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KERAL ELECTRICALS AND ALLIED

ENGINEEING CO. LTD

TRAINING PERIOD : 25-08-2012

PLACE : KOCHIN, KERALA

Kerala Electricals is fully owned by the Government of Kerala. This company has four 

Manufacturing units and the corporate office is in Panampilly Nagar, Kochi. The company is

manufacturing Hi-tech industrial electrical products, viz. Brushless Alternators, Transformers,

HRC Fuses etc. The unit that we visited is Mamala Unit, where transformers are manufactured.

TRANSFORMER 

A transformer is a device that transfers electrical energy from one circuit to another 

through inductively coupled conductors — the transformer's coils. A varying current in the first

or primary winding creates a varying magnetic flux in the transformer's core and thus avarying magnetic field through the secondary winding. This varying magnetic field induces a

varying electromotive force (EMF), or  "voltage", in the secondary winding. This effect is

called mutual induction.

TYPES OF TRANSFORMERS

Step up transformer

The secondary voltage is higher than the primary voltage is known as step up transformer 

Step down transformer

The secondary voltage is lower than the primary voltage is known as step down

transformer 

Auto Transformer

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An autotransformer is an electrical transformer with only one winding. The auto prefix

refers to the single coil acting on itself rather than any automatic mechanism. In an

autotransformer portions of the same winding act as both the primary and secondary. The

winding has at least three taps where electrical connections are made. An autotransformer can be

smaller, lighter and cheaper than a standard dual-winding transformer however the

autotransformer does not provide electrical isolation. Autotransformers are often used to step up

or down between voltages in the 110-117-120 volt range and voltages in the 220-230-240 volt

range, e.g., to output either 110 or 120V (with taps) from 230V input, allowing equipment from a

100 or 120V region to be used in a 230V region.

PARTS OF TRANSFORMER 

1.  Transformer tank 

2.  Breather 

3.  Conservator 

4.  Explosion vent

5.  Transformer oil

6.  Buccholz relay

7.  Method of cooling transformer 

a)  Cooling Methods by Dry Type Transformers

 b)  Cooling Methods for Oil Immersed Transformers

c)  Oil Forced Methods with Heat Exchangers

Transformer Tank 

Transformer tanks are tanks used for housing the active part of the transformer which is

immersed in oil tanks are constructions made of welded thin steel sheets basically; there are three

types of transformer tanks

Tank and tank cover jointed with bolted connection: where the tank cover is a flat

 plate reinforced with ribs, or where the tank and the tank cover are welded constructions. This

type is adjusted for road transportation on special vehicles or for rail road transportation when it

it placed on a wagon.

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Bell-type tank: has a construction reverse to the version under above the cover is

designed as a bell and the lower part is either a flat plate or a shallow vessel joined with the

cover either with welded or bolted connection. This type is also designed for larger transformer 

 because it enables opening of a transformer using a crane with a lower load-bearing capacity

Tank for railroad transportation this version of tank is generally used for larger 

transformers where dimensions and mass represent a limiting factor for road transportation. This

type of tank requires special reinforcements such as suspensions on the train.

Breather

Breathing is the process where the transformer Breather-In or Breather-out the air from its body

due to thermal contraction & expansion of oil mass. When the transformer is loaded or unloaded,

the oil temperature inside the transformer tank rises or falls accordingly the air pressure inside

the tank changes by either breathing in or breathing out the air. This phenomenon is called

"Breathing" of the transformer. Now, the air which is being breathed in contains either dust

 particles and/or humidity that change the dielectric strength of the oil. For proper functioning of 

transformer, it is absolutely necessary that dielectric strength of transformer oil remains

unimpaired. Hence, it is necessary that, the air entering into the transformer is free from moisture

& dust particles.

Function of Silica Gel Breather: 

The sole function of Silica Gel Breather is to dehydrate (remove moisture) the air & to

remove dust particles of the air breathed in by the transformer.

Operation & Working of Silica Gel Breather :

Silica Gel (Air dehydrating) breathers are transparent hollow cylinder tubes which

contain chemically pure silicium salt (Silica Gel) with cobalt indicator. This grade of Silica Gel

is called as ―indicating grade‖ of Silica Gel. 

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The indicating grade of Silica Gel, which is filled in the breather, is hard blue crystals, which

has considerable absorption power for moisture. Silica Gel absorbs moisture signaling the

saturation degree by changing color as follows. 

  Deep Blue Silica Gel completely dry

  Light Blue Silica Gel partly humid. (Absorbed water for about 15% of its weight)

  Pink Silica Gel saturated with moisture. (Absorbed water for about 30 - 40% of its

weight)

For proper functioning of transformer dehumidification of Silica Gel crystals & removal of 

dust particles from breathed air is necessary.

