The Perfect Time: An Examination of Time- Synchronization Techniques

A combination of circumstances has made this the perfect time to look at time-synchronization techniques for power system monitoring devices: • Time synchronization of power system monitoring devices is now center stage because of the problems experienced with post-disturbance analysis following the August 14, 2003 blackout in the northeastern United States and southeastern Canada. NERC recommendations, based on the lessons learned from this postdisturbance analysis, place a high priority on implementing time-synchronization techniques that will eliminate or reduce the effort involved with comparing event information from distributed intelligent disturbance recording devices. • Maturing synchrophasor measurement technology is also creating a demand for very accurate time synchronization of devices in widely separated locations. • Global Positioning System (GPS) high-accuracy synchronized time code signals are now accessible through economical clock receivers that are designed for simple and reliable installation in the harsh environment of electric power facilities. • Time synchronization can be accomplished through communications network technologies that are becoming more prevalent in utility and industrial power facilities. This paper discusses available techniques to acquire synchronized time signals, various standard time signal formats, techniques for distributing these time signals to monitoring and recording devices, and the relative accuracy required by various power system timing and recording applications. Emphasis is given to the accuracy requirements of monitoring and recording devices, the relative accuracy of time signal sources, the effect that time signal distribution has on the accuracy of the signal, and the effect that device sampling and processing have on the accuracy of time tags applied to sampled data. Time-synchronization signal sources include earth-based radio transmission (WWV, WWVH, WWVB, and LORAN-C), satellite-based signal transmission (GOES, GPS, and GLONASS), and time-setting messages via communications networks and dial-up modems. All of these timesynchronization signal and time-setting sources can usually be traced back to common or coordinated precision time references operated and provided by government standards organizations. Coordinated Universal Time (UTC) has become the defacto standard time reference for many applications, including most electric power industry applications. This time is now the accepted standard because of its worldwide availability and relatively economical access through GPS satellite clock receivers. This paper focuses on the use of GPS satellites as a synchronized time source and compares and discusses various clocks and clock signal distribution methods to meet electric power industry time-synchronization applications. The other time-synchronization sources mentioned above are discussed in the appendix of this paper for reference purposes.