Triplen harmonics are odd harmonics divisible by 3 and are problematic on which circuits?

• A. Single-phase circuits
• B. Three-phase, four-wire circuits
• C. DC circuits
• D. All circuits

Answer: (B)

Triplen harmonics are the 3rd, 9th, 15th, and so on—odd harmonics that are multiples of three. The important point is that in every phase of a three-phase system, these harmonics have the same phase angle. In a three-phase, four-wire (wye) system, the neutral is the sum of the three phase currents. Because the triplen harmonics line up in phase across all three phases, they don’t cancel in the neutral; they add up. That can push neutral current well above what the neutral conductor and connected equipment are rated for, causing overheating, increased losses, and voltage distortion.

In DC circuits there are no harmonics at all, so this issue doesn’t arise. In single-phase circuits, there isn’t a neutral current path that combines harmonics from all three phases, so the specific problem of neutral overloading isn’t the same. That’s why triplen harmonics are particularly problematic on three-phase, four-wire networks.