Transformer is a static magnetically coupled device. It works on the principle of faraday law of electromagnetic induction. In this article, we will only discuss the various parts of transformer. If you want to read about the working of transformer, kindly read the following article:
Various parts of transformer is as follows:
Core
Windings
Transformer Oil
Conservator
Breather
Buchholz Relay
Cooling Tubes
Tap Changer
Explosion Vent/ Valve
Core:
The transfer of power from primary to secondary winding occurs through common magnetic circuit known as core of transformer. Transformer core material has high permeability and it provides easy flow of magnetic flux from primary to secondary side. It is made up of very thin silicon steel laminations or soft laminated iron material in order to reduce eddy current and hysteresis losses of transformer. Thin laminated sheets are insulated from each other by thin layer of varnish. The horizontal (top and bottom) part of the core is called yoke while the perpendicular part is known as limb of the transformer. Each of the limb carries the primary and secondary winding of each phase.
Windings :
Transformer windings are made of conducting material. They generally consists of several turns of copper or rarely aluminum coils wound on common magnetic iron core. Winding turns are isolated from each other.
Primary Winding:
Winding connected with the AC input supply is called primary winding of transformer.
Secondary Winding:
Winding to which an electrical load is connected and output voltage is drawn is called secondary windings of transformer.
Transformer Oil:
Core and windings of transformer is completely immersed in the oil. When the temperature of winding increases during loaded condition, oil act as a coolant and reduce the temperature. It also act like an insulator between the transformer windings. Moreover, transformer oil works as a fault detector indicator for the buchholz relay. Buchholz relay senses the fault from the oil level present inside the transformer.
Conservator :
Conservator is present on the top of transformer tank and conserves the transformer oil. Oil filled in the conservator is almost half of the available space. The vacant space is available for the expansion of oil during the increase in oil temperature. Conservator is connected with the main tank through a pipe line. Main tank is completely filled with transformer oil. When the load on transformer increases, windings of the transformer gets heated due to the increase of current. Moreover, temperature of the oil present in the main tank also heated and expands. Conservator provides the available vacant space for the expansion of oil. Hence heated oil of the main tank expands in the conservator tank which is fitted above the transformer.
Breather:
Conservator tank provides adequate empty space for the expanded oil. Once the vacant space is occupied by the expanded oil, the air present in that space flows from the conservator tank to the breather (can be seen in the above fig.) during that period. When the oil inside the transformer contracts during the low temperature, the outside air again enters in the conservator tank through breather.
Lets come to the breather. Breather is a cylindrical container which is present in all oil immersed transformers that have conservator tank. In the above fig, you can see that a bent pipe is attached with the the conservator. A breather is attached at the end of that bend pipe in such a way that the atmospheric air enter and exits the conservator tank through it. The problem is that the air coming from the atmosphere contains dust particles and humidity which changes the dielectric strength of transformer oil. Hence it is necessary that the air enters in the conservator tank should be free from moisture and dust particles.
Breather contains silica gel crystals in it which absorbs the moisture from the atmospheric air. However, one issue with the silica gel crystals is that it doesn’t remove the dust particles from the air. In order to remove the dust particles from the air, a small oil box is placed below the breather cylinder(you can see in the above fig). Silica gel crystals absorbs the moisture content whereas Oil removes the dust particles from the air. In this way, the air enter in the conservator tank is free from moisture and dust particles. Hence breather acts as a medium for providing moisture free air to the conservator tank.
Buchholz Relay :
Buchholz relay is a protective device which is used to detect the internal faults in oil-filled transformers. It is installed in the pipeline that runs from the main tank to the conservator tank. When incipient (internal) fault occurs, heat produce during the fault causes the decomposition of transformer oil. Gases emitted during the decomposition of oil escape towards the conservator and during that process gets entrapped in the relay chamber. When predetermined amount of gases accumulated in the upper part of relay chamber, it causes the oil level to fall. Decrease of oil level causes the float to sink and close the contacts of mercury switch to generate an alarm. In this way, Buchholz relay senses the fault and protects the transformer from internal faults. It must be noted that decrease of oil level doesn’t means immediate tripping so these contacts generate an alarm only.
Explosion Vent/ Valve :
If sudden excessive pressure of oil is developed in the main tank during heavy internal faults, it may result in the explosion of transformer. In order to avoid the explosion of transformer, Explosion Vent is provided to expel the boiling oil during heavy internal faults. It is generally installed above the level of conservator tank. Explosion vent is a metallic pipe provided with a aluminum diaphragm fitted at the exposed end of the vent. When excessive pressure of oil is developed in the conservator tank, diaphragm will rupture and oil in the main tank escapes through the vent. In this way, explosion vent protects the transformer tank during the sudden build of oil pressure.
Cooling Tubes:
Cooling tubes (radiator) are used for cooling the transformer oil. When the temperature of oil increases and gets heated, oil in the main tank naturally moves to the upper portion of tank and enters into the upper valve of cooling tubes. Hot oil naturally circulates through the cooling tubes(radiator) and dissipates the generated heat. Once the oil gets cooled, it again enters the main tank through the lower valve of cooling tubes. It should be noted that when heated oil moves to the upper portion of tank, cold oil takes its vacant place at the downward. In this way, convectional flow of oil continues due to which the oil kept at a cool temperature in the tank. For large heavy transformers, fans are also being inserted in the outer walls of cooling tubes for fast cooling.
Tap Changer:
When the load on transformer increases, the output voltage decreases. Similarly, the voltage on the output terminal increases during light loaded or no loaded conditions. In order to regulate the output voltage of transformer, tap changers are used. Tapings are always installed on the high voltage side. Tap changer select the tapings and alters the number of turns in one winding due to which the turns ratio of transformer is changed. When the number of turns in a transformer varies, the output voltage of transformer is changed. Let understand by this way. Turn ratio of transformer is termed as
Turn Ratio=
Or, we can write as:
Since V1 is the initial supply voltage and it is constant. Moreover, the tap changer is installed on the primary side, so N2 is also constant. Hence the output voltage V2 is regulated by changing the N1 turns.
From the above fig, we can analyze that tap changer select any of the tapping point to get the required turn ratio. If the primary turns of transformer is decreased, the voltage at the secondary side is increased. Similarly, the output voltage V2 can be decreased by increasing the number of primary turns N1. In this way, the desired output voltage can be achieved.
It should be noted that there are two types of tap changers. An on load tap changer(OLTC) and de- energized tap changer(DETC). In case of on load tap changer, tapings can be changed without isolating the transformer from the supply. On the other hand, de-energized tap changer can only change the tapings when the transformer is de-energized.
Terminals and Bushings:
Transformer takes and provides the supply through terminals. Terminals are taken out from the main tank through bushings. Transformer bushings are insulators that provides an insulation between the current carrying line conductor and the transformer tank. They are mounted on the top of transformer main tank. High voltage (HV) bushings and low voltage(LV) bushings are used as per the voltage ratings.
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