“Survival of the fittest” — EMC in electric power substations

摘要

When a short circuit occurs in a transmission substation (usual definition — voltages above 100,000 volts), the resulting fault current is spectacular. In some substations, it may be as high as 80,000 amperes. Let''s assume the fault was caused by the flashover of a porcelain insulator supporting a section of the high voltage power line a few towers away from the substation, due to a buildup of sea salt. The protective relays monitoring this transmission line detect this abnormally high current, and close their “trip” contacts — which then cause both the high voltage circuit breakers connected to this transmission line to open — and thus “clear the fault”. But while the fault exists, this high current is flowing from the tower to ground, then through the earth back to the substation. And since there is a finite (non-zero) resistance to “true earth ground”, this causes the entire substation ground matt voltage to rise. However, this is a 60 Hertz voltage, and adequate insulation — in control circuits and electromechanical protective relays — for this well recognized “ground potential rise” has been defined for years in protective relay standards (e.g. ANSI C37.90).