What Is the Cause of Switching Power Transformer Ringing

1 What are the advantages and disadvantages of machine winding and manual winding in the manufacture of transformer?The advantages of machine wound transformer are high efficiency and beautiful appearance, but winding the ring transformer with tall and small holes is more troublesome, and it is not as reliable as manual winding in terms of insulation treatment process. Manual winding can make the magnetic leakage of the transformer very small. During the winding process, it can be adjusted at any time according to the layout of coil turns. Therefore, the real hi-end transformer must be wound purely manually. The only disadvantage of pure manual winding is low efficiency and slow speed.2 Which of the ring type, EI type, R type and C type power transformers is the best?They have their own advantages and disadvantages, and there is no saying who is the best, so strictly speaking, any kind of transformer can be done best. In terms of structure, the ring type can minimize the magnetic flux leakage, but the EI type can do better in the sense of intermediate frequency density. In terms of magnetic saturation alone, the EI type is stronger than the ring type, but the ring type is better than the EI type in efficiency. Nevertheless, the key to the problem is whether you can give full play to their respective advantages, which is the most fundamental to do a good job of transformer.In the current imported amplifiers, the application of ring transformer is still the mainstream, which basically explains a problem. Enthusiasts should be objective and fair about the * price of transformers. You can't refer to something that isn't done well and say it's bad. Some people say that the ring transformer is easy to magnetic saturation, so why don't you find a way to make it not easy to magnetic saturation? This could have been achieved by technical means. Without enough effort or blindly in order to save cost, it is of course prone to magnetic saturation. Similarly, as long as you make it carefully, the efficiency of EI transformer can be very high.The quality of the transformer has a great impact on the sound, because the transmission energy of the transformer is closely related to the iron core and coil, and its transmission rate plays a decisive role in the influence of the sound. Like EI transformer, people usually think its medium frequency is thicker and high frequency is thinner. Why? Because its transmission speed is relatively slow. And the ring? The low frequency is relatively fierce, while the medium and high frequency are slightly weaker. Why? Because its transmission speed is relatively fast, but if you can make the ring type and EI type perfect through effective structural changes, the key depends on what you do.However, at least one thing is certain that R-type transformer is not easy to do well. It can be used as the power supply of low current front-stage power amplifier and CD player. If it is used as the power supply of rear-stage power amplifier, it has serious defects. Because the structural form of R-type transformer itself is not easy to change, while ring type and EI type are relatively easy to achieve the purpose of beautiful sound by changing the structure. The power amplifier power supply made of R-type transformer usually has a hard sound and lacks aura, and the low-frequency power amplifier is often hard without bouncing force.III. is the greater the silicon content of silicon steel sheet of transformer core, the better?It has not been found that the silicon content of silicon steel sheet has little effect on the quality of transformer, while the oriented and non oriented are related to the model of iron core. Secondly, even if the iron core of the same model is not handled well, the quality difference is also very large, and sometimes the difference is as high as 40% or 50%.For a good iron core and the same material, the heat treatment and wire winding process are very key. A good heat treatment only needs a small 10mA excitation current to reach 15000 Gauss, while a bad heat treatment may need 50mA excitation current to reach the corresponding 15000 Gauss. There is a great difference between the two. Judging whether the iron core is good or not from a professional point of view is mainly through several indicators such as excitation current, iron loss and saturation parameters.IV. if the splicing process is adopted for the belt silicon steel sheet of the ring transformer, does it mean that the quality is certainly not good?It can not be generalized, but it is not easy to splice too many broken heads, because one more broken position will have one more magnetic flux leakage point, so it is best not to have more than 2-3 joint points. In terms of manufacturing process, all broken ends and splices shall be pickled first, but for the ring transformer manufacturing high-grade audio equipment, strictly speaking, it is best to use silicon steel without splicing, and its process quality will be more guaranteed.V. what is particular about the silicon steel sheet material in the transformer?Because the loss of silicon steel in alternating magnetic field is very small, silicon steel sheet is mainly used as magnetic material in transformers. Silicon steel sheet can be divided into hot rolling and cold rolling. Cold rolled silicon steel strip has the advantages of small volume, light weight and high efficiency because of its high magnetic conductivity and low loss. The performance of hot rolled silicon steel strip is slightly inferior to that of cold rolled silicon steel strip.Common EI type transformers are made of silicon steel plate into E-type and I-type sheets with a thickness of 0.35-0.5mm, which are inserted into the winding package after heat treatment. Most of these iron cores use hot-rolled silicon steel sheets (the models of high-quality silicon steel sheets with high silicon content are d41, d42, d43 and D301). The iron core of ring and C-type transformers is formed by winding cold-rolled silicon steel strip. The C-type transformer is cut after heat treatment and paint dipping.The leakage inductance of the transformer is generated by the magnetic flux that does not pass through the primary and secondary coils. These magnetic fluxes pass through the air and form a closed magnetic circuit. The leakage inductance can be reduced by enhancing the coupling density between the primary and secondary of the transformer. The leakage inductance of a good transformer shall not exceed 1 / 100 of the primary coil inductance, and that of a high fidelity hi fi output transformer shall not exceed 1 / 500.

