Part 5 of IEC 61850 identifies the functions of a substation and groups them into six categories: system support, configuration and maintenance, operation and control, local process automation, distributed automatic support and distributed process automation. Each function is described by its task, start criteria, result, performance, decomposition and interaction with others, and is divided into logical nodes that can reside in different devices. The standard does not standardize the functions: it standardizes how the logical nodes that make them up communicate.
Which functions the standard identifies
Part 5 of IEC 61850, in its edition 1, starts from a premise: to know what a substation automation system must communicate, you first have to identify its functions. The standard recognizes that any list will be incomplete, but assumes that the identified functions representatively cover the communication requirements of a substation. The functions are the tasks of controlling, monitoring and protecting the substation equipment and its outgoing circuits, plus those needed to maintain the system itself: configuration, communication management or software management.
Important: the standard does not standardize the functions. It says so expressly. What it does standardize is the communication of the logical nodes into which they are decomposed, so that whoever uses any of those functions communicates it in an interoperable way.
Six categories of functions
The order of part 5 (clause 8) groups the functions as follows. The standard warns that belonging to a category is only a convention and that a function can fit in more than one.
| Category | Functions included |
|---|---|
| System support | Network management, time synchronization, self-checking of the physical device |
| Configuration or maintenance | Node identification, software management, configuration management, logical node operating mode control, settings, test mode, system security management |
| Operation or control | Access security management, control, operational use of spontaneous indication changes, synchronized switching (point-on-wave), parameter set switching, alarm management, event recording, retrieval of data and of disturbance or fault records |
| Local process automation | Protection function (generic), distance protection as an example, bay interlocking, measurement, metering and power quality monitoring |
| Distributed automatic support | Station-wide interlocking, distributed synchrocheck |
| Distributed process automation | Breaker failure, adaptive protection, reverse blocking, load shedding, load restoration, voltage and reactive power control, infeed transfer and transformer changeover, automatic switching sequences |
Bay interlocking belongs to local automation; station-wide interlocking belongs to distributed support, because it uses data from several bays. Interlocking between bays with GOOSE messages is explained in GOOSE interlocking with CILO and CSWI. Breaker failure and load shedding are in the last category because they act on several devices; see breaker failure 50BF.
How the standard describes a function
Each function is described with the same template (clause 7.1 and annex G):
- the task of the function;
- the start criteria;
- the result or impact;
- the performance required of it;
- the decomposition into logical nodes and how many typical decomposition sets exist;
- the interaction with other functions.
Decomposition is the most important part, because communication is based on interacting logical nodes. From the description of the function, the standard describes each logical node (application group, short text of the functionality, IEEE C37.2 function number when applicable, abbreviation) and the data it exchanges.
The logical node as the basic building block
To allow free distribution of functions, the standard decomposes all functions into logical nodes (LN) that can reside in one or several physical devices. Two ideas complete the concept:
- Logical connection. Nodes are linked by logical connections to exchange data. Every logical node is part of a physical device and every logical connection is part of a physical connection. A function is distributed when its nodes reside in different devices.
- “Device” node (LLN0). Some data belongs to no function but to the device itself, such as the name plate or the result of its self-supervision. For these, each physical device has a special logical node, LLN0.
As an example, part 5 shows a protection function made up of three logical nodes in three physical devices: the operator interface node (IHMI) in the station computer, the protection node (P class) in the relay, and the node of the circuit breaker it trips (XCBR) in the remote process interface, together with the current transformer (TCTR). Most functions are made up of at least three nodes: the function core, the process interface and the operator interface. The complete catalog of nodes and their organization by groups is in part 7-4; the standard makes clear that, if any of those functions is used, its communication must be based on the logical node structure.
- Operator interface. The IHMI node in the station computer gives humans access to the function
- Protection node. A P-class node in the relay is the core of the function
- Process interface. The TCTR and XCBR nodes represent the current transformer and the breaker that receives the command
What the standard requires of the receiving node
Part 5 sets a design condition that explains why the data model carries quality and time: each receiving node must know which data it needs, and verify that it arrived complete, valid and with adequate quality. In real-time systems the most important validity criterion is the age of the data. The sending node can assign most of the quality attributes; deciding that a data item is “old” is the receiving node’s task. That way a missing data item is covered: there is simply no data of acceptable age.
If data is corrupted or lost, the receiving node cannot operate normally but can switch to a degraded mode. The standard requires that behavior in normal and degraded mode be well defined, but leaves the design of the degradation to each function. The other nodes of the distributed function and the system supervision must be informed through a standardized message or the appropriate quality attributes. If there is time, a retransmission of the data can even be requested. The detailed sequential behavior of distributed functions cannot be standardized.
The standard gives a clear example of why this is enough: the different interlocking algorithms, such as the Boolean one or the topology-based one, can run with the same set of data, the positions of the switching devices. Interoperability lies in the data, not in the algorithm.
Formal description of the system
The static structure of the communication, where data can come from and where it goes, must be engineered or negotiated in the assembly phase. Opening and closing channels at run time always refers to that static structure. To control free allocation and create interoperable systems, the standard requires a formal description of the device and the system, which is the configuration language of part 6. See SCL files.
What to do with this in a project
- When specifying a substation, list the functions by category and decide in which device each logical node lives; this is what is later reflected in the configuration file.
- For each distributed function, define in writing what each node does if a data item or a link is missing: block, degrade or retry.
- Check that each function has its three pieces: process, core and operator.
- Test each function end to end, not just the relay: the data originates in the process and ends at a circuit breaker or an alarm.
Part 5 also classifies messages by speed; see message types and performance classes. And for an overview of the standard, IEC 61850: what it is and its parts.
Functions and logical nodes with Tiza Group
Do you need to map your substation functions to logical nodes and configure the relays that run them? At Tiza Group we do it through our remote relay configuration service and our IEC 61850 engineering, at medium, high and extra high voltage, in substations and industry. The five substation types of IEC 61850-1 and how the bus grows with size are explained in Substation Types D1 to T2 in IEC 61850-1.
Related guides
- IEC 61850: what it is, parts of the standard and data model
- IEC 61850 levels and interfaces: station, bay, process
- PICOM in IEC 61850-5: what it is and what it is for
- GOOSE interlocking with CILO and CSWI
- Breaker failure protection (50BF)
- Substation automation (SAS): what it is and its levels
- Substation Types D1 to T2 in IEC 61850-1
References
- IEC 61850-5 (edition 1, 2003): communication requirements for functions and device models, clauses 5 to 9 and 11, annexes F and G.
Frequently asked questions
Does IEC 61850 define how a protection or an interlocking must work?
No. The standard makes clear that standardizing functions is not its purpose. What it defines are the logical nodes into which each function is decomposed and the data they exchange, so that devices from different manufacturers can cooperate. The algorithm and the settings are defined by each manufacturer and each project.
What is a logical node and how many are there per function?
It is the smallest part of a function that exchanges data. Most functions are made up of at least three logical nodes: the one that contains the function itself, the process interface and the operator interface. Each physical device also has a special logical node for its own data, called LLN0.
What happens to a distributed function if a data item or a link is lost?
The standard requires the receiving node to verify that the data is complete, valid and of the right age, and that its behavior in normal mode and in degraded mode is well defined. How each function degrades is designed by the manufacturer or the project, and the other nodes and system supervision must be told about the degradation.