**2.5. Voltage swells**

of optical severity at which 50% of persons might sense a flicker in a 60-W incandescent lamp. Excessive light flicker can cause a severe headache and can lead to the so-called 'sick building

4 Power System Harmonics - Analysis, Effects and Mitigation Solutions for Power Quality Improvement

Voltage unbalance problem is an important power quality issue that can be defined as "a condition in a three-phase system in which the root-mean-square (rms) magnitudes and/or phase angles of the fundamental components of the phase voltages are not all equal" [7]. The principal reason of voltage unbalance in a system is the unbalanced loads among the three phases of the network. This asymmetric loading causes unequal phase currents to flow through the electrical network, and causes unsymmetrical voltage drop on system feeders [8]. Voltage unbalances result in additional power losses in the system and cause more losses in electric motors, so that it cannot be completely loaded up to its nominal power. In addition, excessive voltage unbalances can lead to protection system tripping and cause electrical sup-

The IEEE 112 [9] defines the voltage unbalance using a factor called the phase voltage unbalance rate (PVUR), is given in (2), where *Vdev* expresses the phase voltage variation from the

*Vaverage*

Voltage sags or (American English says sag while British English says dip) According to the IEEE-1159 [11], voltage sag is defined as "a reduction in the rms voltage from 0.1 to 0.9 per unit (pu) for a period of 0.5 cycles to 1 minute." Voltage sag can be categorized into three types, according to its time duration, to instantaneous, momentary, and temporary

Voltage sag results from sudden system faults and switching events of large loads having excessive starting currents such as large motors. Voltage sags impact on sensitive electrical devices such as personal computers and communication equipment, as well as excessive sag events may cause loss of data and nuisance operation of protection devices. In addition, programmed industrial processes such as paper-making industries, chip-making machinery, etc.

Voltage sags can be calculated using various formulas. For example, Detroit Edison's sag score (SS) method defines the "sag score" from the amplitudes of the three-phase voltages. A

*VA* <sup>+</sup> *VB* <sup>+</sup> *<sup>V</sup>* \_\_\_\_\_\_\_\_\_*<sup>C</sup>*

can suffer from power supply shutdown in case of severe voltage sags.

larger *SS* indicates the more the severity of the event [13].

*SS* = 1 − (

× 100 (2)

<sup>3</sup> ) (3)

syndrome.'

**2.3. Voltage unbalance**

ply interruption.

**2.4. Voltage sags**

sag [12].

average line voltage (*V*average) [10].

*PVUR* <sup>=</sup> (*Vdev*)*Max* \_\_\_\_\_\_\_\_

Voltage swell can be defined as a rise in the root mean square (rms) voltage for periods that range from 0.5 cycles to 1 minute. Swells are usually produced by electric faults (single lineto-ground), upstream supply failures, heavy load rejection events, and switching off shunt capacitor banks. Voltage swell is categorized, according to time duration, into three types: instantaneous, momentary, and temporary swells. In addition, voltage sags and swells are produced when loads are shifted from one supply source to another such as the transfer from the utility source to the standby emergency generator during a loss of the normal utility power source [14].

Voltage swell has harmful effects on electrical power system operation as it leads to aging of electrical connections, flickering of lights, semiconductor damage in power-electronic devices, and insulation deterioration of the equipment.
