type train transport tank, lift car transport tank.

2) According to the enhanced iron style: corrugated type, bent and strengthened iron type, plate-reinforced iron type

3) According to the seal of the edge of the box: the box edge is welded or not welded.

6.3 The Seal of the Transformer

6.3.1 Basic Knowledge of the Seal

The seal of the transformer belongs to the contact static seal. It is a gasket with a soft material (usually oil-resistant nitrile rubber) between the two flange surfaces that meet each other, and stuff the gap between the two flange surfaces and prevents a sealed medium (transformer oil or insulating gas) with a certain pressure from leaking from the inside of the tank to the outside of the transformer.

6.3.2 Factors Affecting Seal Reliability Include:

1) The shape of the sealed surface: the surface should be smooth and free of defects such as blisters.

2) The size of the pressure of the sealed medium: the friction generated on the contact surface between the sealing component and the sealed surface resists the pressure of the medium acting on it, so that the sealing component maintains a certain stability. If there is no other measures, when the medium pressure reaches a certain value, the frictional resistance will be overcome, and the sealing component will be displaced in the direction in which the medium pressure acts, and the original sealed state will be destroyed. The solution is to add a closed sealed groove.

3) Type of sealed medium

4) Effect of temperature: Since the linear thermal expansion coefficient of rubber is generally an order of magnitude higher than the linear thermal expansion coefficient of metal, it may cause additional thermal stress in the sealing gasket. In addition, with the long-term effect of high temperature, the rubber pad will gradually age and lose its original elasticity, thus affecting the sealing effect; if the rubber pad is in a relatively low temperature state for a long time, it will become brittle and lose its original high elastic energy.

6.4 Body Positioning Structure of the Tank

The factors that determine the positioning structure of the transformer body are mainly the various impact forces that the transformer is subjected to during transportation from the manufacturer to the installation site and the seismic forces that the transformer may be subjected to during operation. The positioning structure is to ensure that the body does not want to produce an unacceptable displacement to the fuel tank under the various forces described above, including the upper and lower positioning of the body.

6.5 The Difference between the Magnetic Shielding and the Electric Shielding of the Fuel Tank

The principle of magnetic shielding is to use the high magnetic permeability of the silicon lamination to form a magnetic shunt with a lower reluctance, so that most of the leakage flux of the transformer is no longer closed through the transformer tank, which can be said to be based on the principle of "sparse". Electrical shielding is the use of eddy current counter-magnetic fields generated by the high conductivity of shielding materials (usually copper or aluminum) to prevent the leakage flux of the transformer from entering the wall of the box. Its foothold is based on "blocking".


VII Transformer Assembly Accessories

7.1 The Main Components of the Transformer:

7.1.1 Bushing: Creepage DistanceCurrent and Other Parameters

Bushing: The bushing is the link between the transformer windings and the power system, and achieves the power transmission between different voltage levels. Its installation makes the transformer an indispensable part of the power transmission and transformation equipment. It can be made into various insulation and current-carrying structure types according to the voltage level and current of the transformer. The bushing is connected to the winding, and the voltage level of the winding determines the insulation structure of the bushing. The current used by the bushing determines the cross section of the conductive portion and the structure of the terminal. Therefore, the bushing is composed of a live portion and an insulating portion.

7.1.2. Switch: Parameters of Current and Voltage Level

Switch: There are two types of voltage regulation methods: non-excitation voltage regulation and on-load voltage regulation. When there is non-excitation voltage regulation, it is not the transformer that does not carry the load twice, but disconnects each side of the transformer from the grid, and changes the tap of the winding without the excitation of the transformer. When there is on-load voltage regulation, the transformer performs the change of the tap of the winding without interrupting the load. In general, the appropriate tap is extracted on the high-voltage winding, because the high-voltage winding is often sheathed outside for easily pull our the tap; in addition, the current on the high-voltage side is small, and the cross-sectional area of the current-carrying portion of the tap-changing lead and the tap-changer is small, and the switch contact portion is also easily solved.

7.1.3 Oil Conservator

1) Definition of oil conservator: When the transformer is operating, the internal transformer oil also changes in volume due to temperature changes. In order to ensure that the insulation and electrical parts of the transformer are still soaked (protected) by oil at the lowest temperature, and the oil does not overflow at the highest temperature, it is the oil conservator that sets up a container that can be used to accommodate this volume change.

2) The role of the oil conservator: When the volume of the transformer oil expands or decreases with the temperature of the oil, the oil conservator plays the role of adjusting the oil to ensure that the transformer tank is often filled with oil.

7.1.Cooler / Radiator

1) Cooler: divided into two types according to the cooling medium: air cooler and water cooler

2) Radiator: Large-capacity transformers generally use a chip radiator, and small-capacity distribution transformers use a tube radiator.

7.1.5 Control Box and Terminal Box

7.1.6 Gas Relay

7.1.7 Pressure Relief Valve

7.1.8 Quick Action Hydraulic Relay

7.1.9 Thermometer


VIII Test and Transportation of the Transformer

8.1 Test Purposes

The transformer test mainly verifies whether the performance of the transformer product meets the regulations and requirements of relevant standards or technical conditions, and finds whether there are defects affecting the normal operation of the transformer in the structure and manufacturing of the transformer. Tests can verify that whether the transformer can operate for a long period of time under rated conditions and can withstand the expected various overvoltages and overcurrents without affecting the life of the transformer.

8.2 Test Classification

8.2.1 Routine Test

The experiment to be carried out for each transformer. Its purpose is to verify the quality of design, process and manufacturing.

1) Winding resistance measurement;

2) Voltage ratio measurement and joint group label verification;

3) Short-circuit impedance and load loss measurement;

4) Measurement of no-load current and no-load loss;

5) Measurement of the dielectric loss factor (tanδ) of the winding-to-ground insulation resistance and/or the insulation system capacitance;

6) Insulation routine test;

7) On-load tap-changer test;

8) Insulating oil test

8.2.2 Type Test

A test carried out on a representative transformer product to prove that the transformer represented is also in compliance with the specified requirements. The purpose of the type test is to check whether the structural performance meets the standard and technical conditions.

1) Temperature rise test;

2) Insulation type test

8.2.3 Special Tests

In addition to type tests and routine tests, tests are carried out in accordance with the manufacturer's and user's agreement.

1) Special test for insulation;

2) Determination of the capacitance between the winding and the ground;

3) Determination of transient voltage transmission characteristics;

4) Three-phase transformer zero-sequence impedance measurement;

5) Short-circuit withstand capability test;

6) Sound level measurement;

7) No-load current harmonic measurement;

8) Power measurement by fan and oil pump motor

8.3 The Transportation Mode of the Transformer

1) Railway transportation;

2) Road transportation;

3) Waterway transportation.

8.4 Small Oil-immersed Power Transformers

Due to the small size, generally the body is equipped with a suitable amount of transformer oil (usually about 100mm under the lid). Large-scale power transformers, because of the heavy weight of transportation, generally take out the transformer oil in the body and transport it separately (transporting oil tanks and oil drums). At the same time, the body is filled with the required dry gas, and the positive pressure is always maintained during transportation. It is 20 kPa to 30 kPa.

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