The IEC 61850 Communication Protocols

The main idea behind the IEC 61850 standard is that the data object model and services are separated from the communication ISO/OSI layers’ stack. This ap-proach offers the opportunity to implement the state-of-the-art of communica-tion technologies. The mainstream technologies are the communicacommunica-tion schemes that have been used for the ISO/OSI reference model. The ISO/SOI stack is based on the concept of layering communication functionality, consisting of seven lay-ers as illustrated in Figure 16. Laylay-ers one and two are the Ethernet physical and link layer, layers three and four comprise the TCP/IP layer, and layers six and seven comprise the MMS layer. The IEC 61850 object models are mapped over different layers in terms of their services and requirements. The object model based on the client/server services ACSI is mapped to the five-seven MMS layers, whereas the high-speed time-critical messages, such as SV, the status indications blocking the trip commands and GOOSE, are mapped directly to the Ethernet link layer (IEC 61850-8-1 2003) (Brand 2004).

Figure 16. IEC 61850 application messages mapping to the OSI layers.

1.3.8.1 The Abstract Communication Service Interface

The IEDs are described by the standardized method that allows all of them to share data by means of an identical structure that is related to their functions.

From the network behaviour perspective, the ACSI provides the specification for the basic model that represents the definition for the substation-specific

infor-SV GOOSE ACSI

SCSM

MMS

IP TCP

Ethernet Link Layer

Ethernet Physical Layer with Priority tagging

Real Time Requirements OSI

Communication Technology

Model (Objects, Services)

mation models. Further, it specifies a set of information exchange service models and the response to those services. This specification allows various IEDs to ex-hibit identical behaviour. The abstraction technique that has been adopted by IEC 61850 is one of the most significant features which separates the SAS appli-cation from the underlying communiappli-cation protocol and operating systems, as illustrated in Figure 17. (Adamiak etc. 2009).

Figure 17. Abstract communication service interface concept.

An ACSI concept has two approaches. Firstly, based on the basic information model, only aspects of real devices or real functions that are visible and accessible over the network are modelled, resulting in hierarchical class models, such as LOGICAL-DEVICE, LOGICAL-NODE, DATA and DataAttribute. Secondly, based upon the exchange service model, the abstraction can be defined from the way in which the devices are able to share the information in terms of the definition, focusing on aspects of the purpose of the services instead of describing how the services are built (IEC 61850-7-2 2003). In a real implementation, the basic in-formation model and services’ model are mapped into an existing communica-tion stack. The mapping schemes are achieved through the SCSM. In IEC 61850, two mapping schemes are specified (IEC 61850-9-1 2003) for the transmission of the SV (IEC 61850-8-1 2003) and for the transmission of wide station events and all other communication services. Further, the ACSI provides abstract interfaces that describe communications between client and server. This type of interface can be used for real-time applications, such as data access, data recovery, device control, publisher/subscriber applications, event reporting and transferring, self-description, self-healing, data typing and data reading. Further, it describes communications between applications on one device as a publisher to many ap-plications on various devices as a subscriber for fast and reliable system-wide event distributions, such as GOOSE, generic substation events (GSEs) and SV.

The ACSI interfaces which are defined above allowed the client to observe the data model, to get and set data, to manipulate data-sets, to log, etc., by means of a calling method such as GetDataValues or SetDataValues. These methods

em-SAS Function

ployed in the programing language are reasonably traditional methods, with as-signed arguments leading to returned output values (Pedersen 2010). Conse-quently, the ACSI model defined the services set within the client application while the server application defined the response to the requested services. It also defined the concept of “application associations”. This feature represents the con-trolling access mechanisms to an object within a device. In order to restrict a par-ticular devices’ visibility, different access-control schemes can be used.

1.3.8.2 GSSE, GOOSE and SV

The GSE service model is one of the main features of IEC 61850 that offers fast and reliable real-time applications to deliver SAS data values over the communi-cations system network. The GSE service model is based on the concept of inde-pendency decentralization. It uses a multicast/broadcast services model based on an efficient method. This multicast/broadcast services model provides for the simultaneous distribution of the SAS event values to all of the SAS subscriber IEDs. The generic substation event distributions also support peer-to-peer and client/server communication models. IEC 61850-7-2 defines two control classes and the structures of two messages, such as,

1 GOOSE, which supports a wide variety of the SAS’s common data, such as analogue, binary and integer value data-types grouped by Da-ta-Set.

