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## EPFL
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The distributed measurement of synchrophasors via Phasor Measurement
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Units (PMUs) represents one of the most advanced sensing layers in the
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domain of Wide Area Monitoring and Control (WAMC) systems. According to
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the IEEE Std. C37.118.1-2011, a PMU is a measurement device synchronized
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to a common Coordinated Universal Time (UTC) reference, which reports
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phase-aligned and time-stamped measurements of frequency, amplitude and
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phase angle of the voltage and current phasors of a power system.
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In general, the time synchronization of PMUs relies on the Global
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Positioning System (GPS) as it represents a good tradeoff between
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performance and cost. However, this synchronization system has three
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main drawbacks: (i) accuracy, (ii) accessibility and (iii) security.
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Concerning the first point, it is worth noting that, in order to
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correctly support synchrophasor based applications, PMUs must be
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characterized by high accuracy levels in estimating the synchrophasors,
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especially when power distribution systems applications are envisaged.
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Modern PMUs are adopting synchrophasor estimation algorithms exhibiting
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steady state phase accuracies in the order of tens of ppm of radians.
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Since these values correspond to time jitter that typically
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characterizes the GPS units usually adopted by the PMU hardware, we may
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conclude that one of the barriers to improve the steady state PMU
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accuracy is the uncertainty of the time dissemination technology.
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Concerning the second point listed before, i.e., the accessibility of
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the GPS, we can observe that such a time source might not provide a
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stable and reliable time reference especially in cases where underground
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substations, with limited or no access to the sky, need to be equipped
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with PMUs. Therefore, a more suitable time dissemination technique,
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deployable over the legacy power system’s telecom infrastructure, might
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be required. Concerning the third point, security, recent works have
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shown that, since civilian GPS satellite signals are not authenticated,
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they can be spoofed by superimposing a fake signal with a higher
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signal-to-noise ratio, which would enable an attacker to manipulate the
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GPS clock.
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Among the possible alternatives to the GPS, White Rabbit (WR) represents
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an excellent candidate as time synchronization protocol in view of its
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superior peculiarities. In this project, we have developed a PMU
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integrating WR technology. Using a WR network composed of a WR switch
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and WR-cRIO modules, we assessed the performance of the WR-PMU by means
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of a PMU calibrator. Moreover, we compared the performance of the WR-PMU
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with the one of a GPS-based PMU. The two PMUs were characterized by the
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same synchrophasor estimation algorithm and by the same hardware
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platform, with the exception of the time synchronization module and
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technique. The results exhibit similar performance between the WR-PMU
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and the GPS-PMU, showing WR’s applicability for PMUs. The developed
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system can be used as an alternative for specific conditions where the
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GPS signal is not available.
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