by Christoph Brunner, Switzerland and Alex Apostolov, USA
Christoph Brunner graduated as
an Electrical Engineer at the Swiss
Federal Institute of
Technology in 1983.
He is President and
Chief Technology
Officer of UTInnovation in Zug,
Switzerland. Before,
he worked as a
project manager
at ABB Switzerland
Ltd in the business
area Power Technology Products in
Zurich where he
was responsible for
the communication
architecture of the
substation automation system. He is
Convenor of working group (WG) 10
and member of WG
17, 18 and 19 of IEC
TC57. As a member
of IEEE-PES and
IEEE-SA, he is active
in several working
groups of the IEEEPSRC. He is International Advisor to the
board of the UCA
International Users
Group.
IEDs with station
and process bus
interfaces use
copper only for power supply.
The IEC 61850 Standard for Communication
Networks and Systems for Utility Automation allows utilities
to consider new designs for substations applicable for both
new substation and refurbishments. The levels of functional
integration and flexibility of communications based solutions
bring significant advantages in costs in all stages of a project.
This integration affects not only the design of the substation,
but almost every component and/or system in it - protection,
monitoring and control, by replacing the hardwired
interfaces with communication links. The use of high-speed
peer-to-peer communications using Generic Substation Event
(GSE) messages and sampled values from non-conventional or
conventional sensors, allows the development of distributed
applications. In addition the use of optical local area networks
leads in the direction of copper-less substations.
This article is addressing the issues only from the
viewpoint of the communication network. The principles,
applications and benefits of data models and the configuration
language will be discussed in a future article.
Interoperability or Interchangeability
One of the main reasons for the success of IEC 61850
is that for the first time it introduced a set of tools that
allow the development and engineering of state-of-the-art
high performance, adaptive and self-healing substation
protection, automation and control systems based on the
latest communications technology and multifunctional
devices from different suppliers. Substation automation
systems (SAS) and distributed protection schemes are not
something new. However, they have typically been designed
and implemented as single-vendor solutions, using for
the protection schemes hard wiring or proprietary vendor
communication protocols. The requirements of the users
for new communications based solutions without any
degradation in functionality and performance while not
being restricted to the use of single vendor’s devices, led to
the development of the peer-to-peer communications models
and services known as GOOSE (Generic object oriented
system event) and Sampled value transmission in the IEC
61850 domain. It is important to understand that the goal
of IEC 61850 is to ensure interoperability – i.e. the ability of
two or more IEDs from the same vendor, or from different
vendors, to exchange information and use that information
for correct execution of specified functions. In some cases
it may be even possible to achieve interchangeability – the
ability to replace a device supplied by one manufacturer with
a device supplied by another manufacturer, without making
changes to the other elements in the system.
The requirements of different applications resulted in
the definition of IEC 61850 models and services that can
support all substation protection, automation and control
applications. One of the differentiating contributions of the
standard compared to other communications protocols is
that it goes beyond the typical SAS communication types
and specifies methods that can be used for time-critical
functionality typically provided through hard-wired signals
exchange. Another important contribution is the definition of
standardized object models where the information available
from the different applications is defined both with syntax as
well as with semantic.
An important issue to understand when considering the
use of different devices and tools is that even though a device
can be compliant to the definitions of the standard, it may not
be able to communicate with devices from other suppliers.
This is due to the fact that in order to support innovation
and flexibility IEC 61850 limits the number of mandatory
data objects and attributes, as well as it leaves the selection of
implemented services to the supplier of any specific device.
The price to pay is the issues with interoperability. That is why
during the process of selection of devices and tools to be used
in a substation protection, automation and control system, it
is very important to perform interoperability testing between
all devices intended for use in a project.
Communication Types in Substation Automation
Systems
In order to understand the need for the development of
peer-to-peer communications messages, we need to consider
the communications within substations. One of the most
1 Multicast communications
2 Distributed function definition in IEC 61850
IED
Bay
computer
IED
switch
IEC 61850
cover story
20
multicast <values>
Data
Protection IED
P...
IF 8
LC1
IF 8
LC2
R...
P...
R...
IED
Protection IED
P...
Distributed
function
PAC.SUMMER.2009
21
commonly used is Client – Server. The client in this domain
can be defined as a device or function that sends a message
to a server device or function, requesting from the server to
perform a specific task (service).
The client application usually manages the user-interface
portion of the application, validates data entered by the user
or sends requests to the servers. The client-based process
is the front- end of the application that the user sees and
interacts with in the substation automation system.
A server device or function fulfills the client request by
performing the requested task. Most IEDs in the substation
operate as servers while interfacing with substation level
devices or applications.
