IEC 61850

IEC 61850 is an international standard which provides a detailed specification of a layered substation communication architecture. The communication architecture is composed of abstract definitions of classes and services which are independent of underlying concrete protocol stacks and deployment platforms. The IEC 61850 standard was originally developed for electrical substation automation systems. In addition to intrasubstation communications, IEC 61850 information models are applicable to substation to substation communications, substation to control center communications, distributed automation communications, metering, electrical equipment condition monitoring and diagnosis, and intelligent electronic device (IED) to engineering systems communications. A series of extensions to IEC 61850 have been developed for use in hydroelectric power plants and wind power plants.

History of IEC 61850

In 1988 EPRI and IEEE initiated the Utility Communications Architecture (UCA) project under the Integrated Utility Communication (IUC) program. The objective of the UCA project was to make provision for interoperability between control systems employed to monitor and control the electric power utilities. Initially the UCA project focused on communications between control centers, and communications between substations and control centers. EPRI and IEEE carried out the UCA project in collaboration with the Pacific Gas and Electric company and Houston Light and Power company. The result of this collaboration was a standard communications architecture referred to as UCA version 1.0.

UCA version 1.0 didn’t provide a detailed description of how the UCA communication architecture was to be practically implemented and used in field devices, therefore the adoption of UCA architecture in the electrical power industry was limited. EPRI and IEEE continued with their efforts to improve the UCA architecture by sponsoring a number of research projects such as the substation integrated protection, control, and data acquisition, and the MMS Forum Working Groups. These efforts lead to thorough specifications of object models of field devices, i.e. definitions of data and control functions provided by these field devices, in what became UCA version 2.0.

In 1997 EPRI and IEEE joined efforts with Working Group 10 (WG10) of the IEC Technical Committee 57 (TC57) to build a common international standard for electrical utility communications. These efforts were based on concepts and definitions of the UCA architecture and lead to the creation of a standard named IEC 61850 which was designed to provide for interoperability, fast communications among field devices, guaranteed data delivery time, and configuration support.

Salient Functional Features

In IEC 61850 application functions are decomposed into small entities referred to as logical nodes (LNs). LNs are virtual representations of physical devices and their applications. For instance, IEC 61850 defines the XCBR, XSWI, TCTR, and TVTR LNs to virtually represent circuit breakers, circuit switches, current transformers, and voltage transformers, respectively. LNs are organized in groups identified by single uppercase letters. Examples of LN groups include protection functions (P), sensors (S), switchgear (X), instrument transformers (T), etc.

By convention the name of each LN begins with the letter which represents the logical node’s group. A LN is composed of data objects. Data objects carry status information, settings, measured values, and controls related to real equipment and real applications. A LN typically contains up to 30 data objects. A data object in turn is composed of data attributes. These attributes are the ultimate carrier of status, settings, measurement, and control information. Each data object typically may contain up to, or in same cases more than, 20 data attributes.

The composition of defined sets of LNs form logical device (LD) models, while the composition of defined sets of LDs form physical device models. The information exchange services in IEC 61850, i.e. methods used by IEDs to access data stored in logical nodes, are abstract and are defined by an Abstract Communication Service Interface (ACSI). The definition of these information exhange services focuses entirely on a description of what these services provide. In fact ACSI services along with data models exist independently from any other protocols. ACSI services follow either one of two communication models, namely client-server or peer-to-peer.

In a client-server communication model a client invokes services to get data from, or set data in, logical nodes of a server. While the peer-to-peer communication model is used for time-critical purposes and is based on fast and reliable exchanges of information between IEDs. The client-server model typically involves one-to-one communications, while the peer-to-peer model involves one-to-one or one-to-many communications depending on the functions being carried out. Of particular interest are the service models called Generic Object Oriented Substation Event (GOOSE) and Generic Substation State Event (GSSE) which support the exchange of a variety of data sets and state change information, respectively.

Configurations of IEC 61850 in IEDs are formally described via an XML based Substation Configuration Language (SCL). Each IED is supposed to provide an SCL file that describes its configuration.

The IEC 61850 standard is mapped to other communication protocols. The mapping of IEC 61850 to concrete protocols is carried out by the Specific Communication Service Mapping (SCSM). SCSM associates abstract communication services, data objects and parameters with proper elements of communication protocols.

IEC 61850 Implementation

IEC 61850 implementations are commonly coded in C/C++. Implementations are typically deployed in microprocessor-based devices such as IEDs or phasor measurement units (PMUs). Several IEC 61850 implementations are coded in a hardware description language such as Verilog or VHDL. These implementations are deployed in field programmable gate arrays (FPGAs), or are hardwired in application specific integrated circuits (ASICs).

Relation with Other Protocols

IEC 61850 is related to any other protocols which it is directly or indirectly mapped to. IEC 61850 is commonly mapped to the standard ISO 9506, otherwise known as the Manufacturing Messaging Specification (MMS). MMS is an application layer protocol which provides for reliable transmission in real-time of process data between networked devices carrying out data acquisition and control functions. MMS itself runs over TCP/IP and Ethernet.

IEC 61850 is also related to time synchronization protocols such as Simple Network Time Protocol (SNTP). SNTP in turn runs over UDP/IP. Peer-to-peer IEC 61850 profiles are supposed to operate under extreme time constraints, consequently they are mapped directly to a layer 2 protocol such as Ethernet. In several recent deployments IEC 61850 is mapped to various web services protocols.

IEC 61850 Security

IEC 61850 does not come with security features of its own. Like many other protocols, IEC 61850 relies on SSL/TLS for message confidentiality and on commonly deployed hashing algorithms and public key cryptographic algorithms for message integrity and authentication. The SHA family of hashing algorithms and Elliptic Curve Cryptography are commonly used for such purposes.

IEC TC57 has developed the IEC 62351-6 standard which focuses on the security of non routable profiles of IEC 61850. No symmetric encryption is normally applied to these profiles due to tight constraints on their message delivery time. IEC 62351-6 allows for minimal computation requirements for these profiles to only digitally sign exchanged messages.

IEC 61850 enforces identity based access control to restrict the set of data objects accessible by a client. Such a restricted access to logical nodes is referred to as virtual access view. Failed access attempts are detected and recorded. IEC 61850 defines the generic security application (GSAL) logical node to monitor violations regarding authorization, access, service privileges and inactive associations.

External Links

IEC 61850: Communication Networks and Systems in Substations, Parts 1 through 10, August 2007.

IEC 62351 – Data and communications security – Part 6: Security for IEC 61850