Tertiary Winding : Construction & Working

• The three winding transformer as its name suggest consists of three winding : Primary winding, secondary winding and third winding is called as tertiary winding.

Construction

• The construction of three winding transformer is similar to that of core type transformer. 
• It consists of three windings in which High voltage ( HV ) winding should be kept at the outer side for insulation consideration whereas the position of the medium voltage ( MV ) and low voltage ( LV ) are inter changeable. 
• The low voltage and high voltage windings are kept close to each other in order to reduce leakage reactance. 
• The three winding may be either three units of single phase or three phase unit.

Construction

• The working principle of three phase winding transformer is similar to that of two winding transformer. 
• When the supply is given to the primary winding, it produces main flux which links with secondary side which consists of secondary and tertiary winding. 
• The main flux produces induced emf in the secondary side. 
• When a load is connected to secondary and tertiary winding, current in them produces demagnetizing flux therefore the primary ampere – turns ( N₁I₁ ) must be balanced secondary ampere – turns ( N₂I₂ ) and tertiary ampere turns ( N₃I₃ ) and magnetizing ampere – turns ( I₀N₁ ).

        Therefore

                 I₁N₁ = I₂N₂ + I₃N₃ + I₀N₁

Application of tertiary winding

The tertiary winding is provided for the following purposes.

• To supply phase compensating device i.e. condenser which is operated at different voltage that different from primary voltage and secondary voltage. 
• To measure voltage of an HV testing transformer. 
• To interconnected three phase supply systems which is operating at different voltages. 
• To supply small additional load at different voltage. ( The tertiary winding is called as auxiliary winding ) 
• The tertiary winding is connected in delta therefore unbalanced condition is not affected by circulating current flows in the delta during fault. 
• The third harmonic current is small is neglected therefore the zero sequence impedance of the transformer is also reduced.
• The delta connected tertiary winding allows earth fault current to flow for the operation of protective device. It also reduces third harmonic voltages and to limit the voltage unbalance when supply voltage is not balanced. ( The tertiary winding is called as stabilizing winding )

The kVA rating of the three winding transformer is equal is largest kVA rating of any of its windings.

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