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8.1.7 THD Calculation
50
∑
Vharm
n=2
Vthd[i] =
Vharm
i: phase (0; 1; 2)
8.1.8 Calculation of Harmonic Bins
Harmonic bins are calculated by FFT with 16 bit resolution (1024 samples on
4 cycles) without windowing (IEC 1000-4-7). From real and imaginary compo-
nents, each bin ratio is calculated on each phase Vharm[3][51], Uharm[3][51]
and Aharm[3][51] in proportion to the fundamental value and the phase angles
Vph[3][51], Uph[3][51] and Aph[3][51] between each bin and the fundamental.
This calculation is accomplished using the following principle:
module in %:
ck: amplitude of the component with a frequency of
Fs: sampled signal
co: DC component
k:
ordinal number (spectral bin)
82
2
[i][n]
,
Uthd[i] =
[i][1]
n: range (2 to 50)
c
= c
×
k
mod
100
k
1
=
c
b
k
=
b
k
512
with
=
a
k
=
c
0
1024
50
∑
Uharm
[i][n]
n=2
[i][1]
Uharm
angle in degree:
+
=
+
2
ja
a
k
k
k
1024
1
∑
×
sin
F
s
512
=
s
0
1024
1
∑
×
cos
F
s
512
=
0
s
1024
1
∑
F
s
=
0
s
Power Quality Analyzer Model 3945
2
,
Athd[i] =
ϕ
k
2
b
k
π
k
+
ϕ
s
k
π
k
+
ϕ
s
k
512
k
=
f
f
1
k
4
50
∑
2
Aharm
[i][n]
n=2
[i][1]
Aharm
a
=
k
arctan
b
k
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