Faults inside the primary 15KV windings of transformers usually begin as line-to-line faults, not involving ground, and with current magnitudes far lower than three phase bolted faults or the time-over-current or instantaneous feeder circuit settings. The only protection scheme which could quickly detect line-to-line faults was an expensive differential scheme, applied usually only to transformers 10 MVA or larger. The purpose of this paper is to show that an instantaneous tripping scheme using negative sequence current can also reliably detect a line-to-line fault condition in less than a cycle. Communication based tripping schemes that could provide security from false tripping on downstream line-to-line faults will be briefly described, but left to be developed in later papers.