A transmission system Undervoltage condition may arise when there is insufficient reactive power generation to maintain the system voltage profile and the condition must be addressed to avoid the possible phenomenon of system voltage collapse. However, it should be addressed by the deployment of system protection schemes. The generation should not be tripped. The greatest case for Undervoltage protection being required would be for a generator supplying an isolated power system or to meet Utility demands for connection of embedded generation.

Undervoltage occurs when the average voltage of a three-phase power system drops below intended levels and is sometimes referred to as a brown-out. Electromechanical devices, including three phase motors and pumps, are designed to be operated at very specific voltage levels. If these devices are allowed to operate at reduced voltage levels, they will draw higher currents. The increase in current causes increased heat in the winding and coils of the equipment damaging the critical insulation protecting them. Operating in Undervoltage conditions can drastically reduce the life of the electromechanical equipment and lead to premature failure.

Increased system loading. Generally, some corrective action would be taken by voltage regulating equipment such as AVR’s or On Load Tap Changers, in order to bring the system voltage back to its nominal value. If the regulating equipment is unsuccessful in restoring healthy system voltage, then tripping by means of an undervoltage relay will be required following a suitable time delay.

Faults occurring on the power system result in a reduction in voltage of the phases involved in the fault. The proportion by which the voltage decreases is directly dependent upon the type of fault, method of system earthing and its location with respect to the relaying point. Consequently, co-ordination with other voltage and current-based protection devices is essential in order to achieve correct discrimination.

Complete loss of busbar voltage. This may occur due to fault conditions present on the incomer or busbar itself, resulting in total isolation of the incoming power supply. For this condition, it may be a requirement for each of the outgoing circuits to be isolated, such that when supply voltage is restored, the load is not connected. Hence, the automatic tripping of a feeder upon detection of complete loss of voltage may be required.

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.

• 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 range between 10% to 90% of the network nominal voltage and for duration range from 0.5 cycle (10mSec) to one minute. According to IEEE 1159/1995, decreased voltage will range from 10% to 90% of the nominal voltage and for duration range from 3 sec to one minute.

a. Under Voltage Causes

• Load switched on and power factor correction capacitors switched off.

• Poor system voltage regulation.

b. Voltage Sag Causes

• Local and remote faults that happen in transmission and distribution especially faults in parallel feeders and customers installation.

• Connection of heavy loads and starts up of large motors.

These two problems have dangerous effects on electrical equipment and electrical distribution network components as following:

a. Under Voltage Impacts

Under voltage causes many problems with the equipment that required steady state voltage especially modern load, so it causes malfunction, miss operation and in some times full operation stoppage.

b. Voltage Sag Impacts

Voltage sag causes many problems like:

• Malfunction of information technology equipment which called microprocessor-based control systems (PLC, ASD, PCs……etc) that may lead to process stoppage.

• Tripping of contactors and electromagnetic relays.

• Disconnection and less efficiency in rotating machine