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Chapter 9—Power quality PowerLogic™ PM5100 series user guide
THD and thd calculations
The meter supports two methods of calculating total harmonic distortion: THD and thd.
THD is a quick measure of the total distortion present in a waveform and is the ratio of
harmonic content to the fundamental. The meter uses the following equation to
calculate THD:
Where H1 is equal to the fundamental harmonic.
thd is an alternate method for calculating total harmonic distortion. It uses the RMS
value for the total harmonic content rather than the fundamental content. The meter
uses the following equation to calculate thd:
TDD calculation
TDD (total demand distortion) evaluates the harmonic currents between an end user
and a power source. The harmonic values are based on a point of common coupling
(PCC), which is a common point where each user receives power from the power
source. The meter uses the following equation to calculate TDD:
Where ILoad is equal to the maximum demand load on the power system.
Displaying harmonics data
The meter displays the numeric magnitude and angle of the fundamental (first)
harmonic.
Viewing harmonics using the front panel
You can view harmonics data using the front panel.
1. Navigate to Harm. The Harmonics % screen displays, with the following menu
options:
2. Press the voltage or current harmonics you want to view. The fundamental (1st)
harmonic’s numeric magnitudes and angles for all phases are displayed.
3. Press 3-11 or 7-15 to view the graphs for the 3rd to 11th or 7th to 15th harmonics,
respectively. For example, to display the 7th to 15th harmonics screen, press 7-15.
THD
HC
H1
--------
100=
thd
HC
H1
2
HC
2
+
-----------------------------------------
100=
TDD HCIA
2
HCIB
2
HCIC
2
++ILoad 100=
Harmonics % display screens
IEEE mode IEC mode Description
V L-L U Line-to-line voltage harmonics data
V L-N V Line-to-neutral voltage harmonics data
Amps I Current harmonics data
TDD TDD Total demand distortion