Very often, the best way to send a message is to send it directly, without any conversion of the message between the sender and the receiver. Today, for substation communications, there is a large variety of protocols. In order that devices running on different protocols may communicate, protocol converters are used. However, protocol converters can cause errors in the messages and introduce delay. The large number of protocols leads to high development expenditure on the part of the manufacturers and high operating and maintenance costs on the part of the utilities.
Since 1995, about 60 experts from 14 countries have been tackling these issues in three IEC working groups. They responded to all these challenges and created the single, global and future-proof standard for substation communications, and this is IEC 61850. Very high objectives were set for the standard:
To cover all the information in substations, down to small digital units for driving the processes, which thus include digital transducers or sensors, and actuators located close to the processes.
Openness for extension of the information to be communicated in the future according to the principle: All that are known are incorporated, and any future applications can be filled in according to the set rules.
Openness for future high-efficiency data transfer.
To promote the idea on interoperability in systems, which surpasses the specifications of data-coding and communication services like IEC 60870-5. The requirements on TYPEering and on the sustainability of products within the service life of the corresponding system are included in the standard.
The standard is now ready. Utilities and manufacturers have been involved in the standardisation work since the beginning, and have taken part in pilot projects and interoperability tests. First orders of substation automation systems compliant with IEC 61850 have been received by manufacturers.
Martin Baltscheit has written a children’s story, which shows that problems arise when a message is passed indirectly from the sender to the receiver.
Advantages
Defines one protocol for the entire substation
Fully supports all substation automation functions comprising control, protection and monitoring
The architecture is future-proof and facilitates future extensions, therefore it safeguards investments
Is a worldwide applicable and accepted standard - the single key to interoperable solutions
Defines the quality requirements (reliability, system availability, data integrity, security, etc.), environmental conditions, and the auxiliary services of the system
Specifies the TYPEering processes and its supporting tools, system life-cycle and the quality assurance requirements and maintenance for the entire substation automation system
States the conformance tests to be carried out on the products
The flexibility allows optimisation of system architectures (scalable technology)
Uses readily available industrial Ethernet and communication components
Facilitates a utility-wide common communication infrastructure, from the control centre to the switchyard
The Idea behind the standard
Producing one standard for substation communication to achieve interoperability sounded simple but there were two challenges:
Such a standard must be able to adapt itself to the changes in communication technology. In other words, such changes must not lead to a major revision of the standard.
A system integrator must be able to build up and configure a system easily.
The ingredients of a future-proof standard would be
Object-oriented communication
Separation of application related functions from the communication methods
Manufacturer-independent information exchange for configuration of the automation system.
Object oriented communication is the present trend in software and hardware. Each object is a function or part of a function, and the object exchanges data with other objects so that the function may be executed.
Substation automation has benefitted from the computer technology, particularly communication technology. While processor speed doubles every two or three years and when data transmission rate increases by a factor of ten, the underlying automation functions and the data involved hardly change. For example, for decades, an overcurrent function has had the current as input data and the trip signal as output data, accompanied by configuration data such as current and time settings. In order that substation automation may be able to take advantage of technological changes, the data are standardised in IEC 61850 and are independent of the communication method, which is separately standardised in IEC 61850.
Data Model
Models have become an indispensable part in TYPEering but they take many forms. A model generally provides some information about a piece of equipment or a process. For a transformer, the equation
Primary voltage: Secondary voltage = Number of primary turns: Number of secondary turns
is a model which explains in simple terms how a transformer works. A box of length 3m, width 2m, height 3m and weight 20 tonnes is another model of a transformer for other purposes. In substation communications, a model which lists the input and output data of the transformer is called the data model.
For each function, IEC 61850 has a data model. A data item has a name which comprises three standardised parts namely, the logical node, the data object and the attribute. These three parts are illustrated by the example of the circuit breaker. The status of the circuit breaker has the name
XCBR.Pos.stVal
XCBR is the logical node. Pos is the data object. stVal is the attribute. These three parts are in an hierarchical structure similar to the Explorer as shown below.
Under Pos, there are other attributes. For example, when a value is written into it, ctlVal allows the circuit breaker to be switched ON or OFF. Under XCBR, there are other data objects such as
Mod (Mode), i.e. enabled, blocked, disabled, under test etc.
Health (Health), i.e. no problems and in normal operation, minor problems but can operate safely, severe problems and no operation possible
Name plate (NamePlt), i.e. the technical details of the circuit breaker.
There are almost always more than one circuit breaker in a substation and the data name can contain user-specific parts. For example, a circuit breaker of Bay A can have the following name
BayA/XCBR.Pos.stVal
There are approximately 90 logical nodes in IEC 61850. A further example is PTOC, the logical node for time-overcurrent protection.
Service Model
In a substation automation system, some data are time-critical but some are not. Time-critical data include for example trip signals for circuit breakers and start signals for disturbance recorders. An example of non-time-critical data is the changes of device settings. Time-critical data need to be sent quickly while non-time-critical data have lower transmission priority. IEC 61850 defines a set of generic services which can meet the requirements of all kinds of anticipated data in a substation with regard to speed of transfer, accuracy and security.
Generic Object Oriented Substation Event (GOOSE) is a fast service. The high speed is achieved by means of connection-less communication and the security of data transfer is assured by repeating the message many times. File Transfer is a service for moving large data blocks such as programs and fault records.
Mapping
The data models and services are mapped onto a real communication stack. The stack comprises now the Manufacturing Message Specification (MMS), Transmission Control Protocol/Internet Protocol (TCP/IP) and Ethernet. Data models and services models do not change much with time, but communication technology may. When communication technology changes, the data models and service models will be mapped on to a new real communication stack. This leads to the minimum changes not only in the standard but also in the substation automation system.
Configuration
When constructing a substation automation system, a system integrator needs to make devices from different manufacturers talk to each other and function in the way they should.
The system integrator needs to know the basic information of each device such as the IP-address, and its capability such as the supported logical nodes and services. According to IEC 61850, each device has a self-description file called the IED Capability Description (ICD). The file can be read by any configuration tool supporting IEC 61850. The system integrator enters into the configuration tool the information about the substation automation system e.g. the primary equipment, the automation functions, communication methods of the devices. The configuration tool reads in all the ICD files and
assigns the automation functions to the devices
makes each party know whom its communication partners are and where these partners are
The configuration tool generates a file called Substation Configuration Description (SCD). The information in this file is written back into the respective devices.
Conformance Testing
The products of a manufacturer, which were designed to comply with IEC 61850, go through the usual system test, routine test and type test, regardless of the final applications. These products also go through Factory Acceptance Test (FAT), site commissioning and Site Acceptance Test (SAT) to verify that the equipment is fit for the intended application. Independent of these tests, conformance testing is carried out so that the manufacturers may confirm conformity to IEC 61850.
Conformance testing covers
Checking documentation and version designation
Configuration
Abstract services
Data Model
Mapping
and can be carried out for example by independent test laboratories, which are accredited by the UCA International Users Group. |
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