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 causes considerable saturation. This saturation with symmetrical magnetizing current generates all the odd harmonics. The fundamental component is not a problem and all triplen harmonics can be absorbed by delta connection in balanced system. The harmonics generated by symmetrical over excitation are odd harmonics (like 5, 7, 11, 13, 17, 19. . . . etc) i.e. those of orders 6k ±1, where k is an integer.
3. INRUSH CURRENT HARMONICS: When a transformer is switched off, sometimes there exists a residual flux density in the core. When the transformer is re-energized the flux density can reach peak levels of twice the maximum flux density or more. It produces high ampere-turns in the core. This causes magnetizing currents to reach up to 5–10 per unit of the rated value, which is very high as compared to the normal values of a few percentage points. This is known as inrush current. This causes generation of enormous second order harmonic component in the transformer current.
4. D.C. MAGNETIZATION: Under magnetic imbalance, the shape of the magnetizing characteristics and the excitation currents are different from those under no load conditions. When the flux is unbalanced, the core contains an average value of flux, which is equivalent to a direct component of excitation current of the transformer. Under such unbalance conditions, the transformer excitation current contains both odd and even harmonic components.