ENGINEERING ARTICLES

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 ...

Harmonics has effect on transformer in various ways, e.g.: 1. Core loss : Harmonic voltage increases the hysteresis and eddy current losses in the lamination. The amount of the core loss depends on harmonic present in supply voltage design parameter of core materials and magnetic circuit. 2. Copper loss : Harmonic current increases copper loss. The loss mainly depends on the harmonics present in the load and effective ac resistance of the winding. Copper loss increase temperature and create hot spots in that transformer. The effect is prominent in the case of converter transformers these transformers do not benefit from the presence of filters as filter are normally connected on the AC. system side. 3. Stress : Voltage harmonics increase stresses of the insulation, 4. Core vibration : Current and voltage harmonics increase small core vibrations. 5. Saturation problem : Sometimes additional harmonic voltage causes core saturation.