Moisture is removed from Silica Gel crystals by heating it inside a ventilated oven at to 150

C, until the color becomes deep blue again.

Dusts particles are filtered by the oil in the oil cup. Pressure value for air passage into the

 breather are: 0.003 Kg/cm inlet or 0.005 Kg/cm outlet.

CRITERIA FOR SELECTING SILICA GEL BREATHER  

Keeping in view the above functions of Silica Gel Breather the main criteria for selecting the

 breather becomes,

  Gel changes color from deep blue to pinkish white as it absorbs moisture. By reheating,

the Gel becomes free from moisture & hence transformer always breathes in dry air.

Visibility of Gel color is very important to decide when to reheat the Gel.

  Oil in the oil cup allows dust particles of air to settle in the oil. By changing the oil &

cleaning the oil cup air becomes free from dust particles. Visibility of oil level & dust

 particle in the oil cup is very important to decide when to change the oil.

  Transformer should breathe from breathing holes (provided in breather) only. If air enters

through any other portion of the breather except breathing holes it is called leakage of air.

  Leakage of air will lead to entrance of air with moisture & particles in the transformer,

which ultimately may lead to formation of spark, and short circuit in the transformer.

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Regarding the diaphragm vent, we should be very particular about its thickness and

material. Some small transformer manufacturers not possessing adequate mechanical knowledge

cannot ascertain the right kind of diaphragm materials.

In some cases, I have the experience of witnessing 3mm thick Perspex sheet being usedas the diaphragm. A few even use 6mm cork sheet as vent material. The correct material for 

diaphragm for a power transformer is 1/128 inch (0.02mm) thick Hylum sheet, which almost

 breaks at a pressure of 1 kg/sq.cm.

Other materials of equivalent property may also be used as vent materials. Manufacturers

as well as buyers, during their customer’s inspection, must ensure the use of the right kind of 

diaphragm material along with its bursting pressure. The check may be done by applying dry air 

 pressure on a complete transformer filled with oil up the required levels. The diaphragm must

 burst at a pre-determined pressure.

Transformer Oil

For both the designer and the user of an oil-filled transformer it can of value to have

some understand if the composition and the properties of the transformer oil and an appreciation

of the ways in which these enable it to perform its dual functions of providing cooling and

insulation within the transformer.

Such an understanding can greatly assist in obtaining optimum performance from the

transformer throughout its operating life.

That is the main purpose of this section. To increase awareness of the role of insulating

oil, which can often be taken somewhat for granted and to help those having dealings with oil-

filled transformers to recognise the important part which the oil plays in the achievement of 

satisfactory operation.

Bucholz Relay

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Power Transformers are considered to be a highly reliable type of equipment, yet, in

order to ensure the continuity of service that modern conditions demand, protective devices are

required. The purpose of such devices is to disconnect faulty apparatus before large-scale

damage is caused by a fault to the apparatus or to other connected apparatus. Such devices

generally respond to a change in the current or pressure arising from the faults and are used for 

either signaling or tripping the circuits.

Protective devices in the ideal case must be sensitive to all faults, simple in operation,

robust for service and economically feasible. Considering liquid immersed transformers, a near-

ideal 'protective device' is available in the form of Gas and Oil relay described here. The relay

operates on the well-known fact that almost every type of electric fault in a 'liquid immersed

transformer' gives rise to gas. This gas is collected in the body of the relay and is used in some

way or other to cause the alarm or the tripping circuit to operate.

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WORKING

The function of a double element relay will be described here. During normal operation

of a transformer the Buchholz relay is completely filled with oil. Buoyancy and t he moment due

to counterweights keep the floats in their original top positions. In the event of some fault in theinterior of the transformer tank, gas bubbles are produced which accumulate in the Buchholz

relay on the way to the conservator. In consequence, the oil level in the relay enclosure drops

which in turn lowers the upper bucket.

This causes the mercury switch to operate an alarm signal. The lower bucket does not

change its position, because when the gas reaches the upper inside wall of the pipe it can escape

into the conservator. Hence, minor fault in the transformer tank will not trigger the lower 

switching assembly and will not trip the transformer. In case the liquid continues to drop due to

loss of oil, the lower bucket also goes down. In consequence, the lower switching system

operates if the level of oil goes below the bottom level of the pipe connected to the relay.

Alternately in the event the liquid flow exceeds a specific value (which is continuously

adjustable, by means of a flap) the lower bucket is forced down, thus triggering the lower 

switching system to operate.