A typical Client/Server operation is a complete
transaction consisting of a request followed by information
delivery of the requested information. It is important to
note that one server can respond to the requests of multiple
clients and vice versa. Usually, operational data except
commands, flows primarily from the server to the clients.
The Client/Server communications can also be described as
connection-oriented, which means that the devices at the end
points establish an end-to-end connection before any data is
sent. Connection-oriented communications are considered
a more reliable network service, because they guarantee that
data will arrive in the proper sequence. The Transmission
Control Protocol (TCP) is a connection-oriented protocol.
The problem with Client/Server communications is that
due to their principles they can not meet the requirements
for performance, especially for protection applications. This
resulted in the idea for development of different types of
communications based on a Connectionless method. In this
case data is sent from one device to another without prior
arrangement. Connectionless protocols are usually described
as stateless, because the end points have no protocol-defined
3 Logical interfaces in Substation Automation Systems (SAS)
Remote control (NCC)
Technical services
7
10
FCT. A
FCT. B
9
STATION LEVEL
1.6
3
Remote
protection
Transfer time t = ta + tb + tc
3
Contr.
2
Prot.
4.5
Process Interface
4.5
Censors
ta
Contr.
2
BAY/UNIT LEVEL
Alex Apostolov
received MSEE,
MSAM and Ph.D.
degrees from the
Technical University
in Sofia, Bulgaria.
He has more than
30 years experience in protection,
automation and
communications.
He is presently
Principal Engineer at
OMICRON
electronics in Los
Angeles, CA. He
is IEEE Fellow and
Member of the
Power Systems
Relaying Committee
and Substations C0
Subcommittee. He
serves on many IEEE
PES working groups
and is Chairman
of Working Group
C9. He is Member
of CIGRE and is
Convener of CIGRE
WG B5.27. He is US
representative in
IEC TC 57 WG 10,
17, 18. He holds
three patents and
has authored and
presented more
than 300 technical
papers.
4 Transfer time definition
1.6
8
Prot.
way to remember where they are in a "conversation" of
message exchanges. Since an IED transmits data to another
device without first ensuring that the recipient is available
and ready to receive the data, there is no guarantee that the
data will be received. The device sending a message simply
sends it addressed to the intended recipient. Because of that
IEC 61850 has introduced specific mechanisms to ensure
the delivery of the data to the recipients. The connectionless
communication methods used in IEC 61850 are also referred
to as peer-to-peer or publisher / subscriber communication.
Peer-to-peer or publisher / subscriber is the characteristic
communications type for the IEC 61850 based systems.
It is one of the distinguishing features of the standard that
makes it attractive to protection and control specialists. It
describes the ability of arbitrary pairs of IEDs connected to the
substation network to manage the exchange of information as
necessary with all devices having equal rights, in contrast to
the master/slave communication. Unlike the client / server
communication, the publisher / subscriber communication
does not need to establish and maintain an association for the
information exchange (Figure 1).
Publisher / Subsrciber communications in IEC 61850
based systems use multicast for data delivery. It is a method
that allows the sending IED to deliver the information
simultaneously to a group of destination IEDs using the most
efficient strategy - to send the messages over the network
only once. By comparison with multicast, conventional
point-to-single-point delivery is called unicast. Publisher /
Subscriber communications are used to perform protection,
control, monitoring and recording functions. Any function
can be divided into sub-functions and functional elements.
The functional elements are the smallest parts of a function
that can exchange data. These functional elements in IEC
61850 are called Logical Nodes. When a function is executed
based on the exchange of communications messages between
two or more devices, it is called “distributed function”.
The exchange of data is not only between functional
elements, but also between different levels of the substation
functional hierarchy. It should be kept in mind that
functions at different levels of the functional hierarchy can
be located in the same physical device, and at the same time
different physical devices can be exchanging data at the same
functional level. Figure 2 shows Logical Connections (LC) -
tb
GOOSE
tc
transfer time
Remote
protection
Actuators
fi
Communication
processor
Communication
processor
fk
is the time
between
PROCESS LEVEL
fuctions in two
HV Equipment
Physical device PD[n]
Physical device PD[m]
devices
PAC.SUMMER.2009
IEC 61850
cover story
22
The IEC 61850
GOOSE and
sampled values
messages
belong to the
performance
class with
highest
transmission
requirements
- less than
1/4 of a cycle
between
functions.
the communication links between functional elements - in
this case logical nodes of the P and R groups. IEC 61850 also
defines interfaces that may use dedicated or shared physical
connections - the communication links between physical
devices. The allocation of functions between different physical
devices defines the requirements for the physical interfaces,
and in some cases may be implemented into more than
one physical LANs. The functions in the substation can be
distributed between IEDs on the same, or on different levels
of the substation functional hierarchy. IEC 61850 defines
three such levels:
Station Bay/Unit Process
These levels and the logical interfaces are shown by the
logical interpretation of Figure 3. IEC 61850 publisher
/ subscriber communication focuses on a subset of the
interfaces shown in Figure 3 with Interface 8 (shown in red)
being used for high-speed peer-to-peer communications
between IEDs and interface 4 – for transmission of sampled
values between the sensors and the IEDs.
