ABB RELION REX640 Technical Manual page 1621

1MRS759142 C
REX640
Technical Manual
U3P
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Figure 832: Functional module diagram
Distortion measurement
The Distortion measurement module measures harmonics up to the 11
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.
Total harmonic distortion
The total harmonic distortion THD is calculated from the measured harmonic
components with the formula
2
n
U
h
 
THD
= ∑
 
U
1
h 2  
=
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U
th
h harmonic component
h
U
Fundamental voltage (1
1
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
the description for sliding values.
Power quality measurement functions
U_A
Distortion Total
U_B
measure- harmonic
U_C
ment distortion
Harmonic
calculation
st
harmonic)
Section 10
SMNTHD_A
SMNTHD_B
SMNTHD_C
SMNDC_A
SMNDC_B
SMNDC_C
SMNH01_A
SMNH01_B
SMNH01_C
SMNH02_A
SMNH02_B
SMNH02_C
...until hmax
th
harmonic.
(Equation 353)
Figure 833 and
1615