A high-speed differential relay for generator protection

L. F. Kennedy (General Electric Company, Schenectady, N. Y.): The two ideas incorporated by Mr. Sonnemann; namely, the use of a variable-ratio characteristic in a differential relay and its dependence upon phase angle, certainly provides a means for overcoming the tendency toward incorrect operation due to the shortcomings of the current transformers. I do not feel, however, that we can fully accept his statement “that when the restraining current is large, …, the sensitivity of the relay should be reduced to allow for greater shortcomings in current-transformer performance.” A more accurate statement would seem to be one which conveyed the idea that a reduction in relay sensitivity may be an acceptable means of overcoming otherwise unavoidable current-transformer errors. While I feel that there is some real merit to be derived from a variable slope, I do believe that we should avoid giving the impression that it is a definite benefit under all conditions and that we are not interested in obtaining current transformers capable of reasonably good performance.