This document discusses power quality issues, specifically voltage sags. It defines voltage sags as decreases in voltage between 10-90% of nominal voltage lasting from half a cycle to one minute. Common causes of voltage sags include motor starting, faults in the power system, and sudden increases in load. The document discusses various methods for mitigating voltage sags, including power conditioning equipment like static VAR compensators, UPS systems, and custom devices like dynamic voltage regulators and D-STATCOMs. It also describes using an auto-transformer controlled by an IGBT switch as a method for mitigating voltage sags.