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Aug 19 2019

Advantages and Disadvantages of Dry-type Transformer and Oil immersed Transformer

Introduction

In the field of construction engineering, transformer is one of the indispensable equipment in power system transmission and distribution engineering. It is an important equipment to change the voltage level, reduce the loss of electric energy in the process of transmission and ensure the safety of power consumption. Power transformers are divided into oil-immersed transformers and dry-type transformers according to the different cooling media.

Article Core

Transformer

Purpose

Advantages and Disadvantages of Dry-type Transformer and Oil immersed Transformer

Application

Semiconductor industry

Keywords

oil-immersed transformers, dry-type transformers 


Catalog

Introduction

Ⅰ Oil-immersed Transformer

1.1 Structure of oil immersed transformer

1.2Characteristics of oil immersed transformer

 

Ⅱ Dry-type Transformer

 

2.1 The working principle of dry-type transformer

2.2 Characteristics of dry-type transformer  in application

2.3 Deficiency of dry-type transformer relative to oil immersed transformer

III Application of dry Transformer

in Engineering

3.1 Poor heat dissipation performance

3.2 high temperature

3.3 Effects on ambient temperature

3.4 Selection of structure form of dry Transformer

3.5 Selection of insulation and heat resistance grade of dry transformer 


Ⅰ Oil Immersed Transformer

1.1 Structure of oil immersed transformer

oil-immersed transformers consist of magnetic core, windings and bushings. The magnetic core provides a path for magnetic flow. Windings create a magnetic field and consist of a conductor coil, wrapped around the core and insulated with pressboard barriers and screens. Thickness of the winding insulation increases with voltage. Bushings connect transformer windings to a substation. A characteristic feature of an oil-immersed transformer is the oil conservator. It simplifies operation by monitoring the oil level in a tank and providing the space for thermal expansion of the oil.

  Oil Immersed Transformer

 Oil Immersed Transformer

1.2 Characteristics of oil immersed transformer

1.2.1 Oil-immersed transformer is a transformer with transformer oil as heat dissipation medium. the application of oil-immersed transformer in power system has been used for hundreds of years in transmission and distribution engineering. Oil-immersed power transformer has become one of the most widely used electrical equipment in the world due to its simple structure, long history, rich manufacturing and operation, reliable operation. Most of the transformers running in the power system are oil-immersed transformers. Dry-type transformer, as the name implies, is a transformer with air as heat dissipation medium instead of transformer oil.

 Basic structure of transformer  

 Basic structure of transformer 

1.2.2 Transformer oil is used as cooling medium for oil-immersed transformer, and the heat generated by transformer operation is transmitted from transformer oil to metal shell and emitted. It has the advantages of fast heat dissipation, uniform conduction and recoverable insulation performance of oil. However, the oil itself has the disadvantages of low heat resistance, flammability and aging, so there are many specific requirements in the use of transformer oil as cooling medium. For example, the buildings in which transformers are placed must meet the fire resistance grade requirements stipulated in the building fire prevention code; Measures must be put in place to prevent the spread of fire caused by oil flow in the event of fire; There must be fire fighting measures to fight oil fires; There must be oil storage pool facilities to prevent oil leakage from polluting the environment; The daily work should monitor the aging of insulating oil and the regular oil regeneration work, and so on. Moreover, the minimum net distance between the outer profile of the fuel oil immersed transformer and the wall and door of the transformer room also has strict distance requirements. The various civil structures, fire barriers and storage tank of oil-immersed power transformer substation are designed because of the flammability and liquidity of oil. In addition, "when the transformer room is located in the building, the door shall be a Class A fire door." In this way, the construction investment designed to prevent fire in each substation is naturally much larger.

1.2.3 With the continuous development of science and technology and the progress of material science, there have been a series of fully enclosed oil-immersed transformers, such as fully sealed S9-M, S10-M, S11-M and so on. It adopts corrugated fuel tank, and the thermal expansion and cold shrinkage of transformer oil is adjusted by the elasticity of the corrugated sheet, which is isolated from air and basically overcome the problem of oil leakage. The fully enclosed oil immersed transformer has no respirator, which solves the problem of oil oxidation caused by the contact between oil and air, and prolongs the service life. It also has a small footprint, energy-saving type, and leading performance technology. It adopts high-quality high-magnetic-magnetic grain oriented cold-rolled silicon steel sheet. Compared with the same capacity of S9 type, the no-load loss is reduced by 30%, and the no-load current is reduced by 40-70%,and the noise is reduced by 5 ≤ 8 decibels (A). However, because it still uses oil as the cooling medium. The design must also comply with the relevant specifications of oil-immersed transformers. 

