Operation noise analysis of the hottest dry-type t

Noise analysis of dry-type transformer with shell

: Abstract: noise is one of the common faults in the operation of dry-type transformer. Dry type transformer is easy to produce noise during operation, so it needs to be improved and controlled. This paper analyzes the main causes of dry-type transformer, and provides two ways to reduce the noise caused by dry-type transformer by using single orientation high magnetic conductivity silicon steel sheet and reducing the rated working magnetic density of iron core

1 preface

in some developed countries, it has been expressly stipulated that oil immersed transformers are not allowed to be used for indoor transformers. In China, the percentage of dry-type transformer output in distribution transformer is also increasing year by year. Especially in some special fields, such as frequency conversion phase-shifting rectifier transformer, photovoltaic power generation transformer, etc. According to the manufacturing process, it can be divided into non encapsulated air insulated dry-type transformer and resin insulated dry-type transformer. In the world, the two are developing in parallel. In China, there is a wide gap between the two. According to statistics, the annual demand for dry-type transformers in China is about 200000, and the non encapsulated air insulated dry-type transformers account for 4%. First, the resin pouring technology was introduced earlier, and many manufacturers introduced it; Second, many transformer manufacturers compete, and the product cost continues to decline. In addition, the insulation heat-resistant grade is low, and the material cost is low; Third, the technology of non encapsulated air insulated dry-type transformer has not been introduced from many manufacturers in China, and the overall technical level of the transformer has not been improved accordingly

2 the main source of noise of dry-type transformer

2.1 the sound of transformer itself

during the operation of dry-type transformer, the magnetostriction of silicon steel sheet leads to the vibration of iron core. In addition to the problems of the silicon steel sheet itself, the joint part of the silicon steel sheet is prone to magnetic leakage, which may cause core vibration or winding vibration. In this way, noise will be formed. Therefore, once the dry-type transformer is started and operated, it will form noise. After the silicon steel sheet conducts magnetism, its size will change slightly. The silicon steel sheet along or parallel to the direction of the magnetic line of force will expand in size, while the silicon steel sheet perpendicular to the direction of the magnetic line of force will shrink in size. When the magnetic field disappears, its size returns to the initial high and low temperature impact testing machine, which is composed of a box, an air circulation system, a refrigeration system, a heating system and a humidity control system. This change is called magnetostriction. After the occurrence of magnetostriction, the magnet core will be converted into vibration along with the excitation frequency. Because the change period of magnetostriction is only half of the power frequency period, the vibration of the power transformer caused by magnetostriction is twice the power frequency as the fundamental frequency. Therefore, the main reason for the vibration of silicon steel sheet is the magnetostriction of ferromagnetic materials. Because the magnetostriction phenomenon of iron core is nonlinear, and there is a difference according to the length of the magnetic flux path between the inner frame and the outer frame of the iron core, the iron core noise includes not only the fundamental frequency, but also the noise of the high-order harmonic spectrum. Magnetostriction occurs randomly in each silicon steel sheet. Moreover, due to the irregular expansion and contraction of silicon steel sheet, it will also affect the surrounding silicon steel sheet. The noise generated by magnetostriction will increase the transverse amplitude of silicon steel sheet, resulting in high-frequency noise, which has become one of the main factors affecting the noise of dry-type transformer, and also determines the noise of dry-type transformer to a certain extent

both are easy to disassemble as a whole, and it is very easy to recycle iron cores, clamps, etc. The difference lies in the winding. The former winding conductor is exposed, which can be easily removed and its conductor insulation can be stripped off. The conductor insulation accounts for about 2%~3% of the total weight of the conductor, and aromatic polyamide insulation paper is recognized as an environmental protection material. The winding wire of the latter is encapsulated by epoxy resin, so it is difficult to take out the wire. The treatment methods are mainly crushing and incineration. Because the insulating material accounts for a large proportion, accounting for about 25% of the total weight of the conductor, and it also contains silicon, glass fiber and other filling materials in addition to epoxy resin, it will cause harm to the environment in the process of crushing or incineration

2.2 three phase load balancing

the operating environment of the dry-type transformer is also one of the main factors that determine its noise. The electrical voltage of the transformer, the nature of the user's load and the environment of the equipment all affect the noise generated by the dry-type transformer, increasing the noise of the dry-type transformer. Taking the influence of three-phase load imbalance of dry-type transformer on noise as an example, during the operation of dry-type transformer, serious load imbalance may occur, resulting in high value zero sequence current on the low-voltage side. With the continuous change of the imbalance degree of the three loads, the current will also change accordingly, and the two are in a positive proportional relationship, that is, when the imbalance degree increases, the value of zero sequence current will also increase. During the operation of dry-type transformer, if a certain zero sequence current is formed, there will be numerical zero sequence magnetic flux in the equipment core. The sine wave from zero sequence flux to fundamental frequency changes the size and phase of all types of flux, resulting in a significant increase in the single-phase or two-phase magnetic density of the iron core, or even saturation, resulting in a significant increase in transformer noise. In addition, the three-phase load of the dry-type transformer is seriously unbalanced, which is also easy to lead to a significant increase in the local temperature of the steel components, which increases the aging speed of the winding insulation due to the high temperature, and reduces the service life of the dry-type new environmentally friendly plastic bag heated water automatic dissolution transformer

2.3 other reasons

there are many other reasons for the source of noise, such as the noise caused by the vibration of the shell of the box type dry-type transformer, etc., which is not described here

