Construction of Transformer

TRANSFORMER CONSTRUCTION

Main Components of Transformers are:

i. Magnetic Core

ii. Windings (Primary and Secondary)

iii. Insulation for Windings

iv. Conservator (or) Tank

v. Cooling arrangement, Temperature Gauge, Oil Gauge

vi. Buchholz relay

vii. Silica gel breather

viii. Bushings (either of porcelain, oil-filled or capacitor type).

Magnetic Core

In all types of transformers, the core is constructed of transformer sheet steel laminations assembled to provide a continuous magnetic path with a minimum of air- gap included, the steel used is of high silicon content, sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities.

Eddy current loss is minimized by laminating the core the laminations being insulated from each other by a high coat of core-plate varnish or by an oxide layer on the surface. The thickness of laminations varies from 0.35 mm for a frequency of 50 Hz to 0.5 mm for a frequency of 25 Hz. The joints in the alternate layers are stragged in order to avoid the presence of narrow gaps right through the cross-section of the core. Such staggered joints are said to be 'imbricated'.

Constructionally, the transformers are of two types, differentiated from each other by the manner in which the primary and secondary coils are placed around the laminated core. They are:

i. Core Type

ii. Shell Type.

In Core Type transformers winding surround the core or shell. (i.e) it is wounded considerable part of the core.

In Shell Type transformers the core (or) shell surrounds a considerable portion of the windings.

Another recent development is spiral-core or wound-core type. All transformers are shown in Figure 1.4.

Windings

There are two windings in a transformer. They are called primary and secondary windings. Windings are made of copper shown in Figure 1.5.

Insulation for Windings

Paper is used as insulating material widely. Enamel insulation is used as the inter-turn insulation of low voltage transformers. For power transformers enameled copper with paper insulation is also used. Some of the insulation materials are shown in Figure 1.5 and 1.6. Example: paper, oil, varnish, oxides.

Conservators or Expansion Tank

A small auxiliary oil tank may be mounted above the transformer and connected to main tank by a pipe. Main function of the conservator is to maintain oil level in the transformer even though the coil may expands or contracts with the change in temperature. A pipe connection between the gas space in the expansion tank, and the cover of the transformer to pass into the expansion tank, so that the transformer tank will be completely filled with oil. Conservator/Expansion tank is shown in Figure 1.6.

Cooling Arrangements, Temperature Gauge, Oil Gauge

The various methods of cooling employed in a transformer are:

(i) Oil Immersed Natural Cooled Transformers: The coil and core are immersed in insulating oil contained in an iron tank. The heat produced in the shell and windings is transferred or removed by the circulation of oil to the surface which dissipates heat to surroundings. If the size of the transformer is big then No. of tubes used were increased. It is not only for keeping the windings cool but also provides additional insulation some of the cooling systems are shown in Figure 1.7.

(ii) Oil Immersed Forced Air Cooled Transformer: This type also core and windings are immersed in oil and cooling is increased by forced air over the cooling surfaces like tank, tubes and radiators by means of far mounted externally to the transformer.

(iii) Oil Immersed Water Cooled Transformer: Same as previous cooling system, additionally cold water will circulate through the tubes immersed in oil. It will collects the heat from the oil.

(iv) Oil Immersed Forced Oil Cooled Transformer: As name indicate core and windings are immersed in oil and oil is forced to flow through the tubes with the help of centrifugal pump located at the inlet/outlet.

(v) Air Blast Cooling: Here the transformers is cooled by a forced circulation of air through core and windings. It is used in substations located in dusty place where oil is considered as a fire hazard. Filters are used to avoid the entry of dust into the ducts.

Temperature gauge is used to indicate the oil temperature which is shown in Figure 1.7 and hottest spot temperature. It is also having alarm unit which is used alert the worker, if the temperature violates the limit.

Likewise, oil gauge is used to indicate the oil level of the transformer. Alarm circuit gives an alarm when the oil level has dropped beyond permissible height due to oil leak or due to any other reason.

Buchholz Relay

Buchholz Relay in transformer is an oil container housed the connecting pipe from main tank to conservator tank. It has mainly two elements. The upper element consist of a float. The float is attached to a hinge in such a way that it can move up and down depending on the oil level. One mercury switch is fixed on the float. The alignment of mercury switch hence depends upon the position of the float.

The lower element consist of a baffle plate and mercury switch. This plate is fitted on a hinge first in front of the inlet (main tank side) of Buchholz relay in transformer in such a way that when oil enters in the relay from that inlet in high pressure the alignment of the baffle plate along with the mercury switch attached to it, will change.

Working:

It is based on mechanical phenomenons and is mechanically actuated shown in Figure 1.8. Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulation oil will be decomposed in different hydrocabron gases, Co₂ and Co. The gas produced due to decomposition of transformer insulating oil will accumulate in the upper part, and the Buchholz container which causes fall of oil level in it.

When transformer is with load, the temperature of the transformer insulating oil increases, consequently the volume of the oil is increased. As the volume of the oil is increased, the air above the oil level in conservator will come out. Again at low oil temperature, the volume of the oil is then decreased, which causes the volume of the oil to be decreased which again causes air to entire into conservator tank.

Air always consist of moisture and it is mixed up with oil if it is allowed to enter into the transformer. Moisture should be resisted or prevented during the entry of air into the transformer, because moisture is harmful for transformer insulation. A silica gel breather is the most commonly used way of filtering moisture from air.

Silical gel crystal has tremendous capacity of absorbing moisture. When air passes through these crystals in the breather and moisture is absorbed by them.

Bushing is an insulated device that allows an electrical conductor to pass safely through an (usually) earthed conducting barrier wall of a transformer. This bushing is made up of porcelain material with capacitors placed around the wire.