 |
| TRANSFORMER |
Transformer transfer alternating electrical power from one circuit to the other by the law of electromagnetic mutual induction without change in frequency.In this image V1 is AC primary voltage E1 and E2 is induced EMF in primary and secondary side respectively. The path of main flux Ø is laminated core of transformer. Transformer are two types step up and step down in step up transformer the primary winding have less number of turn compared to secondary winding depending on the amount of voltage rise opposite in case of step down.Based on the winding connections transformer are three phase and single phase.The three phase high voltage step up transformer mostly used in generating station and industrial applications. Single phase low voltage step down transformer used for domestic purpose. The connection of three phase transformer mainly star/star,star/delta and delta/star.Scott connection used for phase transformation from three phase input power is transformed to two phase output power.Transformer consist of magnetic laminated core of soft iron the primary and secondary winding windings placed around it and transformer used only alternating voltage application.When AC voltage applied in primary winding EMF induced due to change in flux and also EMF induced in secondary winding by the law of electromagnetic mutual induction where the path of main flux is magnetic laminated core.If secondary winding of transformer is close circuited iron and copper losses both the windings due to flow of current and induced flux. In other case secondary winding of transformer is open circuited secondary current is zero very small amount of exciting current drawn by the primary side and small amount of copper loss.Mainly primary side supplies iron losses in the core due to hysteresis and eddy current. Exciting current or no load current have two component one is active component that supplies iron loss and a small amount of copper loss in primary side of transformer and other reactive component produced flux not consumed any power when secondary is open circuited.Mainly transformer have two losses copper and iron.Copper loss occur due to resistance in windings and eddy current in the conductor is proportional to the square of the load current .Iron losses occur due to hysteresis and eddy current in the core are constant not depend on the load current. The efficiency of transformer is depend on the losses of transformer for more efficiency losses will be less.If the transformer efficiency measured over the period of 24 hours is called all day efficiency.In this case calculating this efficiency energy output to the energy Input over a period of 24 hours.Such transformer design low flux density to reduce iron loss.The transformer have maximum efficiency when iron losses is equal to copper losses practically not possible.The main parts of transformer have a steel tank that consist of oil for cooling and insulation purpose, laminated core, primary and secondary windings.Conservator,Breather,Explosion vent pipe, thermometer and Buchholz relay all safety accessories mounted with tank for the operation of transformer. Conservator also known as expansion tank because transformer oil expand when hot and contract when cold it provide space for oil expansion and contraction it also perform minimises oxidation of oil by limiting the surface area of oil in contact with air.Conservator connected with main oil tank through small pipe.Breather is a small container contains with silica gel which is blue in colour when dry and turns pink when it absorbed moisture reconditioned by heating and drying attached with one end of conservator tank. The function of breather absorb moisture air passing through it so that only dry air can enter to the tank breather also work as dehydrator.Explosion vent pipe simple like a bent pipe fitted on top of the main tank cover with a glass at the end.If any case insulation break down in side the transformer heavy short circuited excessive pressure build up inside the main tank a vent pipe provided protection against excessive pressure on account of the decomposition of oil so that main tank explode if vent pipe absence. Thermometer fitted in transformer winding so that observe the dangerous temperature rises and a temperature sensitive device for alarming sound. Thermometer also fitted for to indicate the oil temperature.Buchholz relay used in a transformer protection against low oil temperature connected between conservator and main tank. Two floats switch top and bottom provided and connecting to the relay for alarming sound and trip circuit respectively.
 |
| SHORT CIRCUIT |
 |
| OPEN CIRCUIT |
Open circuit and short circuit test commonly conducted on transformers for iron losses and copper losses. In open circuit test one winding of transformer open and other connected to the supply of normal voltage and frequency.There will be no copper loss in open circuited winding watt meter reading will give the iron losses. In short circuit test low voltage winding short circuited and reduced voltage of sufficient magnitude applied high voltage side to circulate in the short circuit.The watt meter reading shows copper losses.The iron losses negligible during the test.Other tests of transformer are Ratio and polarity,Impedance test,Insulation resistance test,resistance of windings and core insulation voltage test etc.
 |
| AUTO TRANSFORMER |
To improve voltage for domestic appliances and reduced voltage starters of three phase induction motor a auto transformer used. Such transformer has one winding act as primary and part of it as secondary for saving of copper cost compared to the two winding transformer.Main disadvantage of auto transformer is primary winding not insulated to secondary winding in case of fault the secondary winding energized to the high primary voltage. Booster transformer are used for raising line voltage when excessive voltage drop in the circuit.Current transformer and potential transformer are called instrument transformer used for measuring of large alternating current and large alternating voltage respectively.Current transformer are used for step down AC current from higher value to lower value for measurement with ammeter wattmeter,kwh meters etc.
 |
| PARALLEL OPERATION |
The maximum load in volt ampere(VA) which may be applied across the secondary terminal without the ratio and phase angle exceeding the permissible limits is called burden of CT.Potential transformer step down the voltage connected directly to the power circuit between phase and ground.Volt ampere(VA) at a rated secondary voltage for a particular power factor called burden on potential transformer.Two or more transformer will operate in parallel satisfactorily, if they have voltage ratios,polarity, percentage impedance,phase sequence,vector diagram and phase displacement should be the same.
