RMS VOLTAGE VARIATIONS IN THE ELECTRIC SUPPLY SYSTEM
Sag Causes – Transmission System
Common Causes of Voltage Sag
Voltage Sag or Dip Protection
Voltage Swell
Equipment Behavior
Mitigation of Voltage Sags
Voltage Sag Characterization
Sag Causes – Distribution System
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A decrease in RMS voltage at the power frequency for duration from 0.5 cycles to 1 minute, reported as the remaining voltage.
Voltage sag is defined by the IEEE 519 as a sudden reduction of supply (rms) voltage down from 90% to 10% of nominal.
According to the standard, a typical duration of sag is 10msec to 1 minute
Faults (Symmetrical or Un symmetrical in nature)
Transformer energizing (Un symmetrical in nature)
Motor starting (Symmetrical in nature)
Voltage sag due to motor starting can be reduced by the following remedies:
Using a motor that requires less kilovolt-amperes per horsepower to start
Choosing a low-starting torque motor if the motor starts under light load
Replace the large-size motor with a smaller-size motor or motors
Employ motor starters to reduce the motor inrush current at the start
Using shunt or series capacitors to correct the power factor
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Voltage Swell is defined by IEEE 1159 as the increase in the RMS voltage level to 110% - 180% of nominal, at the power frequency for durations of ½ cycle to one (1) minute.
It is classified as a short duration voltage variation phenomena.
Voltage swell is basically the opposite of voltage sag or dip.
The term "momentary overvoltage" is used as a synonym for the term swell.
Voltage swell happens when a heavy load turns off in a power system.
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Lights blink
Motors slow down and return to normal speed
Computers reboot unexpectedly
Automated equipment stops unexpectedly
Transformers exhibit inrush currents upon initial energization. In this case, the high currents occur to energize the transformer core. The steady-state magnetizing current for a transformer is very low, but the momentary current when the transformer is first energized can be quite high.
When a motor is started directly from the mains, it typically draws a current 6-7 times its full load current for a short duration (commonly called the locked rotor current). During this transient period, the source impedance is generally assumed to be fixed and therefore, a large increase in current will result in a larger voltage drop across the source impedance. This means that there can be large momentary voltage drops system-wide, from the power source (e.g. transformer or generator) through the intermediary buses, all the way to the motor terminals.
Several things can be done by the utility, end user and equipment manufacturer to reduce the sensitivity of equipment against voltage sag