Transformer winding machine is a special equipment used to wind transformer coils. In the process of transformer coil production, winding equipment is divided according to transformer voltage level or capacity, including automatic winding machine for winding distribution transformer high and low voltage coils and foil coil winding machine; There are vertical winding machines and horizontal winding machines for winding large power transformer coils. Selecting appropriate transformer winding equipment is the key to the production of high-performance transformers. According to different needs, transformer winding machines can be divided into the following types. Automatic winding machine automatic winding machine is a special equipment for winding high-voltage coils of distribution transformers. The coil wound by this equipment has the advantages of compact structure, small volume and high strength. In order to further improve the production efficiency, the host speed of the new winding machine is increased from 300 rad / min to 500 rad / min.The wire arrangement mechanism is increased from single head to two to three heads, that is, three coils can be wound at one time. At the same time, multiple wires (up to 4) are used and wound; Add radial pressing mechanism; The tension of each pay off reel is controlled independently, so that the winding wires are arranged neatly, the turns are compact and uniform, and the current density is evenly distributed. At the same time, it is equipped with end insulation uncoiling mechanism and wire tensioning mechanism. The computer can preset a variety of parameters, which can complete the coil winding at one time, greatly improve the degree of automation and reduce the labor intensity of workers.Foil coil winding machineFoil coil winding machine is the key equipment for winding low-voltage foil coil. Tension control and foil strip deviation correction are the key technologies of foil coil winding machine. The tension of the foil strip of the existing domestic or imported foil coil winding machine is generated by mechanical friction, and its tension is not stable enough, which will cause defects such as uneven tightness of the wound coils, inconsistent DC resistance between coils, out of tolerance of coil outer diameter and so on. Domestic manufacturers have developed a new type of digital foil winding machine, which uses DC motor to generate foil belt tension. It is characterized by convenient, accurate and stable tension adjustment, and the tension remains unchanged in the whole winding process, so as to ensure the winding quality of the coil. In particular, it has a wide tension adjustment range and is more suitable for winding thinner foil strips without damaging the end edges of the foil strips. Moreover, the tension motor is in the power generation state in the tension state, and the generated electric energy can be fed back to the winding motor or power grid. The friction energy consumption of friction free braking tension mode is significant in energy saving. Moreover, the control system will automatically calculate the coil diameter of the uncoiler, so as to adjust the motor output torque at any time to ensure the constant tension in the whole winding process, which plays a key role in ensuring the coil quality. At the same time, the deviation adjustment system of the equipment adopts photoelectric analog sensor to non-contact detect the edge of the foil belt, and dynamically correct the deviation through the servo motor. The system has high precision, fast response, high reliability and high deviation adjustment accuracy. According to the existing data, the equipment technology has been at the world leading level. Vertical winding machine transformer winding machine transformer winding machine the larger the transformer capacity used now, the greater the impact on the power grid, and the higher the security is required. The manufacturing quality performance of transformer requires not only reliable electrical performance, but also more reliable mechanical stability and short-circuit resistance. The stability is mainly reflected in the manufacturing process control of the coil, that is, the electromagnetic balance of the high and low voltage coil is required, and the axial turns are close and evenly distributed. Therefore, it is required that the coil winding process should be compressed in the axial direction and tightened in the radial direction. The vertical winding machine just uses the self weight of the wire cake to compress naturally in the axial direction, and the tensioning device is used to control the tension in the radial direction. Therefore, the vertical winding machine is widely used for the winding of high-voltage coils in large transformers, and even the low-voltage spiral coils are produced by the vertical winding machine.

Comparison of power transformers:1 What are the advantages and disadvantages of machine winding and manual winding in the manufacture of transformer?The advantages of machine wound transformer are high efficiency and beautiful appearance, but winding the ring transformer with tall and small holes is more troublesome, and it is not as reliable as manual winding in terms of insulation treatment process. Manual winding can make the magnetic leakage of the transformer very small. During the winding process, it can be adjusted at any time according to the layout of coil turns. Therefore, the real hi-end transformer must be wound purely manually. The only disadvantage of pure manual winding is low efficiency and slow speed.2 Which of the ring type, EI type, R type and C type power transformers is the best?They have their own advantages and disadvantages, and there is no saying who is the best, so strictly speaking, any kind of transformer can be done best. In terms of structure, the ring type can minimize the magnetic flux leakage, but the EI type can do better in the sense of intermediate frequency density. In terms of magnetic saturation alone, the EI type is stronger than the ring type, but the ring type is better than the EI type in efficiency. Nevertheless, the key to the problem is whether you can give full play to their respective advantages, which is the most fundamental to do a good job of transformer.In the current imported amplifiers, the application of ring transformer is still the mainstream, which basically explains a problem. Enthusiasts should be objective and fair about the * price of transformers. You can't refer to something that isn't done well and say it's bad. Some people say that the ring transformer is easy to magnetic saturation, so why don't you find a way to make it not easy to magnetic saturation? This could have been achieved by technical means. Without enough effort or blindly in order to save cost, it is of course prone to magnetic saturation. Similarly, as long as you make it carefully, the efficiency of EI transformer can be very high.The quality of the transformer has a great impact on the sound, because the transmission energy of the transformer is closely related to the iron core and coil, and its transmission rate plays a decisive role in the influence of the sound. Like EI transformer, people usually think its medium frequency is thicker and high frequency is thinner. Why? Because its transmission speed is relatively slow. And the ring? The low frequency is relatively fierce, while the medium and high frequency are slightly weaker. Why? Because its transmission speed is relatively fast, but if you can make the ring type and EI type perfect through effective structural changes, the key depends on what you do.However, at least one thing is certain that the R-type power transformer is not easy to do well. It can be used as the power supply of low current front-stage power amplifier and CD player. If it is used as the power supply of