U_A
U_B
U_C
U3P
Distortion
measure-
ment
Total
harmonic
distortion
Harmonic
calculation
SMNTHD_A
SMNTHD_B
SMNTHD_C
SMNDC_A
SMNDC_B
SMNDC_C
SMNH01_A
SMNH01_B
SMNH01_C
SMNH02_A
SMNH02_B
SMNH02_C
...unl hmax
Figure 964: Functional module diagram
10.2.5.1 Distortion measurement
The Distortion measurement module measures harmonics up to the 11
th
harmonic.
The filter used for harmonics includes a spectrum of frequencies near each
harmonic frequency. FFT for one period is used. The resulting frequency spectrum is
wider than in a harmonic subgroup. An anti-aliasing filter is used.
10.2.5.2 Total harmonic distortion
The total harmonic distortion THD is calculated from the measured harmonic
components with the formula
(Equation 392)
I
h
h
th
harmonic component
I
1
Fundamental voltage (1
st
harmonic)
n Number of harmonics
THD shorter mean value outputs are SMNTHD_A, SMNTHD_B and SMNTHD_C. These
and also the 10-minute mean values LMNTHD_A, LMNTHD_B and LMNTHD_C are
seen in the monitored data. These values are always shown as percentage of
the fundamental frequency's phase-to-earth voltage 3-second mean value. The 10-
minute mean values also follow 10 min ticks. See also
Figure 965
and the description
for sliding values.
Power quality measurement functions
1MRS759142 F
1822 REX640
Technical Manual
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