Abstract: A conversion of a three-phase, four-wire power system to divert all zero-sequence current from the power system grounded-neutral source transformer and its phase and neutral conductors. A zero-sequence filter transformer such as a zig-zag transformer is installed, and the system neutral conductor and ground point are relocated from the neutral terminal of the source transformer to that of the zero-sequence filter transformer. A protection scheme operates to trip a main circuit breaker for zero-sequence filter transformer overloads or short circuits. The source transformer and its neutral circuit are thereby protected from the ill effects of zero-sequence currents.

Abstract: A system for use with electrical machines allows fast transfers to be made from one power source to another or fast reconnections to be made to a momentarily interrupted power source without subjecting the machines to unsafe voltage levels. The system short circuits the electrical machines to bring their residual voltage down quickly to a level at which the machines may be safely connected to a power source, and works with both induction and synchronous machines. No interlocking devices are necessary to the system, thus further speeding up fast transfer while minimizing the effects of inadvertent direct short circuiting of the power source.