rear-stage power amplifier, it has serious defects. Because the structural form of R-type transformer itself is not easy to change, while ring type and EI type are relatively easy to achieve the purpose of beautiful sound by changing the structure. The power amplifier power supply made of R-type transformer usually has a hard sound and lacks aura, and the low-frequency power amplifier is often hard without bouncing force.3 Does the silicon steel sheet of transformer core contain more silicon, the better?It has not been found that the silicon content of silicon steel sheet has little effect on the quality of transformer, while the oriented and non oriented are related to the model of iron core. Secondly, even if the iron core of the same model is not handled well, the quality difference is also very large, and sometimes the difference is as high as 40% or 50%.For a good iron core and the same material, the heat treatment and wire winding process are very key. A good heat treatment only needs a small 10mA exciting current to reach 15000 Gauss, while a bad heat treatment may need 50mA exciting current to reach the corresponding 15000 Gauss. There is a great difference between the two. Judging whether the iron core is good or not from a professional point of view is mainly through several indicators such as excitation current, iron loss and saturation parameters.4 If the splicing process is adopted for the belt silicon steel sheet of the ring transformer, does it mean that the quality is certainly not good?It can not be generalized, but it is not easy to splice too many broken heads, because one more broken position will have one more magnetic flux leakage point, so it is best not to have more than two to three joint points. In terms of manufacturing process, all broken ends and splices shall be pickled first, but for the ring transformer manufacturing high-grade audio equipment, strictly speaking, it is best to use silicon steel without splicing, and its process quality will be more guaranteed.5 What about the silicon steel sheet material in the transformer?Because the loss of silicon steel in alternating magnetic field is very small, silicon steel sheet is mainly used as magnetic material in transformers. Silicon steel sheet can be divided into hot rolling and cold rolling. Cold rolled silicon steel strip has the advantages of small volume, light weight and high efficiency because of its high magnetic conductivity and low loss. The performance of hot rolled silicon steel strip is slightly inferior to that of cold rolled silicon steel strip.Common EI type transformers are made of silicon steel plate into E-type and I-type sheets with a thickness of 0.35-0.5mm, which are inserted into the winding package after heat treatment. Most of these iron cores use hot-rolled silicon steel sheets (the models of high-quality silicon steel sheets with high silicon content are d41, d42, d43 and D301). The iron core of ring and C-type transformers is formed by winding cold-rolled silicon steel strip. The C-type transformer is cut after heat treatment and paint dipping.The leakage inductance of the power transformer is generated by the magnetic flux that does not pass through the primary and secondary coils. These magnetic fluxes pass through the air and form a closed magnetic circuit. The leakage inductance can be reduced by enhancing the coupling density between the primary and secondary of the transformer. The leakage inductance of a good transformer shall not exceed 1 / 100 of the primary coil inductance, and that of a high fidelity hi fi output transformer shall not exceed 1 / 500.fqj

Working principle of power transformerAs for the power transformer, it is a special transformer whose output and input share a set of coils. The step-up and step-down are realized with different taps. The tap voltage of less than the common coil is reduced. The tap voltage of more than the common coil increases. The working principle of power transformer is to use the principle of electromagnetic induction. The alternating current we use generates a magnetic field through coil n, and there is a coil m next to coil n. because the magnetic field generated by coil n is not constant, it changes accordingly according to the change of current. The change of magnetic field causes the corresponding current in coil m, and the frequency of coil m will change according to the different turns ratio of coil N and coil M. Of course, now many power transformers are becoming single coil (called autotransformer), and the principle is basically the same.Autotransformer is a transformer with only one winding. When it is used as a step-down transformer, some wire turns are extracted from the winding as the secondary winding; When used as a step-up transformer, the applied voltage is only applied to part of the wire turns of the winding. Generally, the part of the winding belonging to both primary and secondary is called common winding, and the rest of the autotransformer is called series winding. Compared with ordinary transformers, autotransformer with the same capacity has not only small size, but also high efficiency, and the larger the transformer capacity, the higher the voltage. This advantage is more prominent. Therefore, with the development of power system, the improvement of voltage level and the increase of transmission capacity, self coupled transformer is widely used because of its large capacity, low loss and low cost.Function of power transformerIn addition to the small volume of power transformer, there is no clear boundary between power transformer and electronic transformer. Generally, the power supply of 60Hz power network is very large, which may cover as large a capacity as half a continent. The power limitation of electronic devices is usually limited by the ability of rectification, amplification and other components of the system. Some of them belong to the power amplification, but compared with the power generation capacity of the power system, it still belongs to the scope of small power. Various electronic equipment are commonly used in transformers for the following reasons: providing various voltage levels to ensure the normal operation of the system; Provide electrical isolation of parts operating at different potentials in the system; Provide high impedance for AC current, but low impedance for DC; Maintain or modify the waveform and frequency response at different potentials.Loss of power transformerIn the power supply voltage regulator, when the current passes through the primary coil, a certain amount of heat will be generated. (the magnetic flux generated by the coil flows in the iron core, because the iron core itself is also a conductor, and the potential will be induced on the plane perpendicular to the magnetic line of force. This potential forms a closed loop on the broken surface of the iron core and generates current, like P a vortex, so it is called "eddy current". This "Eddy current" increases the loss of the transformer and heats the iron core of the transformer and increases the temperature rise of the power transformer.) more energy loss is generated in the "eddy current", which is the "iron loss" in the term. In addition, when a large number of copper wires are used in our power transformer, a large amount of heat will be generated when the current passes through, which is what we call "copper loss" The main heat sources of power transformer are "copper loss" and "iron loss" It is also these two phenomena that make the power transformer lose more power. Therefore, the temperature rise of the transformer is mainly caused by iron loss and copper loss. Because the power transformer has iron loss and copper loss, its output power is always less than the input power. Therefore, we introduce an efficiency parameter to describe it, = Output power / input power.

