METREL MI 2893 User Manual

MI 2893 / MI 2892 / MI 2885 Measurement methods
191
FFT
Voltage harmonics and THD
10 periods
t
n
1
2
3 4
5 6
50
U
FFT
10 periods
t
n
1
2
3 4
5 6
50
I
Uhn
Ihn
Current harmonics and THD
Figure 156: Current and voltage harmonics









k
k
k
tf
k
cctu 
1
1024
1
0
2
10
sin)(
(57)
f
1
– frequency of signal fundamental (in example: 50 Hz)
c
0
– DC component
k – ordinal number (order of the spectral line) related to the frequency basis
N
C
T
f
1
1

T
N
– is the width (or duration) of the time window (T
N
= N*T
1
; T
1
=1/f
1
). Time window is that time
span of a time function over which the Fourier transformation is performed.
c
k
– is the amplitude of the component with frequency
1
10
f
k
f
Ck


k
– is the phase of the component c
k
U
c,k
– is the RMS voltage value of component c
k
I
c,k
– is the RMS current value of component c
k
Phase voltage and current harmonics are calculated as RMS value of harmonic subgroup (sg): square
root of the sum of the squares of the RMS value of a harmonic and the two spectral components
immediately adjacent to it.
n
th
voltage harmonic:




1
1
2
)10(,
k
knCnp
UhU
p: 1,2,3
(58)
n
th
current harmonic:




1
1
2
)10(,
k
knCnp
IhI
p: 1,2,3
(59)
Total harmonic distortion is calculated as ratio of the RMS value of the harmonic subgroups to the RMS
value of the subgroup associated with the fundamental:

METREL MI 2893 Specifications

General

Brand	METREL
Model	MI 2893
Category	Measuring Instruments
Language	English