# IEC 61850 — Communication Networks and Systems for Power Utility Automation

> IEC 61850 is the international standard for communication in electrical substations and power utility automation. It defines a unified data model for protection relays, circuit breakers, transformers and other substation equipment, enabling interoperability between IEDs (Intelligent Electronic Devices) from different vendors over Ethernet.

- Source : https://industryhub.site/en/standards/iec-61850
- Body : IEC
- Family : Power Systems
- Status : current
- First published : 2003
- Latest revision : 2013 (Edition 2) + ongoing amendments
- Edition : 2
- Access : Paid (IEC webstore — price per part)
- Language : English

## Document structure

- **IEC 61850-1** — Introduction and overview : Scope, objectives and architecture overview. Introduces the key concepts: logical nodes, data objects, services, and communication stacks. Starting point for understanding the standard.
- **IEC 61850-5** — Communication requirements for functions and device models : Defines the performance requirements — message types (Type 1 to Type 7), transfer time classes, and which messages are time-critical. GOOSE messages must be transferred in <4 ms for tripping functions.
- **IEC 61850-6** — Configuration description language (SCL) : XML-based Substation Configuration Language. Defines four file types: SCD (complete substation), SSD (single-line diagram), ICD (IED capability), CID (configured IED). SCL enables vendor-neutral configuration and import/export between engineering tools.
- **IEC 61850-7-1** — Basic communication structure — Principles and models : Defines the abstract data model: server, logical device, logical node, data object, data attribute hierarchy. All IED data is accessed through this model regardless of the underlying communication protocol.
- **IEC 61850-7-2** — Abstract communication service interface (ACSI) : The abstract service definitions — reporting, logging, GOOSE control, sampled values, setting groups. ACSI is the platform-independent service layer that MMS and other protocols implement.
- **IEC 61850-7-3** — Common data classes (CDC) : Standardised data classes — SPS (single point status), DPS (double point status), MV (measured value), CMV (complex measured value), etc. CDC + logical node names give every piece of data a standard identity across all vendors.
- **IEC 61850-7-4** — Compatible logical node classes and data object classes : The catalogue of logical nodes: XCBR (circuit breaker), XSWI (switch), PDIF (differential protection), POTT (distance protection), PTOC (overcurrent protection), MMXU (measurement unit), etc. These standardised names appear in every IEC 61850-compliant relay.
- **IEC 61850-8-1** — Specific communication service mapping — MMS : Maps ACSI services to MMS (Manufacturing Message Specification, ISO 9506) over TCP/IP. MMS is the station-bus protocol for SCADA access to IED data — reports, logs, control.
- **IEC 61850-9-2** — Specific communication service mapping — Sampled Values (SV) : Merging unit interface — publishes digitised instantaneous current and voltage samples (typically 80 or 256 samples/cycle) over Ethernet multicast. Replaces copper current transformer secondary wiring in digital substations.
- **IEC 61850-10** — Conformance testing : Conformance test requirements and methods for IED manufacturers. Successful conformance testing (by accredited labs like KEMA, CESI) is a prerequisite for interoperability.

## Key concepts

- **Logical Node (LN) (LN)** : The fundamental functional unit in IEC 61850. Each logical node represents a specific function (XCBR = circuit breaker, PDIF = differential protection). Every logical node has a standardised set of data objects and attributes, enabling tools and systems to discover and interpret data automatically regardless of vendor.
- **GOOSE (Generic Object-Oriented Substation Event) (GOOSE)** : High-speed publish-subscribe messaging for protection and interlocking — replaces hardwired copper trip/close signals. GOOSE messages are multicast over IEEE 802.1Q VLAN, achieve <4 ms transfer times, and include a retransmission scheme for reliability. Critical performance path in protection scheme design.
- **Sampled Values (SV) (SV)** : IEEE 802.3 Ethernet multicast of instantaneous digitised current and voltage samples from merging units. Enables process bus architecture where protection relays receive digital inputs rather than hardwired CT/VT secondary signals. IEC 61869-9 covers the merging unit interface.
- **Substation Configuration Language (SCL) (SCL)** : XML-based engineering language (IEC 61850-6) for describing the complete substation. An SCD (Substation Configuration Description) file contains the single-line diagram, all IEDs, all logical nodes, all GOOSE and SV subscriptions. SCL-based engineering tools (DIGSI, PCM600, EcoStruxure) exchange SCD files for multi-vendor configuration.
- **Process bus** : Architecture where merging units and intelligent circuit breaker units are installed in the switchgear yard and communicate with protection relays in the relay room over optical fibre Ethernet. Replaces copper CT, VT and binary I/O wiring. Enabled by IEC 61850-9-2 (SV) and IEC 61850-8-1 (GOOSE). Standard for new GIS substations.
- **Station bus** : Ethernet LAN connecting all IEDs within the substation control building to the station SCADA/HMI. Uses MMS (IEC 61850-8-1) for data access and GOOSE for inter-IED communication. Typically a redundant ring network per IEC 62439-3 (PRP/HSR).

