Power quality is considered a pivotal issue in modern industrial and commercial applications especially for nuclear installations. Voltage sag is a common power quality problem. It has an impact on the sophisticated and sensitive electronic equipment in nuclear installations. To overcome this problem, flexible Alternating Current Transmission Systems (FACTS) are used. One of the most efficient FACTS, namely Dynamic Voltage Restorer (DVR) is used in power distribution networks to mitigate voltage sag. It is a series of connected power electronic based devices that can quickly mitigate the voltage sags and restore the load voltage to the pre-fault value. This work proposes an improvement of conventional (d-q-0) transformation of DVR using Proportional Integral (PI) controller to mitigate the voltage sag in distribution network. Our implementation of the PI controller is different than the proposed work in the literature. This allowed enhancement of the performance. Different types of faults are modeled by using MATLAB/SIMULINK to examine the improvement of the proposed technique over the conventional one.