Composition of switching power transformerThe main materials of switching power transformer: magnetic materials, conductor materials and insulating materials are the core of switching power transformer.Magnetic materials: the magnetic materials used in switching transformers are soft ferrite, which can be divided into MnZn system and NiZn system according to their composition and application frequency. The former has high permeability and high saturation magnetic induction, and has low loss in the medium and low frequency range. There are many shapes of magnetic cores, such as EI type, E type, EC type, etc.

Conductor material - Enamelled Wire: generally used for winding small electronic transformers, there are two kinds of enamelled wires: high strength polyester enamelled wire (QZ) and polyurethane enamelled wire (QA). According to the thickness of paint layer, it is divided into type 1 (thin paint type) and type 2 (thick paint type). The insulation coating of the former is polyester paint, which has superior heat resistance, insulation and electrical strength up to 60kV / mm; The insulation layer of the latter is polyurethane paint, which has strong self-adhesive and self welding performance (380 ), and can be welded directly without removing the paint film.

Pressure sensitive tape: the insulating tape has high electrical strength, convenient use and good mechanical performance. It is widely used in interlayer, group insulation and outsourcing insulation of switching transformer coils. The following requirements must be met: good viscosity, anti stripping, certain tensile strength, good insulation, good pressure resistance, flame retardant and high temperature resistance.

Skeleton material: the switch transformer skeleton is different from the general transformer skeleton. In addition to being the insulation and support material of the coil, it also undertakes the installation, fixation and positioning of the whole transformer. Therefore, the material for making the skeleton should not only meet the insulation requirements, but also have considerable tensile strength. At the same time, in order to bear the welding heat of the pins, It is required that the thermal deformation temperature of the skeleton material is higher than 200 , and the material must be flame retardant, with good processability and easy to be processed into various shapes

How to judge the quality of switching power transformer1. How to judge whether the switching power supply transformer is good or bad? Check whether there are obvious abnormalities by observing the appearance of the transformer. For example, whether the coil lead is broken, desoldered, whether the insulating material is burnt, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.2. How to judge the quality of switching power transformer and insulation test. Multimeter R 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. How to judge the quality of switching power transformer and the detection of coil on-off. Place the multimeter in R Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.Several detection methods of switching power transformerSeveral detection methods of switching power transformer are as follows:

1. Check whether there are obvious abnormalities by observing the appearance of the transformer. For example, whether the coil lead is broken, desoldered, whether the insulating material is burnt, 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 the rx10k gear of the universal meter to measure the resistance values between the iron core and the primary, between the primary and each secondary, between the iron core and each secondary, between the electrostatic shielding layer and the secondary and each winding of the secondary. The pointer of the multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.3. Coil on-off detection. Put the multimeter in rx1. If the resistance value of a winding is infinite during the test, it indicates that the winding has open circuit fault.

4. Distinguish primary and secondary coils. The primary pin and secondary pin of the power transformer are generally led out from both sides, and the primary winding is mostly marked with the word 220V, while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. Then identify according to these marks.

1) Insulation performance: increase the insulation strength of various components in the transformer. The oil insulation strength of the transformer is good and has fluidity. It can fill all parts and any space of the transformer and discharge the air, so as to avoid moisture contact between the parts and the air and affect the insulation strength, so as to maintain good insulation between windings, between windings and iron core, and between windings and oil tank shell.2) Heat dissipation performance: the transformer oil cools the iron core and winding of the transformer. During the operation of the transformer, the temperature of the oil near the winding and iron core increases after heating, the volume expands, the specific gravity decreases and rises, and then enters the bottom of the transformer oil tank after cooling by the cooling device, so as to form oil circulation. During the cycle, heat is radiated to the cooling device, so that the iron core and winding are cooled.3) Anti corrosion performance: transformer oil can keep the original chemical and physical properties of insulating materials (such as wood) and make metals (such as steel) play an anti-corrosion role

General principles for power transformer when supplementary oil is requireda) Transformer oils with different oil bases shall not be used together;b) Oils of different grades (freezing point) of the same oil base shall not be used together;

c) The quality of mixed oil shall comply with the provisions of gb7595;d) When new oil or transformer oil of the same oil base and different grades equivalent to the quality of new oil is mixed, its availability shall be determined according to the measured freezing point of the mixed oil. It cannot be mixed only according to the qualified chemical and electrical properties:e) When the in-process oil is mixed with new oil of the same brand, the oxidation stability test of the mixed oil sample shall be carried out in advance, and the mixed oil can be used only when there is no sediment.

f) When imported oil or oil of unknown source is mixed with operating oil of different brands, the aging test of various oils and mixed oil samples shall be carried out in advance. The mixed oil can be used only when the quality of the mixed oil is not lower than that of the original operating oil; If the mixed oil is new oil, the quality of the mixed oil shall not be lower than the worst new oil.What are the protective functions of power transformerRole of gas protection

The main protection measure in the transformer is gas protection. The reduction of transformer oil level and the faults in transformer oil tank are reflected by gas. When the transformer has a slight fault, the oil level will drop, and the light gas will send a signal. When the gas has a serious fault, a large amount of gas will be generated, and the heavy gas will also trip.When there is a fault in the transformer, there will be heating in the fault part. In this way, the oil expansion will occur in the nearby transformer, the air will be discharged and the bubbles will gradually rise, while other materials and oil will produce gas under the action of discharge, so as to reduce the oil level.When the fault is very serious, after the gas gas is generated, the pressure inside the transformer is increased, so that the oil flows to the conservator. The baffle will overcome the resistance of the spring when the oil flow impacts, so as to move the magnet towards the dry spring and connect the contact of the dry spring. In this way, tripping will occur.

