Dranetz PowerXplorer PX5, User Manual

Page: 123 / 286

6-3
CH 6/ Advanced Setup Options
Transient Limit
Transient
detection
Dranetz-BMI instruments label rms voltage or current variations as either sags (voltage
or current decreases below low limit) or swells (voltage or current increases above high
limit) as per IEEE 1159. Voltage disturbances which are shorter in duration than typical
sags and swells are classified as transients. Two basic types of transients are:
1) impulsive transients characterized by very rapid changes in the magnitude of
measured quantities and commonly caused by capacitors or inductors switching on
line, loose wires, lightning, static, and power failures; and 2) oscillatory transients
defined as a temporary, rapid discontinuity of the waveform. PX5 has extensive
recording capabilities for low and medium frequency transients using the following
trigger mechanisms:
• RMS difference cycle-to-cycle
• Crest or Absolute peak (Instantaneous)
• Waveshape cycle-to-cycle magnitude/duration variation
RMS variations
RMS stands for root mean square, a mathematical formula used to measure the average
voltage and current behaviors. RMS variations result whenever voltage or current
behaviors rise above or fall below the programmed thresholds. See RMS Variation
Limit on page 6-9 for more information on rms variations.
Absolute peak
settings
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.
Waveform
capture settings
Oscillatory transients are types of disturbances usually captured as waveshape faults.
PX5 captures pre-trigger and post-trigger waveform cycles. The waveshape fault
triggering algorithm essentially compares the samples for each cycle to the
corresponding samples from the previous cycle. If the samples differ by more than the
threshold tolerance for a length of time exceeding window percent of the power
frequency cycle, a waveshape fault is registered. A trigger occurs when either
waveshape or rms deviation rise above the programmed thresholds. Waveshape trigger
values include the magnitude and duration of the difference between the present and
previous cycle.
Once a record trigger is detected, instantaneous waveform information is recorded for
the prescribed voltage and current channel(s). Disturbance monitoring requires that
voltage be continuously sampled, and recorded only if the signals exceed specified
values. Waveshape changes are only triggerable for voltage transients as the current
waveshapes are usually continually changing on a normally operating distribution
system. However most types of disturbances require that current be recorded as well to
help determine the source of the disturbance.
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