Introduction to working principle of power transformerThe high-voltage winding used in domestic transformers is generally connected into Y connection method, and the connection method of medium voltage winding and low-voltage winding shall be determined according to the system situation. The so-called system situation refers to the relationship between the voltage phasor of high voltage transmission system and the voltage phasor of medium voltage or low voltage transmission system. If it is a low-voltage distribution system, it can be determined according to the standard.High voltage windings are often connected into Y connection because the phase voltage can be equal to 57.7% of the line voltage and the voltage per turn can be lower.1). The voltage phasors of domestic 500, 330, 220 and 110kV transmission systems are in the same phase. Therefore, for three-phase three winding or three-phase autotransformer with the following voltage ratio, star connection method shall be used for high-voltage and medium voltage windings. When the three-phase three iron core column iron core structure is adopted, the low-voltage winding can also adopt star connection method or angle connection method, which depends on that the voltage phasor of the low-voltage transmission system is in phase with the voltage phasor of the medium voltage and high-voltage transmission system or lags by 30 electrical angle.500 / 220 / lvkv YN, yn0, yn0 or YN, yn0, D11220 / 110 / lvkv YN, yn0, yn0 or YN, yn0, D11330 / 220 / lvkv YN, yn0, yn0 or YN, yn0, D11330 / 110 / lvkv YN, yn0, yn0 or YN, yn0, D112). There are two different phase angles for domestic 60 and 35kV Transmission system voltages.For example, YNd11 connection method is adopted for 220 / 60kV transformer and YN, yn0 and D11 connection method is adopted for 220 / 69 / 10kV transformer. The difference between the two 60kV transmission systems is 30 electrical angle.When the 220 / 110 / 35kV transformer adopts YN, yn0 and D11 connection method and the 110 / 35 / 10kV transformer adopts YN, yn0 and D11 connection method, the voltage phasor of the above two 35kV Transmission systems is also different by 30 electrical angle.Therefore, caution should be taken when determining the connection method of 60 and 35kV windings, which must meet the requirements of voltage phasor of transmission system. The connection method of 60 and 35kV winding is determined according to the relative relationship of voltage phasor. Otherwise, even if the capacity and voltage ratio are right, the transformer cannot be used, the connection method is wrong, and the transformer cannot be connected to the transmission system.3). The phasors of domestic 10, 6, 3 and 0.4kV transmission and distribution systems also have two phases. In Shanghai, there is an electrical angle with a voltage phasor difference of 60 between 10kV and 110kV transmission systems. At this time, the three-phase three winding power transformer with 110 / 35 / 10KV voltage ratio and YN, yn0 and Y10 connection method can be used, but the three-phase three core column core is limited.4). However, it should be noted that when the single-phase transformer is connected into three-phase group connection method, the three-phase group of yny0 connection method cannot be used. Yny0 connection method cannot be adopted for three-phase shell transformer.When YN, yn0 and yn0 connection method must be adopted for three-phase five column iron core transformer, there shall be a fourth winding connected into angular connection method in the transformer, and its outlet shall not be led out (the outlet led out during electrical test in structure is not in this example).5). When transformers of different connection groups operate in parallel, the general rule is that the connection group label must be the same.6). When the distribution transformer is used in thunder prone areas, yzn11 connection method can be adopted. When Z connection method is adopted, the impedance voltage algorithm is different from Yyn0 connection method, and the copper consumption of Z connection winding is more. Yzn11 connected distribution transformer has good lightning protection performance.7). Yny0 connection method cannot be adopted when four wound core frames are adopted for three-phase transformer.8). The above connection methods are used for domestic transformers. For export, appropriate connection methods and connection group labels shall be provided as required.9). Generally, there are taps in the high-voltage winding connected with the tap changer. Therefore, when selecting tap changer (including on load voltage regulating tap changer and non excitation voltage regulating tap changer), attention must be paid to the coordination of transformer connection method and tap changer connection method (including connection method, test voltage, rated current, voltage of each stage, voltage regulation range, etc.). For on load tap changer used in on load tap changer of YN connected on load tap changer, it should be noted that the midpoint must be able to be led out

