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Harmonics and Total
Harmonic Distortion
(THD)
Demand Values
The device calculates the THDs as percentage of the base
frequency for currents and voltages.
The device calculates the harmonics from the 2
phase currents and voltages. (The 17
also be shown partly in the value of the 15
component. This is due to the nature of digital sampling.)
The harmonic distortion is calculated using equation
15
∑
2
h
i
=
=
i
2
THD
, where
h
1
h
= Fundamental value
1
h
= Harmonics
2...15
Example
h
= 100 A
1
h
= 10 A
3
h
= 3 A
7
h
= 8 A
11
+
+
2
2
10
3
8
=
THD
100
For reference the RMS value is:
=
+
+
2
2
RMS
100
10
Another way to calculate THD is to use the RMS value as
reference instead of the fundamental frequency value. In the
example above the result would then be 13.0 %.
The device calculates average i.e. demand values of phase currents
I
, I
, I
and power values S, P and Q. The demand time is
L1
L2
L3
configurable from 10 minutes to 30 minutes with parameter
"Demand time".
Measurement Functions
nd
th
harmonic component will
th
harmonic
2
=
13
2 .
%
+
=
2
2
3
8
100
9 .
A
5-3
th
to the 15
of
857-UM001A-EN-P – July 2009
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