Transformer - Drawings

Contents

Introduction

Core Calculation

Selecting the Dimensions of Rod Pack Plates

Calculation of yoke section

Calculation of windings

Number of LV and HV turns

Calculation of phase currents in windings

Calculation of low voltage winding (axial structure)

Calculation of high voltage winding (axial structure)

Radial structure of windings

Define Active Material Weights

Core weight

Weight of winding material

Calculation of characteristics

Calculation of idling losses and current

Calculation of short circuit losses

Short circuit voltage calculation

Calculation of voltage change

Calculation of efficiency

Transformer Thermal Calculation

Calculation of low voltage winding overheating

Calculation of high voltage winding overheating

Calculation of oil overheating

Calculation of mechanical forces in transformer windings

List of literature used

Introduction

Currently, electric energy for industrial purposes and power supply to cities is produced at large thermal or hydroelectric power plants in the form of a three-phase alternating current system with a frequency of 50 Hz. The voltages of generators installed at power plants are standardized and can have values ​ ​ of 6600,,11,000,,13,800,,15,750,,18,000 or 20,000 in (GOST 72162). In order to transmit electricity over long distances, this voltage must be increased to 110, 220, 330 or 500 kv depending on the distance and the transmitted power. Further, at distribution substations, the voltage must be reduced to 6 or 10 sq (in cities and industrial facilities) or to 35 sq (in rural areas and with a large length of distribution networks). Finally, in order to enter the factory shops and residential apartments, the voltage of the networks must be reduced to 380, 220 or 127 v. In some cases, for example, to illuminate boiler houses or mechanical shops and raw rooms, the voltage must be reduced to a life-safe value - 12, 24 or 36 v.

Increase and decrease of AC voltage and power transformers are performed. Transformers do not produce electric energy themselves, but only transform it, that is, change the magnitude of the electric voltage. At the same time, transformers can be step-up if they are designed to increase voltage, and step-down if they are designed to lower voltage. But fundamentally, each transformer can be used either as a step-up or as a step-down depending on its purpose, that is, it is a reversible apparatus. Power transformers have a very high efficiency (k.p. d.), the value of which is from 95 to 99.5%, depending on the power. The higher power transformer has, respectively, a higher k.p.)

A transformer is a static electromagnetic apparatus for converting one (primary) AC system to another (secondary) having different characteristics. The principle of the transformer is based on the law of electromagnetic induction, discovered by the English physicist Faraday in 1831. The phenomenon of electromagnetic induction is that if magnetic flux changes in time inside the closed conductor circuit, then electromotive force (e. d. p.) and induction current occurs. In order to reduce the resistance along the path of the magnetic flux and thereby enhance the magnetic connection between the primary and secondary coils or, as they are more commonly called, windings, the latter must be located on a closed iron (steel) core (magnetic core). The use of a closed steel magnetic core significantly reduces the relative value of the scattering flow, since the permeability of steel used for magnetic cores is 8001000 times higher than that of air (or in general for diamagnetic materials).

Thus, two (or more) windings mounted on a closed steel magnetic conductor are a transformer. From this definition, it follows that the main principal parts of the transformer are the primary and secondary windings and the magnetic circuit. The transformation coefficient is the ratio of inductees in the primary and secondary windings e. d. c., equal to the ratio of the number of turns of these windings.

Transformer consists of magnetic circuit and windings fitted on it. In addition, the transformer consists of a number of purely structural units and elements, which are a structural part of it. The structural elements serve mainly for ease of use and operation of the transformer. These include insulation structures designed to provide insulation of current-carrying parts, taps and inputs - to connect windings to the power line, switches - to regulate the voltage of the transformer, tanks - to fill them with transformer oil, pipes and radiators - to cool the transformer, etc.

