Step-up transformer design Step-up transformer classification
1 Introduction
Both step-up transformer and on-load voltage regulation refer to the transformer tap changer voltage regulation method. The difference is that there is no excitation voltage regulation. The switch does not have the ability to change gears with load, because this tap changer has a short-term disconnection process during the gear switching process. Disconnecting the load current will cause arcing between the contacts, burning the tap changer or short circuit. Therefore, the transformer must be powered off when adjusting gears. Therefore, it is generally used in transformers that do not have very strict voltage requirements and do not require frequent gear adjustments. The on-load tap-changer can switch gears with load, because there is no short-term disconnection process during the gear-shifting process of the on-load tap-changer. It transitions from one gear to another through a transition resistor. Therefore, there is no arcing process in which the load current is disconnected. Generally used for transformers that have strict voltage requirements and require frequent adjustment.
2 Definition
Definition: Transformer is a common Electrical equipment that can be used to convert a certain value of alternating voltage into another value of the same frequency. A step-up transformer is a transformer used to convert a low-value alternating voltage into another higher-value alternating voltage of the same frequency. It is widely used in high-frequency fields, such as inverter power supplies.
3-category high-frequency type
The high-frequency step-up transformer adopts a high-frequency voltage doubler rectifier circuit and applies the latest PWM pulse width modulation technology and power IGBT devices. And based on the electromagnetic compatibility theory, special technology is used to make the DC generator high-quality and portable. It consists of two parts: a control box and a voltage doubling device. It has a built-in protection resistor and has the functions of voltage zero gate protection, overcurrent and overvoltage protection. It is small in size, light in weight, easy to carry, convenient, safe and reliable. It is suitable for on-site DC high voltage testing in the power sector, DC characteristic testing of arresters and other occasions requiring DC high voltage.
DC type
DC step-up transformer has the characteristics of small size, light weight, compact structure, complete functions, strong versatility and easy use. It is especially suitable for power systems, industrial and mining enterprises, scientific research departments, etc. to conduct insulation strength tests under combined frequency or DC high voltage on various high-voltage electrical equipment, electrical components, and insulating materials. It is an indispensable and important equipment in high-voltage testing.
Energy saving and low noise: using high-quality cold-rolled silicon steel sheets for stacking; all oblique joints; using special processing technology to effectively reduce vibration and noise during operation; as well as new materials and new processes such as computer optimization design , The introduction of new technologies makes transformers more energy-saving,More peaceful.
High reliability: Improving product quality and reliability will be our unremitting pursuit. Conduct a large amount of basic research on quality assurance systems and reliability engineering, and actively conduct reliability certification to further improve the reliability and service life of transformers.
Environmental protection properties: heat resistance, moisture resistance, stability, chemical compatibility, low temperature resistance, radiation resistance and non-toxicity.
AC type
AC step-up transformer is a device that converts AC voltage, current and impedance. When AC current flows through the primary coil, the iron core AC magnetic flux is generated in the secondary coil (or magnetic core), causing voltage (or current) to be 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. The winding connected to the power supply is called the primary coil, and the remaining windings are called secondary coils. AC step-up transformer has the characteristics of small size, light weight, compact structure, complete functions, strong versatility and easy use. It is especially suitable for power systems, industrial and mining enterprises, scientific research departments, etc. to conduct insulation strength tests under combined frequency or AC high voltage on various high-voltage electrical equipment, electrical components, and insulating materials. It is an indispensable and important equipment in high-voltage testing.
Dry-type compared to oil-type transformers, dry-type step-up transformers do not have oil, so there are no problems such as fire, explosion, pollution, etc. Therefore, electrical standards and regulations do not require dry-type transformers to be placed in separate inside the room. Especially for the new series, the loss and noise have been reduced to a new level, creating conditions for the transformer and the low-voltage panel to be placed in the same power distribution room. The safe operation and service life of dry-type step-up transformers largely depend on the safety and reliability of the transformer winding insulation. When the winding temperature exceeds the insulation withstand temperature and causes insulation damage, it is one of the main reasons why the transformer cannot work normally. Therefore, it is very important to monitor the operating temperature of the transformer and its alarm control. According to the characteristics of the use environment and protection requirements, dry-type The step-up transformer is available in different enclosures. IP23 protective shell is usually used to prevent solid foreign objects with a diameter greater than 12mm and small animals such as rats, snakes, cats, finches, etc. from entering, causing short circuits and power outages and other vicious failures, and provides a safety barrier for live parts. If the transformer needs to be installed outdoors, an IP23 protective enclosure can be used. In addition to the above-mentioned IP20 protection function, it can also prevent water dripping within an angle of 60° from the vertical line. However, the IP23 casing will reduce the cooling capacity of the transformer, so attention should be paid to the reduction in its operating capacity when selecting it.
Low frequency type
The core flux of low frequency transformer is related to the applied voltage. In the current, the excitation current does not increase as the load increases. Although the core will not be saturated as the load increases, the resistance loss of the coil will increase. If the rated capacity is exceeded, the heat generated by the coil cannot be dissipated in time, and the coil will be damaged. If the coil you use is made of superconducting material, the current will increase. It will not cause heating, but there is still impedance caused by magnetic leakage inside the transformer. However, as the current increases, the output voltage will decrease.The greater the current, the lower the output voltage, so the output power of the transformer cannot be infinite. If you said again that the transformer has no impedance, then when current flows through the transformer, it will generate a particularly large electric force, which can easily damage the transformer coil. Although you have a transformer with unlimited power, it cannot be used. All we can say is that with the development of superconducting materials and iron core materials, the output power of a transformer of the same volume or weight will increase, but it will not be infinite! [1]
4 Physical PropertiesPhysical Characteristics
Transformer —- a static electromagnetic device, a transformer can convert alternating current energy of one voltage into AC power of another voltage with the same frequency. The main components of a transformer are an iron core and two windings wrapped around the iron core. Transformer Principle The coil connected to the power supply receives AC power and is called the primary winding. The coil connected to the load sends AC power and is called the secondary winding. The primary winding and the secondary winding have voltage phasors U1 and voltage phasors U2. , current phasor I1, current phasor I2, electromotive force phasor E1, electromotive force phasor E2, number of turns N1, number of turns N2, are linked once at the same time, the phasor of the magnetic flux of the secondary winding is φm, and this magnetic flux is called the main magnetic flux.