Ⅱ Dry-type Transformer

2.1 The working principle of dry-type transformer

Dry-type transformer is widely welcomed because of its oil-free and reducing the fire resistance grade requirements of buildings. And in some special environments, only dry transformers can be used like "multi-storey or high-rise main buildings". Therefore, in many places where oil immersed transformers are not allowed, such as underground and high-rise buildings, places with high fire protection requirements can be designed as dry-type transformers. At present, epoxy resin pouring dry-type transformer has overcome the problems of low heat resistance grade of insulation material, easy aging cracking of insulation material and so on. With the continuous development of insulating materials and the improvement of material properties, the insulation heat resistance level can reach the H level, which is the technical index that oil immersed transformer can not reach. Second, compared with traditional oil-immersed transformer, the dry-type transformer has many advantages such as oil, fire resistance, dust-proof, moisture-proof and small partial discharge.

 

The dry-type transformer is mainly composed of Iron core and epoxy resin pouring coil that made of silicon steel sheet. The insulation cylinder is placed between the high and low voltage coils to increase the electrical insulation, and it is supported and constrained by the gasket. The fasteners of the parts and components have the anti-loosening performance.

 Three-dimensional structure drawing of dry-type transformer

 Three-dimensional structure drawing of dry-type transformer

2.2 Characteristics of dry-type transformer in application

2.2.1 Dry-type transformers work in the same way as other transformers, following the physical principles of voltage technology required for the use of electrical and magnetic coupling:When the current flows through the current of the wire, it produces a changing magnetic field or the surrounding "magnetic". The fluctuation of a wire around a magnetic field, and the current it produces on the wire. Therefore, there will be fluctuating magnetic fields in the second line and the first line, and the current will induce flow in the second line. And Electricity is through from two conductors that are not actually in contact with each other.

2.2.2 First of all,  dry-type transformers have the advantage of being oil-free and cumbersome, reducing the level of fire hazard. It can reduce investment in preventing fires; it saves corresponding disaster prevention and civil construction investment. Moreover, since the dry type transformer has no oil leakage problem, the maintenance work of the daily operation of the transformer is reduced, and if oil quality aging is not required to be regularly detected, there is no need to change oil. This has greatly reduced the cost of daily maintenance.

Dry-type transformer 

Dry-type transformer 

2.2.3  Secondly, because the dry transformer has no hanging core operation, the room where the dry transformer is placed can reduce the height of the building and reduce the cost of civil engineering.

2.2.4 Again, according to the specification, “high and low voltage power distribution units without fuel and non-oil-immersed power transformers may be placed in the same room.”We usually share the dry-type transformer with low voltage distribution equipment or high voltage distribution equipment, and cancel the independent transformer room, which can reduce the occupation area and save the overall cost of the project.

Sectional view of dry-type transformer 

Sectional view of dry-type transformer

2.3 Deficiency of dry-type transformer relative to oil immersed transformer 

2.3.1 Compared with oil-immersed transformer, epoxy resin has relatively poor heat dissipation performance. It is not as good as the heat dissipation performance of the oil radiator, and the thermal conductivity is better.

2.3.2The temperature monitoring of dry-type transformer depends on the temperature sensor embedded in the transformer body in advance, and its measuring point is fixed, so the temperature obtained is the temperature of a specific position.For the local heat concentration that can occur, the temperature reflected by the monitor is not the accurate average temperature. The insulating oil of oil-immersed transformer is flowable, and the heat conduction is relatively uniform. The temperature reflected by the monitor can be regarded as the average temperature.

2.3.3 Compared with the repairability and recoverability of oil-immersed transformer insulating oil performance, the dry-type transformer insulation material is an epoxy resin integral casting structure, and its performance damage is irreversible, and the insulation material has aging and defect accumulation. Disadvantages. In the event of a failure, the transformer will be scrapped as a whole.

2.3.4 The manufacturing process of dry-type transformer is complicated, and the overall price is higher than that of oil-immersed transformer.

2.3.5 Finally, after the oil immersed transformer is scrapped, it can be disassembled into original materials for recycling. However, the waste equipment of dry-type transformer has poor recovery performance, and epoxy resin casting dry-type transformer cannot be recovered at present. 


III Application of dry-type transformer in engineering

Dry-type transformer has many advantages, but in order to use it reasonably in engineering, it is necessary to fully understand all its advantages and disadvantages. Because the dry transformer is a new product, and there is still a lack of the test of time. Nowadays, some data of insulation aging are only laboratory data, lack of actual use data statistics. So we must be cautions in engineering application. According to our experience of using dry-type transformer in practical engineering, the following precautions are summarized:

3.1 Poor heat dissipation performance

 The structure of dry-type transformer determines its poor heat dissipation performance, which leads to low overload capacity. The oil-immersed transformer is allowed to run for 15 minutes under the condition of 160% overload, while the maximum operating limit current of the dry-type power transformer should not exceed 1.5 times the nameplate rated current, otherwise it may cause damage to the transformer. When carrying out load calculation, these characteristics of various types of transformers must be considered.

3.2 High temperature

 High temperature is produced in operation of dry-type power transformer, which directly affects transformer life, so special attention should be paid to monitoring. The temperature required to be detected in operation of dry-type transformer is different from that of oil immersed transformer. Even in no load state, the data which have influence on core temperature are required to be recorded, so it shows that some temperature is attached to transformer, and the overall temperature rise of dry transformer is out of proportion to the increase of load current.