In order to implement high-speed protection and
control functions over the substation local area network, we
need to make sure that the system will meet some specific
performance requirements. Different distributed functions
impose different performance requirements that have to be
considered in the design process of substation protection,
control, monitoring and recording systems. There are two
independent groups of performance classes:
For control and protection
For metering or power quality applications
Since the performance classes are defined according to the
required functionality, they are independent from the size of
the substation. The requirements for control and protection
are higher, because of the effect of the fault clearing time on
the stability of the system or on sensitive loads. IEC 61850
defines three Performance Classes for such applications:
P1 - applies typically to the distribution level of the
substation or in cases where lower performance requirements
can be accepted.
P2 - applies typically to the transmission level or if not
otherwise specified by the user.
P3 - applies typically to transmission level applications
with high requirements, such as bus protection.
IEC 61850 transfer time definition is based on Figure 4:
The overall performance requirements also depend
on the message type. Type 1 is defined in the standard as
Fast Messages. Since Trip (Type 1A) is the most important
fast message in the substation, it has more demanding
requirements compared to all other fast messages. The same
performance may be requested for interlocking, intertrips
and logic discrimination between protection functions. For
message type 1A, the following requirements are defined:
For Performance Class P1, the total transmission time
shall be in the order of half a cycle. Therefore, 10 ms is
defined.
For Performance Class P2/3, the total transmission time
shall be below the order of a quarter of a cycle. Therefore, 3 ms
is defined for the high-speed peer-to-peer communications.
PAC.SUMMER.2009
Virtual IED
The IEC 61850 model includes data objects and services.
The communications of data between different
multifunctional IEDs working together in distributed
functions are achieved over the substation LAN. In order to
understand the specific features included in the IEC 61850
design of high-speed peer-to-peer communications, we need
to understand the type of Ethernet frames being used.
IEC 61850 GSE Model
High-speed publisher / subscriber communications
in IEC 61850 based protection and control systems use a
specific method designed to meet a variety of requirements.
It is very important to understand, that the concept of the
Generic Substation Event (GSE) model is not based on
commands, but on the sending indication by a function
that a specific substation event has occurred. It is designed
to support reliable high-speed communications between
different devices or applications and allows the replacement
of hard-wired signals between devices with communication
messages exchange, while improving the functionality of
the protection, automation and control system. The model
5 Publisher/subscriber mechanism
Subscriber
Physical Device
ASCI
Server
Data
Data
Application
Physical Device
Physical Device
ASCI
Server
ASCI
Server
Data
GSE
message
Data
Data
Data
Application
Application
Publisher
Subscriber
23
One of the main goals of IEC 61850
is to ensure interoperability between
devices from different manufacturers.
Devices from
different
manufacturers are
mapped to
a common
data model
- virtual IED
includes several features that can be used to with logical
improve the reliability and availability of the
system. At the same time the proper use of devices and
these features in vendors’ implementation
will allow a reduction in maintenance and logical nodes.
increase in the flexibility of the system. To
understand the reasons for these benefits, we need to look
into some of the details of the Generic Substation Event
model. The GSE method can be considered as a mechanism
for reporting by a logical device. The achievement of speed
performance, availability and reliability depends on the
implementation in any specific device. The generic substation
event model is used to exchange the values of a collection of
Data Attributes defined as a Data Set.
The GSE information exchange is based on a publisher/
subscriber mechanism (Figure 5). The publisher writes
the values in a transmission buffer at the sending side and
multicasts them over the substation local area network to
the different subscribers. The data in the published GOOSE
messages is a collection of values of data attributes defined
as members of a data set. The receiver reads the values from
a local buffer at the receiving side. A GSE control class in the
publisher is used to control the process. If the value of at least
one of the DataAttributes has changed, the transmission
buffer of the Publisher is updated with the local service
“publish” and the values are transmitted with a GOOSE
message.
Specific communication services in the subscribers update
the content of their reception buffers and new values received
are indicated to the related applications. Since the GOOSE
messages replace hard-wired signals used for protection and
control applications, IEC 61850 introduces mechanisms that
ensure the delivery of the required information (Fig ure 6).