Physical formula
U1/U2=N1/N2 =I2/I1 (U1 and U2 are voltages, N1 and N2 are the number of coil turns, I1I2 is the current).
5 Failure Analysis There are many reasons for internal faults and accidents in the transformer due to short circuit at the transformer outlet, and they are also relatively complex. They are related to factors such as structural design, quality of raw materials, process level, operating conditions, etc., but the electromagnetic wire The selection is key. In recent years, the electromagnetic wires selected for dissecting the transformer based on the static theoretical design of the transformer are quite different from the stress acting on the electromagnetic wires during actual operation.
1. The winding is loosely wound, improperly transposed, and too thin, causing the electromagnetic wire to hang in the air. Judging from the location of accident damage, deformation is often seen at transpositions, especially at transpositions of transposed wires.
2. The current calculation programs of various manufacturers are based on idealized models such as uniform distribution of the leakage magnetic field, the same diameter of the wire turns, and equal-phase forces. In fact, the transformer The leakage magnetic field is not uniformly distributed and is relatively concentrated in the iron yoke. The electromagnetic wires in this area are also subject to greater mechanical forces. The transposed conductor will change the direction of force transmission due to climbing at the transposed position, resulting in torque; due to the elasticity of the pad The modulus factor and the unequal distribution of axial pads will cause the alternating force generated by the alternating leakage magnetic field to resonate with delay. This is why it is located at the core yoke, transposition, and voltage regulating tap. The root cause of the line cake deforming first.
3. Improper control of the preload force of the winding causes the conductors of ordinary transposed conductors to be misaligned with each other.
4. The impact of temperature on the bending and tensile strength of the electromagnetic wire is not considered when calculating the short-circuit resistance. The short-circuit resistance designed at normal temperature cannot reflect actual operation.According to the test results, the temperature of the electromagnetic wire has a great influence on its yield limit? 0.2. As the temperature of the electromagnetic wire increases, its bending resistance, tensile strength and elongation all decrease. The bending and tensile strength at 250°C Compared with the drop at 50℃, the elongation drops by more than 40%. In actual operating transformers, under rated load, the average winding temperature can reach 105°C, and the hottest spot temperature can reach 118°C. Generally, there is a reclosing process when the transformer is running. Therefore, 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 impact of the first short-circuit current, The winding temperature increases sharply. According to the regulations of GBl094, the maximum allowed is 250°C. At this time, the short-circuit resistance of the winding has dropped significantly. This is why most short-circuit accidents occur after the Satons transformer is reclosed.
5. The use of soft wires is also one of the main reasons for the poor short-circuit resistance of the transformer. Due to insufficient understanding of this in the early stage, or difficulties in winding equipment and technology, manufacturers were unwilling to use semi-rigid wires or had no requirements in this regard during design. Judging from the faulty transformers, they were all soft wires.
6. External short-circuit accidents are frequent. The cumulative effect of electrodynamic force after multiple short-circuit current impacts causes the electromagnetic wire to soften or internal relative displacement, eventually leading to insulation breakdown.
7. The winding turns or wires are not cured and have poor short-circuit resistance. None of the early paint-impregnated windings were damaged. [2]
6 Daily maintenance
1. In order to protect the insulation performance of the high-voltage generator and the insulating oil installed in the machine head, it should generally not be Open the observation window and loosen the surrounding fixing screws at will to prevent the oil from absorbing moisture or dust and reducing the insulation performance.
2. Check whether the lighting, heat dissipation, and dust removal equipment around the transformer are intact, and wipe off the dust on the transformer body and porcelain bottle with a clean cloth.
3. Check the load switch on the high-voltage side of the transformer to ensure flexible operation, good contact, and lubrication of the transmission part.
4. Open the high-voltage grounding knife and check that the grounding is in the disconnected position. Then close the high-voltage load switch, let the transformer test run, and remove the high-voltage side identification plate. Pay attention to the disconnection or closing. When switching high-voltage load switches on transformers, there must be at least two people on site.
5. When new oil needs to be replaced, assistance from the local power department should be obtained to check the performance of the new oil. Its insulation strength is required to be no less than 25,000 volts/2.5 mm; and the oil in the combined machine head The insulation strength should be above 30000 volts/2.5mm.
6. Use a 2500V megger to measure the insulation resistance of the high and low voltage coils of the transformer (to ground and between phases), and confirm that it meets the requirements (at room temperature of 30°C, the high voltage side of the 1OKV transformer is greater than 20MΩ, and the low voltage side is greater than 13MΩ . Before the test, the grounding wire should be connected, and after the measurement is completed, the discharge should be carried out.
7. High-voltage generator or combined machine headThere must be a good grounding wire. You should always use an ohmmeter to measure whether the shell, console shell and external grounding wire are connected, and tighten the grounding bolt.
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