Three Phase Transformer Construction
The most important parts used in the three phase transformer construction are windings and core. Some other parts such as a tank, conservator, bushing, breather etc. are also used along with the core and windings. Simplified construction of a three phase transformer is shown in the figure.
Parts Used in Three Phase Transformer Construction
Laminated Steel Core
The material used for the construction of three phase transformer core is silicon steel. It is used for its high permeability, low magnetic reluctance and low hysteresis loss.
The core is used in form of stacks of laminated thin steel sheets which are electrically insulated from each other by a thin coat of varnish. The core is laminated in order to reduce the eddy current loss. The laminations are typically 0.35 to 0.5 mm thick.
The cylindrical coils (windings) are placed in a concentric way around the same limb with the low voltage winding placed inside and high voltage winding is placed outside.
The low voltage winding is placed close to the core because it is easy to insulate a low winding from the core. Both the windings are properly insulated from each other and the core. In the construction of three phase transformer, three high voltage and three low voltage windings are used.
A mineral oil obtained by fractional distillation of crude petroleum, known as transformer oil is used in the transformers. This oil carries the heat produced in the transformer by convection from the windings and the core to the transformer tank. This oil also improves and maintains the insulation of transformer windings.
A great disadvantage of the mineral oil is its susceptibility to catch and propagate fire. To overcome this disadvantage, a group of synthetic liquids known as Askarales has been developed which is completely fire-proof.
The Askarales offer greater resistance to acid and sludge formation than the mineral oil and, therefore, the life of an Askarel-filled transformer is much longer. But Askarales should never be mixed with mineral oil. Also, the two cannot be interchanged in a transformer.
It is a small cylindrical tank mounted on top of oil-cooled transformers and connected to the main tank by a small pipe. It performs the following functions:
It provides space for expansion and contraction of oil which expands on heating and contracts on cooling.
It minimizes the oxidation of oil by limiting the surface area of oil in contact with air.
It confines the oxidized oil to conservator only.
The conservator is equipped with a sight-glass at one end to indicate the oil level in the transformer. The level of cold oil should always be maintained up to the gauge mark on this glass.
It is a device through which all movement of air from and into the transformer takes place. Air is expelled when the oil expands due to the heating of transformer and is drawn in on cooling of oil. It is installed at one end of the conservator.
The breather is filled with some dehydrator (generally silica gel), which absorbs moisture from the air passing through it so that only dry air can enter the tank. Silica gel is blue in color when dry and turns pink when it gets moisturized. It can be reconditioned by heating in a pan.
A thermometer is installed on all transformers of large rating to indicate the temperature of hot oil. In very large transformers, temperature indicating devices are installed in the winding itself to record the winding temperature. A temperature sensitive alarm system is also installed to sound the alarm whenever there is dangerous temperature rise.
Explosion Vent Pipe
It is a bent pipe with a glass cover at the end. It is installed on top of transformer tank. It provides protection against excessive pressure build up inside a transformer. The transformer can explode in the absence of this device.
A number of valves are provided on each transformer for filling and draining the transformer oil.
Buchholz relay is a gas relay. It sounds the alarm whenever any fault develops inside the transformer and if the fault is dangerous the relay disconnects the transformer circuit. This relay is fitted in between the main tank and the oil conservator on the transformers of large capacity.
Thanks for reading about three phase transformer construction.
Transformer — 3 | Objective Type Question Answers
#1 In a transformer the tappings are generally provided on
low voltage side
#2 The function of conservator in a transformer is
take care of the expansion and contraction of transformer oil due to variation of temperature of surroundings.
#3 A Buchholz relay call be installed on
oil cooled transformers
#4 Gas Is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds…………… degree Celsius.
#5 The main reason for generation harmonics in a transformer could be
saturation of core
#6 Distribution transformers are generally designed for maximum efficiency around
#7 Star/star transformers work satisfactorily when
load is balanced only
#8 Delta/star transformer works satisfactorily when
on balanced as well as unbalanced load
#9 Buchholz’s relay gives warning and protection against
electrical fault inside the transformer itself
#10 The magnetizing current of a transformer is usually small because it has
small air gap
#11 The path of the magnetic flux in transformer should have
#12 Noise level test in a transformer is a
#13 Which of the following is not a routine test on transformers ?
Radio interference test
#14 A transformer can have zero voltage regulation at
leading power factor
#15 Helical coils can be used on
low voltage side of high kVA transformers
#16 Harmonics in transformer result in
all of the above.
#17 The core used in high frequency transformer is usually
#18 value of flux involved in the E.M.F. equation of a transformer
#19 Silicon steel used in laminations mainly reduces
#20 Which winding of the transformer has less cross-sectional area ?
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- Ideal Transformer
- Construction of Three Phase Transformer
- Types of Transformers
- Equivalent Resistance and Reactance of Transformer
- Equivalent Circuit of Single Phase Transformer
- Power Loss in a Transformer
- Open Circuit Test of Single Phase Transformer
- Short Circuit Test on Single Phase Transformer
- Transformer Efficiency
- Regulation of Transformer
- Instrument Transformers
- Polarity of Transformer Windings
- Significance of Vector Group of Transformer
- Buchholz Relay Construction | Working
- Why current transformer secondary should not be opened