82
Power Quality Analyzer Model 3945
8.1.7 THD
Calculation
Vthd[i] =
[
i
][
n
]
Vharm
[
i
][1]
Vharm
∑
n=2
50
2
Uthd[i] =
,
,
[
i
][
n
]
Uharm
[
i
][1]
Uharm
∑
n=2
50
2
Athd[i] =
[
i
][
n
]
Aharm
[
i
][1]
Aharm
∑
n=2
50
2
i: phase (0; 1; 2) n: range (2 to 50)
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:
cos
sin
=
+
×
=
+
×
=
+
=
+
=
∑
∑
∑
=
=
=
1024
0
0
1024
0
1024
0
2
2
1024
1
512
512
1
512
512
1
s
s
s
k
s
k
s
k
s
k
k
k
k
k
k
F
c
s
k
F
a
s
k
F
b
b
a
ja
b
c
ϕ
π
ϕ
π
100
mod
1
×
=
c
c
k
k
=
k
k
k
b
a
arctan
ϕ
module in %:
with
angle in degree:
ck: amplitude of the component with a frequency of
1
4
f
k
f
k
=
Fs: sampled signal
co: DC component
k: ordinal number (spectral bin)
