Simulation of protective relay performance under short-circuit and transient swing conditions

Most methods of relay coordination are based on fixed values of fault current for setting protective relays, regardless of the fact that fault currents are time-dependent. The results of alternative transient calculation procedures are presented. These take into consideration the decay of current with time and can therefore be used to identify the sensitivity of the coordination setting and potential cases of difficulty. Comparison of the static and dynamic short-circuit calculation procedures shows that the dynamic procedure, with enhanced machine modeling, produces results that agree well with national standards and minimizes the engineering design effort, with only a small increase in computational overhead. The results presented are used to compare calculated time overcurrent relay settings and tripping times, based on the dynamic short-circuit calculation, with fault clearing times obtained by modeling the relays using a traditional step-by-step dynamic analysis procedure. These two methods of calculation yield results that are in close agreement. It is also shown that inclusion of protection setting and transient modeling facilities in power system CAD packages offers the opportunity to include automatically and reliably the effect of the protection in any subsequent transient studies of the system