The Substation section of an SCL file describes the switchyard as a functional hierarchy: substation, voltage level, bay, equipment and sub-equipment. Apparatus are joined through connectivity nodes and terminals, which reproduce the single-line diagram, and logical nodes (LNode) can be attached at every level to state which automation function that apparatus or bay performs. It is the basis of the functional name of every signal.
What the standard says and what it is for
Clause 9.2 of IEC 61850-6 (edition 1, 2004) defines the Substation section. Its purpose is to describe the functional structure of the substation, identify the primary apparatus and their electrical connections and, through logical nodes attached to those elements, state which automation functions the system has. In a specification file (SSD) it does so without assigning devices; in a system file (SCD), where the nodes are already assigned, it relates the functions of each IED to the power system. The subnets, access points and the server, GOOSE and sampled value addresses are in SCL Communication Section.
It is the part of the file that a switchyard engineer recognizes at once: it is the single-line diagram, written so that a tool can read it. It is also the source of the functional name of the signals, as explained in logical node names and signals in SCL. To place it within the complete model, see the SCL object model.
The switchyard hierarchy
The objects, from highest to lowest level, are these:
| Element | What it represents |
|---|---|
| Substation | The complete substation. Its name is mandatory, cannot be empty and is a global identifier. |
| VoltageLevel | Part of the substation electrically connected at one voltage. It may declare its voltage. |
| Bay | Identifiable part of the switchyard within a voltage level, for example a line feeder bay. |
| Equipment | Switchyard apparatus: circuit breaker, disconnector, voltage or current transformer, winding, etc. |
| SubEquipment | Part of an apparatus, typically one phase. |
| ConnectivityNode and Terminal | The junction points and the connections of each apparatus. |
For industrial processes or complete networks there may be several Substation sections, one per substation the system serves. If the section is used as a template inside an IED capability file (ICD), its name must be TEMPLATE.
Uniqueness rules imposed by the standard: the voltage level name is unique within the substation, the bay name is unique within its level and, inside the same container, no direct child element may repeat a name. Logical nodes of the same level must also have different identifiers.
If a busbar has no primary apparatus, the standard indicates that it can be modeled as a bay that contains only connectivity nodes.
Equipment and types
Apparatus are identified with a type code, chosen where possible from the logical node class names of IEC 61850-7-4. Some of the codes in the standard, with the number of terminals each may have:
| Code | Apparatus | Terminals |
|---|---|---|
| CBR | Circuit breaker | 2 |
| DIS | Disconnector or earthing switch | 2 |
| VTR | Voltage transformer | 1 |
| CTR | Current transformer | 2 |
| PTW | Power transformer winding | 1 |
| PTR | Power transformer | Depends on windings |
| LTC | On-load tap changer | Part of the winding |
| CAP, REA | Capacitor bank, reactor | 1 or 2 |
| GEN, MOT | Generator, motor | 1 |
| LIN | Overhead line or line section | 2 |
| IFL | External infeed line; marks the substation boundary | 1 |
The complete list includes more types (battery, bushing, cable, converter, surge arrester and others). Private codes are allowed if they start with the letter E, use uppercase letters only and have at least three letters.
The power transformer is a special piece of equipment: it can sit directly under the substation, the voltage level or the bay, and it contains its windings, to each of which a tap changer can be assigned. All other conducting apparatus can only sit in a bay.
Sub-equipment makes it possible, for example, to tie a logical node to a specific phase (A, B, C, neutral, all three or none). It also has a “virtual” attribute to indicate that the element does not exist in reality: useful when a function calculates the current of one phase from the other two.
The single-line diagram: connectivity nodes and terminals
A connectivity node is the point where apparatus join. Each apparatus has terminals (one or two usually suffice at single-line level) that point to the node they are connected to. The reference to the node is written in two forms at once, both mandatory: the full path and the list of separate attributes. The path allows the consistency of the connection to be validated in the XML schema itself; the list is easier for tools to interpret.
The path is formed from the names of each level, separated by a slash. For example, node L1 in bay Q2 of voltage level E1 of substation Baden is written Baden/E1/Q2/L1. The slash was chosen because a period may appear inside the names of higher levels.
