Transformer magnetic material characteristic is nonlinear. This non linearity is the main reason for harmonics during excitation. Sources of harmonics in transformer may be classified into four categories as follows: 1. NORMAL EXCITATION : Normal excitation current of a transformer is non-sinusoidal. The distortion is mainly caused by zero sequence triplen harmonics and particularly the third present in the excitation current. Presence of the electric path like air, oil or tank for zero sequence components can be used to reduce those harmonics. Their high reluctance tends to reduce them. Delta connection of poly-phase transformer is very effective to reduce triplen harmonics provided the three phase voltages are balanced. 2. SYMMETRICAL OVER EXCITATION : Transformers are designed to make good use of the magnetic properties of the core material. When such transformers are subjected to a rise in voltage, the cores face a considerable rise in magnetic flux density, which often ...