An electric power transmission system that isolates a local AC transmission network from a surrounding AC system, comprising: a local AC load center having a plurality of local AC loads, and at least one distribution feeder serving the plurality of local AC loads; at least one distant electric power generating station for supplying AC power to the local AC load center by means of an AC transmission line; a DC transmission ring having a plurality of DC loads thereon at least partially interposed between the local AC load center and the distant electric power generating station, the DC transmission ring isolating the AC power received from the distant electric power generating station from the local AC load center; a first plurality of AC/DC converters electrically interfaced with the DC transmission ring exterior to the local AC load center for converting the AC power from the distant generating station into DC power and making available the DC power converted at a one of the AC/DC converters to at least some of the other of the plurality of DC loads on the DC transmission ring; a second plurality of AC/DC converters electrically interfaced with the DC transmission ring interior to the DC transmission ring for converting the DC power from the DC transmission ring into AC power; the distribution feeder proximate the local AC load center electrically connected to the DC transmission ring through the second plurality of AC/DC converters for supplying the AC power to the local AC load center while all local AC loads are isolated from the distant electric power generating station; characterised in that the electric power transmission system includes a plurality of isolating DC-DC converters, interposed at predetermined locations on the DC transmission ring for isolating faults on the DC transmission ring to prevent the faults from disturbing the system. a local AC load center having a plurality of local AC loads, and at least one distribution feeder serving the plurality of local AC loads; at least one distant electric power generating station for supplying AC power to the local AC load center by means of an AC transmission line; a DC transmission ring having a plurality of DC loads thereon at least partially interposed between the local AC load center and the distant electric power generating station, the DC transmission ring isolating the AC power received from the distant electric power generating station from the local AC load center; a first plurality of AC/DC converters electrically interfaced with the DC transmission ring exterior to the local AC load center for converting the AC power from the distant generating station into DC power and making available the DC power converted at a one of the AC/DC converters to at least some of the other of the plurality of DC loads on the DC transmission ring; a second plurality of AC/DC converters electrically interfaced with the DC transmission ring interior to the DC transmission ring for converting the DC power from the DC transmission ring into AC power; the distribution feeder proximate the local AC load center electrically connected to the DC transmission ring through the second plurality of AC/DC converters for supplying the AC power to the local AC load center while all local AC loads are isolated from the distant electric power generating station; characterised in that the electric power transmission system includes a plurality of isolating DC-DC converters, interposed at predetermined locations on the DC transmission ring for isolating faults on the DC transmission ring to prevent the faults from disturbing the system.