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1.4.1
Inductive and capacitive impedance
1.4.2
Voltage and Current Monitoring
EPM 2200 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE
collection of higher frequency waveforms. These higher frequency waveforms are referred
to as harmonics. Figure 1.12 shows the content of the harmonic frequencies that make up
the distortion portion of the waveform in Figure 1-11.
250
200
150
100
50
0
-50
-100
-150
-200
-250
FIGURE 1–12:
The waveforms shown in Figure 1-12 are not smoothed but do provide an indication of the
impact of combining multiple harmonic frequencies together.
When harmonics are present it is important to remember that these quantities are
operating at higher frequencies. Therefore, they do not always respond in the same
manner as 60 Hz values.
Inductive and capacitive impedance are present in all power systems. We are accustomed
to thinking about these impedances as they perform at 60 Hz. However, these impedances
are subject to frequency variation.
X
At 60 Hz, ω = 377; but at 300 Hz (5 th harmonic) ω = 1,885. As frequency changes
impedance changes and system impedance characteristics that are normal at 60 Hz may
behave entirely different in presence of higher order harmonic waveforms.
Traditionally, the most common harmonics have been the low order, odd frequencies, such
as the 3 rd , 5 th , 7 th , and 9 th . However newer, non-linear loads are introducing significant
quantities of higher order harmonics.
Since much voltage monitoring and almost all current monitoring is performed using
instrument transformers, the higher order harmonics are often not visible. Instrument
transformers are designed to pass 60 Hz quantities with high accuracy. These devices,
when designed for accuracy at low frequency, do not pass high frequencies with high
CHAPTER 1: THREE-PHASE POWER MEASUREMENT
Waveforms of the harmonics
⁄
=
jωL and X
=
1 jωC
L
C
t
a
(EQ 1.3)
1–15
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