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

A comparison of the static and dynamic short-circuit calculation procedures has shown that the dynamic procedure, with enhanced machine modeling, produces results which agree well with national standards and minimizes the engineering design effort, with only a small increase in computational overhead. Time overcurrent relay settings and tripping times obtained by a dynamic short-circuit calculation are compared with fault clearing times obtained by modeling the relays using a traditional step-by-step dynamic analysis procedure. These two very different methods of calculation yield results which are in close agreement, reaffirming the validity of the dynamic short-circuit calculation procedure and offering evidence that the practice of basing the setting of time overcurrent relays on fault currents relating to time t=500 ms is well founded. The authors also demonstrate how inclusion of protection setting and transient modeling facilities in power system CAD (computer-aided design) packages offers the opportunity to include automatically and reliably the effect of the protection in any subsequent transient studies of the system. The main benefit is that this can highlight omissions or sensitivities in the protection arrangements