A method of fault analysis for A-C aircraft electric power systems

THE increasing electric power requirements in today´s aircraft make necessary the continual search for better methods of protection against faults which may cause the loss of electric power or otherwise endanger the mission. Power-system studies have been made in the past using a mock-up of the electric system composed of actual components, and have proved quite successful. However, it is desirable to obtain information faster, and before the development of complete apparatus. Parameters such as line voltages and currents, positive-, negative-, and zero-sequence voltages and currents must be known in order effectively to apply system protection, and they are of much value in regulator design. Faults occurring where they produce the same effect on all regulators, such as at the bus, can be calculated very simply by the method of symmetrical components. A means for detecting such faults by use of negative-sequence voltage is pointed out in another paper.1 However, in multigenerator systems, load-division circuits are employed in order that the load current may be properly divided between generators. Generator currents are usually sensed in a single phase through a current transformer; consequently, generators are forced to divide load current relative to the current in the current transformer associated with a particular generator feeder. Should a fault occur on the load-division phase at a point between a generator and its load-division current transformer, as shown in Fig. 1, that generator would become overexcited and supply practically all of the fault current.