Introduction

Traditional SCADA systems, which rely on protocols like Modbus, IEC 60870-5-101, or DNP3, struggle to keep up with the advanced features of modern Intelligent Electronic Devices (IEDs). Efforts to bridge this gap using IP-based protocol translation services have limitations, often sticking to outdated serial communications at the Remote Terminal Unit (RTU). To revolutionize how substations communicate and take advantage of Ethernet and IP technologies, the IEC Technical Committee 57 introduced the IEC 61850 standard.

Within the scope of IEC 61850, the standard categorizes devices into three distinct levels, each fulfilling specific roles within the power system:

1. Process Level: This stage includes vital devices like switchgears, circuit breakers, switches, current transformers, and voltage transformers. These devices are crucial parts of the primary equipment that directly connect with the power system.

2. Bay Level: Here, Intelligent Electronic Devices (IEDs) take center stage. Within IEC 61850, there's a definition for a process bus, enabling communication between IEDs and other smart instruments and switchgears. These IEDs manage localized tasks concerning control, protection, and data collection within specific bays or sections.

3. Station Level: At this level, the system integrates SCADA (Supervisory Control and Data Acquisition) and HMI (Human-Machine Interface) systems. These systems are vital for substation control and monitoring. To communicate with IEDs positioned at the bay level, the station level uses a station bus, facilitating communication across different substation tiers.

Each level involves specific devices and functions, playing distinct roles in overseeing, managing, and controlling the power system within the substation environment.

IEC 61850 stands as a global standard outlining communication protocols used by smart electronic devices in electrical substations. It plays a vital role in the reference architecture for electric power systems within the International Electrotechnical Commission's (IEC) Technical Committee 57. The standard's abstract data models can be tailored to different protocols, including:

MMS protocol (IEC 61850-8-1): This enables client/server communications via IP and is primarily used in SCADA systems for monitoring purposes.

GOOSE (IEC 61850-8-1): Relies on Ethernet-based multicast communication, allowing IEDs to exchange data within and between bays. It facilitates the transmission of power measurements, supports protection relays, and manages tripping and interlocking circuits. GOOSE serves as a means for status updates and command requests.

Sampled Measured Values (IEC 61850-9-2): This carries current and voltage values across power lines, commonly applied for bus-bar protection and synchrophasors.

These protocols operate over TCP/IP networks or substation LANs, utilizing high-speed switched Ethernet to achieve response times under four milliseconds for protective relaying purposes.

IEC 61850 protocol streamlines the integration of protection, control, measurement, and monitoring functions under a unified protocol. It supports high-speed substation applications, station-wide interlocking, and communication between IEDs. The detailed data modeling and communication services tailored to modern station needs make this standard a crucial element in contemporary substation systems.

In IEC 61850, communication modeling covers information models for DERs, service modeling, mapping to communication protocols, and telecommunication media. The information models establish hierarchical class models (e.g., logical device, logical node, data, dataset, report control, or log) and define various services (e.g., get, set, report, define, delete), known as ACSI services. Non-time-critical objects and services within ACSI are mapped onto MMS services, while time-critical ACSI services directly connect to Ethernet layers like GOOSE and SVs. This mapping, termed SCSM, is detailed in IEC 61850-8-1.

IEC 61850 Standard

The IEC 61850 standard outlines the substation communication network through ten main sections, as illustrated in the figure below. Its architecture abstracts data item and service definitions by creating separate entities, detached from particular underlying protocols. This abstract setup allows the mapping of data items and services to different protocols that meet the required data and service criteria.

Part 6 of this standard introduces the Configuration Description Language (CDL), which manages communication within electrical substations involving Intelligent Electronic Devices (IEDs). On the other hand, Part 7 outlines the core communication structure. This section includes:

The organization of Part 7 within the IEC 61850 standard is presented in the following manner:

IEC 61850-7-1: Principles and models

IEC 61850-7-2: Abstract communication service interface (ACSI)

IEC 61850-7-3: Common Data Classes (CDC)

IEC 61850-7-4: Compatible logical node classes and data classes

Part 7.2 concentrates on establishing abstract services, while Part 7.4 deals with abstracting data objects, referred to as Logical Nodes. Moreover, Part 7.3 details the definition of common objects.

After defining these abstract data and services, the next phase involves translating these abstract services into specific protocols. Part 8 delves into Specific Communication Service Mapping (SCSM), encompassing:

IEC 61850-8-1: Mappings to MMS (ISO/IEC 9506 – Part 1 and 2) and to ISO/IEC 8802-3

Furthermore, Part 9 further defines Specific Communication Service Mapping to:

IEC 61850-9-1: Sampled Values over Serial Unidirectional Multidrop Point-to-Point Link and Bi-directional multipoint onto an Ethernet data frame

IEC 61850-9-2: Sampled Values over ISO/IEC 8802-3 (Process Bus)

The formal connection between the substation automation system and the switchyard is defined using an XML-based Substation Configuration Language (SCL) outlined in Part 6.

This structured framework within the IEC 61850 standard allows for the abstract depiction of communication services and data objects. Eventually, these representations are mapped onto specific protocols for practical implementation within electrical substations.

IEC 61850 Information Model

The information model in IEC 61850 covers several elements: physical devices, logical devices, logical nodes, and data objects. For a visual representation of this structure, you can refer to the figure below.

In the framework of IEC 61850, the structure involves physical devices containing separate functional modules, each represented as a logical device. These logical devices offer various operations categorized as logical nodes. The IEC 61850-7-4 standard outlines 159 distinct classes of logical nodes, providing a wide array of functionalities.

Within these logical nodes, there exist data objects specifically representing application functionalities. These data objects consist of variables categorized into Common Data Classes, detailed in the IEC 61850-7-3 standard, which defines 40 unique CDCs.

Each data object comprises elements known as data attributes. These attributes correspond to 12 functional constraints, and the values assigned to these attributes are defined by Common Data Attributes (CDA).

Abstract Communication Service Interface (ACSI)

The abstract data and object models defined in IEC 61850 establish a standardized framework for describing power system devices. This uniform approach enables all Intelligent Electronic Devices (IEDs) to present data using identical structures directly linked to their function within the power system.

The Abstract Communication Service Interface (ACSI) governs communication between a client and a remote server, enabling several crucial functionalities:

1. Real-time data access and retrieval

2. Device control

3. Event reporting and logging

4. Setting group control

5. Self-description of devices through a device data dictionary

6. Data typing and discovery of data types

7. File transfer

This wide range of capabilities provided by ACSI ensures effective communication and interaction between devices, supporting various critical tasks required for power system operation and management.

Within IEC 61850, ACSI defines a suite of services and their corresponding responses. This standardization ensures consistent behavior across all Intelligent Electronic Devices (IEDs) from a network behavior perspective. Notably, IEC 61850-8-1 establishes mappings of the abstract objects and services to the Manufacturing Message Specification (MMS) protocol of ISO 9506.

The mapping process involves associating specific MMS services to implement the various services outlined within ACSI. This service mapping enables the translation of IEC 61850's object models to specific MMS objects. For example, during the mapping process, the logical device object in IEC 61850 corresponds to a specific representation in an MMS domain. This alignment facilitates interoperability and communication between devices following different protocols, ensuring smooth information exchange within the network.