## Overview

IEC 61850 is the **standard for substation automation and protection communication**. Before IEC 61850, every relay and IED manufacturer used a proprietary protocol — resulting in expensive, vendor-locked substation engineering with hundreds of copper point-to-point connections.

IEC 61850 introduces:
1. **A common data model** — standardised logical nodes and data objects mean any compliant tool can discover and interpret data from any compliant IED
2. **High-speed Ethernet messaging** — GOOSE replaces copper trip wires; Sampled Values replaces CT/VT secondary copper wiring
3. **XML engineering** — SCL files enable vendor-neutral configuration and document the complete substation in a machine-readable format

## Communication Architecture

```
┌─────────────────── Station Bus (MMS / GOOSE) ────────────────────┐
│    SCADA / HMI      Gateway       Protection     Protection      │
│    (Client)         (IEC→ICCP)    Relay A        Relay B         │
└──────────────────────────────────────────────────────────────────┘
         │                                │              │
┌─────── Process Bus (GOOSE + Sampled Values) ───────────────────────┐
│    Merging Unit A    Merging Unit B    Intelligent CB Unit         │
│    (CT/VT digital)   (CT/VT digital)  (position + GOOSE)          │
└────────────────────────────────────────────────────────────────────┘
```

## Message Types and Transfer Times

| Message type | Protocol | Transfer time | Application |
|-------------|----------|--------------|-------------|
| GOOSE | IEEE 802.1Q | <4 ms (Type 1A) | Protection tripping, interlocking |
| Sampled Values | IEEE 802.3 | <4 ms (Type 4) | Merging unit current/voltage |
| Reporting (BRCB/URCB) | MMS/TCP | <100 ms (Type 2) | Event-driven data to SCADA |
| Logging | MMS/TCP | <1000 ms (Type 3) | Historical sequence of events |
| Control | MMS/TCP | <500 ms (Type 1B) | Breaker open/close |
| Files | MMS/TCP | No time limit | Oscillography, settings |

## GOOSE Network Design Requirements

- Dedicated **VLAN per GOOSE stream** — never mix GOOSE with general IT traffic
- **Redundant network**: PRP (Parallel Redundancy Protocol, IEC 62439-3) — zero recovery time for single link/switch failure
- **IEEE 1588 PTP** time synchronisation — Class C accuracy (1 µs) for Sampled Values; Class T (1 ms) for GOOSE and event timestamps
- Switches must support **IGMP snooping** to control GOOSE multicast flooding
- Maximum switch latency in GOOSE path: **<1 ms per hop**

## SCL File Workflow

1. Each IED vendor supplies an **ICD** file (IED Capability Description)
2. Substation engineer imports ICD files into the SCD tool
3. GOOSE and SV subscriptions are configured in the SCD tool
4. The tool exports **CID** files (Configured IED) for each relay
5. CID files are loaded into relays and validated with conformance test tools

## Applicable industries

energy, oil-and-gas, manufacturing, water-treatment

## References & further reading

- [IEC 61850 series — IEC webstore](https://webstore.iec.ch/searchform&q=61850)
- [IEC 61850 overview — IEC](https://www.iec.ch/iec61850)
- [CIGRE WG B5 — IEC 61850 application guides](https://www.cigre.org)