As the liquid flow rate decreases, or the level of the liquid rises, the bucket returns to its

original position. The single element relay has only Trip element and it responds to only oil

METHOD OF COOLING TRANSFORMER 

When transformer supplies a load, two types of losses occur inside the transformer. The

iron losses occur in the core while copper losses occur in the windings. The power lost due to

these losses appears in the form of heat. This heat increases the temperature of the transformer. A

suitable coolant and cooling method is necessary for each transformer to dissipate the heat,

effectively to the surroundings. Basically there are two types of transformers, dry type

transformers and oil immersed transformers. In dry type, the heat is taken to the walls of tank 

and dissipates to the surrounding air through convection. In oil immersed type, the oil is used as

coolant. The entire assembly including core and windings is kept immersed in suitable oil. The

heat developed is transferred to the walls of tank by convection through oil. And finally heat is

transferred to the surroundings from the tank walls by radiation.

The various cooling methods are designated using letter symbols which depend upon:

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1.  Cooling medium used

2.  Type of circulation employed

The various coolants used along with their symbols are,

1.  Air  – A

2.  Gas - G,

3.  Synthetic oil – L

4.  Mineral oil - O,

5.  Solid insulation – S

6.  6. Water  – W

There are two types of circulations which are,

Neutral - N

In natural cooling, the coolant circulating inside the transformer transfers entire heat to

the tank walls from where it is dissipated to the surroundings and transformers gets cooled by

natural air circulating surrounding the tank walls.

Forced – F

In forced cooling, the coolant circulating inside the transformer gets heated as it comes in

contact with windings and core. The coolant partly transfers heat to the tank walls but mainly

coolant is taken to the external heat exchanger where air or water is used in order to dissipate

heat of the coolant.

COOLING METHODS BY DRY TYPE TRANSFORMERS

The cooling methods of dry type transformers are classified as,

Air Natural (An)

This method uses atmospheric air as cooling medium. The natural air surrounding the

tank walls is used to carry away the heat generated, by natural convection.

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It is used for small voltage transformers. Due to the available insulating materials like

glass and silicon resins nowadays, the method can be used for the transformers up to ratings 1.5

MVA.

Air Blast (AB)

In large transformers, cooling by natural air is inadequate. In such cases, the transformer 

is located above the air chamber and a blast of compressed air is forced on core and windings

with the help of blowers or fans.

This improves the heat dissipation and hence higher specific loadings are allowed in dry

type transformers. This reduced the size of transformers. The air supply must be property filtered

to prevent accumulation of dust particles.

COOLING METHODS FOR OIL IMMERSED TRANSFORMERS

The oil used as a coolant has following advantages,

1.  It is good conductor of heat than air.

2.  It has high coefficient of volume expansion. Due to this, adequate circulation

is easily obtained.

3.  The oil acts as an insulating medium, which increases the insulating strength.

The only limitation of oil immersed transformers is that these transformers cannot be used at

 places like mines where there are chances of fire hazard.

The various cooling methods used for such oil immersed transformers are classified as,

OIL NATURAL (ON)

The transformer is immersed in oil so heat generated in core and windings is passed on to

oil by conduction. The heated oil transfer heat to the tank wall from where it is taken away to the

surrounding air. The assembly of oil immersed transformer is shown.

The tubes are provided on the sides of a transformer tank. The oil in the tank is taken to

the tubes. The circulation of oil through tubes causes the cooling.

The temperature rise of a transformer can be reduced by,

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1. Increasing the area of heat dissipation.

2. Decreasing the cooling coefficient.

As the rating of transformer increases the plain walled tank cannot be used. It is

necessary to reduce the cooling coefficient. This is achieved by use of some improved methods

of cooling.

Fig. Oil immersed transformer 

The transformers upto 30 KVA use plain walled tanks. But transformers with ratings

higher than 30 KVA use corrugations, fins, tubes and radiator tanks. The Fig. 2 shows the fins

and corrugations provided on tank walls.

Fig. Tanks with fins and corrugations

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Fig. Tanks with tubes and radiators

The heat developed inside the transformer is taken outside with the help of oil. The oil is

cooled with the help of fins, tubes or external radiations by natural circulation of air.

Hence these methods are called Oil Natural and Air Natural (ONAN) methods. The tubes are

used for transformers up to ratings 5 MVA.

Oil Natural Air Forced (ONAF)

In this method, the tank is made hollow and compressed air is blown into the hollow

space to cool the transformer. The oil circulating inside takes heat to the tank walls. The method

is effective and can be used for large rating transformers. Another way to force air blast is to use

elliptical tubes separated from tank walls through which air is forced by fans.

