Clock synchronization is a prerequisite for the realization of emerging applications in various domains such as industrial automation and the intelligent power grid. This paper surveys the standardized protocols and technologies for providing synchronization of devices connected by packet-switched networks. A review of synchronization impairments and the state-of-the-art mechanisms to improve the synchronization accuracy is then presented. Providing microsecond to sub-microsecond synchronization accuracy under the presence of asymmetric delays in a cost-effective manner is a challenging problem, and still an open issue in many application scenarios. Further, security is of significant importance for systems where timing is critical. The security threats and solutions to protect exchanged synchronization messages are also discussed.

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A Survey of Clock Synchronization Over Packet-Switched Networks

This paper investigates the possibility of using the PTP (Precision Time Protocol) defined in the Standard IEEE 1588 for applications in electric power system, with reference to one of the most challenging measurement problems, i.e. the measurement of synchrophasors. The proposed approach refers to situations in which many measurement devices, connected to each other by suitable communication links, are located in a geographically limited sub-area of the power system. A laboratory experimental setup has been built to evaluate the performance achievable with this solution in terms of both accuracy and robustness. A detailed analysis of the obtained results is presented.

Application of IEEE 1588 to the measurement of synchrophasors in electric power systems

The present invention is concerned with improved time synchronization among the devices (2, 10, 20, 30) of an industrial process control system, and in particular of a Substation Automation system, during a temporary absence of a system reference time. Hence, disruption of time-critical protection and control functions due to resynchronization following the temporary absence of the system reference time is avoided, and the availability of the time-critical functions configured on the devices is increased. During normal operation, a device of the system records an offset or discrepancy between the system reference time and an internal local clock of the device for a period of several hours. As soon as the system reference time breaks down, the device starts predicting the offset or drift between its local clock and the unavailable system reference time based on the recorded offset history. As a transient clock master, the device then distributes an approximated or transient system reference time, based on the device's local clock corrected for the predicted offset, to other devices of the system that in turn run time-critical protection and control functions.

Time synchronization in industrial process control or automation systems

This paper presents a study on a hybrid architecture considering Global Positioning System (GPS) and Precision Time Protocol (PTP), defined in Standard IEEE 1588, for applications in electric power systems, with reference to one of the most challenging measurement problems, namely the measurement of synchrophasors. The proposed approach refers in particular to situations in which many measurement devices, connected to each other by suitable communication links, are located in a geographically limited subarea of the power system. A laboratory experimental setup has been built to evaluate the performance achievable with this solution. A detailed analysis of the obtained results is presented. Copyright © 2010 John Wiley & Sons, Ltd.

Synchrophasors measurement in a GPS-IEEE 1588 hybrid system

Advanced substation applications, such as synchrophasors and IEC 61850-9-2 sampled value process buses, depend upon highly accurate synchronizing signals for correct operation. The IEEE 1588 Precision Timing Protocol (PTP) is the recommended means of providing precise timing for future substations. This paper presents a quantitative assessment of PTP reliability using fault tree analysis. Two network topologies are proposed that use grandmaster clocks with dual network connections and take advantage of the best master clock algorithm (BMCA) from IEEE 1588. The cross-connected grandmaster topology doubles reliability, and the addition of a shared third grandmaster gives a nine-fold improvement over duplicated grandmasters. The performance of BMCA mediated handover of the grandmaster role during contingencies in the timing system was evaluated experimentally. The 1 μs performance requirement of sampled values and synchrophasors are met, even during network or GPS antenna outages. Slave clocks are shown to synchronize to the backup grandmaster in response to degraded performance or loss of the main grandmaster. Slave disturbances are less than 350 ns provided the grandmaster reference clocks are not offset from one another. A clear understanding of PTP reliability and the factors that affect availability will encourage the adoption of PTP for substation time synchronization.

Quantitative Assessment of Fault Tolerant Precision Timing for Electricity Substations

The IEEE 1588 precision time protocol seems to be a promising way to handle synchronization requirements of tomorrow substation automation. However, one of the remaining issues is its lack of reliability in case of the loss of the GPS signal (e.g. due to atmospheric disturbances or failure of the GPS antenna) which would lead to the desynchronization of the devices inside a substation or between different substations. To keep the devices synchronized without the GPS signal, this paper explores different drift clock prediction techniques. Three main approaches based on statistical, probabilistic and time series techniques are evaluated. On one hand, a statistical-based prediction technique can easily reach an accuracy of less than 10 musec for a prediction duration of a couple of seconds at a minimal computing cost. On the other hand, a time series-based prediction technique can provide an accuracy of 76 musec over a period of 48 hours but at a much higher computing power cost. Finally, we identify the most suitable technique for different type of GPS signal loss that typically occurs in the context of substation automation.

Xiao Yin

Papers

Improving reliability of IEEE1588 in electric substation automation