Impact of IEC 61850 on Protection and Automation

Author: Klaus-Peter Brand

Impact of IEC 61850 on Protection and Automation

IEC 61850, the standard for communication in substations was published three years ago and is already widely used in Substation Automation projects. The goal of the standard is interoperability between devices from different manufacturers. It supports the interconnection of all applications in the substation automation (SA) system from the station level with its HMI and remote control gateway to the protection and control IEDs in the bays (station bus), and from these IEDs down to the switchgear (process bus). It supports also the use of unconventional current and voltage sensors. It may replace all signal wires by serial communication links. The standard goes beyond the definition of communication since it provides additional important features like the domain specific Data Model and the Substation Configuration description Language (SCL). Therefore questions came from the users: What is the impact of IEC 61850 on protection and automation? How introduce IEC 61850 based substation automations system to exploit all benefits but to minimize the risk of this step?

The CIGRE Study Committee B5 had formed the WG 5.11 which created a brochure covering all these topics and, as common, a summary in Electra - both published in fall 2007. This CIGRE brochure cannot replace more than 1000 pages of the standard but is intended as a practical guideline for utilities. This article cannot replace the 110 pages of the CIGRE brochure but explain shortly some findings and highlight its helpful role for utilities.

The chapters in the brochure were written by different authors from utilities and providers. The brochure was compiled by the members of working group 5.11 and crosschecked by the representatives of CIGRE SC B5 36 member countries world-wide. The idea is that each chapter is readable by itself depending on the background and the aim of the reader. Therefore, there is some overlap between the chapter content.

Benefits and justification
Chapter 2 summarizes the features of IEC 61850 and points to the benefits. The combination of all it s discussed features makes the standard unique. The homogeneous and comprehensive abstract data model including all services for the communication in substations is formulated very near to the user's (substation engineer) terminology and independent from any implementation which is left as task for the manufacturers.

The mapping of this model to main stream communication means i.e. MMS, TCP/IP and Ethernet makes the standard future proof.

The inclusion of the sampled values service allows exploiting the benefits of new non-conventional instrument transformers like Rogowski coils, capacitive dividers, and electro-optical sensors for current and voltages, as well as using the common conventional transformer-type ones. The SCL of IEC 61850 provides a comprehensive description of the complete SA system. It was defined to be used by all tools - also from different manufacturers - for configuration, engineering, testing, and maintenance i.e. in any phase of the life-cycle starting from any single compliant product and ending with the maintenance phase of the customer specific SA project (Figure 3). In chapter 3 it is shown how these benefits correlate to operative and cost benefits for the utility justifying the introduction and use of IEC 61850. Examples are the use of SCL and mainstream communication technology, but also the options to replace copper wires by serial fiber optic links transporting GOOSE messages or to use any kind of today's and tomorrow's current and voltage sensors. Last not least, interoperability is not only provided between devices of different suppliers but also between different generations of products.

Concepts and migration
SA systems realized according to IEC 61850 up to now are more or less one-to-one copies of existing ones replacing only the proprietary communication by IEC 61850.

This step is already beneficial since it excludes communication from competitor comparison and facilitates the integration of a third party main 2 protection as needed for transmission lines.

At the beginning of chapter 6 it is recommended to reconsider the system concepts to exploit the benefits of IEC 61850 as much as possible. This is especially important for migration strategies (chapter 4). There are no general strategies because any migration depends on the actual state and the intended goal for the SA system.

Specification of IEC 61850 based Systems
The most sensitive phase for SA systems is the specification phase because corrections later in the implementation phase may either be not possible or very costly.

Guidelines for specification are given in chapter 6 by description and as checklist. The description of the site and the already existing or newly ordered switchgear is essential. The starting point is the single line diagram of the substation and the allocated SA functions as usual. The communication design based on Ethernet is more flexible and scaleable than the previous proprietary ones. Active elements like switches support this flexibility.

To get an optimized SA architecture,requirements for both availability and performance have to be stated. If there are no restrictions in the specification, GOOSE messages may replace all wiring between IEDs. At least for new substation the use of unconventional instrument transformers providing samples via the process bus may be considered. However, these advanced features are not a must for using IEC 61850 but an option for the future. The responsibility that the system composed of interoperable devices from different
suppliers is running as specified has to be taken by the System Integrator and fixed in the specification. This role needs appropriate tools, test equipment and trained staff. Besides the SA system itself the most important item to be delivered is a single SCL-based Substation Configuration Description (SCD)
file - a very cost effective basis for all testing and maintenance tools and,
therefore, for any future upgrades also.

Responsibility in utilities
The project execution (chapter 7) is normal besides the fact that in the engineering process the SCD for the complete system has to be created and reused for system tools. In addition to the specification, the procurement process (chapter 5) and the life-cycle management (chapter 8) are within the responsibility of the utility. The utilities should invest in the knowledge about the standard to understand what they may request and what they will get. The integration of the different functions in the substation to one system may strongly impact the structure of utility organization (see Figures 1 &2).

The introduction of IEC 61850 and its impact on protection and automation within substations

  • Cigre Brochure 326 (produced by SC B5 WG B5.11), 2007, price 75/150 ?,
  • Summary in Electra N°233, August 2007, 21-29
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