It is static device used to step up and step down of voltage level without change in frequency and power. Transformer works on the mutual induction principle. This principle states that when electric current flows in primary winding supply then due to alternating nature of supply voltage, it develops varying nature of flux and whenever varying mutual flux linked to a secondary winding through core then according to faraday law, whenever either conductor cuts a magnetic flux or magnetic flux cuts a conductor, EMF produced in conductor, hence in this way, EMF developed on secondary winding.
The basic phenomenon behind working of transformer is mutual induction between two winding linked by a common magnetic flux. Transformer consist of two inductive coils, LV and HV winding. These windings are electrically separated but magnetically linked to each other. When LV or HV winding are supplied of ac supply then alternating magnetic flux developed in the winding and it get linked to load side by a common path that phenomenon is called mutual induction. Due to this, alternating EMF is produced in load side whenever circuit will be closed the current flows in load side.
- Types of Transformer
According to change in level of voltage, there are two type of transformer –
1.1. Step up transformer
1.2. Step down transformer
- Step up transformer
It is used to step up the level of voltage. This transformer increases voltage from primary to secondary winding. In this transformer, secondary turns are more than primary turns. This transformer has made long distance transmission power practical because it steps up the voltage and corresponding it decrease current at same ratio hence, due to decreases the current level there is much less power losses in line. This is used generally for long distribution.
- Step down transformer
This transformer decreases level of voltage from primary to secondary. In this transformer, secondary turns are less than primary turns. It is used mainly in distribution and load side. This transformer has a wide variety of application like distribution side and electronic devices etc. When it comes to operating voltage, the step up transformer application can be roughly divided into two groups: LV (voltages up to 1kv) and HV (voltages above 1kv).
- Construction of transformer
The construction of transformer is of iron core laminated with steel bands core lamination are constructed from insulated metal thin metal strips. These laminations are separated and wound around the limp using a sheet of coat. The laminations are separated and wound around the limb using a sheet of coat or parchment. The winding consists of two types, main and secondary winding. These winding are isolated from each other and are made by electrical coil.
The core of transformer is typically built with high permeability materials, such as silicon steel laminations. Based upon configuration of main and secondary windings, a transformer core may be formed in two ways.
- Core type construction
In core type construction, as shown in fig, the coils are wound around two limb of rectangular magnetic core. Each limb carries one half of primary winding and one half of secondary winding so as to reduce leakage reactance to minimum possible. The LV is wound on inside nearer to core while the HV winding wound on inside nearer to core while HV winding is wound over LV winding away from the core in order to reduce amount of insulation material required.
- Shell type construction
In shell type construction, the coils are wound on central limb of a three limb core. The entire flux passes through central limb and divides in two parts going to side limbs shown in fig. Sandwich type winding is used in such a construction. This type of construction is popular in low voltage application like transformer used in electronic devices, power electronics converters.
Fig: Shell and core type construction
A cylindrical tank plays a key role in transformer. It is arranged over the main tank roof so that sufficient space can be provided to expand to transformer oil. Once the temperature increases, then oil volume can also be increases, then oil volume can also be increased. Then oil goes to conservator tank after cooling down it returns to main tank. The shape of conservator tank in transformer is cylindrical where both ends of oil container are closed. One side of the container is provided for cleaning purpose.
Fig: Conservator tank
As a temperature of transformer increases, the insulating oil in transformer heats up. When oil heats up and spreads, the transformer breathes air in and oil is cooled down and oil level is consumed. The oil level in chamber rises and reduces as breather brings the air in and out to cool the air. The air carries moisture, which contaminates oil and thus deteriorates the consistency of oil. The breather is packed with silica gel to remove moisture content. The key feature of silica gel is to isolate moisture from oil while preserving the consistency of insulating oil. The color of silica gel becomes pink as it absorbs moisture from oil
The basic function of radiator is to cool down the transformer oil. Oil immersed power transformer is generally provided with detachable pressed sheet radiator with isolating valves. But in case of small size distributing transformer, the radiators are generally integrated parts of transformer body and projected from main tank. The working principle of radiator is very simple. It just increases the surface area for dissipating heat of oil. Under loaded condition, warm oil increases in volume and enter into upper portion of main tank. Then oil enters in radiator through top valve and cools down by dissipating heat through thin radiator wall. This cold oil comes back to main tank through bottom radiator valve.
Bushings are the insulation system in construction of transformer that enables an electrical conductor to safely transfer electrical energy through it. When a significant volume of electrical energy travel through it, it provides electrical field power to withstand insulation of conductor. In small transformer, solid porcelain type bushing is used, and in large transformer, oil filled condense type bushing is used.
- Buchholz relay
In order to protect of transformer from internal short circuit due to oil, buchholz relay is used for oil immersed transformer, buchholz relay is an oil and gas actuated relay which sense the fault occurring in the part immersed in transformer. Whenever short circuit occurs in transformer then oil generates enough heat and become decompose itself into hydrogen and monoxide gases and these gases travel through pipe and relay is also mounted on pipe between main tank and conservator tank and these relay sense the gas and activate the tripping circuit.
Distribution voltages classes and standard ratings of transformer
Transformer are used wide variety of purposes, with complete range of voltage and power ratings as well as many special features for particular applications.
The following cover main types:
- Small transformer
They are used for stationary, portable or hand held power supply units. They may be used to supply three phase power up to 40kva at frequencies up to 1mhz.
- Distribution Transformer
They are used to distribute power to domestic premises. They may be single phase or three phase. They have rating ranging from16kva up to 2500kva and this type of transformer like 11, 6.6, 3.3, 440 sand 230V.
- Supply Transformer
They are used to supply larger industrial premises or distribution substations. Ratings ranging from 4mva to 30mva, with primary winding rated up to 66kv and secondary up to 36kv. The primary winding has a highest voltage ranging from 3.6kv to 36kv, the secondary winding voltage does not exceed 1.1kv.
- Transmission transformer
They are among largest and highest voltage transformer in use. They are used to transmit power between high voltage networks and Rating ranging from 60mva to 1000mva and winding are rated at 33, 66,132, 275 and 400kv.
- Power or step up transformer
Power is usually generated in large power station at typically 11kv-20kv, and this transformer are used to step up voltage. These transformers are usually rated at 400, 500, 630,800 or 1000mva and transmit power at 66, 110,132,220, 400 and 765kv.
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