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

8.4.7.2 Analysis tips
The key to pattern recognition is that few disturbance patterns occur naturallyÑmost are man-made.
Analyzing them involves simply tracking down what might be the cause and how might this cause impact the
operation of other equipment. The following table shows some typical examples of these timing relationships.
8.4.8 Discontinuities
8.4.8.1 Scope
Discontinuities refer to the radical departure of some system component from the norm, which the models
cannot explain. In a sense they are a subset of signatures because they do exhibit a graphical pattern, yet they
are different because they represent the existence of outside inßuencing factors. The two most prominent
outside factors are mitigating devices and alternate sources.
Table 2ÑPattern recognition
Patterns
Possible causes
Time of day
Power factor correction capacitors being turned on automatically
Parking lot lights turning on or off either automatically or with photoelectric switches
HVAC/Lighting systems on automatic control
Duration of disturbance
Cyclical loads such as pumps and motors
Laser printer heating elements cycling on for only 10Ð30 s
Timing controls on process/manufacturing equipment
Frequency of occurrence
Continuous cycling of heating element in laser printer, copiers
Transients from SCR controlled devices occurring every cycle
Vending/Soda machine compressor motor creating transients at turn on
Figure 21ÑZ vs. F example showing resonant frequencies
due to power factor correction capacitor

---

IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
59
8.4.8.2 Analysis tips
The single most important technique to identify discontinuities is an understanding of how the power system
should behave. Departures from normal electrical system response (forced or natural) usually indicate the
existence of some outside factor. The following list is helpful in discovering whether or not there has been
outside inßuence.
Ñ
Did the frequency of the signal abruptly change? 
Ñ
Do the zero crossings of the signal remain continuous?
Ñ
Did a magnitude change occur instantaneously, or did it take a little time to settle?
Ñ
Did the signal suddenly lose a portion of a cycle consistent with loose wiring?