Computer-aided coordination and overcurrent protection for distribution systems

Overcurrent protection and coordination studies for electrical distribution systems have become much easier to perform with the emergence of several commercially available software programs that run on a personal computer. These programs have built-in libraries of protective device time-current curves, damage curves for cable and transformers, and motor starting curves, thereby facilitating the design of a selectively coordinated protection system which is also well-protected. Additionally, design time when utilizing computers is far less than the previous method of tracing manufacturers´ curves on transparent paper. Basic protection and coordination principles are presented in this paper along with several helpful suggestions for designing electrical protection systems. A step-by-step methodology is presented to illustrate the design concepts when using software for selecting and coordinating the protective devices in distribution systems