GE Multilin EPM 2200, Инструкция по эксплуатации

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CHAPTER 1: THREE-PHASE POWER MEASUREMENT POWER QUALITY
EPM 2200 POWER METER – INSTRUCTION MANUAL 1–13
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.
XL = jwL and
XC = 1/jwC
At 60 Hz, w = 377; but at 300 Hz (5th harmonic) w = 1,885. As frequency changes
impedance changes and system impedance characteristics that are normal at 60 Hz
may behave entirely differently in the presence of higher order harmonic waveforms.
Traditionally, the most common harmonics have been the low order, odd frequencies,
such as the 3rd, 5th, 7th, and 9th. 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
accuracy; at frequencies above about 1200 Hz they pass almost no information. So
when instrument transformers are used, they effectively filter out higher frequency
harmonic distortion making it impossible to see.
However, when monitors can be connected directly to the measured circuit (such as
direct connection to a 480 volt bus) the user may often see higher order harmonic
distortion. An important rule in any harmonics study is to evaluate the type of
equipment and connections before drawing a conclusion. Not being able to see
harmonic distortion is not the same as not having harmonic distortion.
It is common in advanced meters to perform a function commonly referred to as
waveform capture. Waveform capture is the ability of a meter to capture a present
picture of the voltage or current waveform for viewing and harmonic analysis.
Typically a waveform capture will be one or two cycles in duration and can be viewed
as the actual waveform, as a spectral view of the harmonic content, or a tabular view
showing the magnitude and phase shift of each harmonic value. Data collected with
waveform capture is typically not saved to memory. Waveform capture is a real-time
data collection event.
Waveform capture should not be confused with waveform recording that is used to
record multiple cycles of all voltage and current waveforms in response to a transient
condition.
1.8 Power Quality
Power quality can mean several different things. The terms “power quality” and
“power quality problem” have been applied to all types of conditions. A simple
definition of “power quality problem” is any voltage, current or frequency deviation
that results in mis-operation or failure of customer equipment or systems. The causes
of power quality problems vary widely and may originate in the customer equipment,
in an adjacent customer facility or with the utility.