Measuring Harmonics - ABB A43 User Manual

Technical Description

5.3.1 Measuring Harmonics

To detect and eliminate the problems related to presence of harmonics, it's gener-
General
ally necessary to measure the harmonics. Meters that have harmonic measure-
ment enabled measure harmonics on all voltages and currents up to the 16th har-
monic and calculates the total harmonic distortion (THD).
Measurement of the harmonics is done sequentially, one at a time, and approxi-
Measuring
mately two harmonic numbers are measured every second.
Each harmonic is calculated according to:
and the total current harmonic distortion for the harmonics measured is
calculated according to:
where I
ber n.
At each measurement the harmonic is set to 0 if the rms value of the current is
below a certain lower limit (normally 5% of the basic current).
Since the meter have limited sampling frequency, presence of harmonics over the
Folding distortion
20:th harmonic (1 kHz at 50 Hz line frequency) will result in folding distortion
and can affect the harmonic measurement accuracy negatively.
Due to the possible presence of folding distortion and the fact that harmonics is
measured sequentially, one at a time, it is recommended that the harmonic mea-
surement results of the meter is used as a tool to detect presence of harmonics and
not as an exact instrument to get exact results.
Measuring harmonics require a valid frequency measurement. If the frequency
Frequency
measurement is uncertain, the harmonic measurement will not be performed. To
measurement
get a valid measurement the meter uses a retry scheme. If the retry scheme does
not give a valid measurement the harmonic will be marked as "not available".
A43/A44
User Manual
is the fundamental current and I
f
52
is the current for harmonics with num-
n
2CMC48001M0201
Revision: C