Application of Precision Time Protocol Standard with multi-vendor Relays over PRP and HSR Architectures: A Real Test of the PTP Standard in project FITNESS

Authors: P. Mohapatra, SP Energy Networks, UK, C. Popescu, ABB, UK, P. Balasubramani, GE, UK, M. Wehinger and Ali Abdulla, OMICRON electronics

The equipment will be deployed at Wishaw 275 kV substation in Scotland. It is the first IEC 61850 multivendor based system with equipment from GE Grid Solutions, ABB Ltd, Synaptec and testing tools from OMICRON electronics.
In the FITNESS project, the innovation is to be found in the way the overall system has been designed in order to prove interoperability - a mixture of protection and control equipment from various manufacturers - at process and station bus, as well as Substation Control System (SCS). This project will enable SP Energy Networks to prove not only interoperability at multiple levels but also within a mixture of old and new technologies. The proposed FITNESS architecture introduces the IEC 61850-9-3 (Power Utility Automation Profile) Precision Time Protocol (PTP) with the two redundancy methods in the communication architecture – the Parallel Redundancy Protocol (PRP) and the High-availability Seamless Redundancy (HSR).

Time synchronization is of paramount importance for maintaining reliability and availability of the process bus and sampled values and consequently, the availability of critical protection and control functions in a digital substation. Depending on the regulations, utilities may not accept loss of protection functionalities for more than 1-3 secs, and such loss cannot be a frequent occurrence. In the FITNESS project time synchronization is achieved predominantly by application of the IEC 61850-9-3 standard. This part of the IEC 61850 standard specifies a PTP power utility automation profile of IEC 61588:2009 | IEEE Std 1588-2008 applicable to power utility automation, which allows compliance with the highest synchronization classes of IEC 61850-5 and IEC 61869-9. There is also application of pulse per second (pps) in FITNESS for one merging unit (derived from a PTP compliant IED) and Simple Network Time Protocol (SNTP) for SCADA systems. This article focuses on the application and testing of PTP in the FITNESS digital substation solution.

In a typical digital substation application, the most demanding network in terms of time synchronization is the process bus level where accuracies as low as 1 microsecond are required to meet the adequate accuracy level for sampled values. Going up in the hierarchy of the substation automation systems (SAS), the bay level requires accuracies in the range of milliseconds and the station in the range of tens/hundreds of milliseconds. Like other important areas within SAS, time synchronization has developed over the years from a standalone system using its own time synch infrastructure to time synch sources that send the clock signal directly over the Ethernet network. By doing so, a significant reduction of the time synch cabling is expected.

Standards Implemented
The proposed architecture for FITNESS project uses the IEC 61850-9-3 PTP- power utility profile. In PTP networks there must be only one recognized active clock at a time, called the Grand Master Clock. If there are two Grand Master Clocks on the network (as it is the case in FITNESS project) then a Best Master Clock Algorithm (BMCA) is required in order to ensure that all client devices will synchronize to the same Grand Master Clock. The other Grand Master Clock that is not detected / selected as Grand Master Clock (GMC) will stop sending the synchronization packets itself and will accept the synchronization from the Best Grand Master Clock (BGMC). (see Figure 1).

All components play a big role in a PTP network and directly influence the level of accuracy that can be achieved by the clients. Asymmetric network connections degrade the accuracy, therefore classic layer 2 and 3 Ethernet switches with their “store and forward” technology are not suitable for PTP networks and should be avoided.
PTP synchronization is LAN (Layer 2 Ethernet Mapping) based, multicast addressing using peer-to-peer delay measurement. An important requirement for PTP based systems is that switches (at both station and process bus levels) and IEDs must be able to act as Transparent Clocks (TC) and as Boundary Clock (BC) respectively.

A Transparent Clock (TC) is capable of measuring the time taken by a PTP message from the ingress port to the egress port and to add the measured time value to a correction cell in the message. In this way, any delays/asymmetry that the device could introduce in the transfer of PTP messages are corrected. Network switches belong to this category of Transparent Clocks. A TC shall introduce less than 50 ns of time inaccuracy, measured between the applied synchronization messages at any ingress port and the produced synchronization messages at any egress port, provided that it is in steady state.

A Boundary Clock (BC) is a multiport device that synchronizes to the reference time provided by a master clock on one port and delivers time on one or more ports. Switches, bridges, routers and some IEDs belong to this category. A BC shall introduce less than 200 ns of time inaccuracy between the port in the SLAVE state and any port in the MASTER state, provided that it is in steady state.

An Ordinary Clock (OC) is a clock that has a single Precision Time Protocol (PTP) port in a domain and maintains the timescale used in the domain. It may serve as a source of time, i.e., be a master clock, or may synchronize to another clock, i.e., be a slave clock. IEC 62439-3 allows 2 for Ordinary Clocks that are doubly attached. When the OC is the best clock of its subdomain, it becomes the Grandmaster Clock (GMC). Its port is in the MASTER state and sends Sync messages to synchronize all slave clocks. Otherwise, its port is in the SLAVE (or PASSIVE) state and it receives synchronization messages.

Hybrid clocks (HC) combine a transparent clock (TC) and an ordinary clock (OC).
The Best Master Clock Algorithm (BMCA) is the basis of PTP functionality. The BMCA specifies how each clock on the network determines the best master clock in its subdomain of all the clocks it can see, including itself. The BMCA runs on the network continuously and quickly adjusts for changes in network topology (HSR or PRP).
The BMCA typically uses the following criteria to determine the best master clock in the subdomain:

  • Priority assigned to the clock (1 and 2) by the user
  • Clock Class: Global when synced to a satellite, Local synced with local oscillator and other states for other levels of holdover
  • Clock’s accuracy to UTC
  • Clock variance dependent on the clock’s oscillator stability
  • Unique Identifier: MAC-Address-based selection

In addition to identifying the best master clock, the BMCA also ensures that clock conflicts do not occur on the PTP network by ensuring that:

  • Clocks do not have to negotiate with one another
  • Master clock identification process ensures no two master clocks or no master clocks
Relion advanced protection & control.
BeijingSifang June 2016