Use of Precision Time Protocol to Synchronize Sampled-Value Process Buses

Transmission smart grids will use a digital platform for the automation of high-voltage substations. The IEC 61850 series of standards, released in parts over the last ten years, provide a specification for substation communication networks and systems. These standards, along with IEEE Std 1588-2008 Precision Time Protocol version 2 (PTPv2) for precision timing, are recommended by both the IEC Smart Grid Strategy Group and the National Institute of Standards and Technology Framework and Roadmap for Smart Grid Interoperability Standards for substation automation. IEC 61850, PTPv2, and Ethernet are three complementary protocol families that together define the future of sampled-value (SV) digital process connections for smart substation automation. A time synchronization system is required for an SV process bus; however, the details are not defined in IEC 61850-9-2. PTPv2 provides the greatest accuracy of network-based time transfer systems, with timing errors of less than 100 ns achievable. The suitability of PTPv2 to synchronize sampling in a digital process bus is evaluated, with preliminary results indicating that steady-state performance of low-cost clocks is an acceptable 300 ns but that corrections issued by grandmaster clocks can introduce significant transients. Extremely stable grandmaster oscillators are required to ensure that any corrections are sufficiently small that time synchronizing performance is not degraded.