VAMP 59 - RMS Values; Harmonics and Total Harmonic Distortion (THD)

4.2 RMS values
4 Measurement functions
Technical description
132
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VM59.EN001
4.2. RMS values
RMS currents
The device calculates the RMS value of each phase current. The
minimum and the maximum of RMS values are recorded and
stored (see chapter 4.5).
2
15
2
2
2
1
...
fffRMS
IIII 
4.3. Harmonics and Total Harmonic
Distortion (THD)
The device calculates the THDs as percentage of the base
frequency for currents.
The device calculates the harmonics from the 2
nd
to the 15
th
of
phase currents. (The 17
th
harmonic component will also be
shown partly in the value of the 15
th
harmonic component. This
is due to the nature of digital sampling.)
The harmonic distortion is calculated using equation
1
15
2
2
h
h
THD
i
i



, where
h
1
= Fundamental value
h
2...15
= Harmonics
Example
h
1
= 100 A
h
3
= 10 A
h
7
= 3 A
h
11
= 8 A
%2.13
100
8310
222


THD
For reference the RMS value is:
ARMS 9.1008310100
2222

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 %.