The magnetic conductor of the transformer is a closed magnetic circuit designed to pass the main magnetic flux coupled to both windings. For power transformers, rod-type magnetic conductors are mainly used. Single-phase transformers have magnetic conductors with two rods carrying windings, and three-phase ones - three rods. Rods are connected by upper and lower yokes. The magnetic conductor of the transformer is assembled from plates of electrical steel sheet with thickness of 0.35 or 0.5 mm. To reduce eddy current losses, the plates are isolated from each other by applying a lacquer or chemical insulation film. Since the magnetic conductors are assembled from rectangular plates, their assembly is carried out overlapping (such an assembly is called batch). At the same time formed joints of plates of one layer are overlapped by plates of adjacent layer. Butt assembly is rarely used. The assembly of magnetic conductors by the charge method has two goals: first, reducing the magnetizing current of the transformer and, second, increasing the mechanical strength of the assembled magnetic conductor. The section of the magnetic core rods on which the windings are placed is given a shape close to a circle (inscribed in a circle). The number of stages is chosen depending on the power of the transformer: the greater the power (and therefore the diameter of the circle), the more the number of stages is taken. Inside the magnetic conductors of high-power transformers, cooling channels are arranged for better removal of heat arising from losses in steel, through which oil or air circulates (for "dry" transformers).

Yoke of magnetic conductor has also stepped shape in section. However, for transformers of lower power, for the purpose of some simplification of design, the number of yoke section stages is often taken less than that of the rod section, or sometimes a yoke is made of rectangular section. In recent cases, to reduce idle current and losses in steel, the cross section of the yoke is selected 510% more than the cross section of the rod.

Windings of power transformers are usually divided into high and low voltage windings (HV and LV), and not into primary and secondary, since any of the windings can be primary or secondary, depending on which one is included in the supply network. The winding of the transformer is part of the electrical circuit (primary or secondary), in connection with which it consists of conductor material (winding copper or aluminum) and insulating parts. The winding set also includes lead ends, voltage control branches, capacitive rings and capacitive surge protection electrostatic shields. Windings are made cylindrical two- or multilayer, coil, screw or continuous. The choice of winding type depends on the number of turns, the size and number of parallel wires, the cooling method, the transformer power and other factors.

Magnetic conductor and windings together with fasteners form active part of power transformer.

The transformer is heated during its operation due to losses in it. In order for the heating temperature of the transformer (mainly its insulation) to not exceed the permissible value, it is necessary to ensure sufficient cooling of the windings and the magnetic conductor. For this, in most cases, the transformer (active part) is placed in a tank filled with transformer oil. When heated, the oil begins to circulate and gives heat to the walls of the tank, and from the latter heat is dissipated in the surrounding air.

The materials used for the fabrication of transformers can be divided into:

-active, which include electrical steel of the magnetic conductor and winding wires;

- electrical insulation, which are necessary for electrical insulation of windings and other current-carrying parts of the transformer, for example, electrical insulation board, cable and telephone paper, lakotkan, getinax, porcelain, transformer oil, etc.;

- structural, required for the manufacture of parts of the frame, tank, cooling devices, various fasteners, etc., and other materials, semi-finished products and devices.

For the manufacture of magnetic cores, thin-sheet alloyed electrical steel is used. This steel is hotter and colder. Initially, hot-rolled steel was used, allowing induction of B to 1,41.45 T. Cold rolled steel has specific losses, 1.52 times less than hot rolled steel, and significantly greater magnetic permeability. It allowed to increase the induction to 1.61.7 T.

The main material for the fabrication of transformer windings is winding copper (or aluminum), which is an insulated copper (or aluminum) wire of round or rectangular section.

Electrical insulation materials used in transformer engineering must have certain properties, of which the most important are electrical and mechanical strength, hygroscopicity and heat resistance. One of the main insulating materials is an electric cardboard with a thickness of 0.5 to 3 mm. It has good electrical characteristics, increased oil supply and mechanical strength. Electric cardboard is used to manufacture various insulating parts. Cable paper with a thickness of 0.12 mm is used as insulation between the layers of windings and for isolating the ends of windings and taps. Silk and cotton varnish is used to isolate the ends of windings and taps, as well as to strengthen the insulation of individual winding places, for example, in places of wire bundles. Cotton belts, boiling and taft, are used for mechanical protection of insulation and in general as an auxiliary fastening material. Paper-bakelite cylinders and tubes are used as frames for winding windings (cylinders) and for isolating tie pins of magnetic conductors and taps (tubes). Getinax sheet thickness up to 50 mm is used for the manufacture of insulating boards and panels, as well as parts of the design of switching devices. Porcelain is used for the manufacture of through insulators (bushings) and some insulating parts of dry transformers. Electrical insulation materials also include various varnishes and enamels.

