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MEMBER AIEE FRANK VON ROESGHLAUB MEMBER AIEEr-pHE REVISION OF THE STANDARD describing JL "Short-Circuit Requirements" for synchronous rotat ing machines is of considerable interest to the relay engi neer. The quantitative statement of current-time capa bility of the generator makes available a basis for co ordination of system and machine relaying on unbalanced faults. The permissible time duration of unbalanced faults is expressed as a variable based on /|/ being constant, where is the negative-sequence component of generator armature current and t is the time in seconds.It is desirable to consider using a relay specifically de signed to prevent rotor failure due to excessive negativesequence current, since adequate relays have been made available and the capabilities of generators have been more closely defined. The generator negative-sequence current relays match the generator thermal-capability curve and permit operation to a point just below that where the possibility of machine damage arises, giving a maximum margin for co-ordination with other relays with a minimum of application effbrt.In general, a generator falls into one of four cases, depending on whether it has a voltage regulator and how 4.0, ii.3.0 o: cr 3 §2.0 < Ο LIJ ζ ζ ΙΫ -1

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the authors care to comment on: 1. the effect of negative-sequence currents in the generator under these abnormal frequency conditions, and 2. the frequency characteristic of the negative-sequence relays which are mentioned by the authors and discussed in the companion papers.