IEC 61850 Communication Networks and Systems In Substations: An Overview for Users
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| Power System Automation Devices | Source: Siemens |
Over the last decade, the “digitization” of the electron enterprise has grown at exponential rates. Utility, industrial, commercial, and even residential consumers are transforming all aspects of their lives into the digital domain. Moving forward, it is expected that every piece of equipment, every receptacle, every switch, and even every light bulb will possess some type of setting, monitoring, and/or control.
In order to be able to manage a large number of devices and to enable the various devices to communicate with one another, a new communication model was needed. That model has been developed and standardized as IEC 61850 – Communication Networks and Systems in Substations. This paper looks at the needs of next-generation communication systems and provides an overview of the IEC 61850 protocol and how it meets these needs.
Communication System Needs
Communication has always played a critical role in the real-time operation of the power system. Consequently, a key component of a communication system is the ability to describe themselves from both a data and services (communication functions that an IED performs) perspective. Other “key” requirements include:
- High-speed IED to IED communication
- Networkable throughout the utility enterprise
- High-availability
- Guaranteed delivery times
- Standards-based
- Multi-vendor interoperability
- Support for Voltage and Current samples data
- Support for File Transfer
- Auto-configurable / configuration support
- Support for security
Scope and Outline of IEC 61850
- Introduction and Overview
- Glossary of Terms
- General Requirements
- System and Project Management
- Communication Requirements for Function Models
- Configuration Description for Communication in Electrical Substations Related to IED's.
- Basic Communication Structure for Substation and Feeder Equipment
- Specific Communication Service Mapping (SCSM) - Mappings to MMS (ISO/IEC 9506 – Part 1 and Part 2) and to ISO/IEC 8802-3
- Specific Communication Service Mapping (SCSM) - Sampled Values over Serial Unidirectional Multidrop Pointto-Point Link.
- Conformance Testing
The major architectural construct that 61850 adopts is that of “abstracting” the definition of the data items and the services, that is, creating data items/objects and services that are independent of any underlying protocols. The abstract definitions then allow “mapping” of the data objects and services to any other protocol that can meet the data and service requirements. From a system perspective, there is a significant amount of configuration that is required in order to put all the pieces together and have them work.
Modelling Approach
Legacy protocols have typically defined how bytes are transmitted on the wire. However, they did not specify how data should be organized in devices in terms of the application. This approach requires power system engineers to manually configure objects and map them to power system variables and low-level register numbers, index numbers, I/O modules, etc.
IEC 61850 is unique. In addition to the specification of the protocol elements (how bytes are transmitted on the wire), IEC 61850 provides a comprehensive model for how power system devices should organize data in a manner that is consistent across all types and brands of devices.
Read: IEC 61850 Logical Nodes and Data Classes in Power System Automation Data Modelling
Mapping to Real Protocols
The abstract data and object models of IEC 61850 define a standardized method of describing power system devices that enables all IEDs to present data using identical structures that are directly related to their power system function. The Abstract Communication Service Interface (ACSI) models of IEC 61850 define a set of services and the responses to those services that enables all IEDs to behave in an identical manner from the network behavior perspective.
The mapping of IEC 61850 object and service models to MMS is based on a service mapping where a specific MMS service/services are chosen as the means to implement the various services of ACSI. For instance, the control model of ACSI is mapped to MMS read and write services.
Process Bus
The Process layer of the substation is related to gathering information, such as Voltage, Current, and status information, from the transformers and transducers connected to the primary power system process – the transmission of electricity. The Merging Units in a station sample the signals at an agreed, synchronized rate. In this manner, any IED can input data from multiple MUs and automatically align and process the data.
There is an implementation agreement that defines a base sample rate of 80 samples per power system cycle for basic protection and monitoring and a “high” rate of 256 samples per power system cycle for high-frequency applications such as power quality and high-resolution oscillography.
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| Overview of IEC 61850 Functionality and Associated Communication Profiles |
- SCL enables off-line system development tools to generate the files needed for IED configuration automatically from the power system design significantly reducing the cost and effort of IED configuration by eliminating most, if not all, manual configuration tasks.
- SCL enables the sharing of IED configuration among users and suppliers to reduce or eliminate inconsistencies and misunderstandings in system configuration and system requirements. Users can provide their own SCL files to ensure that IEDs are delivered to them properly configured.
- SCL allows IEC 61850 applications to be configured off-line without requiring a network connection to the IED for client configuration.
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| IEC Substation Model |
- At the “process” layer, data from Optical/Electronic Voltage and Current sensors as well as status information will be collected and digitized by the Merging Units (MUs).
- At the substation level, a Station Bus will exist. Again, this bus will be based today on 10MB Ethernet with a clear migration path to 100MB Ethernet.
- Finally, this architecture supports remote network access for all types of data reads and writes. As all communication is network enabled, multiple remote “clients” will desire access the wide variety of available information.
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