Sudden increase or decrease in loads and starting of motors are the cause of internal voltage sags. Depending on the magnitude, duration of the voltage sag and sensitivity of equipment, severity of the voltage sag can be evaluated. The sensitive electronic equipment's are such as variable speed drive (VSD) controls, programmable logic controllers (PLC’s), motor starter, power supply controller etc.
A. Voltage Sag Classification and Characterization
In the grid system, voltage sag is considered as an abnormal condition. And they are significant reduction in one or several phases for a very short time. There are several classification methods. One of those is according to magnitude and phase angle jumps . There are several types but following four are the basic types like Type A, B, C and D .
Fig. 1. Voltage sag type A, B, C and D.
According several authors the voltage sag of types B,C and D produces positive and negative sequence voltage which distorts the current and affects the power quality of grid.
This voltage sags are also characterized the positive, negative, and zero symmetric