Global Positioning System (GPS)
GPS is a Global Navigation Satellite System (GNSS) operated by the U.S. Department of Defense. This system enables receivers to compute their location, velocity, and time for a variety of applications. Power utility applications use precise and accurate time derived from the GPS-synchronized clocks along with other sources for time synchronization of devices in the power system.
GPS signals can be disrupted due to natural atmospheric interference (e.g., solar flares), antenna failures, or man-made signal manipulation or denial (e.g., GPS jamming and GPS spoofing). In cases involving solar flares, antenna failures, and jamming, most timing devices will detect the loss of GPS and switch to a "holdover" mode of operation, where they use their internal stable oscillators to maintain accurate time. GPS spoofing involves the intentional manipulation of an authentic GPS signal and the rebroadcasting of it to GPS receivers. If the GPS receivers lock to this signal, they could report incorrect timing information without indicating any anomaly to the connected end device.
There are many applications within the power system that use precise time to improve the operational efficiency and dependability of the power grid. In many cases, GPS-based clocks are used to provide high-accuracy time references for these applications. Precise time is used to improve the operational performance of protection applications and for event analysis, state measurement, and fault locating. There have been claims and suggestions that the electric power system is vulnerable to attack by disabling, jamming, or spoofing GPS receivers. While it is true that the majority of the GPS-based time sources used in the power system are vulnerable to being jammed or disabled, it is not true that the result would lead to a disruption of the power system itself. Most widely used protection schemes do not rely on, nor should they rely on, GPS-based time synchronization. There are some special cases where an accurate wide-area time reference has been incorporated to enhance the protection elements. In these cases, there are Terrestrial Time Distribution (TTD) methods that can be applied without relying on GPS.
SEL relays do not depend on GPS. If GPS is not available for acquiring precise time, SEL relays will fall back to using alternative methods for performing protection operations. SEL relays will not misoperate if GPS clocks are disabled, jammed, or spoofed. Because precise time is useful, SEL provides terrestrial precise time distribution over wide-area networks (WANs) using the SEL ICON Integrated Communications Optical Network solution.
For many years, SEL has been teaching that GPS time is not guaranteed. SEL’s recommended best practice is to implement a layered approach to precise time synchronization systems.
There are detection, mitigation, and time distribution approaches that can used to maintain precise time in the event of a disruption to GPS.
- Terrestrial Time Distribution
Terrestrial Time Distribution (TTD) systems can be used to mitigate GPS signal disruptions and maintain high-accuracy time synchronization across a wide area. Synchronous Optical Networking (SONET) is a wide-area network (WAN) communications technology based on time-division multiplexing (TDM) that uses frequency synchronization across the entire network for high bandwidth data communication. The SEL ICON comes standard with an integrated GPS receiver that is used for network timing and the distribution of a precise time reference for intelligent electronic devices (IEDs) connected through this network. In the event of a GPS outage or antenna failure, precise time distribution is maintained with a relative submicrosecond accuracy using an internal time reference across the entire WAN.
- Redundant GPS Clocks With Time Distribution
When multiple GPS clocks receive GPS signals simultaneously, separated by some distance, they will produce time outputs independently from each other. These time signals can be compared using a system that monitors the health of the signals. Examples include monitoring the loss of signal for IRIG-B outputs from multiple clocks or monitoring the time quality bits in an IRIG-B time code (IEEE C37.118 2011). SEL has a low-cost IRIG-B selection system (using the SEL-3400 IRIG-B Distribution Module) available today that can reliably perform the function of selecting the best IRIG-B source based on time quality or the loss of time signals. This device, along with an SEL GPS clock, such as the SEL-2488 Satellite-Synchronized Network Clock, also provides additional features, like delay compensation so the accuracy of the time outputs is preserved as the signal transits through several devices before it reaches downstream intelligent electronic devices (IEDs).
When there is a single GPS antenna failure due to factors such as a lightning strike, precise time can still be maintained using the clock reference from an alternate GPS clock.
- Multiconstellation GNSS Receivers
Apart from U.S.-based GPS, there are several other GNSS technologies being deployed to provide information, such as location, time, and velocity, for use in various applications. These include the Russian Global Navigation Satellite System (GLONASS) and, in the future, the Chinese BeiDou and the European Galileo systems. The SEL-2488 Satellite-Synchronized Network Clock simultaneously tracks both GPS and GLONASS constellations and independently extracts time signals. The SEL-2488 compares time signals (after converting them to a common reference) received independently from these two constellations and alerts the user if there is a discrepancy between these signals. This approach will validate time signals provided by one GNSS system with another system. As these systems have diversity in carrier frequency, signal coding, etc., this comparison provides an additional layer of security against vulnerabilities like jamming and spoofing.