Which harmonics are additive in the neutral of a three-phase system?

In a three-phase system, triplen harmonics, which are multiples of three (such as the third, sixth, ninth harmonics, etc.), are additive in the neutral conductor. This occurs because the triplen harmonics have a unique phase relationship in a three-phase system.

When you consider the phase angle differences of the voltage waveforms for the three phases (typically 120 degrees apart), the contribution of these specific harmonics adds constructively in the neutral. For example, if you have the third harmonic present in all three phases, these harmonics will all add up in the neutral conductor instead of canceling each other out, leading to increased current flowing through the neutral connection.

In contrast, the first harmonic (fundamental frequency) and even harmonics (such as the second, fourth, etc.) typically do not contribute to the neutral current in the same way due to their phase relationships among the three phases. The triplen harmonics deal predominantly with the third harmonic, which is crucial in understanding the behavior of neutral currents in three-phase systems, especially regarding load balance and harmonic mitigation strategies.