MULTI-SOURCE ELECTRIC POWER DISTRIBUTION SYSTEM WITH FAULT PROTECTION PROVIDED BY NON-DIRECTIONAL OVERCURRENT TRIP UNITS

摘要

A two-source medium voltage electric power distribution system has main circuit breakers in each main bus connected to one of the sources and a tie circuit breaker in a tie bus connecting the main buses. Overcurrent trip units in the main circuit breakers, have one current transformer (CT) sensing current in the associated main bus and a second CT sensing current in the tie bus. The CTs are connected to sum their sensed currents in opposition so that with the tie circuit breaker closed the overcurrent trip unit on the faulted side of the system will see the sum of the fault currents from the two sources and trip the associated main circuit breaker open. The overcurrent trip unit of the main circuit breaker on the opposite side of the system from the fault will have the same current flowing in the oppositely poled CTs and will not trip, while the tie circuit breaker will see the fault current from the source on the opposite side of the system from the fault and will trip. Zone interlock signals from the overcurrent trip units of feeder circuit breakers are applied to the tie-circuit breaker trip unit and the trip unit of only the main circuit breaker on the same side of the system as the fault to allow time for the feeder circuit beaker to respond to faults in the feeder circuits with out blocking the other main circuit breaker from tripping in response to a simultaneous main bus fault on the opposite main bus.