ELECTRICAL TRANSFORMERS PDF

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properly ground a transformer, and the secondary electrical system Figure Control transformers are common in appliances and electrical equipment. This Bachelor thesis is about the basics of Power Transformers, such as . responsible for transformation action in an electrical transformer. A transformer is an electrical apparatus designed to convert alternating Taps are provided on some transformers on the high voltage winding to correct for high .


Electrical Transformers Pdf

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The Transformer. The principle parts of a transformer and their functions are: path for the most lines of flux with the least loss in magnetic and electrical energy. Transformers are electrical devices consisting of two or more coils of wire used to transfer electrical energy by means of a changing magnetic field. One of the. The Transformer as an Isolation. Device. • Transformers are useful in providing electrical isolation between the primary circuit and the secondary circuit because.

Transformer, Construction, Working, Types Application & Limitations

A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance. The working principle of the transformer can be understood from the figure below.

The core laminations are joined in the form of strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core. Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force.

The alternating current supply is given to the first coil and hence it can be called as the primary winding. The energy is drawn out from the second coil and thus can be called as the secondary winding.

In short, a transformer carries the operations shown below: Transfer of electric power from one circuit to another. Transfer with the principle of electromagnetic induction. The two electrical circuits are linked by mutual induction. The two coils are insulated from each other and from the steel core. The device will also need some suitable container for the assembled core and windings, a medium with which the core and its windings from its container can be insulated. In order to insulate and to bring out the terminals of the winding from the tank, apt bushings that are made from either porcelain or capacitor type must be used.

In all transformers that are used commercially, the core is made out of transformer sheet steel laminations assembled to provide a continuous magnetic path with minimum of air-gap included.

The steel should have high permeability and low hysteresis loss. For this to happen, the steel should be made of high silicon content and must also be heat treated. By effectively laminating the core, the eddy-current losses can be reduced.

The lamination can be done with the help of a light coat of core plate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, the thickness of the lamination varies from 0. Types of Transformers Types by Design The types of transformers differ in the manner in which the primary and secondary coils are provided around the laminated steel core. According to the design, transformers can be classified into two: 1. The coils used for this transformer are form-wound and are of cylindrical type.

Such a type of transformer can be applicable for small sized and large sized transformers. In the small sized type, the core will be rectangular in shape and the coils used are cylindrical. The figure below shows the large sized type.

You can see that the round or cylindrical coils are wound in such a way as to fit over a cruciform core section. In the case of circular cylindrical coils, they have a fair advantage of having good mechanical strength. The cylindrical coils will have different layers and each layer will be insulated from the other with the help of materials like paper, cloth, micarta board and so on. The general arrangement of the core-type transformer with respect to the core is shown below.

Both low-voltage LV and high voltage HV windings are shown. Core Type Transformer Cruciform Section Core Type Transformers The low voltage windings are placed nearer to the core as it is the easiest to insulate. The effective core area of the transformer can be reduced with the use of laminations and insulation.

The comparison is shown in the figure below. Core Type and Shell Type Transformer Winding The coils are form-wound but are multi layer disc type usually wound in the form of pancakes. Paper is used to insulate the different layers of the multi-layer discs. The whole winding consists of discs stacked with insulation spaces between the coils. These insulation spaces form the horizontal cooling and insulating ducts. Such a transformer may have the shape of a simple rectangle or may also have a distributed form.

This will help in minimizing the movement of the device and also prevents the device from getting any insulation damage. A transformer with good bracing will not produce any humming noise during its working and will also reduce vibration.

A special housing platform must be provided for transformers.

There are guides available, through standards organizations, for estimating the cost associated with transformers losses. It is typically expressed as a percentage, or per unit, of the rated output voltage at rated load. The value of q is taken to be positive for a lagging inductive power factor and negative for a leading capacitive power factor.

However, this is at the expense of the fault current, which would in turn increase with a reduction in impedance, since it is primarily limited by the transformer impedance. Additionally, the regulation increases as the power factor of the load becomes more lagging inductive. The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled around the core sections.

Core steel can be hot- or cold-rolled, grain-oriented or nongrain oriented, and even laser-scribed for additional performance.

Thickness ranges from 0. The core cross section can be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer.

