DIGITALLY CONTROLLED CIRCUIT INTERRUPTER WITH IMPROVED AUTOMATIC SELECTION OF SAMPLING INTERVAL FOR 50 HZ AND 60 HZ POWER SYSTEMS

摘要

A circuit interrupter with a digital trip unit automatically sets thesampling interval appropriate for the 50 Hz or 60 Hz distribution system to which theinterrupter is connected. In order to avoid falsely shifting the interval in response todistorted currents, the trip unit only changes the interval if the number of zerocrossings of the current within a fixed number of samples is within two zero crossingsof the number of zero crossings for an undistorted current of the other frequency fortwo consecutive time periods in which the fixed number of samples are taken.Furthermore, changes in the interval are only written into an EEPROM for use onpower-up a fixed number of times to preclude disabling the EEPROM. In oneembodiment, a 2 pole filter is utilized in the zero crossing counting circuit to eliminatefalse counts from noise originating in a chopper used in the trip unit power supply.In another embodiment, the zero crossing counting circuit is disabled unless externaldc power is available for the trip unit which idles the chopper.