Operation principle of power transformer1. Operating temperature of transformerDuring the operation of the transformer, copper loss and iron loss will be generated, and these two parts of loss will finally be transformed into heat to raise the temperature of the transformer. Most power transformers in China adopt class a insulation. In the process of heat transmission during transformer operation, the temperature of each part varies greatly. The temperature of winding is the highest, followed by the temperature of iron core, and then the temperature of insulating oil. Moreover, the oil temperature of the upper layer is higher than that of the lower layer. The allowable temperature during transformer operation is determined by the upper oil temperature. Class a insulated transformer shall be adopted. In normal operation, when the surrounding temperature is 40 , it is required that the maximum upper oil temperature of the transformer shall not exceed 85 .2. Temperature rise during transformer operationThe difference between the transformer temperature and the surrounding medium temperature is called the temperature rise of the transformer. Because the temperature of each part of the transformer varies greatly, it will affect the insulation of the transformer. Moreover, when the temperature of the transformer increases, the loss of the winding will increase. Therefore, it is necessary to specify the temperature rise of each part of the transformer at rated load. For transformers with class a insulation, when the ambient temperature is 40 , the allowable temperature rise of upper oil is 55 and that of winding is 65 .3. Voltage variation range during transformer operationIn the power system, due to the voltage fluctuation of the power grid, the voltage added to the transformer winding will also change. When the voltage of the power grid is less than the rated voltage of the tap used by the transformer, there is no damage to the transformer; When the voltage of the power grid is higher than the rated voltage of the tap of the transformer, the temperature of the transformer winding will rise, the reactive power consumed by the transformer will increase, and the waveform of the secondary coil will be distorted. Therefore, it is generally appropriate that the power supply voltage of the transformer does not exceed 5% of the rated voltage of the tap.4. Requirements for parallel operation of transformersThe primary windings of two or more transformers are connected in parallel to the common power supply, and the secondary windings are also connected in parallel to supply power to the load. This method is called transformer parallel operation. In the current power system, with the increase of system capacity, the parallel operation of transformers is very necessary.Rated operation mode of power transformer1. Allowable range of transformer voltage variationWhen the voltage is lower than the rated voltage of the transformer, it has no great impact on the transformer and only reduces the output of the transformer;When the voltage is higher than the rated voltage of the transformer, the transformer core loss and reactive power loss increase, and have a certain impact on the transformer insulation.The voltage deviation is within 5%. When the primary voltage of the transformer is within 5% of the rated value, the secondary side can operate with rated output.2. Heating and cooling of transformerAll the electric energy losses in the winding and iron core are converted into heat, which increases the temperature of each part of the transformer, and the temperature rise of each part is not uniform.Heat dissipation process: the heat is transferred from the winding and iron core to the transformer oil, and the heat of the hot oil is transferred to the surrounding air through the radiator.Characteristics of heat dissipation process: all heat generated by iron core, high-voltage winding and low-voltage winding is transmitted to oil; In the whole heating and heat dissipation process, the temperature difference of each part is large, and the winding temperature is the highest; The large capacity transformer has large loss and heat generation. Forced oil circulation air cooling or forced oil circulation water cooling shall be adopted. The larger capacity transformer adopts guided cooling. A certain oil circuit shall be set inside the high and low voltage winding, which can greatly improve the heat dissipation efficiency.3. Allowable temperature and temperature rise of transformerThe allowable temperature of the transformer depends on the insulating material. Oil immersed transformers are paper and oil insulated.When the maximum temperature of transformer winding is 98 , the service life is 20-30 years. During actual operation, the temperature at the highest point of winding temperature is generally less than 98 , and the service life of transformer can exceed 30 years.During transformer operation, the winding temperature and top oil temperature shall be monitored.The transformer overtemperature protection configuration sends an alarm signal when the temperature exceeds the low temperature setting value, and trips when the temperature exceeds the high temperature setting value.4. Transformer overload capacityDuring normal operation of transformer, the load rate of daily load curve is mostly less than 1. According to the principle of equivalent aging, the specified service life can still be obtained as long as the service life of transformer with more loss during overload and less loss during under load can compensate each other. The normal overload capacity of transformer is formulated on the principle of not sacrificing normal service life. That is, in the whole time interval, as long as the insulation aging rate of transformer is less than or equal to 1, and the following conditions are met:(1) During overload, the temperature of the hottest spot of the winding shall not exceed 140 , and the upper oil temperature shall not exceed 95 ;(2) The maximum overload of the transformer shall not exceed 50% of the rated loadWhen the system fails, the primary task is to ensure uninterrupted power supply, and the aging acceleration of transformer insulation is secondary. Accident overload is at the expense of transformer life. The allowable insulation aging rate is much higher than that under normal overload. Accident overload, also known as emergency load, is to make the transformer carry more load in a short time for emergency use. In order to ensure reliability, when determining the allowable value of transformer accident overload, the temperature of the hottest spot of winding in general accident overload shall not exceed 140 , and the load current shall not exceed twice the rated value.