There is a predefined connectivity node, called grounded, which represents earth. An earthing switch is therefore a disconnector connected on one side to that node. If an apparatus must not be connected, its terminal element is removed.
Where logical nodes are attached
Every piece of equipment and every equipment container is also a container of logical nodes. The LNode element identifies an automation function with these attributes:
| Attribute | Content |
|---|---|
| lnClass | Node class according to IEC 61850-7 (mandatory) |
| lnInst | Instance number; may be empty only for LLN0 |
| iedName | IED that hosts it; None if used only as a specification |
| ldInst | Logical device of the IED where it resides |
| prefix | Node prefix in the IED, if it has one |
| lnType | Node type, to detail required data or parameter values |
The standard gives a placement criterion: a switching control node is associated with the apparatus it operates; a measurement node, with the bay that delivers the quantities; and transformer nodes, with the transformer. When the node is later assigned to an IED, the lnType value can be ignored or used to verify that the node type of the device meets the specification.
With iedName, ldInst, prefix, lnClass and lnInst, the link between the switchyard function and the automation system is established.
Functions that are not part of the power system
To tie logical nodes to things that are not power equipment, such as door supervision or the fire protection system, the section includes the Function and SubFunction elements. They are containers of logical nodes that can host general equipment (battery, auxiliary supply or motor), but they do not define connections between equipment.
How it is implemented, in general terms
Whoever specifies a system starts from the single-line diagram. Each apparatus carries its type and its terminals, and each is assigned the logical nodes that will perform its function: switching control, interlocking, measurement, protection. An example from the standard itself is an SSD file with a transformer between 220 kV and 110 kV, with current measurement and differential protection, and a bay with a circuit breaker and a busbar disconnector where each apparatus has its switching node and its interlocking node, with no IED assigned.
- Single-line diagram. Substation, voltage levels, bays and apparatus with their terminals
- Logical nodes. Attached to apparatus, bays and transformers with iedName set to None
- Assignment to IEDs. The system file fills in iedName, ldInst and prefix for each node
- Functional name. Substation, level and bay become part of the signal name
Two good practices follow from the rules above: keep the names of levels and bays aligned with the substation drawings, and verify that each control node is tied to an apparatus of the right type (a circuit breaker control node should not end up tied to a disconnector). A tool with a viewer, such as our IEC 61850 SCL viewer, helps to check the structure quickly.
We can help with your digital single-line diagram
At Tiza Group we carry the single-line diagram and the substation schematics over to the communication model and verify it against your configuration. You can start with the remote relay configuration; for the engineering of the complete automation system, see our IEC 61850 engineering, and for the design of the associated schematics, secondary engineering.
Related guides
- IEC 61850-6 SCL object model
- SCL IED section: server and logical devices
- Logical node names in SCL: prefix and class
- IEC 61850 SCL files: ICD, SCD, CID and the rest
- Bay and station interlocking
- SCL Communication Section: Subnets and Addresses
References
- IEC 61850-6, edition 1 (2004): clauses 6.2 and 9.2 (including the attribute tables of LNode, Terminal and SubEquipment, and the apparatus type table).
- IEC 61850-5 and IEC 61850-7-4: logical node classes associated with the apparatus.
- IEC 61346-1: functional structure used for designation.
Frequently asked questions
What is a connectivity node in SCL?
It is the point where apparatus of the single-line diagram join: a busbar, or the junction of two pieces of equipment inside a bay. Each apparatus carries terminals that point to those nodes, so how they are connected is described without drawing the diagram.
How is an earthing switch represented?
As a disconnector (type DIS) connected on one side to a predefined connectivity node called grounded, which represents earth potential. The tool decides whether that node is unique for the whole substation or one per bay or voltage level.
What happens if a logical node in the switchyard has no IED assigned yet?
Its iedName attribute is None. This is the case in a system specification (SSD) file, where only the required function is defined and not the device that will run it. When it is assigned to an IED in the system file, the attribute takes the name of the device.