EPQ DataNode Series User’s Guide
3
Operation
3-8
TRANSIENTS tab
Voltage disturbances which are shorter in duration than
sags and swells are classified as transients and include
two basic classes: (1) impulsive transients, often attribut-
able to lightning and load switching, and (2) oscillatory
transients, usually caused by capacitor bank switching.
Utility capacitor banks are often switched into service
early in the morning in anticipation of a higher power
demand period. Transients with a high magnitude and
fast rise times can lead to insulation breakdown in
motors, transformers, capacitors, and switchgear.
Pages 3-8 to 3-9 contain important details on Transients.
See Appendix B page B-5 for Transients tab illustration
and additional notes. Refer to the InfoNode Series 5500
User’s Guide on how to set the parameters found in
Transients tab.
Transient Detection
Transients can be captured using three trigger mecha-
nisms: the high frequency positive/negative peak detec-
tors, crest factor peak (instantaneous), and/or the wave-
shape variation. The high frequency peak detected tran-
sient uses special circuitry to detect and capture impul-
sive transients as small as 1/2 microsecond in duration.
These transients are the positive and/or negative value
above or below the low frequency waveshape. The crest
factor or instantaneous peak is the absolute peak sample
value within one cycle. The Instantaneous peak triggers
on the absolute peak value of the entire waveform.
The high frequency peak detector and instantaneous peak
transients are triggerable for voltage and current. The
waveshape trigger values include the magnitude and
duration of the difference between the present and previ-
ous cycle's waveshapes. Waveshape changes are only
triggerable for voltage transients.
The following information is required to set up the trig-
gering mechanism:
Waveshape Threshold Magnitude or Tolerance
Waveshape Threshold Duration or Window
Peak or Instantaneous trigger magnitude
Hi freq negative/positive peak detectors magnitude (early
versions of DataNode have this value set the same as the
Instantaneous peak)
Detecting Impulses
In each of the 128 "time slices" of the waveform, there are
3 sample points taken. These are a Low Frequency sample
and a pair of High Frequency Peak samples. The Low
Frequency point is used for the calculation of RMS and
harmonics. The pair of Peak samples represents the output
of a set of analog peak detectors that capture the instanta-
neous (in the order of a microsecond) high and low values.
These peak detectors are reset 128 times per cycle, and are
used for the HF Transient measurements. Thus there are
128 Low Frequency samples plus 128 Peak sample pairs
per cycle, for a total of 384 samples per cycle.
A high-frequency transient event is recorded if the devia-
tion between the highest and lowest values of the HF
Transient exceeds the user-specified threshold.
Note that the DataNode measures hi and low peaks in the
interval between low frequency samples (130 µsec at 60
Hz). Thus, although a single 1 µsec HF Transient can be
detected, its time can only be determined to within 130
µsec. This applies to each of the 128 intervals in the
waveform.
The impulse detection circuit records positive and negative
peak magnitude and volt-seconds for transients of 1 - 200
µsec in duration. The 130 µsec (at 60 Hz) or the 156 µsec
(at 50 Hz) limit was selected to prevent low frequency
transients (< 5 kHz) from triggering impulse captures,
which are captured by RMS measuring circuitry.
The transient triggers cause up to 8 pre-trigger cycles of
the waveform, recorded immediately to be saved in memo-
ry. More than one cycle of triggered disturbance can be
recorded. If another disturbance occurs on the cycle imme-
diately following the initial one, the recording is extended
up to a max of 10. After that, the threshold is increased by
50%. When there are no more transient triggers, the post-
trigger cycles are saved and the event ends. This method
permits capturing long transients like transformer ener-
gization.
As shown in Figure 3.6 next page, the impulse will either
add to or subtract from the fundamental frequency voltage,
increasing or reducing the apparent peak voltage magni-
tude. To insure consistent detection of an impulse of a
given magnitude, regardless of where it occurs on the fun-
damental sinusoid, the delta between the Hi Peak and Low
Peak detectors for each of the 128 intervals is measured
before the voltage signal is compared with the threshold
level.
