1 Introduction
Step-up transformer and on-load voltage regulation refer to the voltage regulation mode of transformer tap-changer, but the difference is that the non-excitation voltage regulator does not have the ability to shift gears with load, because this tap-changer has a short-term disconnection process during shifting, and disconnecting the load current will cause arc between contacts to burn the tap-changer or short circuit, so the transformer must be powered off when shifting gears. Therefore, it is generally used for transformers that are not very strict in voltage requirements and do not need frequent gear shifting. The on-load tap changer can switch gears with load, because there is no short-time disconnection process in the shifting process of the on-load tap changer, and it is switched from one gear to another through a transition resistance transition, so there is no arcing process in which the load current is disconnected. It is generally used in transformers with strict voltage requirements and frequent gear shifting.
2 definition
Definition: Transformer is a common electrical equipment, which can be used to convert a certain value of AC voltage into another value of AC voltage with the same frequency. Step-up transformer is used to convert a low-value AC voltage into another high-value AC voltage with the same frequency. Widely used in high frequency fields, such as inverter power supply.
3 classification high frequency type
The high-frequency step-up transformer adopts high-frequency voltage doubling rectifier circuit, the latest PWM pulse width modulation technology and power IGBT devices, and adopts special technology according to electromagnetic compatibility theory, so that the DC generator can achieve high quality and portability. It consists of a control box and a voltage doubler, with built-in protection resistors, and has the functions of voltage zero-gate protection, overcurrent protection and overvoltage protection. Small size, light weight, easy to carry, convenient, safe and reliable, suitable for DC high voltage test in power sector, DC characteristic test of lightning arrester and other occasions requiring DC high voltage.
Direct current type
DC step-up transformer has the characteristics of small volume, light weight, compact structure, complete functions, strong versatility and convenient use. It is especially suitable for power system, industrial and mining enterprises, scientific research departments and so on to conduct insulation strength tests on various high-voltage electrical equipment, electrical components and insulation materials under power frequency or DC high voltage. It is an indispensable and important equipment in high voltage test.
Energy saving and low noise: stacking high-quality cold-rolled silicon steel sheets; Fully inclined joint; Adopt special treatment technology to effectively reduce the vibration and noise during operation; As well as the introduction of new materials, new processes and new technologies such as computer optimization design, the transformer is more energy-saving and quieter.
High reliability: improving product quality and reliability will be our unremitting pursuit. Carry out a lot of basic research on quality assurance system and reliability engineering, and actively carry out reliability certification to further improve the reliability and service life of transformers.
Environmental protection features: heat resistance, moisture resistance, stability, chemical compatibility, low temperature resistance, radiation resistance and non-toxicity.
Ac type
AC step-up transformer is a device for converting AC voltage, current and impedance. When alternating current flows in the primary coil, alternating magnetic flux is generated in the iron core (or magnetic core), and voltage (or current) is induced in the secondary coil. A transformer consists of an iron core (or magnetic core) and a coil. The coil has two or more windings, of which the winding connected to the power supply is called the primary coil and the rest is called the secondary coil. AC step-up transformer has the characteristics of small volume, light weight, compact structure, complete functions, strong versatility and convenient use. It is especially suitable for power systems, industrial and mining enterprises, scientific research departments and so on to conduct insulation strength tests on various high-voltage electrical equipment, electrical components and insulation materials under AC or high voltage. It is an indispensable and important equipment in high voltage test.
Compared with oil-immersed transformer, dry-type step-up transformer has no oil, fire, explosion, pollution and other problems, so electrical codes and regulations do not require dry-type transformer to be placed alone in the room. Especially in the new series, the loss and noise have dropped to a new level, which creates conditions for the transformer and low-voltage screen to be placed in the same distribution room. The safe operation and service life of dry-type step-up transformer largely depend on the safety and reliability of transformer winding insulation. The winding temperature exceeds the insulation tolerance temperature, which leads to insulation damage, which is one of the main reasons why the transformer can not work normally. Therefore, it is very important to monitor the operating temperature of transformer and its alarm control. According to the characteristics of operating environment and protection requirements, dry-type step-up transformer can choose different bushings. IP23 protective shell is usually used to prevent solid foreign bodies with a diameter larger than 12mm and small animals such as rats, snakes, cats and birds from entering, causing malignant faults such as short circuit and power failure, and providing a safety barrier for live parts. If the transformer needs to be installed outdoors, you can choose IP23 protective shell. In addition to the above-mentioned IP20 protection function, it can also prevent water droplets within 60 degrees from falling vertically. However, the IP23 shell will reduce the cooling capacity of the transformer, so attention should be paid to the reduction of its operating capacity when selecting.
