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

8.4.3 Waveshape disturbance analysis
8.4.3.1 Scope
Waveshape disturbances are those phenomena that cause a signiÞcant change in the voltage or current wave-
shape from one cycle to the next, or from some representative wave. Typically, waveshape disturbances are
associated with voltages rather than currents since the dynamic load variations in a facility constantly and
dramatically alter the current waveshape. 
Waveshape disturbances provide information regarding system adequacy and the nature of loads inside a
facility. Some faults help determine the appropriate source of disturbances such as interruptions, while oth-
ers may identify the cause of distortion.
8.4.3.2 Analysis tips
An ac electrical system contains inertia, which requires consideration of system response characteristics.
The power system, and all the loads connected to it at any given time, conduct and consume power on a con-
tinual basis. Major disruptions of this ßow will result in changes to the actual waveshapes, but not instanta-
neously. Current may cease to ßow abruptly, but there may be energy dumped into the system from the
collapsing Þelds. Many loads, such as motors, do not stop instantaneously when supply voltage is inter-
rupted. They can regenerate voltage as they spin down, making an interruption take up to several seconds to
go to 0 V.
8.4.4 Sag/swell analysis
8.4.4.1 Scope
Similar to waveshape disturbances, sags and swells describe variations with the waveshapes of the voltage or
current. However, sags and swells are typically 1/2 cycle to 1 min in duration, and are referred to as rms
events. This means that instead of looking at the instantaneous waveshape, we examine the rms value of the
wave. If this value is 10% below or above nominal, then a sag or swell has occurred. Occurrences of rms
voltage 10% above or below normal that last longer than 1 min are overvoltages or undervoltages.

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
55