B
Appendix
B-4
EPQ DataNode Series User’s Guide
Additional Notes on RMS Variations
Kinds of RMS Variations
RMS variations are classified as short duration or long duration, depending on whether the excursion lasts beyond one
minute.
Short Duration Variations
Short duration variations are further classified as instantaneous (duration ½ - 30 cycles), momentary (duration 30 cycles -
3 seconds), and temporary (duration 3 seconds - 1 minute). Short duration RMS voltage decreases, called sags, are most
frequently caused by faults or changes in load current. Voltage sags are often more significant than power interruptions
because more customers are affected, as illustrated in Figure B.1 below. When a protective device clears a fault, only
customers located downline of the device are interrupted. However, until the fault is cleared, customers on adjacent
feeders are subjected to a voltage sag. Protective devices with automatic reclosing can subject customers to as much as
four sags in succession, each lasting 2 - 10 cycles or longer.
Figure B.1 Area of Vulnerability due to Fault-Induced Sags
The severity of a sag in customer service voltage due to a fault on the distribution system increases with the proximity of
the fault to the distribution substation. Remote faults produce milder sags, but sag duration may increase due to increased
protective device operating times.
While most instantaneous sags are related to faults, motor starting is the chief cause of longer sags. An induction motor
draws 6 - 10 times full load current when starting, which - depending on the motor construction, mechanical load, and
the size of the motor relative to the system - may induce a sag of a few cycles to several seconds in duration.
Single-line-to-ground faults are among the dominant cause of swells - short duration RMS voltage increases. Until the
fault is cleared, the unfaulted phases will be subjected to a swell whose magnitude depends on the system grounding
(X0/X1) ratio. The swell can be as high as 1.73 pu for an ungrounded system. Swells can also occur until the automatic
tap changer can react when loads are suddenly de-energized.
Long Duration Variations
Long duration RMS variations are deviations outside the normal tolerance in the ac voltage for a period exceeding one
minute. Overvoltage and undervoltage are the standard terms employed for a long duration voltage increase and
decrease, respectively.
Long duration voltage variations result from addition or removal of system load or reactive compensation, and can be
controlled by improving the voltage regulation characteristics of the system.
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