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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.

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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

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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)


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

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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)


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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