3.3 Effects on ambient temperature

 Because the copper loss in operation of the transformer will become heat emission, the transformer itself will become an uninterrupted heat source, which will be emitted into the surrounding environment, and the ambient temperature will continue to rise.

3.3.1 For dry-type transformers and low-voltage distribution equipment arranged in the same room, the suitable temperature of thermal protection elements in low-voltage distribution equipment should not exceed 40 °C. When the ambient temperature exceeds 40 °C, the high temperature of the low voltage protection equipment may lead to the wrong operation of the switch and shorten the life of the components. At this time, the switch cannot workat the rated current rating. If the method of capacity reduction is adopted, it will bring the problem that when the ambient temperature returns to normal and the switch protection value returns to the rating value, the electrical equipment can not be protected correctly because the switch value has increased. If the switch value is adjusted according to the ambient temperature at any time, it is not only too much work, but also unoperable to adjust the switch value at any time with the change of temperature.

3.3.2 The building height of the oil-immersed transformer chamber is to meet the core operation of the transformer. The space of the building is high, and the difference of the central height of the inlet and exit air window is generally large. To meet the conditions of air convolution, the indoor temperature of the transformer chamber can be reduced by air convolution. And in the substation of dry-type transformer, because there is no transformer core operation, the required height of the building is relatively low, which can not meet the air convective conditions, and can not use the air convolution principle to dissipate heat.

3.3.3 According to the rules for selection, acceptance, Operation and maintenance of dry-type Power Transformers, "the indoor ventilation system for installing dry power transformers shall be independent. The transformer shall have a temperature control device that controls the air-cooling device to switch at the right time with the winding temperature.”The practice of the actual design is to pay attention to the air circulation problem of the transformer room at the design stage, take corresponding measures, reasonably reserve the air circulation air duct, and ensure that the room is well ventilated, so that the normal heat dissipation of the transformer is ensured, and the ambient temperature can meet the requirements of normal operation of the equipment. In the actual engineering design, the measures that can be taken are as follows: installing wall ventilator, strengthening indoor ventilation and reducing the whole indoor temperature. Or equipped with mechanical ventilation on the transformer, so that the heat generated by the transformer can be directly discharged out of the air through the air duct. In addition, in order to ensure the reliable operation of the ventilation system, it is necessary to ensure the reliability of the ventilator power supply. Therefore, the procedure requires that the dry-type transformer be configured with a "The ventilation system with independent power supply can send a remote signal when the mechanical ventilation is stopped." To ensure that the transformer room is well ventilated.

3.3.4 In a word, the heat emitted by the transformer must be sent outdoors to avoid the rising indoor temperature of the transformer room, which will affect the normal operation of other distribution equipment.

3.4 Selection of structure form of dry-type transformer

For the usage of dry-type transformer, it is best to choose different structure transformer for different capacity transformer. For example, the low power foil winding transformer has the advantages of good heat dissipation performance, reduced longitudinal force of transformer, less magnetic loss, low noise and so on. However, with the continuous increase of transformer capacity, the weakness of poor short circuit resistance of large capacity transformers is becoming more and more prominent. Transformers exceeding 2000 KVA are theoretically not possible to use foil winding.Therefore, in the engineering application, it is necessary to choose different structural forms of transformers according to the specific situation.

dry-type transformer 

3.5 Selection of insulation and heat resistance grade of dry-type transformer

3.5.1 The insulation and heat resistance of dry-type transformer is getting higher and higher. At present, F class products are basically popularized, and H grade products are also produced. As a technical index, this is a kind of scientific and technological progress. But in the practical application of the project, we must consider the comprehensive situation of the use conditions and the surrounding environment. When elected the class H transformer, whether the equipment connected to it and other equipment in the same room can also withstand the temperature of class H insulation. For example, whether the class H insulated cable is selected, and so on. If the relevant equipment does not meet the H level, the equipment is not allowed to reach the H level temperature when running, so that the H class insulation is meaningless. Especially when the transformer is arranged in the same room with other equipment, although the heat resistance temperature of the transformer can reach 180 °C. But when the operating temperature of the transformer really reaches 180 °C, the ambient temperature has far exceeded the allowable temperature of the distribution equipment. Especially for products with thermal protection elements, because the working principle of the product is to detect the temperature change at room temperature, when the room temperature exceeds room temperature, the electrical components will not be able to work properly due to the influence of ambient temperature, resulting in abnormal protection. So the electrical components in the distribution cabinet can never allow the ambient temperature to reach 180 °C. If the transformer and the distribution cabinet can not arranged in the same room for this purpose, and the class H insulated transformer is installed separately in the independent transformer room, then the advantages that the dry-type transformer substation has a small footprint can not be reflected.

3.5.2 In general, In general, only transformers can achieve class H insulation, while other related equipment can not reach class H insulation, it is actually a waste that the transformer can not give full play to its due role even if it reaches grade H insulation alone. In this regard, in actual engineering, no matter which level of insulation is selected for the transformer, the working conditions and heat resistance of other ancillary equipment must be considered, which is not necessarily the higher the better. It's not that the higher the grade, the better the effect.


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