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SICAM Q80 7KG8080
3.2.6
Harmonics, interharmonics
Calculation of the frequency components in voltages, currents and thus also in power is performed by means of a Fast
Fourier Transformation (FFT).
The FFT is computed with a rectangular window as per the standard EN 61000-4-7 "Specifications for the
measurement of harmonics and interharmonics in power supply networks". The harmonics are computed according to
the method of harmonics-subgroups, where the RMS value is determined with the two adjacent frequency lines.
The THD (total harmonic distortion) is determined according to the method of harmonics-subgroups total distortion.
This is the ratio of the RMS of the harmonics subgroups to the RMS of the fundamental oscillation subgroup.
The THD pertains to the fundamental oscillation. For this reason, even slightly distorted currents can yield a relatively
high THD, which is why with the TDD (Total Demand Distortion), the reference frame is not the mean of the
fundamental oscillation amplitude, but the maximum fundamental oscillation amplitude within the interval observed.
Thus, TDD <= THD. Since small currents only have minor effects on the overall system, the TDD is a better metric of
the burden from harmonic distortion.
This causes the number of samples to vary since it is chosen on the basis of the fundamental frequency in order to
calculate exactly the harmonic and interharmonic components of that particular fundamental. Example: According to
the standard, the frequency may deviate within the range 50 Hz ±1%. This means that for a 10 kHz sampling
frequency, a period can vary between 98..102 sample values.
The resulting frequency vector is used to derive the aggregation of the 50 harmonics and 10 interharmonics. Together
with the THD and signal voltage, 64 aggregations are derived for each input signal.
The spectra produced by the FFT can be viewed immediately by means of the online monitoring, for which purpose no
aggregation are computed. The special channel for the signal voltage recorded with data reduction (ripple control
signals) is fed directly from the FFT.
Power Quality Recorder SICAM Q80 7KG8080, System manual
E50417-H1076-C420-A2, Release 12.01.2015
Fig 3-3 Mean values from FFT
Analogous to THD for current, in % of fundamental oscillation
with
= Maximum in the calculation interval
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