**2.3. Voltage unbalance**

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 supply interruption.

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

$$PVLIR = \frac{(V\_{dev})Max}{V\_{anny}} \times 100\tag{2}$$

**2.5. Voltage swells**

power source [14].

**2.6. Transients**

briefed as follows:

**2.7. Interruptions**

**A.** Impulsive transients

**B.** Oscillatory transients

processor-based protection relays.

sified in terms of duration and standards as follows:

**A.** Classification according to prior planning

and insulation deterioration of the equipment.

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

Introductory Chapter: Power System Harmonics—Analysis, Effects, and Mitigation Solutions…

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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,

In general, most power quality problems are thought as transient events if they exist for a short duration. Impulsive and oscillatory are the main categories of transients. They are

Impulsive transients are abrupt high magnitude actions that cause voltage and/or current levels to rise in either a positive or a negative direction for a very short period fewer than 50 ns.

An oscillatory transient is an abrupt variation in the steady-state voltage, current, or both, fluctuating at the natural frequency of the system at both the positive and negative directions. Events causing transients occur from different reasons such as lightning strikes, poor grounding system, electrostatic discharge, inductive load switching, and fault clearance. Transients may lead to probable data loss in computers, malfunction of electronic equipment, and micro-

Interruption is a randomly event that occurs with zero-magnitude voltage or current for a particular time period, where the magnitude of voltage or current is less than 0.1 pu. It is clas-

According to EN 50160 [15], the electrical interruptions can be categorized into two types, namely, pre-organized interruptions at which the customers are informed (planned interruptions) and accidental interruptions at which sudden failure of equipment or transient fault

### **2.4. Voltage sags**

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 sag [12].

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

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

$$SS = 1 - \left(\frac{V\_A + V\_B + V\_C}{3}\right) \tag{3}$$
