IEC 61850 Logical Node Groups, LLN0 and LPHD

What the standard says and what it is for

IEC 61850 describes each function of a substation as a logical node: a piece of software with a name, data and defined services. Part 7-4 is the catalog of those classes. Before going into each one, the standard arranges them in groups, and that classification is the first thing to understand in order to read a configuration file or the signal list of a relay.

Whoever receives the model of an IED from another manufacturer can see at a glance what each node is and where to look for its status. This guide is based on edition 1 of part 7-4 (2003); later editions bring more classes, so the list in this text should not be taken as the complete current catalog. The overall picture of the standard is in the general IEC 61850 guide.

Groups by initial letter

The standard establishes that the name of every logical node starts with the letter of its group. In edition 1 the groups are as follows.

The standard also recommends that other technical committees use reserved letters for their domains: H for hydro, F for fuel cells, W for wind, O for solar, B for batteries and N for power plants. That is why a node starting with one of those letters does not belong to the substation catalog.

One point avoids confusion when reading the model of a device. Groups S, T, X, Y and Z exist as nodes in dedicated devices only if there is a process bus. Without a process bus, those nodes are the hardwired inputs and outputs of the higher-level IED, for example the bay unit, which represents the external equipment through its contacts: the standard calls this the process image. In other words, an XCBR node in a bay unit does not mean the circuit breaker speaks IEC 61850; it means the unit models the hardwired signals of that breaker. To see the difference between the two architectures, see the process bus and the station bus.

How a node name is built

The full name the client sees combines the logical device and the node. According to part 7-2, the node name has three parts: an optional prefix defined by the application, the four-character class name taken from part 7-4 and an instance number. Prefix and instance together cannot exceed seven characters. That is how, for example, the three zones of a distance function are told apart: three instances of the same class, each with its own identifier.

Data categories of a node

Each table in part 7-4 organizes the data of a node in categories, and knowing them saves time when reviewing a model.

• Common logical node information: data independent of the function, such as the mode and the name plate.
• Status information: what the process or the function indicates, for example start and operate. It is produced locally and is not changed from the client, unless substitution exists.
• Settings: values the function needs in order to operate. The standard only standardizes an agreed minimum, because many settings depend on how each manufacturer implements the function. They can be changed remotely, although not often.
• Measured values: analog quantities from the process or calculated.
• Controls: data that change by commands, such as the position of a circuit breaker or tap changer.
• Metered values or energy: quantities accumulated over time.

Each data object is listed as mandatory (M) or optional (O) for instantiating the node, and some are marked as transient (T): momentary states that must be recorded or reported to leave evidence, but whose change from true to false does not generate an event. A letter C indicates conditional: at least one of the data objects marked in that category must be used.

The common node: Mod, Beh, Health and NamPlt

All logical nodes, except LPHD, inherit four mandatory data objects from a common node:

Among the optional ones of that common node are local operation (Loc), the health and name plate of the external equipment, the operation counter and the operation time. Each class can leave those optional ones as they are, make them mandatory or not inherit them.

Mod and Beh

The mode accepts five values. The table summarizes what changes with each one.

In off, the functional data are not visible, but the configuration data are, so the client knows what the node could do. The other four modes keep both types of data visible.

The behavior Beh is not written: the node calculates it. It is determined by the combination of its own mode and the mode of LLN0. The rule reads like this: if either one is off, the behavior is off; if one is in test and the other blocked, the result is test/blocked; if both are on, the result is on. In practice, leaving LLN0 in test turns all nodes into test even if each one is on, and allows a whole logical device to be isolated with a single change. This logic is the basis of test mode and simulation in IEC 61850. How IEC 61850 models setting groups and what to watch when switching groups is covered in IEC 61850 Setting Groups.

Health

Health summarizes the state of the node’s hardware and software with three values: OK (green), warning (yellow) and alarm (red). The standard defines green and red unambiguously; the exact meaning of yellow is left to the manufacturer and the function. In LLN0, the data object reflects the worst health value among the nodes of the logical device, so a client can supervise a device with a single read.

The system nodes: LLN0 and LPHD

Group L contains two nodes that do not represent primary functions.

LLN0 (logical node zero) handles what is common to a logical device. It inherits the mandatory data of the common node and adds local operation of the whole device, operation time, the command to run diagnostics and the reset of the LEDs. According to part 7-2, each logical device has exactly one LLN0 and one LPHD, and in LLN0 live the setting group, the event log and the GOOSE and sampled value control blocks. In addition, its mode modifies the behavior of all the other nodes.

LPHD (physical device information) models the real equipment that hosts the logical devices. It is the only node that does not inherit from the common node. Its mandatory data are the device name plate, its physical health and an indicator of whether the node is a proxy. The optional ones cover communication buffer overflows, number of startups, watchdog restarts, loss or presence of the power supply, external source alarm and statistics reset. These are the data the operations team consults to know whether a device restarted without warning.

How it is used in practice

With this knowledge you can:

• Read a substation description file and know, from the letter, what each node represents, using an SCL viewer and the guide on SCL files.
• Check during commissioning that Health and Beh are green and on before declaring a device delivered, and that none was left in test.
• Cross-reference each ANSI function with its node using the ANSI table.

We help you with your data model

At Tiza Group we review the node model of your IEDs, clean up the signals that reach the SCADA and keep the test behavior under control during FAT and SAT testing. We do it remotely, worldwide, with our remote relay configuration service or on site with our IEC 61850 engineering. How IEC 61850 models wind turbines and distributed resources is explained in IEC 61850 Logical Nodes for Wind and Generation.

Related guides

• IEC 61850: what it is, parts of the standard and data model
• Test mode and simulation in IEC 61850
• IEC 61850 SCL files
• Protection logical nodes: PDIS, PTOC, PTRC, RREC
• Control and automatic logical nodes: CSWI, CILO, ATCC
• Primary equipment logical nodes: XCBR, XSWI, TCTR, TVTR
• IEC 61850 Logical Nodes for Wind and Generation
• IEC 61850 Setting Groups: How the SGCB Changes Them

References

• IEC 61850-7-4, edition 1 (2003): compatible logical node classes and data classes, clauses 5.1 to 5.3 and 6.
• IEC 61850-7-2, edition 1 (2003): abstract communication service interface, structure of the node name.
• IEC 61850-7-1: principles and models.

Frequently asked questions