Transformer Basic.

TRANSFORMER

Working principle of Transformer:
A transformer is a static apparatus which transforms  electric power from one circuit to another circuit with same frequency. It can raise or lower the voltage with a corresponding lower or raise of current.

• Faraday's laws of electromagnetic induction is the base of transformer.
• The basis on which transformer works is the mutual induction between two circuits(coil) linked by a common magnetic flux. 
• When one circuit is connected with an alternating voltage source an alternating flux is obtained in the transformer core.
• Most of the flux is linked with other circuit(coil) and produce mutually induced e.m.f.
• If the second coil is closed then current will flow in it thus electrical energy is transferred from the first coil to the second coil. 

  

  Transformer winding format.

Transformer losses: 

1. Iron or Core loss

• includes both hysteresis loss and eddy current loss.
• found from open circuit test.

2. copper loss

• due to ohmic resistance of the transformer winding.
• It is proportional to the square of current flowing through the winding.
• the value of copper loss is found from the short circuit test.

Percentage Impedance:  

• The percentage impedance of a transformer is the voltage drop on full load due to the winding resistance and the leakage reactance expressed as a percentage of the rated voltage.  
• It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions 
• Has major effect on the system  fault level. it determines the maximum value of current that will flow under fault condition. 
• The transformer with low impedance will lead higher fault level and vice versa.   

Buchholz relay: 

• In the field of electric power transmission and distribution Buchholz relay is a safety device. 
• The protection of oil immersed transformers against all the types of internals faults.
• It is a gas and oil operated device mounted in the pipe connecting the transformer main oil tank and the conservator 

• sensitive to the effect of dielectric failure inside the transformer
• Whenever there will be a minor internal fault in the transformer such as
• •  an insulation faults between turns,
  •  break down of core of transformer, 
  • core heating, 
   
• the transformer insulating oil will be decomposed in different hydrocarbon gases, CO₂ and CO.
  
•  The produced gases  will accumulate in the upper part the Buchholz Container which causes fall of oil level in it. Fall of oil level means lowering the position of float and thereby tilting the mercury switch. 
• The contacts of this mercury switch are closed and an alarm circuit will be energized.
• Sometime due to oil leakage on the main tank air bubbles may be accumulated in the upper part of the Buchholz Container which may also cause fall of oil level in it and alarm circuit will be energized. 
  By collecting the accumulated gases from the gas release pockets on the top of the relay and by analyzing them one can predict the type of fault in the transformer. 
  More severe types of faults, such as 
  • short circuit between phases or to earth.
  • faults in the tap changing equipment,

   are accompanied by a surge of oil which strikes the baffle plate and causes the mercury switch of the lower element to close. 
  This switch energized the trip circuit of the Circuit Breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by inter tripping the Circuit Breakers associated with both LV and HV sides of the transformer