Inspection during operation of power transformer(1) The on-site meter indication of the upper oil temperature and high and low winding temperature of the transformer shall be the same as that of the meter or CRT of the control panel. Check whether each temperature is normal and whether it is close to or exceeds the maximum allowable limit.(2) Whether the oil level on the transformer conservator is normal, each oil level gauge shall be free of dirt or damage, and there shall be no condensation inside.(3) Whether the indication of the transformer oil flow meter is normal, whether the color of the transformer oil becomes sharply darker, and there shall be no oil leakage and oil leakage at all parts of the body.(4) The electromagnetic noise of transformer shall have no abnormal change compared with the past. The body and accessories shall not vibrate, and the temperature of each component shall be normal.(5) Whether the cooling system operates normally; For forced oil circulation air-cooled transformer, whether individual fans stop running; Whether the running fan motor is overheated, abnormal sound and abnormal vibration; Whether the oil pump operates normally.(6) Whether each switch in the transformer cooler control device is in the specified position for operation.(7) Whether the transformer shell grounding, iron core grounding and grounding devices at each point are intact.(8) Whether the insulating parts on the transformer tank cover, such as bushing and porcelain bottle, are damaged, cracked and discharged. Whether the oil level indication of the oil charging sleeve is normal.(9) Whether the connectors of the primary circuit of the transformer are in good contact and whether there is heating.(10) Whether the indication of hydrogen monitoring device is abnormal.(11) Whether the fire water circuit of the transformer is intact and the pressure is normal.(12) Whether the desiccant of the moisture absorber is invalid must be checked regularly, replaced and dried.Maintenance of power transformer in operation(1) The staff shall regularly check the chromatography of transformer insulating oil and check the indicated value of hydrogen monitoring device, so as to find the possible abnormalities in the transformer in time.(2) During normal operation of transformer, the temperature of main transformer, auxiliary high voltage transformer and start / standby transformer shall be processed, output and printed by computer every hour. The temperature of auxiliary transformer shall be recorded during regular inspection.(3) According to the provisions of "equipment regular switching test system", the cooler of main transformer, auxiliary high voltage transformer and start / standby transformer shall be tested and switched for operation once every half a month.(4) According to the provisions of "equipment regular switching test system", the tap lifting remote control test shall be carried out for the on load voltage regulating devices of main transformer, auxiliary high voltage transformer and start / standby transformer once every half a month.(5) Check the main transformer, auxiliary high voltage transformer and start / standby transformer according to the provisions of "equipment regular switching test system".Precautions in operation of power transformer(1) No overload operation: long-term overload operation will cause coil heating, gradually aging insulation and short circuit.(2) Regular inspection of insulating oil quality: the oil quality shall be tested regularly, and the unqualified oil shall be replaced in time or other measures shall be taken.(3) Prevent insulation aging damage of transformer iron core and insulation aging caused by long-term heating of iron core.(4) Prevent the insulation from being damaged due to careless maintenance. If it is found that it is damaged, it shall be handled in time.(5) Ensure good conductor contact and local overheating due to poor contact.(6) To prevent lightning stroke, the transformer will be burned due to insulation breakdown.(7) Short circuit protection: in case of short circuit of transformer coil or load, if the protection system fails or the protection setting value is too large, the transformer may be burned. To do this, install reliable short-circuit protection.(8) Well protected grounding.(9) Ventilation and cooling: if the transformer coil conductor is class a insulation, its insulator is mainly paper and cotton yarn. When the temperature rises by 8 , the insulation life will be reduced by about half; The normal temperature of the transformer is below 90 , and the service life is about 20 years; If the temperature rises to 105 , the service life is 7 years. Keep good ventilation and cooling during transformer operation.

Maintenance technology of on load tap changer of power transformer1 basic principle and structure of on load tap changer on load tap changer is an electrical device that can change tap position under excitation state. The basic principle of voltage regulation of on load tap changer is to change the number of effective turns, that is, to change the voltage ratio of the transformer, so as to achieve the purpose of voltage regulation by switching from one tap to another without interrupting the load current. Therefore, during the operation of on load tap changer, first, the continuity of load current should be ensured; Second, it should have good arc breaking performance in the action of switching and tapping. In the process of changing tap, the on load tap changer must use resistance to realize the transition, so as to limit the circulating current during the transition. Resistance combined on load tap changer is usually used. In practice, the structure of resistance current limiting on load tap changer can be divided into three parts, namely switching switch, selector switch and operation structure. Any of these problems will directly affect the normal operation of the transformer.2 problems in the operation of on load tap changer 2.1 problem 1 2.1.1 frequent voltage regulation of contact heating in the change-over switch will cause serious mechanical wear, electrical corrosion and contact pollution between contacts, especially for transformers with large load current, the thermal effect of current will weaken the elasticity of spring, reduce the contact pressure between dynamic and static contacts and increase the contact resistance, It also increases the heat generation between the contacts. Heating accelerates the oxidation corrosion and mechanical deformation of the contact surface, and forms a vicious cycle, resulting in the damage of the change-over switch. 2.1.2 preventive measures before maintenance and operation, the DC resistance of each tapping position of the switch shall be tested respectively. During maintenance of the hanging cover, the contact resistance of the contact shall be measured to check whether the contact coating and contact are good. Rotate each gear position of the tap changer several times a year in combination with maintenance or test to remove the influence of oxide film or oil stain and make it in good contact. 2.2 problem 2.2.1 the disconnection and looseness of transition resistance will cause the whole transformer to burn out. If the transition resistance is switched with load when it is burnt out, not only the load current will be interrupted, but also all phase voltages will appear on the fracture of the transition resistance and between the breaking openings of dynamic and static contacts.This voltage will not only break through the fracture of the resistance, but also produce a strong arc when the dynamic and static contacts are disconnected, resulting in a short circuit between the transformed two taps and a short circuit burning of the tap section of the high-voltage winding. At the same time, the arc quickly decomposes the oil in the switch oil chamber and produces a large amount of gas. If the safety protection device cannot discharge these gases immediately, the switch will be damaged. The energy of the arc can also burn out the insulating cylinder of the switch, so that the switch cannot be repaired. 