Parallel AC electrical system with differential protection immune to high current through faults

Claim 1

An electrical power system comprising: a plurality of multi-phase generators having outputs each connected with a plurality of phase conductors that are mutually interconnected in parallel for supplying at least one load, each of said generators also having a neutral conductor that is grounded proximate said generator; at least one phase conductor and at least one of said generators having a differential protection network between said phase conductor and said neutral conductor of said one generator; said differential protection network comprising (a) means for deriving currents from said phase conductor and from said neutral conductor proportional to the respective currents in said conductors, (b) voltage limiting means connected across each of said means for deriving current to prevent disablement of said differential protection network and to prevent nuisance tripping of said differential protection network upon occurrence of faults on the parallel connected load bus, and (c) a first resistor in series with the means for deriving current from said phase conductor, a second resistor in series with said means for deriving current from said neutral conductor and a third resistor connected across said means for deriving currents and from between a common point of said first and second resistors, one of said first and second resistors providing an overcurrent voltage signal applied to an overcurrent sensing circuit which is in turn related to a differential protection inhibit circuit, said voltage limiting means having a limiting voltage magnitude of less than that of said overcurrent voltage signal that actuates said overcurrent sensing circuit, said voltage limiting means being directly connected in a circuit with said resistors and said means for deriving currents.