One of the most important and ubiquitous electrical machines is the transformer. It receives power at one voltage and delivers it at another. This conversion aids the efficient long-distance transmission of electrical power form generating stations. Since power lines incur significant power losses, it is important to minimize these losses by the use of high voltages. The same power can be delivered by high-voltage circuits at a fraction of the current required for low-voltage circuits.
The design of the magnetic circuit, the core of the transformer, will first be considered. The significance of the no-load behavior of the transformer is explained and of the magnetizing current which exists under all operating condition.
The common form of transformer involves a ferromagnetic core in order to ensure high value of magnetic flux linkage. This is also true of the rotating machine.
Although transformers are generally associated with power system applications, they also occur in many low – power application including electronic circuits.
Transformers effect changes of voltage with virtually no loss of power.
The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.
An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both primary and secondary coils.
The voltage induced across the secondary coil may be calculated from Faraday’s law of induction, which states that:
Where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Î¦ equals the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals
Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the voltage
An ideal transformer would have no energy losses, and would be 100% efficient. In practical transformers energy is dissipated in the windings, core, and surrounding structures. Larger transformers are generally more efficient, and those rated for electricity distribution usually perform better than 98%.
Experimental transformers using superconducting windings achieve efficiencies of 99.85%,While the increase in efficiency is small, when applied to large heavily-loaded transformers the annual savings in energy losses are significant.
A small transformer, such as a plug-in “wall-wart” or power adapter type used for low-power consumer electronics, may be no more than 85% efficient, with considerable loss even when not supplying any load. Though individual power loss is small, the aggregate losses from the very large number of such devices is coming under increased scrutiny.
The losses vary with load current, and may be expressed as “no-load” or “full-load” loss. Winding resistance dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the no-load loss. The no-load loss can be significant, meaning that even an idle transformer constitutes a drain on an electrical supply, which encourages development of low-loss transformers (also see energy efficient transformer).
Transformer losses are divided into losses in the windings, termed copper loss, and those in the magnetic circuit, termed iron loss. Losses in the transformer arise from:
Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.
Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and Inverse Square of the material thickness.
Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and in turn causes losses due to frictional heating in susceptible cores.
In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.
Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer’s support structure will give rise to eddy currents and be converted to heat. There are also radiative losses due to the oscillating magnetic field, but these are usually small.
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It is common in transformer schematic symbols for there to be a dot at the end of each coil within a transformer, particularly for transformers with multiple windings on either or both of the primary and secondary sides. The purpose of the dots is to indicate the direction of each winding relative to the other windings in the transformer. Voltages at the dot end of each winding are in phase, while current flowing into the dot end of a primary coil will result in current flowing out of the dot end of a secondary coil.
TYPE OF TRANSFORMER
There are many types of transformer is there one of them is Air-cored transformer.
The air-cored transformer may literally consist of two concentric coils which have nothing but air within the coils. This immediately has the advantage that the magnetizing current has exactly the same waveform as the voltage to which it is related. It has the very significant disadvantage that it is difficult to produce the necessary magnetic flux to generate the appropriate e.m.f.
The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses paths that take it outside the windings. Such flux is termed leakage flux, and results in leakage inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not directly a power loss (see “Stray losses” below), but results in inferior voltage regulation, causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Transformers are therefore normally designed to have very low leakage inductance.
However, in some applications, leakage can be a desirable property, and long magnetic paths, air gaps, or magnetic bypass shunts may be deliberately introduced to a transformer’s design to limit the short-circuit current it will supply. Leaky transformers may be used to supply loads that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon signs; or for safely handling loads that become periodically short-circuited such as electric arc welders. Air gaps are also used to keep a transformer from saturating, especially audio-frequency transformers in circuits that have a direct current flowing through the windings.
COOLING METHOD OF TRANSFORMER
Oil is used to cool the transformer. It also provide part of the electrical insulation between internal line parts, transformer oil must remain stable at high temperature for an extended period.
Very large or high-power transformer may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.
Cooling fan is used in transformer.
High temperatures will damage the winding insulation.
Small transformers do not generate significant heat and are cooled by air circulation and radiation of heat. Power transformers rated up to several hundred kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans.
