WELDING OF TRANSFORMER

The over hauling of welding plant transformer is also done her. It is a 440/80 Volt transformer. It has a junction circuit breaker. It has three controllers: one for on/off function, one for slow working and one for fast working.

ELECTRONIC BLAST:

The electronic blast is used in tube lights for its proper working. The electronic blast consists of following parts of specified ratings:

Name of Equipment	Remark	Ratings
Choke	1	11W, 110V   18W, 110V   20W, 110V
Electrolytic Condenser	5	10 µFarad, 450V   11 µFarad, 60V   100 µFarad, 50V   10 µFarad, 50V
Capacitor	5	0.22 µFarad, 400V
Resistor	10	2.2Ω, ¼ W   47Ω, ¼ W   470, ¼ W
Biasing Coil	1	_________________________
Transistor	1	1 MJE,1300S, NPN,   100N, 1NPN
HRC	3	6565
Diode	4	IN4007
Diode	4	IN4148
Resistance	2	16kΩ

PRE COOLING TRANSFORMER:

         

The pre cooling transformer is used in AC coaches. When the train stops at any station for more than 10 mins and is loaded with passengers, then the pre cooling transformer is attached to the main supply of the coach. This is done to maintain the comfort temperature of the coach because the batteries are not designed to bear so much load for such a long time. The pre cooling transformer is a 200 Amp, 440/220volts transformer. A rectifier is attached to this transformer which changes AC to DC. The 3-Ω supply is given to HRC fuse. ML4 contactor is connected to these fuses. The rating of this contactor is 70Amp, 415Volts. The supply is given to the rotary switch which is a 4 pole, 4 way, 63 Amp switch. The primary coil of transformer consists of two type of winding – fine and coarse.

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Three Phase Winding Transformer Advantages

Generally in power system mostly two winding transformers are employed. But three winding transformers are employed because of some advantages: 

• The most common reason for having a three winding transformer is to provide a delta connection tertiary winding
• To limit the fault level on the low voltage system of the transformer by dividing the LV infeed (in order to provide double secondary windings) 
• Providing tertiary winding helps to interconnect different power system operating at different voltages (Three winding transformer helps provide power supply at two different secondary voltages, 220kV/11kV/6.6kV transformer can able to provide power at two different voltage levels (11kV and 6.6kV) 
• To regulate the voltage and reactive power of the system by providing synchronous capacitor connected to one of the terminals of the transformer 

Why Delta winding prefer:-

It is always desirable to have one delta connection winding in the three phase transformer as delta connected three phase winding will offer low impedance path for the three phase currents. Also the presence of delta connected three phase winding allows to circulate the current around the delta winding in the event of unbalance loading condition 

Although power system designers aims to avoid use of star/star transformer in power system but cases will arise when the phase shift between the star/delta and delta/star is not applicable such as in the power station supplying power to auxiliary system. Therefore it is common practice to have a star/star with delta tertiary three winding transformer supplying power to the plant auxiliary system

B/H Curve of the magnetic material (core of the transformer) is not linear. Is a sinusoidal voltage (flux) is applied across the primary winding, the magnetizing current obtained will not be sinusoidal in nature and consists of fundamental component and several harmonics. Third harmonic components predominate with several other higher harmonic components. If there is no delta connected winding, or if the star connections of the transformers are not grounded, the line to earth capacitance currents supply system lines supply the harmonic components. If the harmonic components cannot flow in any one of these paths then, secondary voltage will be distorted.

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TRANSFORMERS

Michael Faraday  propounded  the principle  of electro-magnetic  induction  in 1831. It states that a voltage appears  across the terminals of an electric coil when the flux linked with the same changes. The magnitude  of  the induced  voltage  is  proportional to the rate of  change  of the  flux linkages. This  finding forms the basis  for many  magneto electric  machines. The earliest use of this phenomenon  was in the development of induction coils. These coils were used to generate  high   voltage   pulses to  ignite  the  explosive charges in the mines.  As the d.c.  power system was in use at that  time,   very  little of    transformer principle was made use of.  In the d.c.   supply  system the generating station and  the load center have  to be necessarily close to each other  due to the requirement  of economic transmission  of power. Also the d.c. generators cannot be scaled up due to the limitations of the commutator.  This made  the world look for other  efficient  methods  for bulk  power generation  and  transmis- sion. During the second half of the 19th century the alternators, transformers and induction motors were invented.  These machines  work on alternating power supply.  The role of the transformers became obvious. The transformer which consisted of two electric circuits linked by a common magnetic circuit helped the voltage and current levels to be changed keeping the power invariant.  The efficiency of such conversion was extremely high.  Thus  one could choose a moderate voltage for the generation of a.c. power, a high voltage for the transmis- sion of this power over long distances and finally use a small and safe operating voltage at the user end.  All these are made possible  by  transformers.  The  a.c. power systems thus got well  established.

Transformers  can link two or more electric circuits.  In its simple form two electric circuits can be linked by a magnetic circuit, one of the electric coils is used for the creation of a time varying  magnetic filed.  The  second coil which is made  to link this field has an induced voltage in the same.  The magnitude of the induced emf is decided by the number of turns used in each coil. Thus the voltage level can be increased or decreased by changing the number  of turns.  This  excitation  winding is called a primary  and the output winding

is called a secondary.  As a magnetic medium forms the link between the primary  and the secondary windings there is no conductive connection between the two electric circuits. The transformer thus provides an electric  isolation between  the two circuits.  The frequency on the two  sides will be the same.  As there  is no change in the nature of the power, the re- sulting machine  is called a ‘transformer’ and  not a ‘converter’.  The  electric power at one voltage/current level is only ‘transformed’ into electric power, at the same frequency, to an- other voltage/current level.

Even though most of the large-power transformers can be found in the power systems, the use of the transformers  is not limited  to the power systems.   The  use of the principle of transformers is universal.   Transformers  can be found operating in the frequency range starting from a few hertz going up to several mega hertz.  Power  ratings vary from a few milliwatts to several hundreds  of megawatts.  The use of the transformers is so wide spread that it is virtually impossible to think of a large power system without transformers.  Demand on electric power generation doubles every decade in a developing country.  For every MVA of generation  the installed  capacity  of transformers  grows by about 7MVA. These  figures show the indispensable  nature of power transformers.

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