Transformer oil serves two purposes at the same time: to increase the electrical strength of the transformer insulation and to improve its cooling conditions. The use of transformer oil made it possible, on the one hand, to increase electromagnetic loads (induction and current density) on active materials and thereby reduce their consumption and, on the other hand, build high-power transformers and high voltages.

A properly designed transformer, in addition to meeting certain technical requirements, should be as cheap as possible. The cost of the transformer depends on its size and weight, and primarily on the weight of the active materials as more expensive. The main dimensions that ultimately determine the weight of the active materials are the dimensions of the magnetic core.

To correctly select the main dimensions of the magnetic core, you must first find out the dependence of these dimensions on the nominal power of the transformer. As will be seen further, along with the increase in transformer power, its linear geometric dimensions should also increase. When determining the dependence of dimensions on power, it is necessary to assume that transformers of different capacities should be geometrically similar, that is, the ratios of all three linear dimensions should remain the same, and that electromagnetic loads on active materials, that is, induction and current density in windings, should also remain the same. The weight of the active materials (steel and copper) of the transformer is proportional to their volumes or linear dimensions in the third degree. It follows that the weight of the active materials will be proportional to the power of 3/4. With increasing power, the specific consumption of active materials, expressed in kg/kva, decreases. Therefore, more powerful transformers have a relatively lower cost. Powerful transformers are also more economical in operation, since they have relatively less losses, and therefore a higher efficiency (k.p .).

Power transformers of general purpose, depending on voltage and power, are conditionally divided into groups or dimensions. Transformers with capacity from 25 to 100 kva inclusive refer to dimensions I, capacity from 160 to 630 kva - to dimension II, power from 1000 to 6300 kva - to dimension III, power 10000 kva or more with voltage 35 kv and all transformers with voltage 110 kv of HV winding - to dimension IV, power 40,000 kva or more with voltage 220 kv of HV windings and higher - to size V and capacity 10 0000 kva and higher - to dimension VI. transformers of above mentioned capacities are more often produced by oil, i.e. with active part lowered for the purpose of better cooling and increase of insulation strength into tank with oil. However, transformers with a capacity of up to 1000-1600 kva and a voltage of up to 10-15 kv can also be manufactured dry, that is, with air cooling.

The main and operational parameters of power transformers are determined in the relevant standards, and only transformers that meet the requirements of these standards are produced by mass production. Standardized combinations of nominal linear voltages of HV and LV windings, circuits and connection groups for each nominal power value are given in GOST 1202266 and GOST 1192066.

On the HV side of the power transformer, it should be possible to change the transformation coefficient relative to the nominal one by switching branches from the winding. For power transformers, two types of branch switching are used: PBV (switch without excitation), that is, after disconnecting all the windings of the transformer from the network, and PLL (control under load).

Core Calculation

Preliminary selection of diameter D of core rod is performed by curves (Fig. 2.3) [1]. The active section Pst of the rod, that is, the section of the active steel, will depend on the selected section shape, the number of steps, and the fill factor.

In principle, the number of steps should be as large as possible, because the more steps, the greater the Kz factor of filling the circle area with the geometric shape of the rod section. However, for technological reasons, the number of steps is often preferred to be limited so as not to complicate production by too many plate sizes. Therefore, the number of steps is selected depending on the selected diameter D.

The selected number of steps determines the number of plate packs from which the rod section is added. The largest cross section of the rod (stepped figure) is obtained only with certain ratios of the width of the sp of the bags to the diameter D. These ratios are different for different numbers of stages (Fig. 14.1) [1]. The width of each packet sp is obtained by multiplying the corresponding coefficient by the diameter D.

