Electrical Harmonics and Overheating

Electrical harmonics are voltage or current components whose frequencies are integer multiples of the fundamental supply frequency. They are produced mainly by non-linear loads, and their additional current and loss components can cause overheating even when ordinary load measurements appear acceptable.

Neutral-conductor overheating

In a balanced three-phase, four-wire system, fundamental-frequency phase currents largely cancel in the neutral. However, triplen harmonics, especially the third harmonic and its odd multiples, are zero-sequence components that are in phase in all three phases.

• Triplen harmonic currents add arithmetically in the neutral instead of cancelling, so substantial neutral current can exist even with balanced phase loads.
• The increased root-mean-square current raises I²R loss in the neutral conductor and its terminals.
• Skin effect, poor connections and an undersized or shared neutral can further intensify local heating and insulation damage.

Transformer overheating

Harmonic currents increase transformer load losses beyond those expected from the fundamental current alone. Higher-frequency components particularly increase winding eddy-current losses, while voltage distortion can also increase core losses.

• A higher root-mean-square current increases winding copper loss, even if useful power demand has not increased proportionately.
• Skin and proximity effects raise effective conductor resistance at harmonic frequencies.
• Additional eddy-current and stray losses create hot spots, accelerate insulation ageing and reduce permissible loading.
• Triplen harmonics may also circulate in delta-connected windings, adding internal heating.

Prevention and control

Control requires harmonic measurement and equipment selection based on distorted-current conditions, rather than only fundamental-frequency load.

• Neutral current, true root-mean-square current and total harmonic distortion should be measured under representative loading.
• Adequately sized neutrals, sound terminations, load balancing and suitable transformer derating reduce thermal stress.
• Passive or active filters, line reactors and low-harmonic equipment can limit harmonic currents at source.
• Transformers designed for non-linear loads, including appropriate K-rated transformers, can withstand greater harmonic-related losses.