IEC 61850 Communication Networks and Systems In Substations: An Overview for Users

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

  1. Introduction and Overview
  2. Glossary of Terms
  3. General Requirements
  4. System and Project Management
  5. Communication Requirements for Function Models
  6. Configuration Description for Communication in Electrical Substations Related to IED's.
  7. Basic Communication Structure for Substation and Feeder Equipment
  8. Specific Communication Service Mapping (SCSM) - Mappings to MMS (ISO/IEC 9506 – Part 1 and Part 2) and to ISO/IEC 8802-3
  9. Specific Communication Service Mapping (SCSM) - Sampled Values over Serial Unidirectional Multidrop Pointto-Point Link.
  10. 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.

Overview of IEC 61850 Functionality and Associated Communication Profiles

Substation Configuration Language
IEC 61850-6-1 specifies a Substation Configuration Language (SCL) that is based on the eXtensible Markup Language (XML) to describe the configuration of IEC 61850 based systems. SCL specifies a hierarchy of configuration files that enable multiple levels of the system to be described in unambiguous and standardized XML files. 

The main functionality of SCL includes:

  • 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.

IEC Substation Model
Putting the pieces together results in the substation architecture shown in Figure below, 

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.

Application Software
A variety of commercial products supporting IEC 61850 are already available and the future holds promise for many new innovations that will greatly benefit users. Of particular significance are products that support both the IEC 61850 communications standard. The combination of a standardized protocol and a standardized API is a powerful tool that allows users to dramatically lower their costs to build substation automation systems by enabling products from different vendors to plug together into a complete solution. 

Interface with Legacy Protocols
Electric power systems are designed to last for many years. For any new technology to be successfully applied into a modern power system, there must be some way to accommodate the use of legacy IEDs and protocols from the past. IEC 61850 is no different and there are several methods for accommodating legacy protocols in an IEC 61850 system. IEC 61850 itself is well suited to accommodate legacy protocols with its logical device model.

IEC 61850 is now released to the industry. This standard addresses most of the issues that migration to the digital world entails, especially, standardization of data names, creation of a comprehensive set of services, implementation over standard protocols and hardware, and definition of a process bus. Multi-vendor interoperability has been demonstrated and compliance certification processes are being established. Discussions are underway to utilize IEC 61850 as the substation to control center communication protocol. IEC 61850 will become the protocol of choice as utilities migrate to network solutions for the substations and beyond.

Title: IEC 61850 Communication Networks and Systems In Substations: An Overview for Users
Publisher: General Electric
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