Evaluation of Time Gateways for Synchronization of Substation Automation Systems

The International Electrotechnical Commission (IEC) 61850 standard is widely used in substation automation system (SAS), even if some aspects related to the network-based time synchronization are still under investigation. The latest version of the IEC 61850 standard introduces IEEE 1588 Precision Time Protocol (PTP) for distributing time in the station and process buses, in addition to the previously used Simple Network Time Protocol (SNTP). Some time synchronization problems may arise when mixing old and new IEC 61850 devices in the same system; the IEEE 1588 PTP and SNTP technologies have somewhat different time representations and synchronization schemes, requiring time gateways for smooth integration. This paper introduces and compares the performance of some compact time gateways with different implementation architectures. All the examined gateways have the same external structure: They are transparent two-port devices which are inserted in the last network link between the switch and the end device, in order to perform the time conversion from IEEE 1588 PTP to SNTP. The time gateways are built using the same hardware platform based on a Field Programmable Gate Array that enables the creation of real embedded prototypes. The experimental results show that all the considered time gateways are applicable to SAS, but some of them have better performance than others in terms of synchronization accuracy. Moreover, the authors identify the bottleneck in the SNTP implementation of the time gateway architecture. A careful analysis of the behavior of SNTP is proposed, and useful suggestions for trading off between synchronization accuracy and time gateway complexity are given.