Function of differential protectionDifferential protection is the main protection of the transformer, which mainly reflects the faults of the outgoing line and winding of the transformer. It can trip the circuit breakers on each side of the transformer. According to different devices, differential protection can be divided into the following types: transverse differential protection is often used in shunt capacitor and short-circuit protection. When the equipment adopts double bus and double winding, transverse differential protection will be adopted; Longitudinal differential protection mainly reflects short circuit and inter turn short circuit, and the protection scope mainly includes outgoing line and bushing.Backup protection

The main transformer has the characteristics of large impedance during operation. Therefore, the fault of the main transformer on the low-voltage side will not affect the operation of the high-voltage side. The stability of high voltage side can effectively realize the protection function of voltage locking. However, when the main transformer fault is abnormal during operation, it can not respond in time. Therefore, during the operation of the main transformer, backup protection measures shall be taken. The parallel opening mode of high-voltage side and low-voltage side can be adopted to make the opening of locking circuit flexible.Function of voltage and current protection of transformerWhen a fault occurs outside the transformer, overcurrent will be generated; When there is an internal fault in the transformer, the backup of differential protection and gas protection will be generated. In the transformer, current protection devices should be installed. Different protection methods are selected according to different transformer capacity and system short-circuit current.

The requirements for current transformer for relay protection are: reliable insulation; A sufficiently large accuracy limit factor; Sufficient thermal and dynamic stability. The transformer for protection can meet the requirements of accuracy level under rated load. The maximum primary current is called rated accuracy limit primary current. The accuracy limit factor is the ratio of the rated accuracy limit primary current to the rated primary current. When the primary current is large enough, the iron core will be saturated and can not reflect the primary current. The accurate limit coefficient represents this characteristic. The accuracy grade of transformer for protection is 5p and 10p, indicating the allowable error of 5% and 10% at the rated accuracy limit primary current.

1 Common faults and causes of power transformer1) Short circuit damage accidentThe transformer suffers various short circuit accidents during operation, such as single phase to ground, two phases or two phases to ground and three phases, among which the short circuit at the outlet is the most serious. Daily patrol inspection. During the operation of the transformer, daily patrol inspection shall be carried out. It is mainly to check whether the operation of transformer auxiliary equipment is good and whether the transformer temperature is normal. By measuring with infrared imager, some faults such as overheating and lack of oil in the transformer can be found effectively.

2) Insulation accidentThe insulation damage accident of transformer accounts for about 70% 80% of the total accidents. There are insulation damage caused by mechanical force or overheating, or the insulation strength fails to meet the requirements when leaving the factory, or the insulation is damaged, which reduces the strength and cannot meet the requirements of bearing capacity. The analysis reasons are as follows: Transformer water ingress and moisture

The main problems are loose sealing at the end of the casing, water inflow, water leakage of the water cooler, dew accumulation in the explosion-proof cylinder, ponding in the oil conservator, etc. The existence of moisture will reduce the breakdown strength of transformer insulating oil, resulting in insulation accidents. The most common parts of insulation accidents are windings, leads and screens. The development of creepage leads to inter turn and inter layer short circuits. Foreign matters remaining in the transformerThe residual metal conductor foreign matters and impurities in the transformer body cause insulation breakdown and damage in the event of overvoltage or under normal working voltage due to partial discharge or insulation wear.

Lightning strikeThe low lightning resistance level of lightning protection at the medium and low voltage sides of the transformer and the weak insulation structure of the transformer cause the grounding short circuit accident of the transformer during lightning stroke.3) Overheating fault

There are overheating caused by abnormal current, such as overheating caused by circulating current and eddy current, overheating caused by increased resistance of conductive circuit, overheating caused by blocked heat dissipation, etc.2 Daily patrol inspection, regular test and maintenance of transformer1) On line detection technology

Partial discharge on-line monitoring technology. In case of internal fault or poor operating conditions of transformer, partial discharge will be generated due to high local field strength. In general, the detection of partial discharge can be divided into electrical measurement method and non electrical measurement method. On line analysis technology of gas in oil. For a long time, gas chromatography analysis of gas composition and content in oil immersed transformer oil has been an important means to judge the internal state of transformer. The principle is to infer the cracking conditions of oil or oil paper insulation by using the relative ratio of the collected gas concentration.2) Daily patrol inspection. During the operation of the transformer, daily patrol inspection shall be carried out. It is mainly to check whether the operation of transformer auxiliary equipment is good and whether the transformer temperature is normal. By measuring with infrared imager, some faults such as overheating and lack of oil in the transformer can be found effectively.

3) Periodic test. It is mainly carried out in accordance with the code for handover and preventive test of power equipment. There are 35 items in total, and there are more than 10 routine test items. Adhering to the implementation of some projects can effectively find some faults of transformers and take effective measures in time to avoid the expansion of accidents. It is an effective means in production.

Maintenance technology of on load tap changer of power transformer

1 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.