Is your clock ticking all the time? Characterizing substation-hardened clocks for automation

Modern power systems rely on precise and accurate time signals for efficient operation. Time-based measurement of power system signals is now possible with high-speed signal sampling combined with precise time sources. This paper describes advances in time sources, protocols, and distribution methods supported by modern substation clocks. It also describes how each of these time sources and protocols is characterized for performance. Affordable time technology for power utilities has advanced from milliseconds of accuracy 40 years ago to a few tens of nanoseconds in the past few years. Time distribution capabilities have improved as well with new ways of distributing time over local-area and wide-area networks. Technologies like traveling wave fault location (TWFL) have been in existence for decades but did not advance for years. With advances in precise and accurate time sources as well as distribution, there has been a fresh look at TWFL. Applications based on TWFL, synchrophasors, and Sampled Values can now take advantage of nanosecond timing accuracies. These solutions depend on reliable substation clocks designed, built, and tested with the same rigor as other protection and automation equipment in the substation. This paper also describes some of the common failure modes for substation clocks and their recovery mechanisms, including how these conditions are tested prior to deployment. Substation clocks need to withstand the same electrical and environmental stress conditions that protective relays are designed to withstand. This paper describes test setups with pass/fail criteria to characterize substation-hardened clocks during these conditions.