Quality of the power supply becomes a key issue in factory automation. Especially voltage sags appearing on the electricity supply cause voltage sensitive equipment to shutdown and incurr heavy financial losses by interrupting the manufacturing process. However, effects of voltage sags can be minimized considerably if both the utility and the factory are working with good cooperation. This paper presents a Sri Lankan experience of finding out sag minimization or mitigation techniques through a

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... tudy jointly carried out by the power supply utility and a customer. By analyzing the plant disturbance reordered for a five a year period and measurements taken with a sophisticated power quality disturbance recorder it has been observed that voltage sags appearing on the factory supply are mainly caused by faults and subsequent switching operations taking place on the transmission and distribution network of the power utility. In addition to that switching operations carried out in the factory owned distribution network also result in voltage sags. In this study, investigations were carried out for estimating the severity of voltage sags associated with different types of faults in the utility network and their effect on the equipment installed in the factory. Especially the effects of voltage sags on the equipment vulnerable for production process like large induction motors and vector controlled Variable Speed Drives (VSD) were investigated to find out the possibilities for improving their voltage sag immunity. Simulations are extensively used to describe field observations. Adequate theoretical descriptions are given to justify field observations and simulation results. Even though well proven sophisticated equipment is available for voltage sag mitigation these are not taken into consideration here due to their high cost. Instead, simple solutions such as distribution network rearrangement, reviewing protection settings and activating already embedded control algorithms in sensitive equipment are considered. Most of the solutions discussed here do not incur additional cost. Remarkablygood results have been gained by applying these techniques.