Low frequency type
The magnetic flux of low frequency transformer core is related to the applied voltage. In terms of current, the excitation current will not increase with the increase of load. Although the load increases, the iron core will not be saturated, which will increase the resistance loss of the coil. If the rated capacity is exceeded, the coil will be damaged because the heat generated by the coil cannot be dissipated in time. If the coil you use is made of superconducting material, it will not heat up when the current increases, but there is impedance generated by magnetic leakage inside the transformer. However, with the increase of current, the output voltage will drop. The greater the current, the lower the output voltage, so the output power of the transformer cannot be infinite. If you say that the transformer has no impedance, then when the current flows through the transformer, it will produce great electromotive force, which is easy to damage the transformer coil. Although you have an infinite power transformer, you can't use it. It can only be said that with the development of superconducting materials and iron core materials, the output power of transformers with the same volume or weight will increase, but it is not infinite! [ 1]
4 Physical attributes Physical attributes
Transformer-Static electromagnetic device, which can convert alternating current of one voltage into alternating current of another voltage with the same frequency. The main components of the transformer are an iron core and two windings sheathed on the iron core. The principle of transformer is that the coil connected with power supply receives alternating current, which is called primary winding, and the coil connected with load sends alternating current, which is called secondary winding, primary winding, secondary winding, voltage phasor U 1, voltage phasor U2, current phasor I 1, current phasor I2, electromotive phasor E 1, electromotive phasor E2 and number of turns N65438.
Physics formula
U1/U2 = n 1/N2 = I2/I1(u1and U2 are voltages, n1and N2 are coil turns, and I 1I2 is current).
5 Fault Analysis There are many and complicated reasons for internal faults and accidents of transformer caused by short circuit at the outlet of transformer, which are related to structural design, raw material quality, process level, operating conditions and other factors, but the selection of electromagnetic wire is the key. In recent years, the stress of electromagnetic wire designed and selected based on the static theory of transformer is quite different from that in actual operation.
1, the winding is loose, improperly transposed and too thin, which leads to the suspension of the electromagnetic wire. Judging from the location of accident damage, deformation often occurs at the transposition, especially at the transposition of transposition wires.
2. At present, the calculation programs of various manufacturers are compiled on the basis of idealized models such as uniform distribution of leakage magnetic field, same turn diameter and in-phase force, but in fact, the leakage magnetic field of transformer is not uniformly distributed, and it is relatively concentrated in the yoke part, and the electromagnetic wires in this area are also subjected to greater mechanical force; The transposed conductor will change the direction of force transmission at the transposed position because of climbing, resulting in torque; Due to the elastic modulus of the pad, the axial pad is not equidistantly distributed, which will delay the alternating force vibration caused by the alternating leakage magnetic field, which is also the fundamental reason why the wire cakes at the yoke, the transposition and the corresponding parts of the core with voltage regulating taps are deformed first.
3. Improper control of winding pre-tightening force leads to dislocation of common transposed wires.
4. The influence of temperature on the bending and tensile strength of electromagnetic wire is not considered in the calculation of short-circuit resistance. According to the test results, what is the temperature yield limit of the electromagnetic wire? 0.2 has a great influence. With the increase of temperature of electromagnetic wire, its bending strength, tensile strength and elongation all decrease. The bending strength at 250℃ is lower than that at 50℃, and the elongation decreases by more than 40%. However, under rated load, the average winding temperature can reach 105℃ and the hottest temperature can reach 1 18℃. There is generally a reclosing process in transformer operation, so if the short-circuit point cannot disappear for a while, it will bear the second short-circuit impact in a very short time (0.8s). However, due to the sudden increase of winding temperature after the first short-circuit current shock, according to the provisions of GBl094, the maximum allowable temperature is 250℃. At this time, the short-circuit resistance of the winding has been greatly reduced, which is why most short-circuit accidents occur after the reclosing of Satons transformer.
5. The use of flexible wires is also one of the main reasons for the poor short-circuit resistance of transformers. Due to the lack of understanding in the early stage, or the difficulty of winding equipment and technology, manufacturers are reluctant to use semi-hard conductors or there is no such requirement at all in design. Judging from the faulty transformers, they are all soft conductors.
6. External short-circuit accidents occur frequently, and the accumulation effect of electromotive force after multiple short-circuit current shocks leads to softening of electromagnetic wires or internal relative displacement, which eventually leads to insulation breakdown.
7. The winding turns or wires are not cured, and the short-circuit resistance is poor. None of the windings dipped in paint in the early days were damaged. [2]
6 Daily maintenance
1. In order to protect the insulation performance of the insulating oil contained in the high-voltage generator and the nose, generally do not open the observation window at will, and do not loosen the fixing screws around it to prevent the oil from absorbing moisture or falling ash and reducing the insulation performance.
2. Check whether the lighting, heat dissipation and dust removal equipment around the transformer are in good condition, and wipe off the dust on the transformer body and porcelain insulator with a clean cloth.
3, check the transformer high voltage side load switch, to ensure flexible operation, good contact, transmission parts for lubrication.
4. Pull open the high-voltage grounding knife, check that the grounding is in the off position, close the high-voltage load switch, let the transformer run, and remove the high-voltage side sign. Pay attention to disconnect or close the transformer high voltage load switch, there must be more than two people at the scene.
5. When new oil needs to be replaced, the local electric power department should be assisted to check the performance of the new oil, and its insulation strength should be not less than 25,000 V/2.5 mm; The oil insulation strength in the combination cover should be higher than 30000 volts /2.5 mm.
6. Use a 2500V shaking table to measure the insulation resistance of the high and low voltage coils (to ground and phase) of the transformer, and confirm that it meets the requirements (at room temperature of 30℃, the high voltage side of 10kv transformer is greater than 20MΩ, and the low voltage side is greater than13mΩ. The grounding wire should be connected before the test and discharged after the measurement.
7. The high-voltage generator or combined nose must have a good grounding wire. Be sure to use ohmmeter to measure whether its shell, console shell and external grounding wire are connected, and tighten the grounding bolt.