**2. Power quality disturbances**

There is a wide variety of power quality disturbances which affect the performance of customer equipment. The most common of these are briefly described in this section of the chapter.

### **2.1 Voltage sags**

Voltage Sag is defined as a short reduction in voltage magnitude for a duration of time, and is the most important and commonly occurring power quality issue. The definitions to characterise voltage sag in terms of duration and magnitude vary according to the authority. According to the IEEE defined standard (IEEE Std. 1159, 1995),voltage sag is defined as a decrease of rms voltage from 0.1 to 0.9 per unit (pu), for a duration of 0.5 cycle to 1 minute. Voltage sag is caused by faults on the system, transformer energizing, or heavy load switching.

### **2.2 Interruptions**

Interruption is defined as a 0.9 pu reduction in voltage magnitude for a period less than one minute. An interruption is characterized by the duration as the magnitude is more or less constant. An interruption might follow a voltage sag if the sag is caused by a fault on the source system. During the time required for the protection system to operate, the system sees the effect of the fault as a sag. Following circuit breaker operation, the system gets isolated and interruption occurs. As the Auto-reclosure scheme operates, introduced delay can cause a momentary interruption.

#### **2.3 Transient overvoltages, swells**

Overvoltage is an increase of Root Mean Square (RMS) voltage magnitude for longer than one minute. Typically the voltage magnitude is 1-1.2 pu and is caused by switching off a large load from the system, energizing a capacitor bank, poor tap settings on the transformer and inadequate voltage regulation. Overvoltages can cause equipment damage and failure. Overvoltages with duration of 0.5 cycle to 1 min are called voltage swells. A swell is typically of a magnitude between 1.1 and 1.8 pu and is usually associated with single line to ground faults where voltages of non-faulted phases rise.
