Types of harmonics
How can we classify harmonics? What types of harmonics do we know? That is a subject of this short post.
Different literature sources mention various classification. We just list here several categories based on our own practice.
A) According the relation of the harmonic frequency to the fundamental frequency of signal
- Integer harmonics (f_harm / f_fund = integer)
- Interharmonics (non-integer)
- Subharmonics (< f_fund)
B) With respect to converter pulse number
- Characteristic harmonics
- Non-characteristic harmonics
C) According to the direction of rotation
- Positive sequence
- Negative sequence
- Zero sequence
D) According to the location (for VFD)
- Grid side harmonics
- Machine side harmonics
E) According to the distorted quantity
- Current harmonics
- Voltage harmonics
- Other (e.g. torque harmonics)
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Note 1
Interharmonics may originate e.g. when the machine side harmonics reflect to the grid side. As the machine generally has different fundamental frequency than the grid, these harmonics will not be integer of the grid power frequency.
Interharmonics may be grouped and displayed as integer harmonics. Grouping methods are defined in standards such as IEC 61000.
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Note 2
Characteristic harmonics correspond to a certain pattern defined by the Fourier series. Best known is probably the 6-pulse rectifier bridge that has characteristic harmonics of order 5, 7, 11, 13, 17, 19, 23, 25 and so on.
A generic formula for p-pulse rectifier bridge is
h = k·p ± 1
h … order of characteristic harmonic
k … integer
p … pulse number
Note 3
Direction of rotation is an important characteristic whe decomposing or synhesizing a signal waveform. For example, triplen harmonics belong to the zero sequence. They do not have any phase shift within a three-phase system.
References
[1] Network harmonics – Introduction, https://mb-drive-services.com/net_harm-introduction/
[2] Network harmonics – VFD harmonic spectrum, https://mb-drive-services.com/network_harmonics_vfd_harmonic_spectrum/
