VTdrive - What is Harmonics?0 pages

What is Harmonics?

Harmonics: Harmonics by definition are a steady state distortion of the fundamental
frequency (60 Hz). Harmonic distortion of current occurs when sinusoidal voltage is
applied to a non-linear load (ex. electronic ballast, PLC, adjustable-speed drive, arc
furnace, any ac/dc converter). The result is a distortion of the fundamental current
waveform. This distortion occurs in integer multiples of the fundamental frequency (60
Hz). Hence, the 2nd Harmonic has a frequency = 2 x 60 = 120 Hz, the 3rd Harmonic =
180 Hz and so on. Voltage distortion, on the other hand, is generated indirectly as result
of harmonic currents flowing through a distribution system.
It is important to note that the vast majority of harmonic currents found in a distribution
system are odd-order harmonics (3rd, 5th, 7th, etc.). Secondly, more often than not,
the sources of the harmonic currents in a distribution system are the loads in operation
within that facility. Interestingly, these are frequently the types of loads that are the
most sensitive to distortion in the current and/or voltage.
Triplen Harmonics – The triplen harmonics are defined as the odd multiples of the 3rd
harmonic (ex. 3rd, 9th, 15th, 21st etc.). Triplen harmonics are of particular concern
because they are zero sequence harmonics, unlike the fundamental, which is positive
sequence. The consequence of this fact is that the magnitude of these currents on the 3
phases are additive in the neutral. This can lead to very large currents circulating in the
neutral, and unless the neutral is sufficiently oversized this can present a fire hazard.
These currents can also circulate in the transformer causing significant overheating
there too. Single-phase power supplies for equipment such as electronic ballasts and
PCs are the most significant source of Triplen harmonics.
5th and 11th Harmonics – The 5th and 11th harmonics are also of particular concern
to industry today.

Although the 5th harmonic is much more prevalent, both have a

negative sequence. This means that when distorted voltage containing the 5th or 11th