IEEE 1588v2 precision timekeeping: getting substation automation in sync

As the marine chronometer demonstrated, simply keeping accurate time is a huge competitive advantage: it allows you to precisely coordinate complex systems.

In the age of exploration, the British Empire was able to achieve great maritime feats thanks to a modest new invention: the marine chronometer, a clock that could keep accurate time while at sea. By setting the chronometer to the local time in the port city of Greenwich, then comparing that time to the position of the sun in the sky, British captains could calculate their longitude to extraordinary levels of accuracy. The chronometer was nothing less than a paradigm shift - a revolutionary advantage that allowed British explorers to surpass their contemporary competitors. Though the British Empire no longer spans the globe, the British marine chronometer’s lasting influence can be seen in Greenwich Mean Time (GMT), the benchmark against which all other local time zones are set.

For industrial networks, accurate timekeeping offers similar advantages. This is especially true of power distribution networks, which need accurate timekeeping and network synchronisation in order to coordinate the activities of widely distributed equipment and regulate power transmissions. Higher levels of precision allow power grids to unlock their own revolutionary advantages: a ‘smart grid’ that can achieve new levels of efficiency, security and reliability by autonomously and intelligently distributing power to end users in response to changes in demand.

This article explores some of the limitations that current industrial systems, especially power substations, face in synchronising their networks. It also provides an overview of today’s commonly used timekeeping technologies, such as NTP and GPS, and identifies how IEEE 1588v2 precision time protocol (PTP) can transform how your current industrial network is run.

Historical time synchronisation technologies

In an industrial data network, time synchronisation allows all of the different devices on that network to use a common clock to coordinate their activities. Network integrators currently have a number of different time synchronisation options available. Each has its own advantages and disadvantages, but not all of them are optimal for use in industrial networks.

Inter-range Instrumentation Group (IRIG) The IRIG standard defines a serial time code format for use with serial communications networks. First standardised in 1956, IRIG signals are a legacy technology used with older serial systems. IRIGB 205-87 is the latest update of this standard. Network Time Protocol (NTP) NTP is a time protocol for data networks, first established in 1985. NTP relies on a hierarchical, layered system to promulgate the current time throughout the network. NTP imposes a hierarchical tree architecture on the network to avoid cyclical dependencies. Global Positioning System (GPS) GPS satellites are highly accurate atomic clocks placed in orbit around the earth. Satellite signals carrying timekeeping information can travel at light speed to receivers on the ground. These light-speed signals are also corrected according to the principles of general relativity, which gives each receiver on the ground highly accurate time information.

Figure 1: NTP divides the network into different strata. (source: B. D. Esham for Wikimedia Commons)

Potential time synchronisation pitfalls

Industrial systems, such as substation automation networks, rely on accurate time synchronisation in order to coordinate activity across many different subsystems and devices. However, many existing technologies are inadequate for the needs of industrial automation measurement and control systems.

Accuracy

For industrial networks, every nanosecond counts - but most legacy technologies are simply unable to deliver that level of performance. For example, a substation automation network needs nanosecond-level accuracy on raw data sampled values in order to better support mission-critical applications such as fault recording, remote monitoring and remote control. IRIGB and NTP are an order of magnitude too slow to achieve nanosecond accuracy. Even under ideal, local conditions, NTP’s accuracy can be measured in the hundreds of microseconds.

Figure 2: An IEEE 1588v2 system needs just one GPS receiver to provide highly accurate time information to many devices.

Cost

The GPS network provides highly accurate time data measured by extremely precise atomic clocks, but in order to access that information the network must have a GPS receiver at each node. This is a prohibitive cost that is impractical for industrial networks where each device needs time information and its own GPS receiver. GPS would become more practical if there was some way to reduce the number of nodes in the entire network, or more efficiently use a fewer number of GPS receivers so that the entire network can benefit from the accuracy of GPS timekeeping.

A time protocol for industrial networks

NTP, GPS and IRIGB are capable technologies that simply aren’t suited to the requirements of substation operations. Fortunately, the IEEE 1588v2 Precision Time Protocol (PTP) is designed specifically for industrial networked measurement and control systems. In a network based on IEEE 1588v2, the grandmaster clock determines the reference time for the entire substation automation system. Ethernet switches can act as boundary or transparent clocks, and additional devices (such as merging units, IEDs and protection devices) are designated as ordinary clocks. All of these devices are organised into a master-slave synchronisation hierarchy with the grandmaster clock at the top. As illustrated in the figure below, exchanging PTP packets between master and slave devices, and automatically adjusting the ordinary clocks, effectively synchronises the entire network. Only the grandmaster clock needs a connection to GPS timekeeping; that data can be accurately distributed to the rest of the devices on the network through the synchronisation hierarchy.

An ethernet switch that supports IEEE 1588v2 can guarantee timestamp accuracy to within 1 µs and can be configured for master, boundary or transparent clock functionality. To be truly precise, the rest of the network needs to support IEEE 1588v2 as well: in an industrial computing network, IEEE 1588v2-compliant computers fill the role of the ordinary clock that receives synchronised time data from the ethernet switch.

Figure 3: To be in sync, all of the system components, including the embedded computers, should support IEEE 1588v2.

When the entire network supports IEEE 1588v2, the system can coordinate operations down to the nanosecond level and still keep perfectly in sync. This level of coordination is especially valuable in power substation systems, which is why IEEE 1588v2 is part of the IEC 61850-2 standard specifying communications requirements for power automation networks. The IEC incorporated IEEE 1588v2 into the standard because more precise time synchronisation allows electrical substations and power automation networks to achieve the following benefits:

• Blackout prevention through early detection of grid problems, early location of disturbances and real-time power islands.
• Accurate fault recording and event loggers that enable precise event analysis thanks to event loggers that can be scrutinised down to the nanosecond level.
• More efficient use of assets through congestion relief and equipment condition monitoring.
• Demand response through time-of-use billing, virtual power generators and outage management.

Conclusion

IEEE 1588v2’s cost-effective, nanosecond-level accuracy gives substation and other power utility networks a competitive edge just as formidable as the one Britain’s chronometer-equipped navy possessed over its peers. As part of a smart grid, highly synchronised substations are more efficient, more economical, more sustainable and more responsive. These advantages allow electricity providers to increase the profitability of their operations and decrease their impact on the environment.

By Justin Wu, Product Manager, and Bruce Chen, Project Supervisor, Moxa Inc