The impact of harmonic currents (THDi) on the low voltage distribution circuits

[Genghiz]

Introduction : 

The third harmonic content of the line current is the ratio of the "third harmonic" and the fundamental (first harmonic), and this is expressed in percentage (%). let us see its impact on the low voltage distribution circuits.

Harmonic Currents : 

This is to cover the situation where there is current flowing in the neutral of a balanced three-phase system

Such neutral currents are due to the line currents having a harmonic content, which does not cancel in the neutral. The most significant harmonic which does not cancel in the neutral is usually the third harmonic.

The magnitude of the neutral current due to the third harmonic may exceed the magnitude of the power frequency line current. In such a case, the neutral current will have a significant effect on the current carrying capacity of the cables in the circuit.

Kindly be notes that, when the harmonic content is more than 33 % then the cable size determining is based on the neutral current.

Conclusion : 

The THDi of the circuit leads to bring attention in sizing the neutral conductor, and based on its value (>33%) the neutral become the size determining conductor. In some case, neutral conductor is bigger than the line conductor.