Interharmonics are available to be recorded as individuals (specified by Harmonic if an integer number or by frequency), and/or as Interharmonic or Harmonic Groups and Sub-groups. They may be referenced to the appropriate fundamental grouping if desired. In all there are 12 selection methods:

Interharmonics are recorded in the PM7000 as Detail Troubleshooting Channels using the Adaptive Store or Point Store methods. The Interharmonics Function Group selection screen is populated with the twelve selection possibilities. Choosing any of these functions leads to a choice of relevant parameters, signal and harmonic etc., thus complete flexibility is available.

Today more and more companies are concerned with interharmonics. Interharmonic frequencies are not integer multiples of the fundamental frequency (i.e.- 120hz = 2nd harmonic on a 60hz system), but are all other frequencies that are between the multiples (i.e.- 122hz, 75hz).

Interharmonics exist alongside the fundamental and other harmonics. They cause irregular distortion and can cause strange effects in sensitive equipment .

InterHarmonics Calculations

Harmonics, Harmonics Groups and Sub-groups; InterHarmonics, InterHarmonics Groups and Sub-groups are all available and calculated according to IEC61000-4-30 (which calls IEC 61000-4-7).

IEC 61000-4-7 calls for a Transform input time of ~200ms (10 cycles at 50Hz or 12 cycles at 60Hz) and hence the bandwidth is nominally 5Hz. The diagram below shows the 60Hz case:

Note that as shown in the diagram the spectrum is in fact continuous, and not just a series of lines. However the transform methods usually used for computational convenience only yield results at the above line intervals (50Hz, 60Hz or 5Hz), so the spectrum is often represented as a line spectrum.

Having resolved the spectrum into 5Hz elements, the authors of the Standards defined various groupings of 5Hz elements to describe harmonic energy in the chunks appropriate to different applications. (See the IEC 61000-4-7 Standard for further application information).

The groupings are of six types (A) to (F), as shown below:

These are:

(A) Individual Harmonics. This grouping is just the single line at an integer harmonic frequency. (In this case the 2 nd Harmonic, 120Hz.)

(B) Harmonic Sub-groups comprising integer harmonics plus the two adjacent Interharmonics. This grouping shows the 3 rd Harmonic Sub-group around 180Hz.

(C) Harmonic groups. These Groups comprise all the energy from half-way between the harmonic below the nominal to half-way above. The components at 210 and 270Hz are halved so that between them, the Groups centered on (e.g.) the 3 rd and 4 th harmonics (which both have contributions from the Interharmonic line at 210Hz) use 100% of all the available components. The above three groupings are centered on actual harmonics. In addition to individual Interharmonics ((D) below) there are also two groupings centered halfway between harmonics to deliberately exclude harmonics themselves:

(D) Individual Interharmonics. This grouping is just the single line at a non-integer harmonic frequency. (In this case at 45Hz.). [Individual Interharmonics are sometimes indexed as the Nth Interharmonic of the Mth harmonic group, in this case 0:9 (9 th Interharmonic of the Zeroth harmonic = 0 * 60 Hz + 9 * 5Hz = 45Hz).

Because of the need to accommodate 50 and 60 Hz in the PM7000 and the possibility of this indexing being ambiguous, the frequency itself is used.]

(E) Interharmonic Sub-groups. These Sub-groups contain the lines not included in the Harmonic Sub-groups. The Sub-group shown (E) comprises the 70 Hz to 110 Hz contributions, and for the PM7000 this is described as the Sub-group below the 2 nd Harmonic.

(F) Interharmonic Groups. These Groups contain all the lines between integer harmonics. The Group shown (F) has contributions from 185 to 235 Hz, and in the PM7000 nomenclature is described as the Group below the 4 th Harmonic.

Notice how the Interharmonic Sub-group and Group contributions complement the Harmonic Sub-group and Individual Harmonics respectively. Parameter values for group and sub-groups combinations are calculated from the Root Sum Squares of the relevant individual Interharmonics, and each final result is filtered with 1.5sec time constant first order filter.