The cross section of the yoke, since the magnetic flux in the yoke of the same magnitude as in the rod, theoretically (at least in a geometric sense) would have to repeat the cross section of the rod. However, the yoke does not carry windings and therefore its shape is not due to special requirements in this regard. For type II transformers, the most common is a two-stage (T-shaped) yoke. And only in larger transformers, dimensions III and above make the yoke multistage with a number of stages close to or equal to the number of stages of the rod.

Transformer Thermal Calculation

The loss of electrical energy arising from the operation of the transformer in its magnetic conductor and windings, as well as in the details of the structure, turns into thermal energy and causes heating of the corresponding parts of the transformer. The materials from which the transformer is made, mainly its insulating parts, allow heating only to a known limit. Limits of permissible heating for each type of material are established experimentally, based on reliable long-term operation of the transformer. However, in most cases, more complete use of active materials is obtained by increasing their temperature. The transformer must therefore be designed and designed to provide sufficient cooling during operation.

Heat released in the transformer is dissipated into the environment. This heat is transmitted through the external surface of the transformer - windings and magnetic circuit at dry transformers and external walls of the tank and cooling devices at oil transformers. If this heat did not dissipate, then the temperature of the transformer would continuously increase due to its heat capacity, which would lead to the destruction of its insulation in the first place, and the transformer would soon fail.

The transformer, which has been disconnected for a long time, has a temperature equal to the ambient air temperature. From the moment of switching on, the transformer begins to heat up. As soon as the temperature of its parts becomes higher than the temperature of the ambient air, the heat from the transformer begins to be transferred to the ambient air. From this point, the transformer cooling process begins. But as soon as the transformer begins to give heat to the surrounding air, the increase in the temperature of its parts will slow down, since at the same time cooling will increase, and finally, a steady-state thermal state will occur. In this state, the amount of heat generated in the transformer will become equal to the heat removed from it, so that the excess of the transformer temperature above the ambient air temperature will become unchanged. The temperature excess value for brevity is often called overheating. So, for example, overheating the winding above the air means exceeding the temperature of the winding above the temperature of the ambient air.

Cooling of any heated body in air occurs by dissipating heat from the surface of the body. This heat dissipation occurs in two ways: thermal radiation, convection (heat transfer by heated particles of air or liquid). So the transformer with natural air cooling, or the so-called dry transformer, is cooled. However, air cooling is not intensive and for transformers even medium power is insufficient. In this regard, oil cooling began to be used (since 1889), which made it possible to build large transformers and, moreover, high voltage.

The temperature of the transformer is therefore composed of its overheating above the air and the temperature of the ambient air. But the amount of overheating of parts of the transformer above the air depends on the amount of loss of the transformer, which in turn depends on its load, that is, on the design, mode of operation and practically does not depend on the temperature of the ambient air. Therefore, the thermal calculation of the transformer is reduced to the determination of the overheating of its parts, and not their temperature, since the temperature of the transformer will change with the change in air temperature.

Since the heating of the transformer is limited to a certain value of its temperature, the value of the highest allowable overheating is determined taking into account the highest possible ambient temperature. For Russia, in the conditions of natural seasonal and daily change, a temperature of + 40 ° С is adopted.

According to the requirements of GOST 1167765, the following permissible overheating standards for individual parts of power oil transformers are established, which are given in Table 10.1. [1]. For windings, the overheating is + 65 ° C, for oil in the upper layers + 55 ° C. The overheating rate specified in Table 10.1 [1] for windings is set based on the maximum permissible temperature of 105 - 110 ° С, determined by the class of insulation of the material and confirmed by long-term operating conditions and studies (65 + 40 = 105 ° С). The average winding temperature during the total life of the transformer, taking into account fluctuations in ambient and air temperatures and changes in load, will be significantly lower than 105 ° C. According to GOST 11677-65, losses and short-circuit voltage of oil transformers are assumed to be + 75 ° С for the design (conditional) temperature of windings, to which (according to GOST 348465) losses and short-circuit voltage of oil transformers must be given. Under these conditions, the service life of the transformer insulation is determined for about 15-20 years.

