Impact of Transmission Line Protection Schemes on Voltage Sags

Improving voltage quality, as regards voltage sags, can be achieved by reducing the number of short-circuits and by shortening the fault clearance time through improving the protection schemes used in the network. A methodology previously developed by the authors based on Monte Carlo simulation has been extended by means of introducing adequate probabilistic performance indices. An example is presented, considering the IEEE Reliability Test System and load equipment compliant with SEMI F47-0706 and IEC 61000-4-11/34 standards. Four transmission line primary protection schemes are compared, and results show that obtained improvements have neither a straight relation to site exposure to voltage sags nor to the site short-circuit power. The presented example highlights the importance of applying stochastic prediction methods to this type of assessment, allowing utilities to support capital expenditure decisions in new protection scheme design aiming at improving the quality of supply as regards voltage sags. Moreover, the methodology allows to quantify the customer benefit, which depends on the equipment voltage tolerance curve.