2 GSSE, which supports only the status change information events, fixed structure binary events and bit pairs.

Therefore, the type of shared information is the major difference between the GOOSE and GSSE services. The flexible GOOSE model is used by all new sys-tems, and conveys a wide range of messages and binary and analogue data.

Meanwhile, GSSE is older and only delivers binary values within its messages.

Figure 18 illustrates the GOOSE model. The message is based on the publish-er/subscriber exchange mechanism. From an implementation point of view, at the publisher side the values are written in the local buffer, while at the receiving side the subscribers read the values from the local buffer. The local buffers of the sub-scribers are updated by the communications system where the GSE control class has been used as a controller for the procedure from the publisher side (IEC 61850-7-2 2003).

Figure 18. GOOSE service operation mechanism.

The substation IEDs recognize the changing status as well as when the last status changes occur upon receiving the GOOSE messages, which contain all the needed information. Further, the local timer of the subscriber can be set based on the re-lated time of the latest status change event. At this point, GOOSE has been identi-fied as having one of the fastest times for critical messages within an SAS. There-fore, GOOSE messages are mapped directly to the Ethernet layer in order to sup-port the real-time operation requirements. Typical protection events, such as trip, interlock and status indication, are recognized as high priority services in which the processing time must be less than a quarter of a cycle. For instance, the message transmission time for the 50 Hz cycling frequency system is specified as

<4 ms. Moreover, since the GOOSE messages are directly mapped over the Ethernet layer, which does not guarantee delivering services. The retransmission scheme should be used to ensure that the messages are received based on a mul-ticast transmission. Lastly, since the GOOSE message can serve various applica-tions with different performance requirements and various data types, it may be considered as a flexible tool, (Zhang & Kumal 2008).

Another high-speed time-critical message within the SAS are the SV messages, which are used to deliver measured values from switchyard-to-bay IEDs in a dig-itized form. Multicast is a transmission scheme whereby the measured values at one location can be delivered to any number of subscribers. Either an intelligent instrument or the MU is responsible for the digitalization process. A particular target may require a distinctive sampling rate which can be freely selected based

Forwarding

upon their needs (Hou & Dolezik 2008). More details about SV are provided in Chapter Three.

1.3.8.3 Manufacturing Messaging Specification (MMS)

The object model and service that are defined in IEC 61850-7-x are mapped over the application layer of the MMS (layer seven), which is a part of ACSI that does not utilize time-critical messages. The MMS is an international standardized mes-saging system (ISO 9506) that has been used to exchange real-time data and ser-vices among network devices. The mean feature of the MMS is that it is ind-pendent of the application function being performed as well as the device and software manufacturer. Further, based on the highly generic nature of the mes-sage services provided by MMS, it is appropriate for different types of functions, de-vices and industries - for instance, based on ACSI’s implementation require-ments the information modelling and services that are provided by MMS precise-ly meet its needs (Stalling 2007). When looking at the benefit of implementing MMS messaging services, three major effects that can be evaluated contribute to reduce costs. First, there is interoperability, which allows network applications and IEDs to share their real-time information without the need to create infor-mation environments. Second, there is independence, which provides interoper-ability independently of the developer of the application, network connectivity and functions based on provide common communication services. Last, there is data access, which facilitates applications to provide useful functions by obtain-ing the information that is required through the network application (IEC 61850-7-2 2003). From the implementation perspective as regards MMS services, the object of the ACSI server class is mapped based on IEC 61850-8-1 by SCSM one-to-one over the virtual machine device (VMD) object. The VMD is the key feature of the MMS services, specifying how the server behaves based upon the client application’s point of view. Moreover, it represents that part of an application task that enables the monitoring and control of services by means of a set of re-sources and functionality associated with one or more devices. Generally, the VMD defines the objects that the server consists of. Further, the services enable a client to access and manipulate these objects by means of assigning one or multi-ple communication addresses that create service access points (SAPs) where the MMS services can be exchanged. Figure 19 illustrates the VMD concept (SISCO 1995).

Figure 19. MMS concept.