The path along which the thermal energy emitted in the windings and magnetic conductor of the transformer passes can be divided into several sections. At each of these sites there is a temperature difference, that is, their difference at the boundaries of the sites.

1st section - from internal points of winding or magnetic core to their external surfaces washed with oil. In this section heat transfer is carried out by means of thermal conductivity. When calculating internal temperature differences in the multilayer winding, either empirical formulas or amendments to the design temperature determined by experimental data are used. The thermal calculation of power transformers is somewhat simplified by the fact that since each wire of a layered or continuous winding is directly washed with oil, there is practically no temperature difference inside the winding. Temperature correction is given only in case of application of reinforced (thickness) winding insulation and additional insulation of coils.

The 2nd section is the transfer of heat from the winding to the oil. On the surface of the windings, a temperature difference occurs between the winding and the oil washing it, which depends on the amount of heat released from the surface of the winding, the location of the winding surfaces cooled by the oil, the size of the oil channels and the viscosity of the oil. The temperature difference (overheating) of the winding surface and oil is determined by formulas compiled on the basis of experimental data for each type of winding.

The 3rd section is heat transfer by heated oil from the winding to the tank walls and cooling devices. Oil, washing the transformer windings, carries away heat generated in it from the winding surface. In this case, heat transfer occurs by convection, that is, by moving the oil, which occurs due to the difference in densities of the heated and cold oil. The movement of the oil around the winding itself varies depending on the type of winding, the shape, size and location of the oil channels. The oil heated at the winding surface rises to the upper part of the transformer tank, contacts the tank walls and gives them heat received from the winding, drops to the lower part of the tank, and then returns to the windings. If there are cooling pipes or coolers (radiators) on the walls of the tank, the heated oil enters the pipes or the upper pipe of the radiator and, having cooled in the pipes washed by external air, drops down along them, enters the lower part of the transformer tank and is directed again to the windings. The oil is then heated again to absorb the heat generated in the windings and the magnetic core and rises upwards. Thus, in the operating transformer, a closed convection current of oil in its tank occurs and a continuous process of oil circulation occurs.

The 4th section is the transfer of heat from the oil to the wall of the transformer tank in the presence of a temperature difference between the oil and the wall. This temperature difference is determined by the same laws as the temperature difference between the winding and the oil, that is, it depends on the value of the specific thermal load on the wall of the tank and the cooling device.

The 5th section is the heat transfer through the thickness of the tank wall. The temperature difference in this area does not exceed 1 ° C, and therefore it is usually neglected.

The 6th section (last) is the heat removal from the tank walls and the cooling device to the ambient air. Heat is removed from the outer surface of the tank wall into the ambient air in two ways: part of the heat is removed by convection air flow, the second part - by radiation. Heat transfer by radiation depends on the temperature of the radiating body and the temperature of the air, as well as on the configuration of the tank wall and cooling device and the state of their surface.

Heat transfer by radiation from the surface of smooth tanks painted with paints with non-metallic fillers reaches 50-55% of the total heat transfer. For tubular tanks or for tanks with radiators, it decreases to 15% of the total heat transfer. This is due to the rectilinear propagation of radiant energy. Heat transfer by radiation in this case does not occur from the whole surface, but only from the outer envelope of the surface of the cooling device. Heat transfer by air convection occurs in contrast to heat transfer by radiation from the entire surface of the tank, pipes and coolers. It depends on the temperature difference between the tank walls and the air, the height of the tank, the shape of its surface and the barometric pressure of the air. Heat transfer increases with increasing surface of tank and cooling devices, wall temperature and increasing free access of ambient air to tank walls.

In a practical thermal calculation, two main temperature overheating differences are determined: excess of the winding temperature above the oil temperature and excess of the oil temperature above the air temperature (or, briefly, overheating of the winding above the oil and overheating of the oil above the air). The sum of these overheats gives the winding overheating above the air. If necessary, corrections are added to the overheating of the winding above the oil, depending on the thermal conductivity of the reinforced turn and inter-layer insulation and on the size of the oil channels, and to the overheating of the oil above the air - correction, depending on the ratio of the heights of the loss centers (active part) and the cooling device.

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