Applying digital current differential systems over leased digital service

In the early 1980s, many utilities had current differential pilot wire systems installed on lower voltage lines. These relay systems were applied over either a privately owned communications cable (pilot wire) or a dedicated leased circuit from the local telephone company. In the mid-to-late 1980s, the digital communications boom began and telephone companies started converting their infrastructure to fiber optics. New fiber-optic technologies like SONET and ATM allowed the telephone companies to take advantage of the high bandwidth fiber optics and allowed the greater flexibility of circuit allocation and routing. As with all new technology, this new communications architecture was promoted as great for everybody except, of course, those applications designed to operate specifically over a point-to-point dedicated circuit. Many utilities that relied on these circuits for their pilot wire relaying were advised that these circuits would be obsolete and no longer available. The existing pilot wire relays would not operate over the new circuits, creating a dilemma for the utilities that had relied on them. Faced with this challenge, some utilities converted their pilot wire systems to fiber optic, some converted the protection schemes to distance-based pilot protection, and some held on to the old system as long as possible. In the 1990s, current differential relay systems also evolved from their analog pilot wire roots to completely digital systems with modern high-speed digital communication interfaces. Now, once again, many utilities desired to apply these systems over leased digital communication circuits.