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.
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