Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

8.3.2 Event reality check
A reality check should also be done on the actual graphs and reports depicting an event. Subclause 8.2.2
deÞned a reality check as an assurance that recorded data is reasonable based on the circuit conÞguration and
monitoring hookup method. Figures 17 and 18 demonstrate the need for an event reality check.
Figure 17 might be interpreted as heavy amounts of pulsed current causing extreme voltage ßat-topping. In
reality, it is simply the output voltage of a low-end UPS. The real world tells us that it is virtually impossible,
in our normal systemÕs operation, to distort the utilityÕs voltage to a square wave.

---

IEEE
Std 1159-1995
IEEE RECOMMENDED PRACTICE FOR
50
Figure 16ÑThree graphs describing a local temporary interruption event

---

IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
51
Figure 18 appears to be a fast impulse, perhaps a static discharge. But on closer examination we see that the
impulse, with a magnitude of over 400 V, reaches full scale and returns to zero instantly with no overshoot. It
is highly unlikely, even when using mitigating devices, that the normally linear power system would respond
to an impulse in this fashion. Electrical inertia in the systemÕs impedance would certainly cause some over-
shoot. This impulse fails the reality check and is most likely the result of instrument error.
8.4 Interpreting critical events
Once the critical events have been determined and checked, the next step in interpretation begins. If the anal-
ysis of the summaries identiÞed particular events, these should now be examined. If no speciÞc events were
identiÞed, then each should be inspected based on its chronological order. Keep in mind that an event may
consist of more than one graph or report.
Table 5, shown below, is a reference chart for data interpretation. For the conditions given, it identiÞes the
analysis technique to be used and locates that by subclause number. Each subclause then discusses the char-
acteristics and possible causes.