Power transformer rated up to a few KVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. Some power transformers are immersed in specialized transformer oil that acts both as a cooling medium, thereby extending the lifetime of the insulation transformer. The transformer gets heated due the iron and copper losses occurring in them. It is necessary to dissipate this heat so that the temperature of the winding is kept below the value at which the insulation begins to deteriorate. The cooling of transformer is more difficult than the rotating machines because these machines create a turbulent air flow, which assists in removing the heat generated due losses. Luckily the losses in transformers are comparatively small. According to cooling system there are two types of transformer
1-> ONAN – oil natural air natural
2-> ONAF – oil natural air forced
Oil natural by means a number of radiators with air natural flow. The radiator is fixed to the tank through individual cut-off valves, provided with a handle operate closing device in line with the radiator connector pipe position, indicate open position of the valve.
An ideal transformer would have no energy losses, and would therefore be 100% efficient. In practical transformers energy is dissipated in the windings, core , and surrounding structure.
TRANSFORMER OIL TEST
Transformer oil can give better insulation level to operate the transformer. It also is a coolant to carry the heat away from the electrical transformer windings and iron core to the cooling radiators on the side of the oil filled transformer. If there is no better insulation level, it produced short circuit in the transformer. There for transformer oil should be checked. Normally moisture and other liquids are mixed with oil, the insulation level is reduced. Normally we can see the silica gel for absorb the moisture of transformer. Also there is an equipment to check the insulation level of transformer.
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Transformer oil, or insulating oil, is usually a highly- refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformer, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.
The oils help cool the transformer. Because it also provides part of the electrical insulation between internal live parts, transformer oil must remain stable at high temperatures for an extended period. To improve cooling of large power transformers, the oil-filled tank may have external radiators through which the oil circulates by natural convection. Very large or high-power transformer may also have cooling fans, oil pumps, and even oil-to-water heat exchangers.
Large, high voltage transformers undergo prolonged drying processes, using electrical self-heating, the application of a vacuum, or both to ensure that the transformer is completely free of water of water vapor before the cooling oil is introduced. This helps prevent corona formation and subsequent electrical breakdown under load.
Oil filled transformer with a conservator tend to be equipped with Buchholz relays.
These are safety devices that detect the build of gases inside the transformer and switch off the transformer. Transformer without conservators is usually equipped with sudden pressure relays, which perform a similar function as the Buchholz relay.
The flash point and pour point are 140 degree c and -6 degree c respectively. The dielectric strength of new untreated oil is 12MV/m and after treatment it should be > 24MV/M.
Large transformer for indoor use must either be of the dry type, that is, containing no liquid, or use a less-flammable liquid.
Well into the 1970, polychlorinated biphenyls were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in early 1970, concern about the toxicity of PCBs have led to their banning in many countries.
Today, non-toxic, stable silicone-based or fluorinated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion- resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid, ester is also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than minerals oil.
Transformer oil acts as an insulating and cooling medium in transformers. The insulating oil fills up pores in fibrous insulation and also the gaps between the coil conductors and the spacing between the siding and the tank, and thus increases the dielectric strength of the insulation. A transformer in operation generates heat in the winding, and that heat is transferred to the oil. Heated oil then flows to the radiators by convection. Oil supplied from the radiators, being cooler, cools the winding. There are several important properties such as dielectric strength, flash point, viscosity, specific gravity and pour point and all of them have to be considered when qualifying oil for use in transformers. Normally mineral oil is used, but coconut oil has been shown to possess all the properties needed to function as an environmentally friendly and economic replacement to the mineral oil for this purpose.
DIFFERENCE BETWEEN AIR COOLED TRANSFORMER AND OIL IMMERSED TRANSFORMER
The basic difference between the air cooled transformer and oil immersed transformer is:
If the heat generated in the transformer winding is capable of being swapped by the surrounding ambient air without causing any problem to the performance of the transformer then it’s normally reffered as Naturally Air Cooled transformer or simply air cooled transformer.
However when the capacity of the transformer increases, the heat generated in the winding of the transformer is more and the ambient air normally is inadequate to cool the windings and maintain the performance of the transformer. It’s because of this reason the transformer windings are submerged in an oil tank. It has been observed that the hydrogen ions present in oil help to maintain the temperature of the winding of the transformer and thus overall performance of transformer.
EFFECT OF FREQUENCY
The time-derivative term in Faraday’s Law shows that the flux in the core is the integral with respect to time of the applied voltage. Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time. In practice, the flux would rise to the point where magnetic saturation of the core occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current.
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