2.2.2 Preventive measures strengthen the inspection of transition resistance. (1) before the transformer leaves the factory, before operation and after overhaul, check whether the fastening of transition resistance is loose and whether the resistance wire is mechanically damaged, so as to avoid local overheating and burning during switching. (2) for on load switches with switching times of more than 20000 times or operating for more than 2 years, check whether the material of transition resistance becomes brittle, whether the resistance changes value and whether the fastening is loose. (3) when the transformer is switched at a large current of more than twice the rated current during operation, check whether the transition resistance is burned. (4) when the on load switch does not switch, that is, the main spring of the fast mechanism is tired or broken, the transmission system is damaged, the fasteners are loose, the machinery is stuck, the limit failure, etc., so that the switch cannot be switched, the switch fails halfway, and the time extension of the switching procedure exceeds the specified value, it is necessary to check whether the transition resistance is burned. 2.3 question 3 2.3.1 the oil chamber of the change-over switch is an independent oil tank. During operation, the oil in the oil chamber of the change-over switch is absolutely not allowed to enter the transformer body, because a certain arc is generated during the operation of the change-over switch, resulting in the deterioration of the oil quality in the oil chamber.This oil can only be used in the oil chamber of the change-over switch, but cannot enter the transformer body. Once entered, it will seriously affect the chromatographic analysis of transformer internal oil, that is, the judgment of transformer internal fault. The causes of oil leakage in the oil chamber of the transfer switch are as follows: (1) the oil drain valve at the bottom of the oil tank of the on load tap changer is not fastened, resulting in the mixing of oil in the oil tank of the transformer body and the oil tank of the on load tap changer. (2) the sealing gasket between the two oil tanks has poor material or poor assembly process. During on-site treatment, it was found that most of the rubber pads did not work due to improper selection of rubber pad size and no allowance after compression. In addition, the upper edge of the on load tap changer oil tank is misaligned with the reserved hole of the main oil tank, making it more difficult to seal. In addition, the upper edge of the insulating cylinder of the on load tap changer oil tank was fractured, resulting in oil leakage. (3) the central transmission shaft oil seal is not tight. 2.3.2 preventive measures Pay close attention to the oil level of the tap changer oil conservator during operation.When the oil level of the transformer oil conservator is abnormally increased or decreased, check whether the oil chamber of the switch leaks oil. Take oil samples from the transformer regularly. If the contents of hydrogen, acetylene and total hydrogen in the chromatographic analysis of the main transformer exceed the standard, check whether the oil chamber of the change-over switch leaks oil for timely treatment. 2.4 question 4 2.4.1 the oil quality of on load tap changer is deteriorated. Each time the switch is operated, the oil quality is deteriorated due to arc, and the insulation level of the switch decreases. Transformer oil is the most basic insulating material of tap changer.As insulation and arc extinguishing medium, it also has the functions of cooling, lubrication and corrosion prevention. During tapping and switching, due to the action of electric arc, the insulating oil in the switch oil chamber is decomposed and free carbon, hydrogen, acetylene and other gases and oil dirt are separated. The gas will generally be discharged from the insulating oil, but part of the free carbon particles and oil dirt are mixed in the insulating oil and part of them accumulate on the surface of the insulating parts of the switch. In addition, a small amount of metal particles sputtered after melting the contact material also remain on the surface of the insulator. The increase of these deposits will increase the leakage current and reduce the insulation resistance, which will eventually lead to oil discharge along the insulation surface and damage the switch. 2.4.2 preventive measures after 6-12 months of operation or 2000-4000 times of switching, the oil sample in the switch box shall be taken for test. After 5000 10000 times of switching or when the breakdown voltage of insulating oil is lower than 25kV, the insulating oil of switch box shall be replaced and the surface of insulating parts shall be cleaned.

1. Basic working principle of gas protectionWhen a fault occurs inside the power transformer oil tank (including the grounding short circuit through arc resistance caused by slight turn to turn short circuit and insulation damage), due to the action of fault point current and arc, transformer oil (usually 25# insulating oil) and other insulating materials will decompose due to local heating to produce gas. Because the gas is relatively light, they will flow from the oil tank to the upper part of the conservator. When the fault is serious, the oil will expand rapidly and produce a large amount of gas. At this time, violent gas mixed with oil flow will rush to the upper part of the conservator to make the contact of the relay act, connect the specified control circuit, and send a signal in time or automatically cut off the transformer to form a protection device that acts in response to the above gas, which is called gas protection.Gas relay is the main component of gas protection, such as fj3-80 and QJ series. It is installed on the connecting pipe between the oil tank body and the conservator. In order to increase the sensitivity and reliability of gas protection, all the gas generated by faults in the transformer must pass through the gas relay smoothly, Therefore, when installing the transformer, the oil tank body shall be inclined to the conservator by 1 1.5%, and the oil pipe shall be inclined to the conservator by 2 4%.The operation of the old gas relay (float mercury contact relay) is not stable enough, and its anti vibration ability is poor, which is easy to cause misoperation due to mercury flow due to vibration. The open cup baffle gas relay has high vibration resistance and reliable action. Therefore, the open cup baffle gas relay is widely used in China's power system.During normal operation, the relay is filled with transformer oil. In case of slight internal fault of the transformer, a small amount of gas generated by decomposition of the transformer rises and accumulates in the air chamber at the upper part of the gas relay, forcing the oil level to rotate and fall, so that the upper magnetic steel is close to a pair of dry spring contacts and sends a light gas fault signal after action. If it is a serious fault, a large amount of gas will be generated. At the same time, the oil temperature will rise, the hot oil will expand, and the pressure in the tank will increase sharply, resulting in the rapid impulse of oil and air flow to the lower baffle of the relay, resulting in the lower magnetic steel close to another pair of dry spring contacts and acting on tripping.2. Setting of gas relayThe setting of gas relay can be carried out through differential pressure flow rate test or oil pump oil speed test equipment. The starting value is set according to the oil flow speed. Generally, 0.5 1.5m/s is used for heavy gas. When the transformer has forced oil circulation device, the range of 1 1.5m/s is used; when there is no forced oil circulation device, the range of 0.6 1m / S is used; For light gas, the volume of the space occupied by the gas in the relay is set, and the signal is sent when the range is 250 300CM3.

