Antony Mapfumo

The IEC 61850 standard overview

The IEC 61850 is at the heart of data communications in the smart grid. The IEC (International Electrotechnical Commission) 61850 standard outlines a framework for communication between several single devices in the power system. The standard was designed to separate the data model from method of communication, address the importance of a structured approach to the design of substation automation systems, utilise existing technologies like Ethernet and TCP/IP, simplify system configuration, device measurement sharing and to enable vendor independence. Although the scope of the IEC 61850 standard was originally substation focussed applications are already operating on wide area substation-to-substation communication using various components of IEC 61850.

The standard defines 10 major sections and a total of 13 logical group nodes. IEC 61850 takes advantage of an object oriented data model that can be mapped to number of protocols that can run over TC/IP networks or fast Ethernet. These protocols include GOOSE (Generic Object Oriented Substation Event), MMS (Manufacturing Message Specification), SMV (Selectable Mode Vocoder) and more recently DPWS (Devices Profile for Web Services).

The object-oriented data model

The IEC 61850 is an abstract application layer protocol. It abstracts the definition of data items and services. This allows the mapping of data objects (commonly referred to as Logical Nodes) and services to any protocol that can meet the service requirements. The basic data model structure is application independent. Data items or objects are independent of the underlying protocols. The main advantage of the IEC 61850 modelling approach is that it provides a model of how IEDs (Intelligent Electronic Devices) should organise data in a consistent manner across device brands. The most important point to note is that devices can configure themselves and client applications can extract object definitions over the network resulting in time (no need to manually configure devices) and cost savings. This is crucial in distributed control of energy systems that adhere to the IEC 61850 standard.

Real-time communications using Generic Object Oriented Substation Event (GOOSE)

The IEC 61850 allow peer-to-peer communication model for Generic Substation Events (GSE) between Intelligent Electronic Devices (IEDs) in a substation. A Generic Object Oriented Event (GOOSE) message is one of the messages associated with a GSE. GOOSE messages are mapped directly on the data-link layer resulting in low delay in the network. Time-critical events are usually multicast across a local Ethernet network as GOOSE messages. The importance of GOOSE is that it helps to satisfy the real-time requirements of the smart grid inside a local area network (LAN). GOOSE messages cannot travel beyond the local area network since GOOSE does not have a Network (Layer 3) component. This is to be taken into account when designing a real-time communications system for the distributed control of energy systems.

Real-time communications using Manufacturing Message Specification (MMS)

The Manufacturing Message Specification (MMS) is vendor-neutral international standard that deals with real-time process data and supervisory information transfer between intelligent devices and systems. MMS defines a set of standard objects that must be in every device on which operations like read and write can be executed, a set of standard messages for monitoring and control between clients and servers and finally a set of encoding rules for mapping these messages to bits and bytes when transmitted. The abstract communication service interface (ACSI) definition is at the core of IEC 61850. MMS is one (others include Web Services) of the mappings implementing these services. As previously discussed GOOSE messages are limited to communications within a LAN (local Area Network) due to lack of Layer 3 information so when we want support for transfer of real-time process data and supervisory information over a WAN (Wide Area Network) we can use MMS as it resides on top of the TC/IP stack.