Precise timekeeping for 'Smart Grid'
March 2012, Electrical Power & Protection
For industrial networks, accurate timekeeping offers 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 white paper 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 1588 v2 precision time protocol (PTP) can transform how your current industrial network is run.
An overview of 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.
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 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.
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 are not suited for 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. The Ethernet switch acts as the boundary or transparent clock, 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. 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.
An Ethernet switch that supports IEEE 1588v2 can guarantee time-stamping accuracy to within 1 μs, and 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.
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.
* Accurate fault recording and event loggers.
* More efficient use of assets.
* Demand response.
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.
The Moxa advantage: getting substation operations in sync
Moxa’s PTP-7728-PTP IEC 61850-3 Fast Ethernet switches support the latest version of IEEE 1588v2 technology to deliver precise time synchronisation for substation network and related applications. The PT-7728-PTP offers:
* Up to 14 100BaseFO (Multi-mode ST connector) or 100BaseTX, ports and 1 BNC connector, support for IEEE1588 v1 and v2 hardware time stamping on each port, and pulse outputs (pps) on one BNC port.
* 1 and 2-step for both transparent and boundary clock operation with accuracy under 1 μs in End to End mode.
* 2-step for both transparent and boundary clock operation with accuracy under 1 μs in Peer to Peer mode.
* Network clock synchronisation accuracy in the nanosecond range.
* Clock synchronisation to support large and distributed substation networks.
* Low-cost implementation in multicast messaging networks such as Ethernet.
* Fast re-synchronisation when system changes occur.
* Simple installation and maintenance.