Rectangular cores use a single width of strip steel, while circular cores use a combination of different strip widths to approximate a circular cross-section, such as in Figure 2. The type of steel and arrangement depends on the transformer rating as related to cost factors such as labor and performance. Just like other components in the transformer, the heat generated by the core must be adequately dissipated.

While the steel and coating may be capable of withstanding higher temperatures, it will come in contact with insulating materials with limited temperature capabilities. In larger units, cooling ducts are used inside the core for additional convective surface area, and sections of laminations may be split to reduce localized losses. The core is held together by, but insulated from, mechanical structures and is grounded to a single point in order to dissipate electrostatic buildup.

The core ground location is usually some readily accessible point inside the tank, but it can also be brought through a bushing on the tank wall or top for external access. This grounding point should be removable for testing purposes, such as checking for unintentional core grounds.

There are two basic types of core construction used in power transformers: core form and shell form. In core-form construction, there is a single path for the magnetic circuit. For single-phase applications, the windings are typically divided on both core legs as shown. In three-phase applications, the windings of a particular phase are typically on the same core leg, as illustrated in Figure 2.

In shell-form construction, the core provides multiple paths for the magnetic circuit. Due to advantages in short-circuit and transient-voltage performance, shell forms tend to be used more frequently in the largest transformers, where conditions can be more severe.

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The terms winding and coil are used interchangeably in this discussion. Copper and aluminum are the primary materials used as conductors in power-transformer windings. While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper.

Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength. The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors.

Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop s created by joining the strands at the ends.

Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents.

Transformer

A variation of this technique, involving many rectangular conductor strands combined into a cable, is called continuously transposed cable CTC , as shown in Figure 2.

A schematic of coils arranged in this three-phase application was also shown in Figure 2.

Shell-form transformers use a similar concentric arrangement or an interleaved arrangement, as illustrated in the schematic Figure 2. The coils are typically connected with the inside of one coil connected to the inside of an adjacent coil and, similarly, the outside of one coil connected to the outside of an adjacent coil. Sets of coils are assembled into groups, which then form the primary or secondary winding.

When considering concentric windings, it is generally understood that circular windings have inherently higher mechanical strength than rectangular windings, whereas rectangular coils can have lower associated material and labor costs.

Electric power transformer engineering

In some special cases, elliptically shaped windings are used. Concentric coils are typically wound over cylinders with spacers attached so as to form a duct between the conductors and the cylinder. Figures 2. A variety of different types of windings have been used in power transformers through the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils; or they can be wound horizontally and placed upright upon completion.

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As mentioned previously, the type of winding depends on the transformer rating as well as the core construction. Several of the more common winding types are discussed here. However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers.

The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically.

This type of winding lends itself to the interleaved arrangement previously discussed Figure 2. Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination. Several strands can be wound in parallel if the current magnitude so dictates.

Variations of this winding are often used for applications such as tap windings used in load-tap-changing LTC transformers and for tertiary windings used for, among other things, third-harmonic suppression. A helical winding consists of a few to more than insulated strands wound in parallel continuously along the length of the cylinder, with spacers inserted between adjacent turns or discs and suitable transpositions included to minimize circulating currents between parallel strands.

The manner of construction is such that the coil resembles a corkscrew. Helical windings are used for the higher-current applications frequently encountered in the lower-voltage classes.

Each disc comprises multiple turns wound over other turns, with the crossovers alternating between inside and outside. Most windings of kV class and above used in coreform transformers are disc type. Given the high voltages involved in test and operation, particular attention is required to avoid high stresses between discs and turns near the end of the winding when subjected to transient voltage surges.

Numerous techniques have been developed to ensure an acceptable voltage distribution along the winding under these conditions. This task can be accomplished by several means. Most transformers are provided with a means of changing the number of turns in the high-voltage circuit, whereby a part of the winding is tapped out of the circuit. In many transformers, this is done using one of the main windings and tapping out a section or sections, as illustrated by the schematic in Figure 2.

With larger units, a dedicated tap winding may be necessary to avoid the ampere-turn voids that occur along the length of the winding.It sounds a bit weird though.

Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. Lucien Gaulard and John Dixon Gibbs first exhibited a device with an open iron core called a 'secondary generator' in London in , then sold the idea to the Westinghouse company in the United States.

A drawback of toroidal transformer construction is the higher labor cost of winding. Current carrying conductors produces magnetic field when current passes through it.

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