Logical Node (LN)

Each LN represents a well-defined, self-contained function within a physical IED. The standard pre-defines the data objects and behavior for common utility functions, ensuring interoperability between devices from different manufacturers.

• XCBR: Circuit breaker control and monitoring
• PDIS: Distance protection element
• MMXU: Measurement unit for currents and voltages
• PTRC: Protection trip conditioning logic

Every LN instance is uniquely identified by a structured reference path that reflects its physical location in the substation hierarchy. This path concatenates the IED name, access point, and logical device.

• Format: IEDNameLDName/LNClassNameInstanceID
• Example: Q1SB1PDIS1 refers to instance 1 of distance protection in bay Q1
• Enables precise addressing for GOOSE publisher-subscriber bindings and SCADA data mapping

The IEC 61850-7-4 standard defines a strict schema of mandatory (M) and optional (O) data objects within each LN class. This guarantees a baseline of functionality while allowing manufacturers to differentiate.

• Mandatory: Mod (Mode), Beh (Behavior), Health (Health), NamPlt (Name Plate) are common to all LNs
• Optional: Advanced diagnostic or control points specific to a vendor's implementation
• Ensures that critical operational data is always present and uniformly accessible

All LNs inherit from a common logical node class (LLN0), which provides fundamental attributes for self-supervision and mode control. This inheritance enforces a consistent management interface across every function in the substation.

• Mode (Mod): Controls operational state—On, Off, Test, Test/Blocked
• Behavior (Beh): Reports the actual current functional state
• Health (Health): Reflects internal self-diagnostic status (Ok, Warning, Alarm)
• Name Plate (NamPlt): Provides vendor, version, and configuration metadata

LNs expose their data through standardized Abstract Communication Service Interfaces (ACSI) , which are mapped to specific protocols like MMS. These services define how clients read, write, report, and log data.

• GetDataValues/SetDataValues: Immediate read/write of data object attributes
• Reporting: Buffered and unbuffered change-driven event reporting
• Logging: Time-stamped historical data storage for disturbance analysis
• GOOSE Control: Direct multicast publishing of status changes for peer-to-peer tripping

LNs are grouped into Logical Devices (LDs) within a physical IED. An LD typically represents a coherent protection or control domain, such as a bay controller or a measurement unit, and always contains an LLN0 for management and an LPHD for physical device information.

• LLN0: Manages the LD's datasets, reports, and GOOSE control blocks
• LPHD: Maps to the physical hardware, providing nameplate and health data
• Allows a single IED to host multiple independent functional domains, such as separate bays

Standardized templates that define the structure and semantics of data objects contained within a Logical Node. Each CDC specifies a set of mandatory and optional attributes with defined functional constraints (e.g., ST for status, MX for measurements).

• SPS (Single Point Status): Used for binary states like circuit breaker position.
• MV (Measured Value): Carries analog measurements with deadbanding and range attributes.
• ACT (Protection Activation): Models the output of protection functions, including general, phase-specific, and neutral signals.

A container that groups Logical Nodes and provides a communication access point for an Intelligent Electronic Device (IED). An IED typically hosts one or more Logical Devices, each representing a distinct functional subsystem.

• Contains exactly one LLN0 (Logical Node Zero) for managing common device-level data.
• Contains exactly one LPHD (Physical Device Information) for hardware health and nameplate data.
• Enables modular organization of protection, control, and measurement functions within a single physical device.

IEC 61850-7-4 organizes Logical Nodes into functional groups based on their application domain. Each group is identified by a leading character in the LN class name.

• L Group (System LNs): Physical device information (LPHD) and logical node zero (LLN0).
• P Group (Protection): Distance protection (PDIS), overcurrent (PTOC), differential (PDIF).
• R Group (Protection Related): Auto-recloser (RREC), synchrocheck (RSYN).
• X Group (Switchgear): Circuit breaker (XCBR), disconnector (XSWI).
• T Group (Instrument Transformers): Current transformer (TCTR), voltage transformer (TVTR).

The full path to a data attribute follows a strict hierarchical concatenation that uniquely identifies every signal in a substation automation system.

Structure: IEDNameLDName/LNName.DataObjectName.DataAttributeName

Example: T1CTRL/XCBR1.Pos.stVal

• T1CTRL: The IED controlling Transformer 1.
• XCBR1: The circuit breaker Logical Node instance.
• Pos: The data object for switch position (type DPC).
• stVal: The status value attribute (e.g., 0 for off, 1 for on).

Logical Nodes produce data that is published via Generic Object Oriented Substation Event (GOOSE) messages. Receiving IEDs subscribe to these messages by mapping incoming data to input references in their own LN instances.

• A PDIS (distance protection) LN may subscribe to the Pos.stVal output of an XCBR LN to implement breaker failure logic.
• Subscription is configured in the Substation Configuration Language (SCL) file using ExtRef elements.
• GOOSE allows peer-to-peer communication between LNs across different physical IEDs on the station bus.

Each Logical Node instance within a system must have a unique reference composed of a standardized class name, an instance number, and an optional prefix.

• Class: The four-character standardized function name (e.g., XCBR, MMXU).
• Instance Number: A numeric suffix distinguishing multiple instances of the same class (e.g., XCBR1, XCBR2).
• Prefix: An optional user-defined string added for further functional grouping (e.g., Q0XCBR1 for a specific bay).
• The combination ensures unambiguous identification across the entire substation project.