Time Synchronization in Substations

Guest Article by
Tor Skeie, ABB Corporate Research

Ethernet Communication in Substations

ABB

Project description

At this point in time, the Substation Automation (SA) business is on the edge of migrating towards open solutions. A proof of this new trend is the upcoming IEC 61850 standard on Communication networks and systems in substations issued by Technical Committee 57. The ultimate vision is twofold:

	Cost savings by deploying commercial off-the-shelf components.
	Achieving interoperability between products from different vendors on all levels within the substation automation field.

The communication within Substation Automation systems requires very time-critical data exchanges. The high-speed properties of switched Ethernet together with its dominant position in LAN networking makes Ethernet the prime communication candidate for substation automation use.

In order to prove the usability of switched Ethernet relative developing the Communication in Substations was established. The primary goal of the project has been:

	To demonstrate that precise time synchronization of Intelligent Electrical Devices (IED) over switched Ethernet is possible.
	To prove that switched Fast Ethernet has sufficient real-time characteristics to meet the Substation Automation demands.

Highlights

SA time synchronization requirements are associated with four levels of accuracy defined as the IEC 61850 classes 1 to 5. This project has challenged the hardest requirements and succeeded to present general solutions for IEC classes 4/5 (4ms/1ms) and IEC class 3 (25 ms) in multi-traffic LAN environments.

	The IEC class 3 solution is a complete off-the-shelf concept based on standard Ethernet switches and timeservers. We have shown that using low-level software time stamping of the synchronization packets outside the real-time operating system is indistinguishable from time stamping using special-purpose hardware.
	The IEC class 4/5 requirements can hardly tolerate any inaccuracy within the infrastructure. OnTime Networks, which is a collaborating partner in the project, provides an elegant solution integrating the timeserver into industrial-class Fast Ethernet switches. By this integral approach OnTime Networks has made IEC classes 4/5 synchronization over switched Ethernet possible.
	By advanced simulations of real substations we have furthermore proved that switched Fast Ethernet has sufficient real-time properties for being a common network infrastructure in Substation Automation.

Impact for substation automation

Traditionally, realization of SA systems has mirrored the three logical levels of the functionality both from a device and network perspective. The proven real-time properties of switched Ethernet means that Substation Automation can break down the traditional bus levels migrating to a common single network concept achieving substantial cost savings.

By demonstrating an implementation of the most critical IEC classes of synchronization accuracy over switched Fast Ethernet the final obstacle of fully migrating to Ethernet in SA is removed. This step would significantly reduce the cabling and transceiver cost, since today dedicated (separate) links are used for this purpose.

The results from the project are proactively applied by ABB in the IEC standardization.

For more information please contact
Tor Skeie, ABB Corporate Research
tor.skeie@no.abb.com

Figure: Switched Ethernet as a common network infrastructure in a future substation automation system. Process bus communication links (access points) pin pointed both in a real substation and a conceptual picture.

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Last modified: Sunday August 01, 2004 .