Harmonics can have a significant impact on the temperature rise of transformers. Temperature rise in transformers is primarily caused by losses occurring during the operation of the transformer. These losses can be broadly categorized into two types: load losses (also known as copper losses) and no-load losses (also known as iron losses).
Copper Losses (Load Losses):
Copper losses occur due to the resistance of the windings in the transformer when current flows through them. These losses increase with the square of the current. Harmonics can cause an increase in the effective current flowing through the windings, even if the fundamental current remains the same. Since higher-order harmonics have frequencies that are multiples of the fundamental frequency, they can result in additional copper losses. This increased copper loss can lead to higher temperature rise in the transformer.
Iron Losses (No-Load Losses):
Iron losses occur due to the hysteresis and eddy current losses in the transformer core when it is subjected to varying magnetic fields. Harmonics in the current waveform can result in additional iron losses as they cause more rapid and frequent changes in the magnetic flux, leading to higher core losses. Like copper losses, increased iron losses can contribute to higher overall temperature rise in the transformer.
Skin Effect:
Harmonics can also exacerbate the skin effect in conductors. The skin effect causes current to be concentrated near the surface of the conductor at higher frequencies, reducing the effective cross-sectional area available for conducting current. This increases the effective resistance of the windings, leading to higher copper losses and, consequently, higher temperature rise.
Resonance:
In certain cases, harmonics can create resonance conditions in the transformer's windings, leading to even higher currents and losses at specific frequencies. Resonance can significantly increase the heating effect and cause temperature rise beyond what would be expected from individual harmonic contributions.
To mitigate the effects of harmonics on transformer temperature rise, it's essential to consider harmonic filtering and proper transformer design. Special attention should be given to the selection of transformers with higher harmonic capacity and the implementation of filtering or inductors to reduce harmonic content in the system. Additionally, system designers and operators should be aware of the harmonic levels in the electrical system and consider their impact on transformer operation and temperature rise.