Undervoltage/overcurrent coordination in Smart Grids

Smart Grid requires a high level of automation and coordination in order to reduce the intercommunications to reasonable economical and technical limits. Under-voltage/over-current coordination is one of the stringent requirements in order to meet such limits. A methodology is presented that allows the translation of the ride-through curves (time/voltage characteristics) into the time/current domain, where both characteristics can be coordinated. In order to perform the characteristic transformation, only the magnitude of short-circuit current at the point of common coupling (PCC) is needed. The methodology is described when fuses and circuit breakers are used as overcurrent protective devices, coordinated with voltage sags immunity curves. The voltage-sag probability of occurrence is included in the analysis to take into account the sensitive equipment dropout. The methodology changes when the circuit includes distributed generation in order to allow the study when the overcurrent and undervoltage are not of constant magnitude and current reversal may take place.