Planning and Protection Implications of Reduced Fault-Withstand Time of Power Transformers

The recent revisions (1980-1983) to the standards for power transformers in both the U.S.A. and Canada call for corresponding revisions in many related planning and protection practices. Particularly, it is found that the reduction to 2 seconds for maximum through faults on transformers and the necessity to include a value for system impedance in the specification of the larger transformers will have significant effects on previous practices for planning, protection, operations, loading and specification. This paper discusses a number of these effects, with an emphasis from a protection point of view.