Frequency trend and discrete underfrequency relaying practices in India for utility and captive power applications

A typical application procedure for a coordinated load shedding scheme using both frequency trend relays and normal discrete underfrequency relays is described. By using such a combination of relays it is possible to develop comprehensive protection against frequency disturbances, both sudden and gradual. It is emphasized that in a system where the causes of frequency disturbances are quite varied and complex, a fully automatic scheme may not be the ideal solution and manual load shedding as a backup may be necessary. A typical procedure for a captive power plant system operating in parallel with the grid is described as applicable to a continuous process plant industry. The strategy for islanding such industrial power systems from the grid during grid disturbances is described