3 ways to reduce noise

3.1 use single orientation high magnetic conductivity silicon steel sheet

it is recommended to use single orientation high magnetic conductivity silicon steel sheet as dry-type transformer core material, for example, jnex series super core produced by JFE steel in Japan. The high magnetic conductivity silicon steel sheet greatly improves the integrity of the crystalline orientation, and the performance of the insulating layer is also well improved, which strengthens the tension resistance of the coating, so as to avoid the large-scale expansion of the silicon steel sheet size due to the influence of magnetostriction, and greatly reduce the noise formed by dry-type transformers. When the magnetic flux density is set to 1.5T, compared with the general silicon steel sheet, the size of high grain oriented silicon steel sheet affected by magnetostriction decreases by 40%. Therefore, when the single oriented high magnetic conductivity silicon steel sheet and the general silicon steel sheet are in the condition of magnetic density, the magnetostriction amplitude of the single oriented high magnetic conductivity silicon steel sheet is small, the vibration is also small, and the noise level naturally decreases, which can generally be reduced by 2dB (a) ~4db (a). However, because the silicon content in the high magnetic conductivity silicon steel sheet is significantly higher than that in the general silicon steel sheet, the hardness value of this type of silicon steel sheet is higher, and the tool loss is more serious in the core shear process. Moreover, compared with ordinary silicon steel sheets, the price of single oriented high magnetic conductivity silicon steel sheets is more expensive, which greatly increases the cost of products and naturally increases the investment of enterprises

3.2 reducing the rated working magnetic density of iron core and changing the clamping force of iron core

one of the main methods to reduce the noise of dry-type transformer is to reduce the rated working magnetic density of iron core. Compared with the no-load noise of the two pole joint, the multi-stage joint iron core used in the iron core has less noise, which reduces the noise by 4dB (a) ~5db (a) compared with the original. The root cause of this phenomenon is that after the use of grade 4 or above joints, the magnetic flux distribution in the joint part is more uniform, and at the same time, the magnetic density in the air gap is greatly reduced, which reduces the noise caused by magnetic attraction in the joint part. Not only that, reducing the width of the joint can reduce the frequency of vibration, but also reduce the excitation capacity and excitation current of the dry-type transformer, and the no-load noise will also be reduced. During the experiment, the enterprise found that if the set value of rated working magnetic density is not less than 1.45t, when the magnetic density is reduced by 0.1t, the core noise will be reduced by 2dB (a) ~3db (a). However, the magnetic density cannot be reduced indefinitely. With the reduction of magnetic density, the production cost of products will also increase, and the volume of iron core will have to increase. Moreover, when the rated working magnetic density of the iron core is too low, the advantage of low loss of high-quality silicon steel sheet cannot be brought into play. Therefore, the reduction of magnetic density shall not be less than 10% of the standard magnetic density. As shown in Figure 2, the increase of noise is in direct proportion to the change of iron core size and magnetic flux density. When the core diameter is 200mm, the noise level is 60lp/db; when the core diameter is 300mm, the noise level is 70lp/db. Therefore, with the increase of iron core diameter, the noise level also increases. When the magnetic flux density is 1.2b/t, the per unit value of the core noise is displayed as 0.7; when the magnetic flux density is 1.4b/t, the per unit value of the core noise is displayed as 0.9. If the transformer noise is wavy, it may be caused by the self resonance of the iron core. Technicians can properly adjust the clamping force of the iron core. In other words, it is to adjust the torque of the bolt on the transformer. It not only includes the tightness of the bolts at both ends of the clamp, but also needs to adjust the tightness of the through bolt and the cushion block pressing bolt, so as to achieve the purpose of reducing noise

3.3 other improvement methods

enterprises can also achieve the purpose of reducing the noise of dry-type transformers through the following ways: first, when installing, adopt elastic materials, such as damping pads, and increase sound insulation and attraction materials. Taking the vibration damping pad as an example, the staff can pad 3mm vibration damping rubber between the iron core pad foot and the bottom of the box during the installation process, so that it can withstand the pressure of 2.7mpa to 3.0MPa, so as to reduce the noise of the transformer. Second, improve the stacking quality of silicon steel sheets and reduce the machining burrs of silicon steel sheets. Workers can apply resin coating to the shear section to eliminate part of the stress formed at the edge, so as to reduce the internal stress and increase the magnetostriction. The coating thickness should be at 50 μ m~100 μ m。 Third, control the natural frequency of the iron core to avoid being close to the vibration frequency of magnetostriction, so as to avoid resonance. During the design process, the designer needs to make the natural frequency avoid the fundamental frequency and the 2nd ~ 4th harmonic frequency band caused by magnetostriction, so as to avoid the resonance of the iron core

4 conclusion

dry type transformer is one of the transformer types widely used at present, which is indeed conducive to the production and work of enterprises. However, the noise generated by it is also one of the problems that must be solved. Otherwise, it will have different degrees of adverse effects on the lives of surrounding residents and the daily production of enterprises. However, enterprises need to pay attention to how to transform the system concept into a mass production ready process by reducing the magnetic density and using high-quality high magnetic conductivity silicon steel sheets? Klaus Maffei gives the answer with technologies such as fiberform process, which can effectively reduce noise, but also greatly increase the manufacturing cost of products. Therefore, the noise control work needs to be comprehensively considered, and the most cost-effective "hand" section should be selected to solve the problem and reduce the noise formed by the equipment