Switching power transformer is a power transformer with switch tube. In the circuit, in addition to the voltage conversion function of ordinary transformer, it also has the functions of insulation isolation and power transmission. It is generally used in switching power supply and other occasions involving high-frequency circuits.Function and classification of switching power transformerThe switching power supply transformer and the switch tube form a self-excited (or other excited) intermittent oscillator to modulate the input DC voltage into a high-frequency pulse voltage. It plays the role of energy transfer and conversion. In the flyback circuit, when the switch is on, the transformer converts the electric energy into magnetic energy for storage, and releases it when the switch is off. In the forward circuit, when the switch is on, the input voltage is directly supplied to the load and the energy is stored in the energy storage inductor. When the switch is cut off, the energy storage inductor is used for continuous flow to the load.Switching power transformer is divided into single excitation switching power transformer and double excitation switching power transformer. The working principle and structure of the two switching power transformers are not the same. The input voltage of the single excitation switching power supply transformer is a unipolar pulse, and it is also divided into positive flyback voltage output; The input voltage of double excitation switching power supply transformer is bipolar pulse, which is generally bipolar pulse voltage output.Principle of switching power transformerFor switching power supply, the working principle of switching transformer is different from that of ordinary transformer. The positive and negative half cycle waveforms of the input AC voltage or current of the ordinary transformer are symmetrical, and the input voltage and current waveforms are generally continuous. Within a cycle, the average value of the input voltage and current is equal to 0, which is the basic feature of the working principle of the ordinary transformer; Switching transformers generally work in the switching state, and their input voltage or current is generally not continuous, but intermittent. Most of the average value of input voltage or current in a cycle is not equal to 0. Therefore, switching transformers are also called pulse transformers, which is the biggest difference between switching transformers and ordinary transformers in working principle.PWM (pulse width modulation) controls the switch tube to turn on the rectified DC voltage, so that the high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply, and the induced current is generated at the secondary side of the transformer. After rectification, the required voltage or multi-channel voltage can be obtained.Test details of switching power transformer1. Check whether there are obvious abnormalities by observing the appearance of the transformer: such as whether the coil lead is broken, desoldered, whether the insulating material has scorched traces, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.2. Insulation test: use multimeter R & times; 10K gear shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and secondary and secondary windings respectively. The pointer of multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.3. Coil on-off detection: place the multimeter in R & times; Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.4. Discrimination of primary and secondary coils: the primary pin and secondary pin of power transformer are generally led out from both sides, and the primary winding is mostly marked with 220V, while the secondary winding is marked with rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.5. Detection of no-load current: A. open all secondary windings by direct measurement method, and place the multimeter in AC current gear (500mA, connected in series into the primary winding. When the plug of the primary winding is plugged into 220V AC mains power, the multimeter indicates the no-load current value. This value shall not be greater than 10% 20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment shall be about 100mA. If it exceeds too much, it indicates that the transformer has short-circuit fault. B. the indirect measurement method is used in voltage transformation A 10 / 5W resistor is connected in series in the primary winding of the power transformer, and the secondary is still no-load. Turn the multimeter to the AC voltage gear. After power on, use two probes to measure the voltage drop u at both ends of the resistor R, and then use Ohm's law to calculate the no-load current I empty, i.e. I empty = u / R. connect the primary stage of the power transformer to 220V mains power, and use the AC voltage of the multimeter to measure the no-load voltage of each winding in turn (U21, u22, U23 and u24) shall meet the required values, and the allowable error range is generally: high voltage winding 10%, low voltage winding 5%, and the voltage difference between two groups of symmetrical windings with center tap shall be 2%.6. Test the temperature range of power transformer: generally, the allowable temperature rise of low-power power transformer is 40 50 . If the insulating material used is of good quality, the allowable temperature rise can be improved.7. Detect and judge the same name end of each winding: when using the power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the power transformer by series method, the same name end of each winding in series must be connected correctly without mistake. Otherwise, the transformer cannot work normally.8. Comprehensive detection and discrimination of short-circuit fault of power transformer: the main symptoms of short-circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Generally, the more inter turn short-circuit points in the coil, the greater the short-circuit current, and the more serious the heating of transformer. The simple method to detect and judge whether there is short-circuit fault of power transformer is to measure no-load power Current. For the transformer with short-circuit fault, its no-load current value will be much greater than 10% of the full load current. When the short-circuit is serious, the transformer will heat rapidly within tens of seconds after no-load power on, and it will feel hot to touch the iron core by hand. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.What is the difference between switching power supply and transformer in use?Switching power supply: the switching power supply can stably turn the voltage within a certain range into a very accurate low voltage or high voltage (for example, 110v-250 input, the output voltage can be stably controlled at the required voltage positive and negative difference of 0.5V).Transformer: the output voltage of the transformer changes with the input voltage, that is, the input voltage increases, the output voltage increases, the input voltage decreases, and the output voltage decreases.To sum up, it can be concluded that the switching power supply first changes the AC into DC, and then changes the DC into AC with higher frequency through the power switch tube. The voltage conversion through the high-frequency power transformer not only improves the efficiency, but also greatly reduces the volume after the frequency is high, and saves the copper and iron loss. Because it is controlled by the power switch tube, the switch tube is conductive at low current The power on time is short and the output voltage can be maintained. When the load is large, the switch tube works continuously to maintain the output voltage. Therefore, the output voltage of the switching power supply is stable and can be selected as high-precision instruments such as LED display screen. fqj