Trends in distribution transformer protection

Until the 1960s, ground-mounted, cable connected distribution transformers were usually supplied from oil circuit-breakers with trip coils operated directly from current transformers in the connections to the transformer. Time grading was provided by short-circuiting the trip coils by special fuses, usually referred to as time limit fuses. The development of the HV fuse in the late 1950s led to the creation of the fuse-switch and this brought about a cheaper solution to the protection of distribution transformers. In more recent times the technology of switching devices is undergoing change with the introduction of SF₆ as the insulating and arc control medium in place of oil. These new designs of equipment introduce the possibility of re-examining the techniques and economics of transformer protection which the authors attempt to do