TABLE OF CONTENTS The SCR will probably have a long design life in applications calling for short pulses of high currents. Its ability to latch means it cannot be forced out of a saturated condition by overload, as is the case with the insulated gate bipolar transistors (IGBT). When used as an AC power source, the SCR is able to supply fault currents sufficient to permit selective breaker tripping for minimum system disturbance. The design of high-power pulsed converter systems is an art of its own, and some of the principles are discussed in this chapter. The author has had the privilege of working on a number of fascinating applications in this specialized area.
Most switchgear items carry a short time rating that must be respected for pulsed service. Protective relays may be inverse-time types to permit carrying high currents for a short time without tripping a breaker. Instantaneous elements must be set above the maximum current, of course. A wide variety of overcurrent relay characteristics is available inverse, very inverse, and extremely inverse in both short-time and long-time versions. Within a given type, current taps and a time dial allow adjustment over a very wide range of tripping times and currents. For example, if an extremely inverse-time overcurrent relay is selected to trip a breaker at a given current after a very long time, the breaker will remain closed at 5 times this current for 1 sec, 10 times this current for 0.3 sec, and 40 times this current for...
