SOLUTION

The term harmonic is derived from acoustics and is defined as the content of the electrical signal whose frequency is an integral multiple of fundamental or system frequency produced by generators.

Transformer as Source of Harmonics

Transformers can be considered as sources of harmonic voltages, which arise from magnetic distortion and magnetic saturation due to the presence of a DC component in its secondary. The magnitude of these harmonics depends upon the operating flux density. Converter transformers are usually operated at high flux densities than conventional 3-phase transformers, and therefore the possibility of generation of harmonics is more in the former than in the latter.

Effects of Harmonics on Transformers

Harmonics have an effect on transformers in various ways

1. Core loss: Harmonic voltage increases the hysteresis and eddies current losses in the laminations. The amount of the core loss depends on the harmonics present in the supply voltage design parameter of core materials and magnetic circuits. Eddy current losses are of large concern when harmonic current is present in the network. These losses increase approximately with the square of the frequency. Total eddy current losses are normally about 10% of the losses at full load.
2. Hysteresis loss: Hysteresis loss increases because voltage harmonics set up the hysteresis loop on the fundamental loop. So each loop represents a higher magnetization power requirement and higher core losses in the transformer.
3. Copper loss: Harmonic current increases copper loss. The loss (I²R,) mainly depends on the harmonics present in the load and the effective ac resistance of the winding. Copper loss increases temperature and creates 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 filters are normally connected on the a.c. system side.
4. Stress: Voltage harmonics increase the stresses of the insulation.
5. Core vibration: Current and voltage harmonics increase small core vibration.
6. Saturation problem: Sometimes additional harmonic voltage causes core saturation.
7. Interference in communication line:- The third harmonic current may cause interference in the communication circuit running parallel to power lines. If a tertiary delta winding is provided, the third harmonic currents will be reduced but not completely eliminated.