Once a new value of a date attributed has resulted in
the multicasting of a new GOOSE message, the repetition
mechanism ensures that the message is sent with a changing
time interval between the repeated messages until a new
change event occurs. As shown in Figure 6, at the beginning
after a change the interval is very short – a few milliseconds,
which later increases until it reaches a value of a few seconds.
This method achieves several important tasks:
Ensures that a loss of a single message is not going to
affect the functionality of the system
Allows any new device to inform all subscribing devices
about its state
Allows any new device to learn the state of all publishing
devices it subscribes to
Allows any device requiring information from a
publisher to supervise that the publisher is still alive, even if
no change event occurs
The GOOSE messages contain information that allows the
receiving devices to know not only that a status has changed,
but also the time of the last status change. This allows a
receiving device to set local timers relating to a given event.
As already mentioned, the content of the GOOSE
message allows the receiving devices to perform processing
of the data in order to execute required actions. Some of the
attributes in the GOOSE message related to these functions
are the time stamp representing the time at which the current
state number changed for the first time, sequence number –
the value of a counter that increments each time a GOOSE
message with the same values has been sent and the Test - a
parameter that indicates that the GOOSE message is used for
test purposes.
IEC 61850 Process Bus
IEC 61850 based connection to switchgear and
sensors is typically called process bus. While much of this
communication can be based on the communication services
mapped in IEC 61850-8-1, the connection of instrument
transformers with a digital interface requires the transmission
of an analogue waveform as a stream of sampled values. For
that purpose, the service "transmission of sampled values"
has been introduced. The mapping of that service is not
defined in IEC 61850-8-1, but in IEC 61850-9-2. The use
of a digital interface to conventional or non conventional
instrument transformers result in further improvements and
can help eliminate some of the issues related to the conflicting
requirements of protection and metering IEDs.
The interface of the instrument transformers (both
conventional and non-conventional) with different types
of substation protection, control, monitoring and recording
equipment is through a device called a Merging Unit. This is
defined in IEC 61850-9-1 as: “Merging unit: interface unit
PAC.SUMMER.2009
High-speed
peer-to-peer
communications in
IEC 61850
are based on
a publisher/
subscriber
mechanism.
IEC 61850
cover story
24
Some of the built-in features
in GOOSE can be used to
improve the security of
substation PAC systems.
that accepts multiple analogue CT/VT and binary inputs, and
produces multiple time synchronized serial unidirectional
multi-drop digital point to point outputs to provide data
communication via the logical interface 4. It is important to be
able to interface with both conventional and non-conventional
sensors in order to allow the implementation of the system in
existing or new substations.
The Merging unit (as can be seen from Figure 7) is
comparable to the analog input module of a conventional
protection or other multifunctional IED. The difference is
that in this case the substation LAN performs as the digital
data bus between the input module and the protection
or functions in the device. They are located in different
devices, just representing the typical IEC 61850 distributed
functionality (Figure 7).
There are several important differences between the data
sampling in a microprocessor based relay and the process bus
as defined in IEC 61850:
While in the relays the sampling is controlled by the IED
and may use frequency tracking, in IEC 61850 all interface or
merging units are time synchronized with accuracy better
than 1 microsecond and use a fixed number of samples per
cycle at the nominal frequency
The sampled values in the IED are exchanged directly
between the A/D converter and the processor, while in IEC
61850 they are transmitted using typically multicast from the
merging unit (publisher) to all IEDs (subscribers) that need
these sampled values
Interoperability between merging units and IEDs is ensured
through the standard. A certain level of interchangeability
is ensured through documents providing implementation
guidelines. Two modes of sending sampled values between a
merging unit and a device that uses the data are defined. For
protection applications the merging units send 80 samples/
cycle in 80 messages/cycle, i.e each Ethernet frame has the
MAC Client Data contain a single set of V and I samples. For
waveform recording applications, such a sampling rate may
not be sufficient. That is why 256 samples/cycle can be sent
in groups of 8 sets of samples per Ethernet frame sent 32
times/cycle .
The sampled analogue values model applies to the
exchange of values of a DATA-SET. The data of the data set
are of the common data class SAV (sampled analogue value as
defined in part IEC 61850-7-3). A buffer structure is defined
for the transmission of sampled values from the instrument
transformer logical nodes TCTR and TVTR (Figure 8).
