IEC 61850 International Standard

Communication The IEC 61850 series is meant to supply interoperability between all devices in
substations. Communication between these devices has got to fulfil tons of requirements imposed
by all the functions to be performed in substations. counting on the philosophy both of the
vendor and of the user and on the state-of-the-art in technology, the allocation of functions to
devices and control levels isn’t commonly fixed. This leads to different requirements for the
different communication interfaces within the substation. The IEC 61850 series shall support
any allocation of functions.
The IEC 61850 series should have an extended lifetime but be ready to follow the fast changes in
communication technology by both technical as well as technical approach. Figure 1
shows the connection of this a part of the IEC 61850 series to subsequent parts of the IEC
61850 series. The IEC 61850 series has been organized in order that changes to at least one part don’t
require a big rewriting of another part, i.e. the parts are supported the communication
requirements during this a part of the IEC 61850 series; the derived modelling requirements in
subsequent parts won’t change the wants of this a part of the IEC 61850 series. The
general parts, the need specification and therefore the modelling parts are independent from any
implementation. The implementation needed for the utilization of the IEC 61850 series is defined in
some dedicated parts.

Terms and definitions
For the aim of this a part of IEC 61850, the subsequent terms and definitions also as those
given in IEC 61850-24, apply.

Task which are performed by substation automation systems Generally, a function consists
of subparts called logical nodes, which exchange data with one another . By definition, only
logical nodes exchange the data and, therefore, a function which exchanges data with other
functions must have a minimum of one logical node. As a consequence, only data contained in
logical nodes are often exchanged within the context of the IEC 61850 series.

Distributed Function

Function which is performed in two or more logical nodes that are located in several physical
devices. Since all functions communicate in how , the definition of an area or a distributed
function isn’t unique but depends on the definition of the functional steps to be performed
until the function is completed. just in case of the loss of 1 LN or one related communication
link, the function could also be blocked completely or show a graceful degradation, if applicable.

Set of interacting entities which perform a standard functionality. Its backbone is a few
communication mechanism.

Logical system
Communicating (via its logical nodes) set of all fuctions of application performing some overall
task like “management of a substation” within the context of IEC 61850

Physical system
interaction set of all devices hosting these functions and therefore the interconnecting physical
communication network. The boundary of any system is defined by its logical or physical
interfaces. Examples are industrial, management, information systems and
within the scope of the IEC 61850 protocol, substation automation systems. The backbone of
physical system is its communication system.

Substation automation system
System which operates, protects, monitors, etc. the substation, i.e. the first system.
For this purpose, it uses complete numerical technology and serial communication protocols.
(communication system).

Primary system
common term for all power grid equipment and switchgear

Secondary system
Interaction set of all components and systems within the substation for operation, protection,
monitoring, etc, i.e. the first system. just in case of full application of numerical technology,
the secondary system is similar to the substation automation system.

Communication system
Interconnected set of all communication links

Mechanism or piece of kit designed to serve a purpose or perform a function, for
example a breaker, relay, or substation computer. Communication relevant properties are
described during a proper device related model.

Intelligent device
Is any device incorporating one or more processors with the potential to receive or send
data/control from or to an external source, for instance electronic multifunction meters, digital
relays, controllers. An entity capable of executing the behavior of 1 or more specified
logical nodes during a particular context and delimited by its interfaces. If not stated otherwise
intelligent electronic devices have an indoor clock by definition providing for instance time
tags. This adds the need of a system wide time synchronization of of these clocks
if applicable.

Physical device
Equivalent to an intelligent device as utilized in the context of the IEC 61850 series

Logical Node
Smallest a part of a function that exchanges data. A Logical Node (LN) represents the function
within a physical device; it performs some of the operations for that particular function. A LN is an object
defined by its data and methods. Logical nodes associated with primary equipment aren’t the
primary equipment itself but its intelligent part or image within the secondary system, i.e. local
or remote I/Os, intelligent sensors and actuators, etc.

