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Voltage sag analysis and its reduction to improve power system performance

Voltage sag may represent one of the major power quality problems. The abnormal operating conditions (such as heavily loading conditions, frequently starting of large induction motors and transmission system faults...) in electric system utility are considered the main reasons of voltage sag problem. In this paper, the various main reasons of voltage sag problem are studied on 4 bus system and 14 bus systems. The obtained results of the used simulation Alterative Transient Program (ATP) show the magnitude and duration of the voltage sag arising from the faults is more severe than that of the other causes, especially in heavily loaded network. The reduction of the voltage sag requires either the use of storage energy or an end user solution such as. uninterruptible power supplies or over saturated transformers. The effect of simple compensator connected in series (with transmission lines) or in shunt (at the system busbars); to improve the system performance (reduce the voltage sag) is studied in this paper. The obtained results of hundred runs of the simulation program show the best type of compensator. The more effective buses or lines (best location of compensators) may be determined using the proposed system performance indices. The use of proposed Unified Power Flow Controller (UPFC) to reduce the voltage sag problems has been studied in this work. Complete simulation results are represented. A good comparison between various compensation techniques and voltage sag reduction algorithms is also presented as well as the study of the sensing and intervention times.