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PowerLogic™ PM5300 series user guide Chapter 11—Power quality
© 2014 Schneider Electric All Rights Reserved 89
Chapter 11—Power quality
This section describes the meter’s power quality features and how to access power
quality data.
The meter measures voltage and current harmonics up to the 31st harmonic, and
calculates Total Harmonic Distortion (THD) and Total Demand Distortion (TDD and
tdd).
Harmonics overview
Harmonics are integer multiples of the fundamental frequency of the power system.
Harmonics information is valuable for power quality analysis, determining properly
rated transformers, maintenance and troubleshooting.
Harmonics measurements include per-phase magnitudes and angles for the
fundamental and higher harmonics relative to the fundamental frequency. The meter’s
power system setting defines which phases are present and determines how line-to-line
or line-to-neutral voltage harmonics and current harmonics are calculated.
Harmonics data provide information to determine how non-linear loads affect the power
system. For example, power system harmonics can cause current flow on the neutral
conductor, increase heating in electric motors, and eventually damage connected
equipment. Power conditioners or harmonic filters can be used to minimize unwanted
harmonics.
Total Harmonic Distortion and Total Demand Distortion
Total Harmonic Distortion (THD) is a measure of the total per-phase voltage or current
harmonic distortion present in the power system. It provides a general indication of the
quality of a waveform. THD is calculated for each phase of both voltage and current.
Total Demand Distortion (TDD) is the per-phase harmonic current distortion against the
full load demand of the electrical system. TDD indicates the impact of harmonic
distortion in the system. For example, if your system is showing high THD values but a
low demand, the impact of harmonic distortion on your system might be insignificant.
However at full load, the THD value for the current harmonics is equal to TDD, so this
could negatively impact your system.
The meter uses the following series of equations to calculate THD and TDD.
Harmonic content calculations
1. Calculate harmonic content (HC).
HC (harmonic content) is equal to the RMS value of all the non-fundamental harmonic
components in one phase of the power system.
2. Calculate the harmonic content for current (HCI).
HCI (harmonic content current) is equal to the RMS value of all the non-fundamental
current harmonic components (HI2…HIn) in one phase of the power system.
HC H2
2
H3
2
H4
2
++=
HCI HI2
2
HI3
2
HI4
2
++=