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Abstract-- Legacy substation automation protocols and
architectures typically provided basic functionality for power
system automation and were designed to accommodate the
technical limitations of the networking technology available for
implementation. There has recently been a vast improvement in
networking technology that has changed dramatically what is
now feasible for power system automation in the substation.
Technologies such as switched Ethernet, TCP/IP, high-speed
wide area networks, and high-performance low-cost computers
are providing capabilities that could barely be imagined when
most legacy substation automation protocols were designed.
IEC61850 is an important new international standard for
substation automation that will have a very significant impact on
how electric power systems are designed and built for many
years to come. IEC61850 is a part of the International
Electrotechnical Commissions (IEC) Technical Committee 57
(TC57) architecture for electric power systems. The model-
driven approach of the TC57 standards, including IEC61850, is
an innovative approach that requires a new way of thinking
about substation automation that will result in very significant
improvements in both costs and performance of electric power
systems.
I. C
OMMUNICATION
S
YSTEM
N
EEDS

ommunication has always played a critical role in the
real-time operation of the power system. In the
beginning, the telephone was used to communicate line
loadings back to the control center as well as to dispatch
operators to perform switching operations at substations.
Telephone-switching based remote control units were
available as early as the 1930s and were able to provide
status and control for a few points. As digital
communications became a viable option in the 1960s, data
acquisition systems (DAS) were installed to automatically
collect measurement data from the substations. Since
bandwidth was limited, DAS communication protocols were
optimized to operate over low-bandwidth communication
channels. The cost of this optimization was the time it took
to configure, map, and document the location of the various
data bits received by the protocol.

As we move into the digital age, literally thousands of
analog and digital data points are available in a single
Intelligent Electronic Device (IED) and communication
bandwidth is no longer a limiting factor. Substation to master
communication data paths operating at 64,000 bits per second
are now commonplace with an obvious migration path to
much higher rates. With this migration in technology, the
cost component of a data acquisition system has now
become the configuration and documentation component.
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

Given these requirements, work on a next generation
communication architecture began with the development of
the Utility Communication Architecture (UCA) in 1988. The
result of this work was a profile of recommended protocols
for the various layers of the International Standards
Organization (ISO) Open System Interconnect (OSI)
communication system model. This architecture resulted in
the definition of a profile of protocols, data models, and
abstract service definitions that became known as UCA. The
concepts and fundamental work done in UCA became the
foundation for the work done in the IEC Technical Committee
Number 57 (TC57) Working Group 10 (WG10) which
resulted in the International Standard IEC 61850
Communication Networks and Systems in Substations.


II. S
COPE AND
O
UTLINE OF
IEC

61850
The stated scope of IEC 61850 was communications within
the substation. The document defines the various aspects of
the substation communication network in 10 major sections as
shown in Table 1 below.



Technical Overview and Benefits of the
IEC 61850 Standard for Substation Automation
C
Ralph Mackiewicz
SISCO, Inc.
Sterling Heights, MI
USA

2
TABLE

I
S
TRUCTURE OF THE
IEC

61850

S
TANDARD

Part #
Title
1
Introduction and Overview
2
Glossary of terms
3 General
Requirements
4
System and Project Management
5
Communication Requirements for Functions and
Device Models
6
Configuration Description Language for
Communication in Electrical Substations Related to
IEDs
7
Basic Communication Structure for Substation and
Feeder Equipment
7.1 - Principles and Models
7.2 - Abstract Communication Service Interface
(ACSI)
7.3 - Common Data Classes (CDC)
7.4 - Compatible logical node classes and data classes
8
Specific Communication Service Mapping (SCSM)
8.1 - Mappings to MMS(ISO/IEC 9506 Part 1 and
Part 2) and to ISO/IEC 8802-3
9
Specific Communication Service Mapping (SCSM)
9.1 - Sampled Values over Serial Unidirectional
Multidrop Point-to-Point Link
9.2 - Sampled Values over ISO/IEC 8802-3
10 Conformance
Testing

Parts 3, 4, and 5 of the standard start by identifying the
general and specific functional requirements for
communications in a substation (key requirements stated
above). These requirements are then used as forcing functions
to aid in the identification of the services and data models
needed, application protocol required, and the underlying
transport, network, data link, and physical layers that will
meet the overall requirements.

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. The definition of the abstract services is
found in part 7.2 of the standard and the abstraction of the
data objects (referred to as Logical Nodes) is found in part
7.4. In as much as many of the data objects are made up of
common pieces (such as Status, Control, Measurement,
Substitution), the concept of Common Data Classes or
CDC was developed which defined common building
blocks for creating the larger data objects. The CDC elements
are defined in part 7.3.

Given the data and services abstract definitions, the final
step was one of mapping the abstract services into an actual
protocol. Section 8.1 defines the mapping of the abstract data
object and services onto the Manufacturing Messaging
Specification MMS and sections 9.1 and 9.2 define the
mapping of the Sample Measured Values (unidirectional
point-to-point and bi-directional multipoint accordingly) onto
an Ethernet data frame. The 9.2 document defines what has
become known as the Process Bus.

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. In order to facilitate this
process and to eliminate much of the human error component,
an XML based Substation Configuration Language (SCL) was
defined in part 6. It allows the formal description of the
relations between the substation automation system and the
substation (switchyard). At the application level, the
switchyard topology itself and the relation of the switchyard
structure to the SAS functions (logical nodes) configured on
the IEDs can be described. Each device must provide an SCL
file that describes the configuration of itself.

Although the scope of 61850 was originally focused
inside the substation, discussions are underway to look at
defining 61850 for the Substation to Master communication
protocol (already in service in several installations). In
addition, applications are in service that uses various
components of 61850 for wide area substation-to-substation
communication.

Finally, part 10 of the document defines a testing
methodology in order to determine conformance with the
numerous protocol definitions and constraints defined in the
document.

The rest of this paper goes into some focused details of the
various parts of the IEC 61850 standard.
III. M
ODELING
A
PPROACH

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