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Power Transformer

INTRODUCTION

Transformer is a vital link in a power system which has made possible the power generated at low voltages (6600 to 22000 volts) to be stepped up to extra high voltages for transmission over long distances and then transformed to low voltages for utilization at proper load centers.

With this tool in hands it has become possible to harness the energy resources at far off places from the load centers and connect the same through long extra high voltage transmission lines working on high efficiencies. At that, it may be said to be the simplest equipment with no motive parts. Nevertheless it has its own problems associated with insulation, dimensions and weights because of demands for ever rising voltages and capacities.

In its simplest form a Transformer consists of a laminated iron core about which are wound two or more sets of windings. Voltage is applied to one set of windings, called the primary, which builds up a magnetic flux through the iron. This flux induces a counter electromotive force in the primary winding thereby limiting the current drawn from the supply. This is called the no load current and consists of two components- one in phase with the voltage which accounts for the iron losses due to eddy currents and hysteresis, and the other 90° behind the voltage which magnetizes the core.

This flux induces an electro-motive force in the secondary winding too. When load is connected across this winding, current flows in the secondary circuit. This produces a demagnetising effect, to counter balance this the primary winding draws more current from the supply so that

IP.NP = IS.NS

Where Ip and Np are the current and number of turns in the primary while IS and NS are the current and number of turns in the secondary respectively. The ratio of turns in the primary and secondary windings depends on the ratio of voltages on the Primary and secondary sides. The magnetic core is built up of laminations of high grade silicon or other sheet steel which are insulated from each other by varnish or through a coating of iron oxide. The core can be constructed in different ways relative to the windings.



CONSTRUCTION

1- Transformer core
Construction in which the iron circuit is surrounded by windings and forms a low reluctance path for the magnetic flux set up by the voltage impressed on the primary. Fig (1), Fig. (6) and Fig. (7) Shows the core type


Fig (1) core type]

The core of shell type is sh
own Fig.(2), Fig.(3), Fig.(4), and Fig.(5), in which The winding is surrounded by the iron Circuit Consisting of two or more paths through which the flux divides. This arrangement affords somewhat Better protection to coils under short circuit conditions.

In actual construction there are Variations from This simple construction but these can be designed

With such proportions as to give similar electrical characteristics.


Fig (2) shell type



Fig.(3) Single phase Transformer













Fig. (4) Single phase Transformer .



Fig. (5) 3- phase Transformer Shell type



Fig. (6) 3- phase Transformer core type


Fig. (7) Cross section of a three-phase Distribution Transformer (Core Type)


Three-phase Transformers usually employ three-leg core. Where Transformers to be transported by rail are large capacity, five-leg core is used to curtail them to within the height limitation for transport.

Even among thermal/nuclear power station Transformers, which are usually transported by ship and freed from restrictions on in-land transport, gigantic Transformers of the 1000 MVA class employ five-leg core to prevent leakage flux, minimize vibration, increase tank strength, and effectively use space inside the tank.

Regarding single-phase Transformers, two-leg core is well known. Practically, however, three leg cores is used, four-leg core and five-leg core are used in large capacity Transformers. The sectional areas of the yoke and side leg are 50 % of that of the main leg; thus, the core height can be reduced to a large extent compared with the two leg core.

For core material, high-grade, grain oriented silicon steel strip is used. Connected by a core leg tie plate fore and hind clamps by connecting bars. As a result, the core is so constructed that the actual silicon strip is held in a sturdy frame consisting of clamps and tie plates, which resists both mechanical force during hoisting the core-and-coil assembly and short circuits, keeping the silicon steel strip protected from such force.

In large-capacity Transformers, which are likely to invite increased leakage flux, nonmagnetic steel is used or slits are provided in steel members to reduce the width for preventing stray loss from increasing on metal parts used to clamp the core and for preventing local overheat. The core interior is provided with many cooling oil ducts parallel to the lamination to which a part of the oil flow forced by an oil pump is introduced to achieve forced cooling.
When erecting a core after assembling, a special device shown in Fig. (8) Is used so that no strain due to bending or slip is produced on the silicon steel plate.


Fig (8)


Fig (9)

The steel strip surface is subjected to inorganic insulation treatment.

All cores employ miter-joint core construction. Yokes are jointed at an angle of 45° to utilize the magnetic flux directional characteristic of steel strip.

A computer-controlled automatic machine cuts grain-oriented silicon steel strip with high accuracy and free of burrs, so that magnetic characteristics of the grain-oriented silicon steel remains unimpaired. Silicon steel strips are stacked in a circle-section. Each core leg is fitted with tie plates on its front and rear side, with resin-impregnated glass tape wound around the outer circumference. Sturdy clamps applied to front and rear side of the upper and lower yokes are bound together with glass tape.

And then, the resin undergoes heating for hardening to tighten the band so that the core is evenly clamped Fig. (9). Also, upper and lower clamps are connected by a core leg tie plate; fore and hind clamps by connecting bars.



As a result, the core is so constructed that the actual silicon strip is held in a sturdy frame consisting of clamps and tie plates, which resists both mechanical force during hoisting the core-and-coil assembly and short circuits, keeping the silicon steel strip protected from such force.



In large-capacity Transformers, which are likely to invite increased leakage flux, nonmagnetic steel is used or slits are provided in steel members to reduce the width for preventing stray loss from increasing on metal parts used to clamp the core and for preventing local overheat.



The core interior is provided with many cooling oil ducts parallel to the lamination to which a part of the oil flow forced by an oil pump is introduced to achieve forced cooling.



When erecting a core after assembling, a special device shown in Fig. (8) Is used so that no strain due to bending or slip is produced on the silicon steel plate.
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