Under Voltage and Voltage Sag

They are two popular and important power quality problems present in electrical distribution network that strongly affect the proper functioning of the loads in modern society.
What do under voltage and voltage sag (dip) mean?
Under Voltage: is the decrease in AC RMS voltage at power frequency to less than 90% of the network nominal voltage and for duration longer than one minute. According to IEEE 1159/1995, decreased voltage will range from 80% to 90% of the nominal voltage and for duration longer than one minute.
Voltage Sag: is the decrease in AC RMS voltage at power frequency to the rang between 10% to 90% of the network nominal voltage and for duration rang from 0.5 cycle (10 ms) to one minute. According to IEEE 1159/1995, decreased voltage will range from 10% to 90% of the nominal voltage and for duration rang from 3 sec to one minute.
What are the causes of under voltage and voltage sag?
Under Voltage Causes:
(1) Load switched on and power factor correction capacitors switched off.
(2) Poor system voltage regulation.
Voltage Sag Causes:
(1) Local and remote faults that happen in transmission and distribution especially faults in parallel feeders and customers installation.
(2) Connection of heavy loads and starts up of large motors.
What are the impacts of under voltage and voltage sag?
These two problems have dangerous effects on electrical equipments and electrical distribution network components as following:
Under Voltage Impacts:
Under voltage causes many problems with the equipments that required steady state voltage especially modern load so it causes malfunction, miss operation and in some times full operation stoppage.
Voltage Sag Impacts:
Voltage sag causes many problems like:
(1) Malfunction of information technology equipments which called microprocessor based control systems (PLC, ASD, PCs……etc) that may lead to process stoppage.
(2) Tripping of contactors and electromagnetic relays.
(3) Disconnection and less efficiency in rotating machines.

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