Transformers_part2

Special alloy steel of high resistance and low hysteresis loss is used as transformer core


As the flux in the cores is a pulsating one, the magnetic circuit must be lamianted and the separate laminations insulated in order to retain the advantage
of subdivision


Cut the transformer sheets as far as possible along the grain which is the direction in which the material has a higher permeability


For large three phase transformer, the core is five-limbed core than , which needs a cross section in the yokes less than that required in usual three limbed core but core losses increase




Cross over coils are suitable for currents not exceeding about 20A. They are used for HV windins in comparatively small transformers, and comprise wires of small circular section with double cotton covering


Disc coils are made up of a number of flat sections, generally with rectangular wire


In Disc coils every turn is contact with oil hence better cooling


Sandwich windings , commonly employed for shell-type transformers, allow of easy control over reactance.


Conservators are required to take up the expansion and contraction of the oil with changes of temperature in service without allowing the oil to come in contact with air , from which it is liable to take up moisture


The displacement of air due to change of oil volume takes place through a breather containing calcium chloride or silica gel, which extracts the moisture from the air.


Oil in transfomer serves the double purpose of cooling and insulating.


Sludging means slow formation of semi solid hydrocarbons, due to heat and oxidation. It will cause over heating of transformer by blocking the cooling ducts


Sludge formation is more in the presence of bright copper surfaces


Three phase transformer connections representations:

>Group 1: Zero phase displacement--Yy0,Dd0,Dz0
>group 2: 180⁰ Phase displacement--Yy6,Dd6,Dz6
>group 3: 30⁰ lag Phase displacement--Dy1,Yd1,Yz1
>group 4: 30⁰ lead Phase displacement--Dy11,Yd11,Yz11

Star-Star connection is most economical connection for small, high voltages transformers. In this connection third harmonic voltages are absent from the line voltages; unless there is a fourth wire no third harmonic currents will flow. Best suitable for core type construction, for shell type need tertiary winding to stabilize the neutral condition.


Delta-Delta connection is an economical connection for large , low voltage transformers.


In Delta - Delta, even though one phase faulty the transformer can operate at 58 percent of normal. This connection called Vee(VV) connection.