CIGRE 2008 B5, preferential topic 1

Paris, France. 24 - 29 August

The architecture of a substation automation system may significantly be changed, when a process bus based on IEC 61850 is added. Issues to be considered include testing, reliability and time synchronization. To achieve a cost reduction is one of the major drivers to introduce a process bus. A fully distributed system may offer the most flexibility. However, to address concerns on reliability and availability, optimized system architectures are proposed. This is a controversial topic. Some consider an optimization to address the availability issues as needed and not providing limitations. That may be restricted to the grouping of functions, but not necessarily to the communication links. Other contributions state that by using real time Ethernet, the availability issues can be solved using a flexible architecture. Availability figures were presented for different architectures. From the selected architectures, all redundant solutions fulfill the requirements.

There was an agreement, that an interoperable solution for time synchronization is required as part of the standard. Currently, this is a 1 PPS signal; however, other solutions like IEEE 1588 are proposed as part of a new edition of the standard. Several aspects of time synchronization that need to be considered where mentioned. Samples need to be synchronized at source and the standardization of sample rate is important for interoperability. Synchronization islands may improve availability.

Cyber security for a process bus was not discussed in the papers, but the issue was raised in the discussion during the conference. All contributors agreed about the relevance, but had different opinions on the methods to be used. One proposal was, to use physical security methods; i.e. to build a fence around the substation and keep it simple inside. Another proposal was to avoid security issues by choosing a point to point architecture. The third proposal referred to SW methods based on IEC 62351, but it was not recommended to use cryptography for GOOSE and sampled value messages.

When introducing a new technology within a system, it is not sufficient to consider only the technical factors. Business benefits like the overall installation costs versus payback are another factor. The process bus helps to do more projects faster, with less resources and less costs. During the session, the key features that reduce the life cycle costs of a process bus based substation automation system have been discussed. These features include an easier implementation, the elimination of copper wires and the self-describing documentation. Extensions and upgrades of the system can be done as SW modifications and the maintenance on the system can be done safely. Another important aspect is the possibility to easily integrate non conventional CTs and VTs with lower costs compared to conventional devices.

Based on the experience with projects using a process bus, there is an agreement, that conformance testing is not sufficient to guarantee interoperability. The experience with interoperability of devices from multiple vendors is various. The freedom offered by the standard in several areas can result in lacking interoperability between IEDs and requires flexibility from client applications.

There is still some hesitation concerning the market acceptance of IEC 61850. A simple architecture, scalability and testability are important for an acceptance. The use of the process bus may be first accepted for new stations or extension of stations with new bays rather than for retrofit projects. In migration scenarios, mixed mode may be required, which means that a relay has to deal at the same time with digital signals from a new sensor system and analog signals from an old sensor system. Experience with mixed mode is available from pilot projects. A different solution to designing protection relays with both analog as well as digital inputs, can be to add instead merging units to the old sensor system doing the analog to digital conversion outside the relay.

Condition monitoring of primary equipment is done in order to either extend the life of the equipment or to change from a time based maintenance to a condition based maintenance. New logical nodes will be defined in edition 2 of IEC 61850 to model the condition monitoring information.

IEC 61850 in general will have an impact on testing. Standardized configuration files can be used to configure test equipment. With the introduction of a process bus new concepts are required, in particular concerning the isolation of the device under test from the rest of the system. The standard offers several features that can help, but specifications on how to use and interpret these features are required to achieve interoperability. The protection relay, the communication network including communication links and the merging unit and primary equipment are three components that need to be tested independently. Using the substation configuration language, an automation of the tests is possible.

To conclude, we can note that significant progress has been made since the publication of IEC 61850. Different approaches of system architectures exist that address the availability of process bus based solutions. There are some concerns about the impact of Ethernet switches on the availability of the system.

Some further standardization work is required to facilitate the use of the process bus. Introduction of a time synchronization method that uses the communication network may help to reduce costs and improve availability. The use of the different features for testing support provided by IEC 61850 may need further agreements to achieve interoperability. In general, it may be appropriate to have more agreements that limit some of the flexibility of IEC 61850 for the benefit of easier integration of multi vendor systems. Nevertheless, the technology has been proven, the solutions exist and the experience is available to successfully implement process bus in real projects.

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