The information exchange for sampled values is based
on a publisher/subscriber mechanism. The publisher writes
the values in a local buffer at the sending side (see Figure 5),
while the subscriber reads the values from a local buffer at the
receiving side. A time stamp is added to the values, so that
the subscriber can check the timeliness of the values and use
them to align the samples for processing. The communication
system shall be responsible to update the local buffers of the
subscribers. A sampled value control (SVC) in the publisher is
used to control the communication procedure. The currents
and voltages from TCTR and TVTR accordingly are delivered
as sampled values over the substation LAN. In this case the
network becomes the data bus that provides the interface
between the instrument transformer logical nodes and the
different logical nodes that are used to model the functional
elements of the IED. Depending on the requirements, the user
can design it with different communications architectures as
described in the next section of the article.
7 Merging unit block diagram
8 Bus differential based on sampled analog values
6 GOOSE repetition mechanism
max.
Repetition
Interval
fast
Repetitions
max.
Repetition
Interval
Time
new Event: Data Change
Merging
units can be
considered as
shared analog
input modules.
TCTR
Merging Unit
Amplifiers, Filters
Analog circuit
Group Delay D1
Calibrator
Time
Synchronization
1 pps
PAC.SUMMER.2009
Analog to
Digital
Converter
Digital Signal
Processing
Group Delay D2
IEC 61859 9-2
SAV
TVTR
Data Set and
SAV
Formatting
Merging Unit
Bus Differential
PDIF
SAV
61859-9-2
25
IEC 61850 Substation Architectures
IEC 61850 is being implemented gradually by starting
with adaptation of existing IEDs to support the new
communications standard over the station bus and at the
same time introducing some first process bus based solutions.
Full advantage of all the features available in the new
communications standard can be taken if both the station and
process bus are used.
IEC 61850 communications based distributed applications
involve several different devices connected to a substation
local area network as shown in the simplified block diagram
in Figure 9. Merging Units (MU) process the sensor inputs
while binary input/output units can be used to monitor the
status of the breaker and trip or close it when necessary based
on the GOOSE messages it receives from the different IEDs.
The merging unit and the input/output unit can be
combined in a single device – a process interface unit (PIU) as
shown in Figure 9.
All multifunctional IEDs then receive sampled values
messages and binary status messages, process the data
(including re-sampling in most of the cases), make a decision
and operate by sending a GSE message to the IOU to trip the
breaker or perform any other required action. Figure 10 is an
illustration of how the substation design changes when the
full implementation of IEC 61850 takes place. All copper
cables used for analog and binary signals exchange between
devices are replaced by communication messages over fiber. If
the DC circuits between the substation battery and the IEDs
or breakers are put aside, “copper-less” substation is a fact.
The next possible step when using station and process
bus is the optimization of the switchgear. In order for the
protection, control and monitoring functions in a substation
to operate correctly several instrument transformers are
placed throughout the high voltage installation. However
with the capability to send voltage and current measurements
as sampled values over a local area network it is possible
to eliminate some of these instrument transformers. One
example is the voltage measurements needed by distance
protections. Traditionally voltage transformers are installed
in each outgoing feeder. However if voltage transformers
are installed on the busbar, the voltage measurements can
be transmitted over the local area network to each function
requiring these measurements.
IEC 61850 Process Bus Benefits
Process bus based applications offer some important
advantages over conventional hard wired analog circuits. The
first very important one is the significant reduction in the cost
of the system due to the fact that multiple copper cables are
replaced with a small number of fiber optic cables.
Using a process bus also results in the practical elimination
of CT saturation because of the elimination of the current
leads resistance.The impedance of the merging unit current
inputs is very small, thus resulting in a significant reduction
in the possibility for CT saturation and all associated with it
protection issues. Process bus based solutions also improve
the safety of the substation by eliminating one of the main
9 Station and process bus functional architecture
Substation
HMI
Gateway to
SCADA
Ethernet Network - Station Bus
IED
IED
IED
IED
IED
Ethernet Network - Process Bus
PIU
PIU
PIU
PIU
PIU
safety related problems - an open current circuit condition.
Since the only current circuit is between the secondary of a
current transformer and the input of the merging unit located
right next to it, the probability for an open current circuit
condition is very small. It becomes non-existent if optical
current sensors are used.
Last, but not least, the process bus improves the flexibility
of the protection, monitoring and control systems. Since
current circuits can not be easily switched due to open circuit
concerns, the application of bus differential protection, as
well as some backup protection schemes becomes more
complicated. The above is not an issue with process bus,
because any changes will only require modifications in the
subscription of the protection IEDs receiving the sampled
analog values over IEC 61850 9-2.
10 Substation design with process and station bus
The use of IEC
61850 station
and process
bus practically
IU
IU
IU
eliminates
copper cables
from the
Ethernet Switch
substation.
Multifunctional Device
PAC.SUMMER.2009