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Solutions
Volume 6, Issue 2 Transients in Power Systems
How the sampling rate of a monitoring device can affect event detection and waveform analysis
How many samples are enough? To represent an analog signal (such as voltage and current), a monitoring
device takes discrete snapshots of the analogsignal and converts them to their approximate
digital equivalent. Sample rate is the number of
samples of the analog signal that are taken per
cycle to represent the signal digitally. Obviously,
the more samples taken per cycle, the more
accurate the digital representation of the signal
will be.
The concern is that many transient events
are missed or not accurately approximated
because some meters use fewer samples than
are necessary to accurately depict the analog
signal. In general, most meters will sample
between 64 and 512 samples per cycle. While
this may be quick enough to detect the majority
of longer duration events, faster events such as
transients may either be missed completely or not
accurately represented.
By denition, transient events last less than one cycle. Because of their
short duration and often unpredictable pattern of occurrence, capturing
and analyzing transient events requires the use of more sophisticated
monitoring devices or meters. These meters sample the analog signal at
a much higher frequency than standard meters. The POWERLOGIC
Series 4000T Circuit Monitor (CM4000T) will sample at 5 MHz or
83,333 samples per cycle (based on a 60-Hz system) during a high-speed
event, compared to 512 samples per cycle (or less) in a standard meter.This results in data that has 162 times more resolution accuracy than the
standard available data. While longer duration events may be properly
diagnosed using lower sample rates, many transient events cannot.
PowerLogic
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(Part 2 in Transient Series)
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Examples To illustrate the benet of using higher data
sampling rates when troubleshooting power
disturbances, this section shows CM4000T
waveform captures of transient events at
different sampling rates. It also demonstratesthe operation and effectiveness of transient
voltage surge suppressors (TVSSs). An
impulsive transient was “injected” onto the
source voltage with a capacitive load. The
resulting waveforms were captured, with and
without TVSSs, using three different data
sampling rates: 64 samples per cycle, 512
samples per cycle, and 83,333 samples per
cycle.
Example 1: Transient events captured
at 64 samples per cycle
The effects of a lower data sample rate (64points per cycle) are illustrated in Figure A and
Figure B. While the occurrence of a -cycle
event is apparent, accuracy of the information
becomes questionable when the two waveform
captures are compared. Notably, both events
were caused by the same device at roughly
the same point in the waveform, and therefore,
should have the same initial voltage polarity.
However, Figure A shows the initial polarity
of the event to be out of the voltage signal,
while Figure B shows the initial polarity to be
into the voltage signal. This information can
be misleading and costly, should the wrong
solution be employed to correct the anomaly.
Because the event occurred at slightly different
times on the waveform, the poor resolution of
the sampling rate could miss the initial polarity
of the event. Because the initial polarity is
extremely important in ascertaining the source
of (and solution to) the transient, it is important
to obtain a high enough resolution to correctly
determine the initial polarity of the event. In
this case, the event was caused by a transient
generator, which should have the effect of
adding voltage to the original signal.
When the polarity of an event is into the
voltage signal, the source of the event is taking
energy out of the system. An example of this
effect is a capacitor switching event: when
the capacitor charges, a large inrush current
is created. This results in an initial notch into
the voltage signal (with subsequent oscillation
or “ringing”). Alternatively, the polarity of
an event out of the voltage signal signies
POWERLOGIC
Solutions
that energy is being added to the system. An
example of this effect may be lightning strikes,
switching inductive loads, or, as illustrated in
this example, a transient generator.
The two waveform captures also provide
information about the TVSS operation. The
waveform shows that the TVSS attenuated the
transient event, which lowered the amplitude
of the event and shortened the ringing period.
However, with this lower sampling, it is a
mistake to assume the true magnitude of
attenuation is reected in Figures A and B. This
is illustrated in the following examples.
Figure A: Event captured using the 64 Sample/Cycle Monitor (without a TVSS on the load)
Figure B: Event captured using the 64 sample/cycle monitor (with a TVSS on the load)
Example 2: Transient events captured
at 512 samples per cycle Figure C and Figure D illustrate the same
type of event that was described in Example
1, but with a sample rate of 512 samples per
cycle. The polarity of the transient event isout of the waveform as expected, but lack of
data points in Figure D cause the waveform to
barely indicate the correct polarity. Even at 512
samples per cycle, the troubleshooter might
erroneously assume that the polarity of the
event is accurately shown.
Although 512 samples per cycle is one sample
every 32.5 microseconds, a meter sampling
PowerLogic Solutions is produced by
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presents a common
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problem, and offers
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solve it.
The transient is outof the voltage signal,which signies thatenergy is added to thesystem.
The transient is intothe voltage signal,which signies thatenergy is beingremoved from thesystem
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This is typical of an RLC (resistive, inductive, capacitive) circuit that
has been injected with energy from some source. In this case, the load
was later revealed to be a capacitor (with the leads being resistive and
inductive).
Figure E shows the same event shown in Figures A, B, C, and D.
However, Figure E indicates that the transient is much greater
magnitude than one would assume from looking at the other gures.
The table below illustrates this.
Figure E: Event captured using a CM4000T
Table 1: Transient Magnitude vs. Data Sampling Rate
64 samples
per cycle
512
samples
per
cycle
83,333
samples
per cycle
Without
TVSS
750 volts 750
volts
>2,500
voltsWith TVSS 200 volts 200
volts
700 volts
Much more energy was present in the voltage transient than was rst
indicated by the slower sampling rates. The conclusion to be drawn
is that faster sample rates give truer pictures of the magnitude of
transients, and thus, the potential damage to equipment.
Summary By sampling the waveform at 83,333 samples per cycle in the
CM4000T (based on a 60 Hz system), a troubleshooter can
conclusively determine the magnitude, duration, and initial polarity
of an extremely fast voltage event. The higher sampling rate also
gives a troubleshooter a detailed prole of an event higher and more
accurate level of detail that would not be available at normal sampling
rates, allowing better diagnosis of (and solutions to) transient voltage
events. In many cases, more damaging energy is behind a transient
than can safely be assumed based on normal sampling rates. In this
particular case, not only were the characteristics of the transient voltage
event accurately shown, but the CM4000T was able to conclusively
demonstrate the chosen solution, a TVSS, was effective in mitigating
the problem at the load.
at this rate may not detect many causes of
short duration events; and these events may
disrupt equipment and processes. Bouncing
mechanical contacts, removing inductive loads,
and electrostatic discharge (ESD) impulsescan all exhibit short duration occurrences
with potentially damaging effects to adjacent
equipment. Even at 512 samples per cycle,
these rapid events may go undetected.
The waveforms in Figure C and Figure
D illustrate that the TVSS attenuated the
transient event. Once again, the sampling
rate does not allow conclusions to be made
regarding the actual magnitude of attenuation.
Figure C: Event captured using the 512 sample/cycle monitor (without a TVSS on the load)
Figure D: Event captured using the 512 sample/cycle monitor (with a TVSS on the load)
Example 3: Transient events captured
at 5 MHz Figure E shows that much more information
can be gathered by the higher sampling rateof the CM4000T (83,333 samples per cycle,
based on a 60 Hz system). This waveform
illustrates a voltage transient event with and
without a TVSS. In this case, the polarity of
the transient event is clearly shown to be out
of the waveform (that is, something is putting
energy into the system). Zooming into these
waveforms shows that the impulse contains
two ringing signals superimposed on each
other at approximately 25 kHz and 250k Hz.
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POWERLOGIC
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2002 Schneider Electric All Rights ReservedDocument # 3000HO0204 September 2002
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