Voltage sag studies based on simulation in large power customers

Voltage sag is one of the most important power quality problems affecting large power customers. It can cause misoperations of customer´s sensitive devices and severe economic losses. However, most studies focused on the general principles and characteristics of voltage sag but not specific problems in the given customers. In this paper, two large power customers are taken into consideration. The objective is to try to find out the causes of voltage sags in them. Voltage sags recorded by customers are analyzed, including their magnitude, duration and frequency of sags. Then simulation is applied to exploring the causes. It focuses on the performance of voltage sags in customers during different faults. The work indicates that the most voltage sags are caused by single-phase-to-ground faults in over-head line systems. The frequency of voltage sags will be reduced when using underground cables in stead of over-head lines. Then double- phase-short-circuit faults will become the main causes.