Semiconductor-magnetic overvoltage and underfrequency protection circuits

THE ELECTRIC POWER requirements of advanced manned aircraft are such that 3-phase 400-cycle a-c electric power systems are used on many of the recently developed aircraft. Airborne equipment, such as radar and navigational aids, is of such a nature that the performance requirements for the a-c power systems are more stringent than those normally encountered for d-c power systems. If the limits for system voltage or frequency are exceeded as a result of a malfunction in one or more of the system components, some of the equipment supplied by the system, as well as the system components themselves, would be damaged. Such damage can be eliminated or minimized through proper isolation of the affected area of the system. This isolation, and thereby protection, is obtained through the use of supervisory panels which incorporate fault sensing, time delays, and appropriate relay circuitry. The stringent requirements plus the need for tolerance in normal operating conditions for a-c generators, regulators and constant-speed drives necessitates protection circuits which can sense an out-of-limits condition within ±1 or 2% and initiate proper isolation. This sensing accuracy must be maintained over temperature ranges of ¿55 to +120 C (degrees centigrade) up to 70,000 feet of altitude, and under other severe environmental conditions. To accomplish this end, circuits utilizing the latest circuit components and techniques must be employed.