The links between entities

Logical connection
Communication link between logical nodes

Physical connection
Communication link between physical devices

The possibility to exchange a tool from an equivalent vendor, or from different vendors, utilizing
the same communication interface and as a minimum, providing an equivalent functionality, and
with no impact on the remainder of the system. If differences in functionality are accepted, the
exchange can also require some changes somewhere within the system. Interchangeability
requires standardization of functions and, during a strong sense, of devices also. Both such
requirements are not present in the scope of the IEC 61850 series.

Closely connected parts of the substation with some common functionality. Examples are
the switchgear between an incoming or outgoing line and therefore the busbar, the buscoupler with its
circuit breaker and adjacent isolators and earthing switches, the transformer with its related
switchgear between the 2 busbars representing the 2 voltage levels, the diameter (see
definition) during a 1½ breaker arrangement, virtual bays in ring arrangements (breaker and
adjacent isolators), etc.

Logical allocation of functions and interfaces
The functions of a substation automation system could also be logically allocated on three different
levels (station, bay/unit, or process). These levels are defined by the logical interpretation of
Figure 2 along side the logical interfaces 1 to 10.
a) Process level functions are all functions interfacing to the method . These functions
communicate through the logical interfaces 4 and 5 to the bay level.
b) Bay level functions (see bay definition in Clause 3) are functions using mainly the info of
one bay and acting mainly on the first equipment of 1 bay. These functions
communicate through the logical interface 3 within the bay level and via the logical interfaces 4
and 5 to the method level, i.e. with any quite remote I/Os or intelligent sensors and
actuators. Interfaces 4 and 5 can also be hardwired, but hardwired interfaces are beyond
the scope of the IEC 61850 series.
c) There are two classes of functions at station level:
1) Process related station level functions are functions using the info of quite one
bay or of the entire substation and working on the first equipment of quite
one bay or of the entire substation. These functions communicate mainly via the
logical interface 8.
2) Interface related station level functions are functions which define the interface of the
SAS to the local station operator HMI (Human Machine Interface), to a foreign control
center TCI (TeleControl Interface) or to the remote engineering system for monitoring and
maintenance TMI (TeleMonitoring Interface). These functions communicate via the
logical interfaces 1 and 6 with the bay level and via the logical interface 7 and therefore the
remote control interface to the surface world.

Logical nodes and logical connections
To fulfill all the wants stated above, especially the free distribution and allocation of
functions, all functions are decomposed into logical nodes (LN) which will reside in one or
more physical devices. There are some data to be communicated which refer to not any
function but to the physical device itself like nameplate information or the results of device
self-supervision. Therefore, a logical node “device” is required and can be introduced as LLN0.
The LNs are linked by logical connections (LC) for a fanatical exchange of knowledge in between.
Therefore, the IEC 61850 protocol series shall define the communication between these LNs.
This approach is shown in Figure 3. The logical nodes (LN) are allocated both to functions (F)
and physical devices (PD). The logical nodes linkings are with logical connections (LC), the
devices by physical connections (PC). Any logical node is a component of a physical device; any
logical connection is a component of a physical connection. The logical node “device” dedicated for
any physical device is displayed as LN0 (in the four letter code explained in Figure 3 for all
logical nodes, LLN0).
Since it’s impossible to define all functions for present and future use, or their distribution
and interaction, it’s vital to specify and standardize the interaction between the
logical nodes during a generic way.

Examples of decomposition of common functions into logical nodes
In Figure 4, there are samples of common functions given
a) synchronized breaker switching;
b) distance protection;
c) overcurrent protection.
The functions are decomposed into logical nodes listed in Figure 4, the allocated physical
devices are described by numbers
1) Station computer.
2) Synchronized switching device.
3) Distance protection unit with integrated overcurrent function.
4) Bay control unit.
5) Current instrument transformer.
6) Voltage instrument transformer.
7) Busbar voltage instrument transformer.
The logical node “device” (LLN0) as contained in any physical device isn’t shown. 

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