Oil Natural Water Forced (ONWF)

In this method, the copper cooling coils or pipes are fitted above the core but below the

oil surface. The cool water is forced through these coils or pipes which provides the additional

cooling where natural water head is available, this method is very cheap.

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The pipes are provided with fans to increase conduction of heat from oil to pipes. The

major disadvantage of this method is, in case of leakage of water. The water can contaminate the

oil reducing the dielectric strength of oil.

Oil Forced Methods With Heat Exchangers 

In this method, forced circulation oil (OF) is the main feature. The motor driven pump is

used to force the oil from top of transformer to the external heat exchanger.

In the heat exchanger, the oil is cooled with some methods like use of air blast, water 

 blast etc. The cold oil is circulated back to the transformer from the bottom.

The oil forced methods are classified depending on how the oil is cooled in the heat

exchangers. These methods are,

Oil Forced Air Natural (Ofan) 

The oil is circulated with the help of pump and in the heat exchanger it is cooled with the

help of natural air. This method is rarely used in practice.

Oil Forced Air Forced (Ofaf) 

In the external heat exchanger the compressed air is blasted with the help of fans to cool

is the oil. The advantage of this method is at low loads when losses are less there is no need to

use the fans to cool the oil.

The natural air is sufficient. At higher loads, both fans and pump are switched on by

sensing the temperature which improves the cooling. Hence efficiency of this system is higher.

The scheme is shown

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Fig. Oil forced air forced cooling method

Oil Forced Water Forced (Ofwf)

In this method, in the heat exchanger instead of air blast, water blast is used to cool the

oil. The pressure oil is kept higher than water so oil mixes with water in case of leakage but

water dose not mix with oil. Due to this method, smaller transformer size is sufficient as it is not

necessary to employ water tubes inside the transformer tank. The method is suitable for 

transformers having ratings more than 30 MVA. The method is used for the transformers at

hydroelectric stations as large water supply with appropriate water head is easily available. The

scheme is shown

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Fig.Oil forced water forced

Fig. Oil Cooled

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DESINGNING DEPARTMENT 

Designing indicates positioning of the components inside the canopy, which taken as an

instruction & carried throughout.

Design varies only in length, breadth and thickness & differs in size, based upon the

capacity of the generators.

Canopy is designed using the software such as,

  CAD

  Hi-CAD

  AUTOCAD

  RADAN

Canopy (enclosure) enclosing the generators and acting as proofing material (acoustic).

Types of designing:

  Air Cooled Type

  Oil Cooled Type

Air Cooled Type:

  Air coolant - to absorb the high temperature.

  Foam – reducing the sound to 75 db.

Oil Cooled Type:

  Radiator  – absorb the high pressure and temperature.

  Foam (Rockwool) – reducing the sound to 75 db by absorbing it.

PRESS SHOP 

Processes undergone in press shop are,

1.  Shearing:

   NC cut the sheet metal into required size length & thickness.Shearing machine:

  Machine Ref: SBG/PSMO6

  Model-GS=3106

  Machine-NC Shearing Machine 

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  Cutting capacity – Maximum thickness 6 mm. 

2.  Punching:

   Numeric computerized shearing machines (CNS Machine) punch the holes.

3.  Slitting:

  Removing the unwanted pieces from the sheet metal i.e. notching.

4.  Press Brake:

  Sheet metal is bended at the required place.

Types of sheet metal:

  CRCA ( cold role )

  HRCA ( hot role )

Types of Punching machines: 

  Laser Punch

  Turret punch

  Accute

  ASR 

Types of cutting:

  Standard cutting

  Economic cutting

  Wax cutting

Process:

Design of canopy is fed to the CNS machine. Based upon the instruction, machine starts

its work.

FABRICATION DEPARTMENT

Types of machines used in Fabrication:

  Grinding machine

  Drilling Machine

  Chop Saw Cutting Machine.

  Jigsaw Machine.

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Welding:

Types of welding used:

1.  MIG Welding

  Metal Inert gas welding (MIG).

  Inert gas – CO2 gas.

  Pressure – 5 Bar.

  MS coil of 0.8 mm.

2.  TIG welding

  Tungsten inert gas welding (TIG)

  Metal of thickness 5mm – Tig welding.

3.  Spot welding

  Used for non-similar metals.

4.  Gas welding

  Used for unsimilar metals.

  Oxygen + acetylene gases composition – gas welding.

  Instead of MS coil here electrode is used.

  Less strengthen than MIG welding.

  Two types of uses:

  Welding (Acetylene gas)

  Cutting (Oxygen + LPG)

Grinding Machine

  4‖sanding Machine

  AG7-Grinding Machine

Drilling Machine

  Hand drilling machine

  Sensible drill